Neurotransmitter

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Structure of a typical chemical synapse
Postsynaptic density Voltage- gated Ca++ channel Synaptic vesicle Neurotransmitter transporter Receptor Neurotransmitter Axon terminal Synaptic cleft Dendrite

A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a synapse. The cell receiving the signal, any main body part or target cell, may be another neuron, but could also be a gland or muscle cell.

Neurotransmitters are released from synaptic vesicles into the synaptic cleft where they are able to interact with neurotransmitter receptors on the target cell. The neurotransmitter's effect on the target cell is determined by the receptor it binds. Many neurotransmitters are synthesized from simple and plentiful precursors such as amino acids, which are readily available and often require a small number of biosynthetic steps for conversion.

Neurotransmitters are essential to the function of complex neural systems. The exact number of unique neurotransmitters in humans is unknown, but more than 100 have been identified. Common neurotransmitters include glutamate, GABA, acetylcholine, glycine and norepinephrine.

Mechanism and cycle

See also: Neurotransmission

Synthesis

Neurotransmitters are generally synthesized in neurons and are made up of, or derived from, precursor molecules that are found abundantly in the cell. Classes of neurotransmitters include amino acids, monoamines, and peptides. Monoamines are synthesized by altering a single amino acid. For example, the precursor of serotonin is the amino acid tryptophan. Peptide transmitters, or neuropeptides, are protein transmitters that often are released together with other transmitters to have a modulatory effect. Purine neurotransmitters, like ATP, are derived from nucleic acids. Other neurotransmitters are made up of metabolic products like nitric oxide and carbon monoxide.

	Examples
Amino Acid	glycine, glutamate
Monoamines	serotonin, epinephrine, dopamine
Peptides	substance P, opioids
Purines	ATP, GTP
Other	nitric oxide, carbon monoxide

Synaptic vesicles containing neurotransmitters

Storage

Neurotransmitters are generally stored in synaptic vesicles, clustered close to the cell membrane at the axon terminal of the presynaptic neuron. However, some neurotransmitters, like the metabolic gases carbon monoxide and nitric oxide, are synthesized and released immediately following an action potential without ever being stored in vesicles.

Release

Generally, a neurotransmitter is released at the presynaptic terminal in response to an electrical signal called an action potential in the presynaptic neuron. However, low level 'baseline' release also occurs without electrical stimulation. Neurotransmitters are released into and diffuse across the synaptic cleft, where they bind to specific receptors on the membrane of the postsynaptic neuron.

Receptor interaction

After being released into the synaptic cleft, neurotransmitters diffuse across the synapse where they are able to interact with receptors on the target cell. The effect of the neurotransmitter is dependent on the identity of the target cell's receptors present at the synapse. Depending on the receptor, binding of neurotransmitters may cause excitation, inhibition, or modulation of the postsynaptic neuron. See below for more information.

Elimination

Acetylcholine is cleaved in the synaptic cleft into acetic acid and choline

In order to avoid continuous activation of receptors on the post-synaptic or target cell, neurotransmitters must be removed from the synaptic cleft. Neurotransmitters are removed through one of three mechanisms:

1. Diffusion – neurotransmitters drift out of the synaptic cleft, where they are absorbed by glial cells. These glial cells, usually astrocytes, absorb the excess neurotransmitters. In the glial cell, neurotransmitters are broken down by enzymes or pumped back into
2. Enzyme degradation – proteins called enzymes break the neurotransmitters down.
3. Reuptake – neurotransmitters are reabsorbed into the pre-synaptic neuron. Transporters, or membrane transport proteins, pump neurotransmitters from the synaptic cleft back into axon terminals (the presynaptic neuron) where they are stored for reuse.

For example, acetylcholine is eliminated by having its acetyl group cleaved by the enzyme acetylcholinesterase; the remaining choline is then taken in and recycled by the pre-synaptic neuron to synthesize more acetylcholine. Other neurotransmitters are able to diffuse away from their targeted synaptic junctions and are eliminated from the body via the kidneys, or destroyed in the liver. Each neurotransmitter has very specific degradation pathways at regulatory points, which may be targeted by the body's regulatory system or medication. Cocaine blocks a dopamine transporter responsible for the reuptake of dopamine. Without the transporter, dopamine diffuses much more slowly from the synaptic cleft and continues to activate the dopamine receptors on the target cell.

Discovery

See also: History of catecholamine research

Until the early 20th century, scientists assumed that the majority of synaptic communication in the brain was electrical. However, through histological examinations by Ramón y Cajal, a 20 to 40 nm gap between neurons, known today as the synaptic cleft, was discovered. The presence of such a gap suggested communication via chemical messengers traversing the synaptic cleft, and in 1921 German pharmacologist Otto Loewi confirmed that neurons can communicate by releasing chemicals. Through a series of experiments involving the vagus nerves of frogs, Loewi was able to manually slow the heart rate of frogs by controlling the amount of saline solution present around the vagus nerve. Upon completion of this experiment, Loewi asserted that sympathetic regulation of cardiac function can be mediated through changes in chemical concentrations. Furthermore, Otto Loewi is credited with discovering acetylcholine (ACh) – the first known neurotransmitter.

Identification

There are four main criteria for identifying neurotransmitters:

1. The chemical must be synthesized in the neuron or otherwise be present in it.
2. When the neuron is active, the chemical must be released and produce a response in some targets.
3. The same response must be obtained when the chemical is experimentally placed on the target.
4. A mechanism must exist for removing the chemical from its site of activation after its work is done.

However, given advances in pharmacology, genetics, and chemical neuroanatomy, the term "neurotransmitter" can be applied to chemicals that:

• Carry messages between neurons via influence on the postsynaptic membrane.
• Have little or no effect on membrane voltage, but have a common carrying function such as changing the structure of the synapse.
• Communicate by sending reverse-direction messages that affect the release or reuptake of transmitters.

The anatomical localization of neurotransmitters is typically determined using immunocytochemical techniques, which identify the location of either the transmitter substances themselves or of the enzymes that are involved in their synthesis. Immunocytochemical techniques have also revealed that many transmitters, particularly the neuropeptides, are co-localized, that is, a neuron may release more than one transmitter from its synaptic terminal. Various techniques and experiments such as staining, stimulating, and collecting can be used to identify neurotransmitters throughout the central nervous system.

Actions

Neurons form elaborate networks through which nerve impulses – action potentials – travel. Each neuron has as many as 15,000 connections with neighboring neurons.

Neurons do not touch each other (except in the case of an electrical synapse through a gap junction); instead, neurons interact at contact points called synapses: a junction within two nerve cells, consisting of a miniature gap within which impulses are carried by a neurotransmitter. A neuron transports its information by way of a nerve impulse called an action potential. When an action potential arrives at the synapse's presynaptic terminal button, it may stimulate the release of neurotransmitters. These neurotransmitters are released into the synaptic cleft to bind onto the receptors of the postsynaptic membrane and influence another cell, either in an inhibitory or excitatory way. The next neuron may be connected to many more neurons, and if the total of excitatory influences minus inhibitory influences is great enough, it will also "fire". That is to say, it will create a new action potential at its axon hillock, releasing neurotransmitters and passing on the information to yet another neighboring neuron.

Modulation

A neurotransmitter may have an excitatory, inhibitory or modulatory effect on the target cell. The effect is determined by the receptors the neurotransmitter interacts with at the post-synaptic membrane. Neurotransmitter influences trans-membrane ion flow either to increase (excitatory) or to decrease (inhibitory) the probability that the cell with which it comes in contact will produce an action potential. Synapses containing receptors with excitatory effects are called Type I synapses, while Type II synapses contain receptors with inhibitory effects. Thus, despite the wide variety of synapses, they all convey messages of only these two types. The two types are different appearance and are primarily located on different parts of the neurons under its influence. Receptors with modulatory effects are spread throughout all synaptic membranes and binding of neurotransmitters sets in motion signaling cascades that help the cell regulate its function. Binding of neurotransmitters to receptors with modulatory effects can have many results. For example, it may result in an increase or decrease in sensitivity to future stimulus by recruiting more or less receptors to the synaptic membrane.

Type I (excitatory) synapses are typically located on the shafts or the spines of dendrites, whereas type II (inhibitory) synapses are typically located on a cell body. In addition, Type I synapses have round synaptic vesicles, whereas the vesicles of type II synapses are flattened. The material on the presynaptic and post-synaptic membranes is denser in a Type I synapse than it is in a type II, and the type I synaptic cleft is wider. Finally, the active zone on a Type I synapse is larger than that on a Type II synapse.

The different locations of type I and type II synapses divide a neuron into two zones: an excitatory dendritic tree and an inhibitory cell body. From an inhibitory perspective, excitation comes in over the dendrites and spreads to the axon hillock to trigger an action potential. If the message is to be stopped, it is best stopped by applying inhibition on the cell body, close to the axon hillock where the action potential originates. Another way to conceptualize excitatory–inhibitory interaction is to picture excitation overcoming inhibition. If the cell body is normally in an inhibited state, the only way to generate an action potential at the axon hillock is to reduce the cell body's inhibition. In this "open the gates" strategy, the excitatory message is like a racehorse ready to run down the track, but first, the inhibitory starting gate must be removed.

Neurotransmitter actions

As explained above, the only direct action of a neurotransmitter is to activate a receptor. Therefore, the effects of a neurotransmitter system depend on the connections of the neurons that use the transmitter, and the chemical properties of the receptors.

• Glutamate is used at the great majority of fast excitatory synapses in the brain and spinal cord. It is also used at most synapses that are "modifiable", i.e. capable of increasing or decreasing in strength. Modifiable synapses are thought to be the main memory-storage elements in the brain. Excessive glutamate release can overstimulate the brain and lead to excitotoxicity causing cell death resulting in seizures or strokes. Excitotoxicity has been implicated in certain chronic diseases including ischemic stroke, epilepsy, amyotrophic lateral sclerosis, Alzheimer's disease, Huntington disease, and Parkinson's disease.
• GABA is used at the great majority of fast inhibitory synapses in virtually every part of the brain. Many sedative/tranquilizing drugs act by enhancing the effects of GABA. Correspondingly, glycine is the inhibitory transmitter in the spinal cord.
• Acetylcholine was the first neurotransmitter discovered in the peripheral and central nervous systems. It activates skeletal muscles in the somatic nervous system and may either excite or inhibit internal organs in the autonomic system. It is distinguished as the transmitter at the neuromuscular junction connecting motor nerves to muscles. The paralytic arrow-poison curare acts by blocking transmission at these synapses. Acetylcholine also operates in many regions of the brain, but using different types of receptors, including nicotinic and muscarinic receptors.
• Dopamine has a number of important functions in the brain; this includes regulation of motor behavior, pleasures related to motivation and also emotional arousal. It plays a critical role in the reward system; Parkinson's disease has been linked to low levels of dopamine and schizophrenia has been linked to high levels of dopamine.
• Serotonin is a monoamine neurotransmitter. Most is produced by and found in the intestine (approximately 90%), and the remainder in central nervous system neurons. It functions to regulate appetite, sleep, memory and learning, temperature, mood, behaviour, muscle contraction, and function of the cardiovascular system and endocrine system. It is speculated to have a role in depression, as some depressed patients are seen to have lower concentrations of metabolites of serotonin in their cerebrospinal fluid and brain tissue.
• Norepinephrine which is synthesized in the central nervous system and sympathetic nerves, modulates the responses of the autonomic nervous system, the sleep patterns, focus and alertness. It is synthesized from tyrosine.
• Epinephrine which is also synthesized from tyrosine is released in the adrenal glands and the brainstem. It plays a role in sleep, with one's ability to become and stay alert, and the fight-or-flight response.

Types

There are many different ways to classify neurotransmitters. Dividing them into amino acids, peptides, and monoamines is sufficient for some classification purposes.

Major neurotransmitters:

• Amino acids: glutamate, aspartate, D-serine, gamma-Aminobutyric acid (GABA), glycine
• Gasotransmitters: nitric oxide (NO), carbon monoxide (CO), hydrogen sulfide (H₂S)
• Monoamines: dopamine (DA), norepinephrine (noradrenaline; NE, NA), epinephrine (adrenaline), histamine, serotonin (SER, 5-HT)
  • Catecholamines: dopamine, norepinephrine (noradrenaline), epinephrine (adrenaline)
• Trace amines: phenethylamine, N-methylphenethylamine, tyramine, 3-iodothyronamine, octopamine, tryptamine, etc.
• Peptides: oxytocin, somatostatin, substance P, cocaine and amphetamine regulated transcript, opioid peptides
• Purines: adenosine triphosphate (ATP), adenosine
• Others: acetylcholine (ACh), anandamide, etc.

In addition, over 100 neuroactive peptides have been found, and new ones are discovered regularly.^[25][26] Many of these are co-released along with a small-molecule transmitter. Nevertheless, in some cases, a peptide is the primary transmitter at a synapse. Beta-Endorphin is a relatively well-known example of a peptide neurotransmitter because it engages in highly specific interactions with opioid receptors in the central nervous system.

Single ions (such as synaptically released zinc) are also considered neurotransmitters by some, as well as some gaseous molecules such as nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H₂S). The gases are produced in the neural cytoplasm and are immediately diffused through the cell membrane into the extracellular fluid and into nearby cells to stimulate production of second messengers. Soluble gas neurotransmitters are difficult to study, as they act rapidly and are immediately broken down, existing for only a few seconds.

The most prevalent transmitter is glutamate, which is excitatory at well over 90% of the synapses in the human brain. The next most prevalent is gamma-Aminobutyric Acid, or GABA, which is inhibitory at more than 90% of the synapses that do not use glutamate. Although other transmitters are used in fewer synapses, they may be very important functionally: the great majority of psychoactive drugs exert their effects by altering the actions of some neurotransmitter systems, often acting through transmitters other than glutamate or GABA. Addictive drugs such as cocaine and amphetamines exert their effects primarily on the dopamine system. The addictive opiate drugs exert their effects primarily as functional analogs of opioid peptides, which, in turn, regulate dopamine levels.

List of neurotransmitters, peptides, and gaseous signaling molecules

Neurotransmitters Category Name Abbreviation Metabotropic Ionotropic Small: Amino acids (Arg) Arginine Arg, R α2-Adrenergic receptors, imidazoline receptors NMDA receptors Small: Amino acids Aspartate Asp, D – NMDA receptors Small: Amino acids Glutamate Glu, E Metabotropic glutamate receptors NMDA receptors, kainate receptors, AMPARs Small: Amino acids Gamma-aminobutyric acid GABA GABAB receptors GABAA receptors, GABAA-ρ receptors Small: Amino acids Glycine Gly, G – NMDA receptors, glycine receptors Small: Amino acids D-serine Ser, S – NMDA receptors Small: Acetylcholine Acetylcholine ACh Muscarinic acetylcholine receptors Nicotinic acetylcholine receptors Small: Monoamine (Phe/Tyr) Dopamine DA Dopamine receptors, trace amine-associated receptor 1 – Small: Monoamine (Phe/Tyr) Norepinephrine (noradrenaline) NE, NAd Adrenergic receptors – Small: Monoamine (Phe/Tyr) Epinephrine (adrenaline) Epi, Ad Adrenergic receptors – Small: Monoamine (Trp) Serotonin (5-hydroxytryptamine) 5-HT Serotonin receptors (all except 5-HT3) 5-HT3 Small: Monoamine (His) Histamine H Histamine receptors – Small: Trace amine (Phe) Phenethylamine PEA Human trace amine-associated receptors: hTAAR1, hTAAR2 – Small: Trace amine (Phe) N-methylphenethylamine NMPEA hTAAR1 – Small: Trace amine (Phe/Tyr) Tyramine TYR hTAAR1, hTAAR2 – Small: Trace amine (Phe/Tyr) octopamine Oct hTAAR1 – Small: Trace amine (Phe/Tyr) Synephrine Syn hTAAR1 – Small: Trace amine (Trp) Tryptamine hTAAR1, various serotonin receptors – Small: Trace amine (Trp) N-methyltryptamine NMT hTAAR1, various serotonin receptors – Lipid Anandamide AEA Cannabinoid receptors – Lipid 2-Arachidonoylglycerol 2-AG Cannabinoid receptors – Lipid 2-Arachidonyl glyceryl ether 2-AGE Cannabinoid receptors – Lipid N-Arachidonoyl dopamine NADA Cannabinoid receptors TRPV1 Lipid Virodhamine Cannabinoid receptors – Small: Purine Adenosine Ado Adenosine receptors – Small: Purine Adenosine triphosphate ATP P2Y receptors P2X receptors Small: Purine Nicotinamide adenine dinucleotide β-NAD P2Y receptors P2X receptors Neuropeptides Category Name Abbreviation Metabotropic Ionotropic Bombesin-like peptides Bombesin BBR1-2-3 – Bombesin-like peptide Gastrin releasing peptide GRP – – Bombesin-like peptide Neuromedin B NMB Neuromedin B receptor – Bradykinins Bradykinin B1, B2 – Calcitonin/CGRP family Calcitonin Calcitonin receptor – Calcitonin/CGRP family Calcitonin gene-related peptide CGRP CALCRL – Corticotropin-releasing factors Corticotropin-releasing hormone CRH CRHR1 – Corticotropin-releasing factors Urocortin CRHR1 – Galanins Galanin GALR1, GALR2, GALR3 – Galanins Galanin-like peptide GALR1, GALR2, GALR3 – Gastrins Gastrin Cholecystokinin B receptor – Gastrins Cholecystokinin CCK Cholecystokinin receptors – Granins Chromogranin A ChgA – – Melanocortins Adrenocorticotropic hormone ACTH ACTH receptor – Melanocortins Proopiomelanocortin POMC Melanocortin 4 receptor – Melanocortins Melanocyte-stimulating hormones MSH Melanocortin receptors – Neurohypophyseals Vasopressin AVP Vasopressin receptors – Neurohypophyseals Oxytocin OT Oxytocin receptor – Neurohypophyseals Neurophysin I – – Neurohypophyseals Neurophysin II – – Neurohypophyseals Copeptin – – Neuromedins Neuromedin U NmU NmUR1, NmUR2 – Neuropeptide B/W Neuropeptide B NPB NPBW1, NPBW2 – Neuropeptide B/W Neuropeptide S NPS Neuropeptide S receptors – Neuropeptide Y Neuropeptide Y NY Neuropeptide Y receptors – Neuropeptide Y Pancreatic polypeptide PP – – Neuropeptide Y Peptide YY PYY – – Opioids Enkephalins δ-Opioid receptor – Opioids Dynorphins κ-Opioid receptor – Opioids Neoendorphins κ-Opioid receptor – Opioids Endorphins μ-Opioid receptors – Opioids Endomorphins μ-Opioid receptors – Opioids Morphine μ-Opioid receptors – Opioids Nociceptin/orphanin FQ N/OFQ Nociceptin receptors – Orexins Orexin A OX-A Orexin receptors – Orexins Orexin B OX-B Orexin receptors – Parathyroid hormone family Parathyroid hormone-related protein PTHrP – – RFamides Kisspeptin KiSS GPR54 – RFamides Neuropeptide FF NPFF NPFF1, NPFF2 – RFamides Prolactin-releasing peptide PrRP PrRPR – RFamides Pyroglutamylated RFamide peptide QRFP GPR103 – Secretins Secretin Secretin receptor – Secretins Motilin Motilin receptor – Secretins Glucagon Glucagon receptor – Secretins Glucagon-like peptide-1 GLP-1 Glucagon-like peptide 1 receptor – Secretins Glucagon-like peptide-2 GLP-2 Glucagon-like peptide 2 receptor – Secretins Vasoactive intestinal peptide VIP Vasoactive intestinal peptide receptors – Secretins Growth hormone–releasing hormone GHRH Growth hormone–releasing hormone receptor – Secretins Pituitary adenylate cyclase-activating peptide PACAP ADCYAP1R1 – Somatostatins Somatostatin Somatostatin receptors – Tachykinins Neurokinin A – – Tachykinins Neurokinin B – – Tachykinins Substance P – – Tachykinins Neuropeptide K – – Other Agouti-related peptide AgRP Melanocortin receptor – Other N-Acetylaspartylglutamate NAAG Metabotropic glutamate receptor 3 (mGluR3) – Other Cocaine- and amphetamine-regulated transcript CART Unknown Gi/Go-coupled receptor[31] – Other Gonadotropin-releasing hormone GnRH GnRHR – Other Thyrotropin-releasing hormone TRH TRHR – Other Melanin-concentrating hormone MCH MCHR 1,2 – Gasotransmitters Category Name Abbreviation Metabotropic Ionotropic Gaseous signaling molecule Nitric oxide NO Soluble guanylyl cyclase – Gaseous signaling molecule Carbon monoxide CO – Heme bound to potassium channels Gaseous signaling molecule Hydrogen sulfide H2S – –

Brain neurotransmitter systems

Neurons expressing certain types of neurotransmitters sometimes form distinct systems, where activation of the system affects large volumes of the brain, called volume transmission. Major neurotransmitter systems include the noradrenaline (norepinephrine) system, the dopamine system, the serotonin system, and the cholinergic system, among others. Trace amines have a modulatory effect on neurotransmission in monoamine pathways (i.e., dopamine, norepinephrine, and serotonin pathways) throughout the brain via signaling through trace amine-associated receptor 1. A brief comparison of these systems follows:

Neurotransmitter systems in the brain

System	Pathway origin and projections	Regulated cognitive processes and behaviors
Noradrenaline system [34][35][36][37][38][39]	Noradrenergic pathways: Locus coeruleus (LC) projections LC → Amygdala and Hippocampus LC → Brain stem and Spinal cord LC → Cerebellum LC → Cerebral cortex LC → Hypothalamus LC → Tectum LC → Thalamus LC → Ventral tegmental area Lateral tegmental field (LTF) projections LTF → Brain stem and Spinal cord LTF → Olfactory bulb	anxiety arousal (wakefulness) circadian rhythm cognitive control and working memory (co-regulated by dopamine) feeding and energy homeostasis medullary control of respiration negative emotional memory nociception (perception of pain) reward (minor role)
Dopamine system [36][37][38][40][41][42]	Dopaminergic pathways: Ventral tegmental area (VTA) projections VTA → Amygdala VTA → Cingulate cortex VTA → Hippocampus VTA → Ventral striatum (Mesolimbic pathway) VTA → Olfactory bulb VTA → Prefrontal cortex (Mesocortical pathway) Nigrostriatal pathway Substantia nigra pars compacta → Dorsal striatum Tuberoinfundibular pathway Arcuate nucleus → Median eminence Hypothalamospinal projection Hypothalamus → Spinal cord Incertohypothalamic pathway Zona incerta → Hypothalamus	arousal (wakefulness) aversion cognitive control and working memory (co-regulated by norepinephrine) emotion and mood motivation (motivational salience) motor function and control positive reinforcement reward (primary mediator) sexual arousal, orgasm, and refractory period (via neuroendocrine regulation)
Histamine system [37][38][43]	Histaminergic pathways: Tuberomammillary nucleus (TMN) projections TMN → Cerebral cortex TMN → Hippocampus TMN → Neostriatum TMN → Nucleus accumbens TMN → Amygdala TMN → Hypothalamus	arousal (wakefulness) feeding and energy homeostasis learning memory
Serotonin system [34][36][37][38][44][45][46]	Serotonergic pathways: Caudal nuclei (CN): Raphe magnus, raphe pallidus, and raphe obscurus Caudal projections CN → Cerebral cortex CN → Thalamus CN → Caudate-putamen and nucleus accumbens CN → Substantia nigra and ventral tegmental area CN → Cerebellum CN → Spinal cord Rostral nuclei (RN): Nucleus linearis, dorsal raphe, medial raphe, and raphe pontis Rostral projections RN → Amygdala RN → Cingulate cortex RN → Hippocampus RN → Hypothalamus RN → Neocortex RN → Septum RN → Thalamus RN → Ventral tegmental area	arousal (wakefulness) body temperature regulation emotion and mood, potentially including aggression feeding and energy homeostasis reward (minor role) sensory perception
Acetylcholine system [34][36][37][38][47]	Cholinergic pathways: Forebrain cholinergic nuclei (FCN): Nucleus basalis of Meynert, medial septal nucleus, and diagonal band Forebrain nuclei projections FCN → Hippocampus FCN → Cerebral cortex FCN → Limbic cortex and sensory cortex Striatal tonically active cholinergic neurons (TAN) TAN → Medium spiny neuron Brainstem cholinergic nuclei (BCN): Pedunculopontine nucleus, laterodorsal tegmentum, medial habenula, and parabigeminal nucleus Brainstem nuclei projections BCN → Ventral tegmental area BCN → Thalamus	arousal (wakefulness) emotion and mood learning motor function motivation (motivational salience) short-term memory reward (minor role)
Adrenaline system [48][49]	Adrenergic pathways: Rostral ventrolateral medulla (RVLM) projections RVLM → Spinal cord RVLM → Brain stem RVLM → Hypothalamus	medullary control of respiration sympathetic nervous system feeding and energy homeostasis arousal stress

Further information: Neural pathway, Neuromodulation, and List of regions in the human brain

Drug effects

Understanding the effects of drugs on neurotransmitters comprises a significant portion of research initiatives in the field of neuroscience. Most neuroscientists involved in this field of research believe that such efforts may further advance our understanding of the circuits responsible for various neurological diseases and disorders, as well as ways to effectively treat and someday possibly prevent or cure such illnesses.

Drugs can influence behavior by altering neurotransmitter activity. For instance, drugs can decrease the rate of synthesis of neurotransmitters by affecting the synthetic enzyme(s) for that neurotransmitter. When neurotransmitter syntheses are blocked, the amount of neurotransmitters available for release becomes substantially lower, resulting in a decrease in neurotransmitter activity. Some drugs block or stimulate the release of specific neurotransmitters. Alternatively, drugs can prevent neurotransmitter storage in synaptic vesicles by causing the synaptic vesicle membranes to leak. Drugs that prevent a neurotransmitter from binding to its receptor are called receptor antagonists. For example, drugs used to treat patients with schizophrenia such as haloperidol, chlorpromazine, and clozapine are antagonists at receptors in the brain for dopamine. Other drugs act by binding to a receptor and mimicking the normal neurotransmitter. Such drugs are called receptor agonists. An example of a receptor agonist is morphine, an opiate that mimics effects of the endogenous neurotransmitter β-endorphin to relieve pain. Other drugs interfere with the deactivation of a neurotransmitter after it has been released, thereby prolonging the action of a neurotransmitter. This can be accomplished by blocking re-uptake or inhibiting degradative enzymes. Lastly, drugs can also prevent an action potential from occurring, blocking neuronal activity throughout the central and peripheral nervous system. Drugs such as tetrodotoxin that block neural activity are typically lethal.

Drugs targeting the neurotransmitter of major systems affect the whole system, which can explain the complexity of action of some drugs. Cocaine, for example, blocks the re-uptake of dopamine back into the presynaptic neuron, leaving the neurotransmitter molecules in the synaptic gap for an extended period of time. Since the dopamine remains in the synapse longer, the neurotransmitter continues to bind to the receptors on the postsynaptic neuron, eliciting a pleasurable emotional response. Physical addiction to cocaine may result from prolonged exposure to excess dopamine in the synapses, which leads to the downregulation of some post-synaptic receptors. After the effects of the drug wear off, an individual can become depressed due to decreased probability of the neurotransmitter binding to a receptor. Fluoxetine is a selective serotonin re-uptake inhibitor (SSRI), which blocks re-uptake of serotonin by the presynaptic cell which increases the amount of serotonin present at the synapse and furthermore allows it to remain there longer, providing potential for the effect of naturally released serotonin.^[51] AMPT prevents the conversion of tyrosine to L-DOPA, the precursor to dopamine; reserpine prevents dopamine storage within vesicles; and deprenyl inhibits monoamine oxidase (MAO)-B and thus increases dopamine levels.

Agonists

Main article: Agonist

An agonist is a chemical capable of binding to a receptor, such as a neurotransmitter receptor, and initiating the same reaction typically produced by the binding of the endogenous substance. An agonist of a neurotransmitter will thus initiate the same receptor response as the transmitter. In neurons, an agonist drug may activate neurotransmitter receptors either directly or indirectly. Direct-binding agonists can be further characterized as full agonists, partial agonists, inverse agonists.

Direct agonists act similar to a neurotransmitter by binding directly to its associated receptor site(s), which may be located on the presynaptic neuron or postsynaptic neuron, or both. Typically, neurotransmitter receptors are located on the postsynaptic neuron, while neurotransmitter autoreceptors are located on the presynaptic neuron, as is the case for monoamine neurotransmitters; in some cases, a neurotransmitter utilizes retrograde neurotransmission, a type of feedback signaling in neurons where the neurotransmitter is released postsynaptically and binds to target receptors located on the presynaptic neuron. Nicotine, a compound found in tobacco, is a direct agonist of most nicotinic acetylcholine receptors, mainly located in cholinergic neurons. Opiates, such as morphine, heroin, hydrocodone, oxycodone, codeine, and methadone, are μ-opioid receptor agonists; this action mediates their euphoriant and pain relieving properties.

Indirect agonists increase the binding of neurotransmitters at their target receptors by stimulating the release or preventing the reuptake of neurotransmitters. Some indirect agonists trigger neurotransmitter release and prevent neurotransmitter reuptake. Amphetamine, for example, is an indirect agonist of postsynaptic dopamine, norepinephrine, and serotonin receptors in each their respective neurons; it produces both neurotransmitter release into the presynaptic neuron and subsequently the synaptic cleft and prevents their reuptake from the synaptic cleft by activating TAAR1, a presynaptic G protein-coupled receptor, and binding to a site on VMAT2, a type of monoamine transporter located on synaptic vesicles within monoamine neurons.

Antagonists

Main article: Receptor antagonist

An antagonist is a chemical that acts within the body to reduce the physiological activity of another chemical substance (as an opiate); especially one that opposes the action on the nervous system of a drug or a substance occurring naturally in the body by combining with and blocking its nervous receptor.

There are two main types of antagonist: direct-acting Antagonist and indirect-acting Antagonists:

1. Direct-acting antagonist- which takes up space present on receptors which are otherwise taken up by neurotransmitters themselves. This results in neurotransmitters being blocked from binding to the receptors. The most common is called Atropine.
2. Indirect-acting antagonist- drugs that inhibit the release/production of neurotransmitters (e.g., Reserpine).

Drug antagonists

An antagonist drug is one that attaches (or binds) to a site called a receptor without activating that receptor to produce a biological response. It is therefore said to have no intrinsic activity. An antagonist may also be called a receptor "blocker" because they block the effect of an agonist at the site. The pharmacological effects of an antagonist, therefore, result in preventing the corresponding receptor site's agonists (e.g., drugs, hormones, neurotransmitters) from binding to and activating it. Antagonists may be "competitive" or "irreversible".

A competitive antagonist competes with an agonist for binding to the receptor. As the concentration of antagonist increases, the binding of the agonist is progressively inhibited, resulting in a decrease in the physiological response. High concentration of an antagonist can completely inhibit the response. This inhibition can be reversed, however, by an increase of the concentration of the agonist, since the agonist and antagonist compete for binding to the receptor. Competitive antagonists, therefore, can be characterized as shifting the dose–response relationship for the agonist to the right. In the presence of a competitive antagonist, it takes an increased concentration of the agonist to produce the same response observed in the absence of the antagonist.

An irreversible antagonist binds so strongly to the receptor as to render the receptor unavailable for binding to the agonist. Irreversible antagonists may even form covalent chemical bonds with the receptor. In either case, if the concentration of the irreversible antagonist is high enough, the number of unbound receptors remaining for agonist binding may be so low that even high concentrations of the agonist do not produce the maximum biological response.^[61]

Precursors

Biosynthetic pathways for catecholamines and trace amines in the human brain L-Phenylalanine L-Tyrosine L-DOPA Epinephrine Phenethylamine p-Tyramine Dopamine Norepinephrine N-Methylphenethylamine N-Methyltyramine p-Octopamine Synephrine 3-Methoxytyramine AADC AADC AADC primary pathway PNMT PNMT PNMT PNMT AAAH AAAH brain CYP2D6 minor pathway COMT DBH DBH In humans, catecholamines and phenethylaminergic trace amines are derived from the amino acid L-phenylalanine.

While intake of neurotransmitter precursors does increase neurotransmitter synthesis, evidence is mixed as to whether neurotransmitter release and postsynaptic receptor firing is increased. Even with increased neurotransmitter release, it is unclear whether this will result in a long-term increase in neurotransmitter signal strength, since the nervous system can adapt to changes such as increased neurotransmitter synthesis and may therefore maintain constant firing. Some neurotransmitters may have a role in depression and there is some evidence to suggest that intake of precursors of these neurotransmitters may be useful in the treatment of mild and moderate depression.

Catecholamine and trace amine precursors

L-DOPA, a precursor of dopamine that crosses the blood–brain barrier, is used in the treatment of Parkinson's disease. For depressed patients where low activity of the neurotransmitter norepinephrine is implicated, there is only little evidence for benefit of neurotransmitter precursor administration. L-phenylalanine and L-tyrosine are both precursors for dopamine, norepinephrine, and epinephrine. These conversions require vitamin B6, vitamin C, and S-adenosylmethionine. A few studies suggest potential antidepressant effects of L-phenylalanine and L-tyrosine, but there is much room for further research in this area.

Serotonin precursors

Administration of L-tryptophan, a precursor for serotonin, is seen to double the production of serotonin in the brain. It is significantly more effective than a placebo in the treatment of mild and moderate depression. This conversion requires vitamin C. 5-hydroxytryptophan (5-HTP), also a precursor for serotonin, is more effective than a placebo.

Diseases and disorders

Diseases and disorders may also affect specific neurotransmitter systems. The following are disorders involved in either an increase, decrease, or imbalance of certain neurotransmitters.

Dopamine:

For example, problems in producing dopamine (mainly in the substantia nigra) can result in Parkinson's disease, a disorder that affects a person's ability to move as they want to, resulting in stiffness, tremors or shaking, and other symptoms. Some studies suggest that having too little or too much dopamine or problems using dopamine in the thinking and feeling regions of the brain may play a role in disorders like schizophrenia or attention deficit hyperactivity disorder (ADHD). Dopamine is also involved in addiction and drug use, as most recreational drugs cause an influx of dopamine in the brain (especially opioid and methamphetamines) that produces a pleasurable feeling, which is why users constantly crave drugs.

Serotonin:

Similarly, after some research suggested that drugs that block the recycling, or reuptake, of serotonin seemed to help some people diagnosed with depression, it was theorized that people with depression might have lower-than-normal serotonin levels. Though widely popularized, this theory was not borne out in subsequent research. Therefore, selective serotonin reuptake inhibitors (SSRIs) are used to increase the amounts of serotonin in synapses.

Glutamate:

Furthermore, problems with producing or using glutamate have been suggestively and tentatively linked to many mental disorders, including autism, obsessive compulsive disorder (OCD), schizophrenia, and depression. Having too much glutamate has been linked to neurological diseases such as Parkinson's disease, multiple sclerosis, Alzheimer's disease, stroke, and ALS (amyotrophic lateral sclerosis).

CAPON Binds Nitric Oxide Synthase, Regulating NMDA Receptor–Mediated Glutamate Neurotransmission

Neurotransmitter imbalance

Generally, there are no scientifically established "norms" for appropriate levels or "balances" of different neurotransmitters. It is in most cases pragmatically impossible to even measure levels of neurotransmitters in a brain or body at any distinct moments in time. Neurotransmitters regulate each other's release, and weak consistent imbalances in this mutual regulation were linked to temperament in healthy people . Strong imbalances or disruptions to neurotransmitter systems have been associated with many diseases and mental disorders. These include Parkinson's, depression, insomnia, Attention Deficit Hyperactivity Disorder (ADHD), anxiety, memory loss, dramatic changes in weight and addictions. Chronic physical or emotional stress can be a contributor to neurotransmitter system changes. Genetics also plays a role in neurotransmitter activities. Apart from recreational use, medications that directly and indirectly interact with one or more transmitter or its receptor are commonly prescribed for psychiatric and psychological issues. Notably, drugs interacting with serotonin and norepinephrine are prescribed to patients with problems such as depression and anxiety—though the notion that there is much solid medical evidence to support such interventions has been widely criticized. Studies shown that dopamine imbalance has an influence on multiple sclerosis and other neurological disorders.

Sociolinguistics

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Sociolinguistics 

Sociolinguistics is the descriptive study of the effect of any and all aspects of society, including cultural norms, expectations, and context, on the way language is used, and society's effect on language. It differs from sociology of language, which focuses on the effect of language on society. Sociolinguistics overlaps considerably with pragmatics and is closely related to linguistic anthropology.

Sociolinguistics' historical interrelation with anthropology can be observed in studies of how language varieties differ between groups separated by social variables (e.g., ethnicity, religion, status, gender, level of education, age, etc.) and/or geographical barriers (a mountain range, a desert, a river, etc.). Such studies also examine how such differences in usage and differences in beliefs about usage produce and reflect social or socioeconomic classes. As the usage of a language varies from place to place, language usage also varies among social classes, and it is these sociolects that sociolinguistics studies.

Sociolinguistics can be studied in various ways such as interviews with speakers of a language, matched-guise tests, and other observations or studies related to dialects and speaking.

Sociolinguistics in history

Beginnings

The social aspects of language were in the modern sense first studied by Indian and Japanese linguists in the 1930s, and also by Louis Gauchat in Switzerland in the early 1900s, but none received much attention in the West until much later. The study of the social motivation of language change, on the other hand, has its foundation in the wave model of the late 19th century. The first attested use of the term sociolinguistics was by Thomas Callan Hodson in the title of his 1939 article "Sociolinguistics in India" published in Man in India.

Western contributions

The study of sociolinguistics in the West was pioneered by linguists such as William Labov in the US and Basil Bernstein in the UK. In the 1960s, William Stewart and Heinz Kloss introduced the basic concepts for the sociolinguistic theory of pluricentric languages, which describes how standard language varieties differ between nations (e.g. American/British/Canadian/Australian English; Austrian/German/Swiss German; Bosnian/Croatian/Montenegrin/Serbian Serbo-Croatian). Dell Hymes is another sociolinguist credited with building the foundation of the study of sociolinguistics and is the founder of the journal Language in Society. His SPEAKING method, an acronym for setting, participants, ends, act sequence, keys, instrumentalities, norms, and genres, is widely recognized as a tool to analyze speech events and measure linguistic competence in a speech event.

Applications

A sociolinguist might study how social attitudes determine what is considered appropriate language use or inappropriate language use in a particular setting. Sociolinguists might also study the grammar, phonetics, vocabulary, and other aspects of various sociolects. Sociolinguists also study language on a national level among large populations to find out how language is used as a social institution. William Labov, a Harvard and Columbia University graduate, is often regarded as one of the founders of the study of sociolinguistics. He focuses on the quantitative analysis of variation and change within languages, making sociolinguistics a scientific discipline.

Studies in the field of sociolinguistics typically take a sample population and interview them, assessing the realization of certain sociolinguistic variables.

A commonly studied source of variation is regional dialects. Dialectology studies variations in language based primarily on geographic distribution and their associated features. Sociolinguists concerned with grammatical and phonological features that correspond to regional areas are often called dialectologists.

Another Method is the Matched-guise test. This technique has the listener listen to a pair of words and evaluate them based on personality and dialect, as some groups have shared views on language attitude.

Sociolinguistic interview

The sociolinguistic interview is the foundational method of collecting data for sociolinguistic studies, allowing the researcher to collect large amounts of speech from speakers of the language or dialect being studied. The interview takes the form of a long, loosely-structured conversation between the researcher and the interview subject; the researcher's primary goal is to elicit the vernacular style of speech—i.e., the register associated with everyday, casual conversation. This goal is complicated by the Observer's Paradox: the researcher is trying to elicit the style of speech that would be used if the interviewer were not present. To this end, a variety of techniques may be used to reduce the subject's attention to the formality and artificiality of the interview setting. For example, the researcher may attempt to elicit narratives of memorable events from the subject's life, such as fights or near-death experiences; the subject's emotional involvement in telling the story is thought to distract their attention from the formality of the context. Some researchers interview multiple subjects together, in order to allow them to converse more casually with each other than they would with the interviewer alone. The researcher may then study the effects of style-shifting on language by comparing a subject's speech style in more vernacular contexts, such as narratives of personal experience or conversation between subjects, with the more careful style produced when the subject is more attentive to the formal interview setting. The correlations of demographic features such as age, gender, and ethnicity with speech behavior may be studied by comparing the speech of different interview subjects. Interviews with native language speakers can be used in an attempt to study dying languages as well. This is depicted in the documentary The Linguists.

Fundamental concepts

While the study of sociolinguistics is very broad, there are a few fundamental concepts on which many sociolinguistic inquiries depend.

Speech community

Main article: Speech community

Speech community is a concept in sociolinguistics that describes a distinct group of people who use language in a unique and mutually accepted way among themselves. This is sometimes referred to as a Sprechbund.

To be considered part of a speech community, one must have a communicative competence. That is, the speaker has the ability to use language in a way that is appropriate in the given situation. It is possible for a speaker to be communicatively competent in more than one language.

Demographic characteristics such as areas or locations have helped to create speech community boundaries in speech community concept. Those characteristics can assist exact descriptions of specific groups' communication patterns. 

Speech communities can be members of a profession with a specialized jargon, distinct social groups like high school students or hip hop fans, or even tight-knit groups like families and friends. Members of speech communities will often develop slang or specialized jargon to serve the group's special purposes and priorities. This is evident in the use of lingo within sports teams.

Community of Practice allows for sociolinguistics to examine the relationship between socialization, competence, and identity. Since identity is a very complex structure, studying language socialization is a means to examine the micro-interactional level of practical activity (everyday activities). The learning of a language is greatly influenced by family, but it is supported by the larger local surroundings, such as school, sports teams, or religion. Speech communities may exist within a larger community of practice.

High prestige and low prestige varieties

Main article: Prestige (sociolinguistics)

Crucial to sociolinguistic analysis is the concept of prestige; certain speech habits are assigned a positive or a negative value, which is then applied to the speaker. This can operate on many levels. It can be realized on the level of the individual sound/phoneme, as Labov discovered in investigating pronunciation of the post-vocalic /r/ in the North-Eastern USA, or on the macro scale of language choice, as realized in the various diglossia that exist throughout the world, where Swiss-German/High German is perhaps most well known. An important implication of the sociolinguistic theory is that speakers 'choose' a variety when making a speech act, whether consciously or subconsciously.

The terms acrolectal (high) and basilectal (low) are also used to distinguish between a more standard dialect and a dialect of less prestige.

It is generally assumed that non-standard language is low-prestige language. However, in certain groups, such as traditional working-class neighborhoods, standard language may be considered undesirable in many contexts. This is because the working class dialect is generally considered a powerful in-group marker. Historically, humans tend to favor those who look and sound like them, and the use of non-standard varieties (even exaggeratedly so) expresses neighborhood pride and group and class solidarity. There will thus be a considerable difference in use of non-standard varieties when going to the pub or having a neighborhood barbecue compared to going to the bank. One is a relaxed setting, likely with familiar people, and the other has a business aspect to it in which one feels the need to be more professional.

Prestige in Pittsburgh

In a book by Barbara Johnstone, she refers to a study by Christina Gagnon in which she analyzed perceptions of the Pittsburghese dialect from Pittsburgh, Pennsylvania compared to standard English. Pittsburghers were asked to read short passages in their own dialect and in standard English without the Pittsburgh accent, then participants listening to the voice recordings were asked to rate the speakers on level of success, education, if they were neighborly, etc. all based on their voice and way of speaking. The results showed that the participants, who were Pittsburghers, preferred the standard English in terms of social status. The stereotype that Pittsburghers are poor, uneducated, and less motivated showed through in the participants' answers. However, when asked to rate the readers in terms of friendliness, trustworthiness, and community involvement, participants rated the Pittsburghese dialect higher, likely because people generally find more trust among those who sound more like them, as did those involved in the study.

Social network

Understanding language in society means that one also has to understand the social networks in which language is embedded. A social network is another way of describing a particular speech community in terms of relations between individual members in a community. A network could be loose or tight depending on how members interact with each other. For instance, an office or factory may be considered a tight community because all members interact with each other. A large course with 100+ students would be a looser community because students may only interact with the instructor and maybe 1–2 other students. A multiplex community is one in which members have multiple relationships with each other. For instance, in some neighborhoods, members may live on the same street, work for the same employer and even intermarry.

The looseness or tightness of a social network may affect speech patterns adopted by a speaker. For instance, Sylvie Dubois and Barbara Horvath found that speakers in one Cajun Louisiana community were more likely to pronounce English "th" [θ] as [t] (or [ð] as [d]) if they participated in a relatively dense social network (i.e. had strong local ties and interacted with many other speakers in the community), and less likely if their networks were looser (i.e. fewer local ties).

A social network may apply to the macro level of a country or a city, but also to the interpersonal level of neighborhoods or a single family. Recently, social networks have been formed by the Internet through online chat rooms, Facebook groups, organizations, and online dating services.

Differences according to class

Further information: Linguistic insecurity

Sociolinguistics as a field distinct from dialectology was pioneered through the study of language variation in urban areas. Whereas dialectology studies the geographic distribution of language variation, sociolinguistics focuses on other sources of variation, among them class. Class and occupation are among the most important linguistic markers found in society. One of the fundamental findings of sociolinguistics, which has been hard to disprove, is that class and language variety are related. Members of the working class tend to speak less of what is deemed standard language, while the lower, middle, and upper middle class will, in turn, speak closer to the standard. However, the upper class, even members of the upper middle class, may often speak 'less' standard than the middle class. This is because not only class but class aspirations, are important. One may speak differently or cover up an undesirable accent to appear to have a different social status and fit in better with either those around them, or how they wish to be perceived.

Class aspiration

Studies, such as those by William Labov in the 1960s, have shown that social aspirations influence speech patterns. This is also true of class aspirations. In the process of wishing to be associated with a certain class (usually the upper class and upper middle class) people who are moving in that direction socio-economically may adjust their speech patterns to sound like them. However, not being native upper-class speakers, they often hypercorrect, which involves overcorrecting their speech to the point of introducing new errors. The same is true for individuals moving down in socio-economic status.

In any contact situation, there is a power dynamic, be it a teacher-student or employee-customer situation. This power dynamic results in a hierarchical differentiation between languages.

Non-standard dialect (associated with lower classes)	Standard dialect (associated with higher classes)
It looks like it ain't gonna rain today.	It looks as if it isn't going to rain today.
You give it to me yesterday.	You gave it to me yesterday.
Y'gotta do it the right way.	You have to do it the right way.

Social language codes

Basil Bernstein, a well-known British socio-linguist, devised in his book, 'Elaborated and restricted codes: their social origins and some consequences,' a method for categorizing language codes according to variable emphases on verbal and extraverbal communication. He claimed that factors like family orientation, social control, verbal feedback, and possibly social class contributed to the development of the two codes: elaborated and restricted.

Restricted code

According to Basil Bernstein, the restricted code exemplified the predominance of extraverbal communication, with an emphasis on interpersonal connection over individual expression. His theory places this code within environments that operate according to established social structures that predetermine the roles of their members, in which the commonality of interests and intents due to a shared local identity creates a predictability of discrete intent and therefore a simplification of verbal utterances. Such environments may include military, religious, and legal atmospheres, criminal and prison subcultures, long-term married relationships and friendships between children. Due to the strong bonds between speakers, explicit verbal communication is often rendered unnecessary and individual expression irrelevant. However, simplification is not a sign of a lack of intelligence or complexity within the code; rather, communication is performed more through extraverbal means (facial expression, touch, etc.) in order to affirm the speakers' bond. Bernstein notes the example of a young man asking a stranger to dance: there is an established manner of asking, and yet communication is performed through physical graces and the exchange of glances. As such, implied meaning plays a greater role in this code than in the elaborated code. Restricted code also operates to unify speakers and foster solidarity.

Elaborated code

Basil Bernstein defined 'elaborated code' according to its emphasis on verbal communication over extraverbal. This code is typical in environments where a variety of social roles are available to the individual, to be chosen based upon disposition and temperament. Most of the time, speakers of elaborated code utilize a broader lexicon and demonstrate less syntactic predictability than speakers of restricted code. The lack of predetermined structure and solidarity requires explicit verbal communication of discrete intent by the individual in order to achieve educational and career success. Bernstein notes, with caution, the association of this code with upper classes (while restricted code is associated with lower classes), where the abundance of available resources allows persons to choose their social roles, warning, however, that studies associating the codes with separate social classes used small samples and were subject to significant variation. He also asserts that elaborated code originates due to differences in social context rather than intellectual advantages; as such, elaborated code differs from restricted code according to the context-based emphasis on individual advancement over assertion of social/community ties.

The codes and child development

Bernstein explains language development according to the two codes in light of their fundamentally different values. For instance, a child exposed solely to restricted code learns extraverbal communication over verbal, and therefore may have a less extensive vocabulary than a child raised with exposure to both codes. While there is no inherent lack of value to restricted code, a child without exposure to elaborated code may encounter difficulties upon entering formal education, in which standard, clear verbal communication and comprehension is necessary for learning and effective interaction both with instructors and other students from differing backgrounds. As such, it may be beneficial for children who have been exposed solely to restricted code to enter pre-school training in elaborated code in order to acquire a manner of speaking that is considered appropriate and widely comprehensible within the education environment.

Additionally, Bernstein notes several studies in language development according to social class. In 1963, the Committee for Higher Education conducted a study on verbal IQ that showed a deterioration in individuals from lower working classes ages 8–11 and 11–15 years in comparison to those from middle classes (having been exposed to both restricted and elaborated codes). Additionally, studies by Bernstein, Venables, and Ravenette, as well as a 1958 Education Council report, show a relative lack of success on verbal tasks in comparison to extraverbal in children from lower working classes (having been exposed solely to restricted code).

Contradictions

The idea of these social language codes from Bernstein contrast with famous linguist Noam Chomsky's ideas. Chomsky, deemed the "father of modern linguistics," argues that there is a universal grammar, meaning that humans are born with an innate capacity for linguistic skills like sentence-building. This theory has been criticized by several scholars of linguistic backgrounds because of the lack of proven evolutionary feasibility and the fact that different languages do not have universal characteristics.

Sociolinguistic variations

Main articles: Variation (linguistics), Dialectology, and Language and gender

The study of language variation is concerned with social constraints determining language in its contextual environment. The variations will determine some of the aspects of language like the sound, grammar, and tone in which people speak, and even non-verbal cues. Code-switching is the term given to the use of different varieties of language depending on the social situation. This is commonly used among the African-American population in the United States. There are several different types of age-based variation one may see within a population as well such as age range, age-graded variation, and indications of linguistic change in progress. The use of slang can be a variation based on age. Younger people are more likely to recognize and use today's slang while older generations may not recognize new slang, but might use slang from when they were younger.

Variation may also be associated with gender. Men and women, on average, tend to use slightly different language styles. These differences tend to be quantitative rather than qualitative. That is, to say that women use a particular speaking style more than men do is akin to saying that men are taller than women (i.e., men are on average taller than women, but some women are taller than some men). Other variations in speech patterns of men and women include differences in pitch, tone, speech fillers, interruptions, use of euphemisms, etc. 

Variations in language can also come from ethnicity, economic status, level of education, etc.

Further information: Complimentary language and gender

Rayleigh sky model

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Rayleigh_sky_model 

The Rayleigh sky model describes the observed polarization pattern of the daytime sky. Within the atmosphere, Rayleigh scattering of light by air molecules, water, dust, and aerosols causes the sky's light to have a defined polarization pattern. The same elastic scattering processes cause the sky to be blue. The polarization is characterized at each wavelength by its degree of polarization, and orientation (the e-vector angle, or scattering angle).

The polarization pattern of the sky is dependent on the celestial position of the Sun. While all scattered light is polarized to some extent, light is highly polarized at a scattering angle of 90° from the light source. In most cases the light source is the Sun, but the moon creates the same pattern as well. The degree of polarization first increases with increasing distance from the Sun, and then decreases away from the Sun. Thus, the maximum degree of polarization occurs in a circular band 90° from the Sun. In this band, degrees of polarization near 80% are typically reached.

Degree of polarization in the Rayleigh sky at sunset or sunrise. The zenith is at the center of the graph.

When the Sun is located at the zenith, the band of maximal polarization wraps around the horizon. Light from the sky is polarized horizontally along the horizon. During twilight at either the vernal or autumnal equinox, the band of maximal polarization is defined by the north-zenith-south plane, or meridian. In particular, the polarization is vertical at the horizon in the north and south, where the meridian meets the horizon. The polarization at twilight at an equinox is represented by the figure to the right. The red band represents the circle in the north-zenith-south plane where the sky is highly polarized. The cardinal directions (N, S, E, W) are shown at 12-o'clock, 9 o'clock, 6 o'clock, and 3 o'clock (counter-clockwise around the celestial sphere, since the observer is looking up at the sky).

Note that because the polarization pattern is dependent on the sun, it changes not only throughout the day but throughout the year. When the sun sets toward the South, in the winter, the North-Zenith-South plane is offset, with "effective" North actually located somewhat toward the West. Thus if the sun sets at an azimuth of 255° (15° South of West) the polarization pattern will be at its maximum along the horizon at an azimuth of 345° (15° West of North) and 165° (15° East of South).

During a single day, the pattern rotates with the changing position of the sun. At twilight, it typically appears about 45 minutes before local sunrise and disappears 45 minutes after local sunset. Once established it is very stable, showing change only in its rotation. It can easily be seen on any given day using polarized sunglasses.

Many animals use the polarization patterns of the sky at twilight and throughout the day as a navigation tool. Because it is determined purely by the position of the sun, it is easily used as a compass for animal orientation. By orienting themselves with respect to the polarization patterns, animals can locate the sun and thus determine the cardinal directions.

Theory

Geometry

The geometry representing the Rayleigh sky

The geometry for the sky polarization can be represented by a celestial triangle based on the sun, zenith, and observed pointing (or the point of scattering). In the model, γ is the angular distance between the observed pointing and the sun, Θ_s is the solar zenith distance (90° – solar altitude), Θ is the angular distance between the observed pointing and the zenith (90° – observed altitude), Φ is the angle between the zenith direction and the solar direction at the observed pointing, and ψ is the angle between the solar direction and the observed pointing at the zenith.

Thus, the spherical triangle is defined not only by the three points located at the sun, zenith, and observed point but by both the three interior angles as well as the three angular distances. In an altitude-azimuth grid the angular distance between the observed pointing and the sun and the angular distance between the observed pointing and the zenith change while the angular distance between the sun and the zenith remains constant at one point in time.

The angular distances between the observed pointing and the sun when the sun is setting to the west (top plot) and between the observed pointing and the zenith (bottom plot)

The figure to the left shows the two changing angular distances as mapped onto an altitude-azimuth grid (with altitude located on the x-axis and azimuth located on the y-axis). The top plot represents the changing angular distance between the observed pointing and the sun, which is opposite to the interior angle located at the zenith (or the scattering angle). When the sun is located at the zenith this distance is greatest along the horizon at every cardinal direction. It then decreases with rising altitude moving closer toward the zenith. At twilight the sun is setting in the west. Hence the distance is greatest when looking directly away from the sun along the horizon in the east, and lowest along the horizon in the west.

The bottom plot in the figure to the left represents the angular distance from the observed pointing to the zenith, which is opposite to the interior angle located at the sun. Unlike the distance between the observed pointing and the sun, this is independent of azimuth, i.e. cardinal direction. It is simply greatest along the horizon at low altitudes and decreases linearly with rising altitude.

The three interior angles of the celestial triangle.

The figure to the right represents the three angular distances. The left one represents the angle at the observed pointing between the zenith direction and the solar direction. This is thus heavily dependent on the changing solar direction as the sun moves across the sky. The middle one represents the angle at the sun between the zenith direction and the pointing. Again this is heavily dependent on the changing pointing. This is symmetrical between the North and South hemispheres. The right one represents the angle at the zenith between the solar direction and the pointing. It thus rotates around the celestial sphere.

Degree of polarization

The Rayleigh sky model predicts the degree of sky polarization as:

${\displaystyle \delta ={\frac {\delta _{max}\sin ^{2}\gamma }{1+\cos ^{2}\gamma }}}$

The polarization along the horizon.

As a simple example one can map the degree of polarization on the horizon. As seen in the figure to the right it is high in the North (0° and 360°) and the South (180°). It then resembles a cosine function and decreases toward the East and West reaching zero at these cardinal directions.

The degree of polarization is easily understood when mapped onto an altitude-azimuth grid as shown below. As the sun sets due West, the maximum degree of polarization can be seen in the North-Zenith-South plane. Along the horizon, at an altitude of 0° it is highest in the North and South, and lowest in the East and West. Then as altitude increases approaching the zenith (or the plane of maximum polarization) the polarization remains high in the North and South and increases until it is again maximum at 90° in the East and West, where it is then at the zenith and within the plane of polarization.

The degree of sky polarization as mapped onto the celestial sphere.

 

The degree of polarization. Red is high (approximately 80%) and black is low (0%).

Click on the adjacent image to view an animation that represents the degree of polarization as shown on the celestial sphere. Black represents areas where the degree of polarization is zero, whereas red represents areas where the degree of polarization is much larger. It is approximately 80%, which is a realistic maximum for the clear Rayleigh sky during day time. The video thus begins when the sun is slightly above the horizon and at an azimuth of 120°. The sky is highly polarized in the effective North-Zenith-South plane. This is slightly offset because the sun's azimuth is not due East. The sun moves across the sky with clear circular polarization patterns surrounding it. When the sun is located at the zenith the polarization is independent of azimuth and decreases with rising altitude (as it approaches the sun). The pattern then continues as the sun approaches the horizon once again for sunset. The video ends with the sun below the horizon.

Polarization angle

The polarization angle. Red is high (approximately 80%) and black is low (0%).

The scattering plane is the plane through the sun, the observer, and the point observed (or the scattering point). The scattering angle, γ, is the angular distance between the sun and the observed point. The equation for the scattering angle is derived from the law of cosines to the spherical triangle (refer to the figure above in the geometry section). It is given by:

${\displaystyle {\cos \gamma =\sin \theta _{s}\sin \theta \cos {(\psi _{s}-\psi )}+\cos \theta _{s}\cos \theta }}$

In the above equation, ψ_s and θ_s are respectively the azimuth and zenith angle of the sun, and ψ and θ are respectively the azimuth and zenith angle of the observed point.

This equation breaks down at the zenith where the angular distance between the observed pointing and the zenith, θ_s is 0. Here the orientation of polarization is defined as the difference in azimuth between the observed pointing and the solar azimuth.

The angle of polarization (or polarization angle) is defined as the relative angle between a vector tangent to the meridian of the observed point, and an angle perpendicular to the scattering plane.

The polarization angles show a regular shift in polarization angle with azimuth. For example, when the sun is setting in the West the polarization angles proceed around the horizon. At this time the degree of polarization is constant in circular bands centered around the sun. Thus the degree of polarization as well as its corresponding angle clearly shifts around the horizon. When the sun is located at the zenith the horizon represents a constant degree of polarization. The corresponding polarization angle still shifts with different directions toward the zenith from different points.

The video to the right represents the polarization angle mapped onto the celestial sphere. It begins with the sun located in a similar fashion. The angle is zero along the line from the sun to the zenith and increases clockwise toward the East as the observed point moves clockwise toward the East. Once the sun rises in the East the angle acts in a similar fashion until the sun begins to move across the sky. As the sun moves across the sky the angle is both zero and high along the line defined by the sun, the zenith, and the anti-sun. It is lower South of this line and higher North of this line. When the sun is at the zenith, the angle is either fully positive or 0. These two values rotate toward the west. The video then repeats a similar fashion when the sun sets in the West.

Q and U Stokes parameters

The q and u input.

The angle of polarization can be unwrapped into the Q and U Stokes parameters. Q and U are defined as the linearly polarized intensities along the position angles 0° and 45° respectively; -Q and -U are along the position angles 90° and −45°.

If the sun is located on the horizon due west, the degree of polarization is then along the North-Zenith-South plane. If the observer faces West and looks at the zenith, the polarization is horizontal with the observer. At this direction Q is 1 and U is 0. If the observer is still facing West but looking North instead then the polarization is vertical with him. Thus Q is −1 and U remains 0. Along the horizon U is always 0. Q is always −1 except in the East and West.

The scattering angle (the angle at the zenith between the solar direction and the observer direction) along the horizon is a circle. From the East through the West it is 180° and from the West through the East it is 90° at twilight. When the sun is setting in the West, the angle is then 180° East through West, and only 90° West through East. The scattering angle at an altitude of 45° is consistent.

The input stokes parameters q and u are then with respect to North but in the altitude-azimuth frame. We can easily unwrap q assuming it is in the +altitude direction. From the basic definition we know that +Q is an angle of 0° and -Q is an angle of 90°. Therefore, Q is calculated from a sine function. Similarly U is calculated from a cosine function. The angle of polarization is always perpendicular to the scattering plane. Therefore, 90° is added to both scattering angles in order to find the polarization angles. From this the Q and U Stokes parameters are determined:

${\displaystyle Q_{in}=\sin(2\theta +90)}$

and

${\displaystyle U_{in}=\cos(2\theta +90)}$

The scattering angle, derived from the law of cosines is with respect to the sun. The polarization angle is the angle with respect to the zenith, or positive altitude. There is a line of symmetry defined by the sun and the zenith. It is drawn from the sun through the zenith to the other side of the celestial sphere where the "anti-sun" would be. This is also the effective East-Zenith-West plane.

The q input. Red is high (approximately 80%) and black is low (0%).

 

The u input. Red is high (approximately 80%) and black is low (0%).

The first image to the right represents the q input mapped onto the celestial sphere. It is symmetric about the line defined by the sun-zenith-anti-sun. At twilight, in the North-Zenith-South plane it is negative because it is vertical with the degree of polarization. It is horizontal, or positive in the East-Zenith-West plane. In other words, it is positive in the ±altitude direction and negative in the ±azimuth direction. As the sun moves across the sky the q input remains high along the sun-zenith-anti-sun line. It remains zero around a circle based on the sun and the zenith. As it passes the zenith it rotates toward the south and repeats the same pattern until sunset.

The second image to the right represents the u input mapped onto the celestial sphere. The u stokes parameter changes signs depending on which quadrant it is in. The four quadrants are defined by the line of symmetry, the effective East-Zenith-West plane and the North-Zenith-South plane. It is not symmetric because it is defined by the angles ±45°. In a sense it makes two circles around the line of symmetry as opposed to only one.

It is easily understood when compared with the q input. Where the q input is halfway between 0° and 90°, the u input is either positive at +45° or negative at −45°. Similarly if the q input is positive at 90° or negative at 0° the u input is halfway between +45° and −45°. This can be seen at the non symmetric circles about the line of symmetry. It then follows the same pattern across the sky as the q input.

Neutral points and lines

Areas where the degree of polarization is zero (the skylight is unpolarized), are known as neutral points. Here the Stokes parameters Q and U also equal zero by definition. The degree of polarization therefore increases with increasing distance from the neutral points.

These conditions are met at a few defined locations on the sky. The Arago point is located above the antisolar point, while the Babinet and Brewster points are located above and below the sun respectively. The zenith distance of the Babinet or Arago point increases with increasing solar zenith distance. These neutral points can depart from their regular positions due to interference from dust and other aerosols.

The skylight polarization switches from negative to positive while passing a neutral point parallel to the solar or antisolar meridian. The lines that separate the regions of positive Q and negative Q are called neutral lines.

Depolarization

The Rayleigh sky causes a clearly defined polarization pattern under many different circumstances. The degree of polarization however, does not always remain consistent and may in fact decrease in different situations. The Rayleigh sky may undergo depolarization due to nearby objects such as clouds and large reflecting surfaces such as the ocean. It may also change depending on the time of the day (for instance at twilight or night).

In the night, the polarization of the moonlit sky is very strongly reduced in the presence of urban light pollution, because scattered urban light is not strongly polarized.

Light pollution is mostly unpolarized, and its addition to moonlight results in a decreased polarization signal.

Extensive research shows that the angle of polarization in a clear sky continues underneath clouds if the air beneath the cloud is directly lit by the sun. The scattering of direct sunlight on those clouds results in the same polarization pattern. In other words, the proportion of the sky that follows the Rayleigh Sky Model is high for both clear skies and cloudy skies. The pattern is also clearly visible in small visible patches of sky. The celestial angle of polarization is unaffected by clouds.

Polarization patterns remain consistent even when the sun is not present in the sky. Twilight patterns are produced during the time period between the beginning of astronomical twilight (when the sun is 18° below the horizon) and sunrise, or sunset and the end of astronomical twilight. The duration of astronomical twilight depends on the length of the path taken by the sun below the horizon. Thus it depends on the time of year as well as the location, but it can last for as long as 1.5 hours.

The polarization pattern caused by twilight remains fairly consistent throughout this time period. This is because the sun is moving below the horizon nearly perpendicular to it, and its azimuth therefore changes very slowly throughout this time period.

At twilight, scattered polarized light originates in the upper atmosphere and then traverses the entire lower atmosphere before reaching the observer. This provides multiple scattering opportunities and causes depolarization. It has been seen that polarization increases by about 10% from the onset of twilight to dawn. Therefore, the pattern remains consistent while the degree changes slightly.

Not only do polarization patterns remain consistent as the sun moves across the sky, but also as the moon moves across the sky at night. The moon creates the same polarization pattern. Thus it is possible to use the polarization patterns as a tool for navigation at night. The only difference is that the degree of polarization is not quite as strong.

Underlying surface properties can affect the degree of polarization of the daytime sky. The degree of polarization has a strong dependence on surface properties. As the surface reflectance or optical thickness increase, the degree of polarization decreases. The Rayleigh sky near the ocean can therefore be highly depolarized.

Lastly, there is a clear wavelength dependence in Rayleigh scattering. It is greatest at short wavelengths, whereas skylight polarization is greatest at middle to long wavelengths. Initially it is greatest in the ultraviolet, but as light moves to the Earth's surface and interacts via multiple-path scattering it becomes high at middle to long wavelengths. The angle of polarization shows no variation with wavelength.

Uses

Navigation

Many animals, typically insects, are sensitive to the polarization of light and can therefore use the polarization patterns of the daytime sky as a tool for navigation. This theory was first proposed by Karl von Frisch when looking at the celestial orientation of honeybees. The natural sky polarization pattern serves as an easily detected compass. From the polarization patterns, these species can orient themselves by determining the exact position of the sun without the use of direct sunlight. Thus under cloudy skies, or even at night, animals can find their way.

Using polarized light as a compass however is no easy task. The animal must be capable of detecting and analyzing polarized light. These species have specialized photoreceptors in their eyes that respond to the orientation and the degree of polarization near the zenith. They can extract information on the intensity and orientation of the degree of polarization. They can then incorporate this visually to orient themselves and recognize different properties of surfaces.

There is clear evidence that animals can even orient themselves when the sun is below the horizon at twilight. How well insects might orient themselves using nocturnal polarization patterns is still a topic of study. So far, it is known that nocturnal crickets have wide-field polarization sensors and should be able to use the night-time polarization patterns to orient themselves. It has also been seen that nocturnally migrating birds become disoriented when the polarization pattern at twilight is unclear.

The best example is the halicitid bee Megalopta genalis, which inhabits the rainforests in Central America and scavenges before sunrise and after sunset. This bee leaves its nest approximately 1 hour before sunrise, forages for up to 30 minutes, and accurately returns to its nest before sunrise. It acts similarly just after sunset.

Thus, this bee is an example of an insect that can perceive polarization patterns throughout astronomical twilight. Not only does this case exemplify the fact that polarization patterns are present during twilight, but it remains as a perfect example that when light conditions are challenging the bee orients itself based on the polarization patterns of the twilight sky.

It has been suggested that Vikings were able to navigate on the open sea in a similar fashion, using the birefringent crystal Iceland spar, which they called "sunstone", to determine the orientation of the sky's polarization. This would allow the navigator to locate the sun, even when it was obscured by cloud cover. An actual example of such a "sunstone" was found on a sunken (Tudor) ship dated 1592, in proximity to the ship's navigational equipment.

Non-polarized objects

Both artificial and natural objects in the sky can be very difficult to detect using only the intensity of light. These objects include clouds, satellites, and aircraft. However, the polarization of these objects due to resonant scattering, emission, reflection, or other phenomena can differ from that of the background illumination. Thus they can be more easily detected by using polarization imaging. There is a wide range of remote sensing applications in which polarization is useful for detecting objects that are otherwise difficult to see.