Reticular formation

From Wikipedia, the free encyclopedia

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

 

Reticular formation
Coronal section of the pons, at its upper part. (Formatio reticularis labeled at left.)
Traverse section of the medulla oblongata at about the middle of the olive. (Formatio reticularis grisea and formatio reticularis alba labeled at left.)
Details
Location	Brainstem
Identifiers
Latin	formatio reticularis
MeSH	D012154
NeuroNames	1223
NeuroLex ID	nlx_143558
TA98	A14.1.00.021 A14.1.05.403 A14.1.06.327
TA2	5367
FMA	77719

The reticular formation is a set of interconnected nuclei that are located throughout the brainstem. It is not anatomically well defined, because it includes neurons located in different parts of the brain. The neurons of the reticular formation make up a complex set of networks in the core of the brainstem that extend from the upper part of the midbrain to the lower part of the medulla oblongata. The reticular formation includes ascending pathways to the cortex in the ascending reticular activating system (ARAS) and descending pathways to the spinal cord via the reticulospinal tracts.

Neurons of the reticular formation, particularly those of the ascending reticular activating system, play a crucial role in maintaining behavioral arousal and consciousness. The overall functions of the reticular formation are modulatory and premotor, involving somatic motor control, cardiovascular control, pain modulation, sleep and consciousness, and habituation. The modulatory functions are primarily found in the rostral sector of the reticular formation and the premotor functions are localized in the neurons in more caudal regions.

The reticular formation is divided into three columns: raphe nuclei (median), gigantocellular reticular nuclei (medial zone), and parvocellular reticular nuclei (lateral zone). The raphe nuclei are the place of synthesis of the neurotransmitter serotonin, which plays an important role in mood regulation. The gigantocellular nuclei are involved in motor coordination. The parvocellular nuclei regulate exhalation.

The reticular formation is essential for governing some of the basic functions of higher organisms and is one of the phylogenetically oldest portions of the brain.

Structure

A cross section of the lower part of the pons showing the pontine reticular formation labeled as #9

The human reticular formation is composed of almost 100 brain nuclei and contains many projections into the forebrain, brainstem, and cerebellum, among other regions. It includes the reticular nuclei, reticulothalamic projection fibers, diffuse thalamocortical projections, ascending cholinergic projections, descending non-cholinergic projections, and descending reticulospinal projections. The reticular formation also contains two major neural subsystems, the ascending reticular activating system and descending reticulospinal tracts, which mediate distinct cognitive and physiological processes. It has been functionally cleaved both sagittally and coronally.

Traditionally the reticular nuclei are divided into three columns:

• In the median column – the raphe nuclei
• In the medial column – gigantocellular nuclei (because of larger size of the cells)
• In the lateral column – parvocellular nuclei (because of smaller size of the cells)

The original functional differentiation was a division of caudal and rostral. This was based upon the observation that the lesioning of the rostral reticular formation induces a hypersomnia in the cat brain. In contrast, lesioning of the more caudal portion of the reticular formation produces insomnia in cats. This study has led to the idea that the caudal portion inhibits the rostral portion of the reticular formation.

Sagittal division reveals more morphological distinctions. The raphe nuclei form a ridge in the middle of the reticular formation, and, directly to its periphery, there is a division called the medial reticular formation. The medial RF is large and has long ascending and descending fibers, and is surrounded by the lateral reticular formation. The lateral RF is close to the motor nuclei of the cranial nerves, and mostly mediates their function.

Medial and lateral reticular formation

The medial reticular formation and lateral reticular formation are two columns of nuclei with ill-defined boundaries that send projections through the medulla and into the midbrain. The nuclei can be differentiated by function, cell type, and projections of efferent or afferent nerves. Moving caudally from the rostral midbrain, at the site of the rostral pons and the midbrain, the medial RF becomes less prominent, and the lateral RF becomes more prominent.

Existing on the sides of the medial reticular formation is its lateral cousin, which is particularly pronounced in the rostral medulla and caudal pons. Out from this area spring the cranial nerves, including the very important vagus nerve. The lateral RF is known for its ganglions and areas of interneurons around the cranial nerves, which serve to mediate their characteristic reflexes and functions.

Function

The reticular formation consists of more than 100 small neural networks, with varied functions including the following:

1. Somatic motor control – Some motor neurons send their axons to the reticular formation nuclei, giving rise to the reticulospinal tracts of the spinal cord. These tracts function in maintaining tone, balance, and posture—especially during body movements. The reticular formation also relays eye and ear signals to the cerebellum so that the cerebellum can integrate visual, auditory, and vestibular stimuli in motor coordination. Other motor nuclei include gaze centers, which enable the eyes to track and fixate objects, and central pattern generators, which produce rhythmic signals of breathing and swallowing.
2. Cardiovascular control – The reticular formation includes the cardiac and vasomotor centers of the medulla oblongata.
3. Pain modulation – The reticular formation is one means by which pain signals from the lower body reach the cerebral cortex. It is also the origin of the descending analgesic pathways. The nerve fibers in these pathways act in the spinal cord to block the transmission of some pain signals to the brain.
4. Sleep and consciousness – The reticular formation has projections to the thalamus and cerebral cortex that allow it to exert some control over which sensory signals reach the cerebrum and come to our conscious attention. It plays a central role in states of consciousness like alertness and sleep. Injury to the reticular formation can result in irreversible coma.
5. Habituation – This is a process in which the brain learns to ignore repetitive, meaningless stimuli while remaining sensitive to others. A good example of this is a person who can sleep through loud traffic in a large city, but is awakened promptly due to the sound of an alarm or crying baby. Reticular formation nuclei that modulate activity of the cerebral cortex are part of the ascending reticular activating system.

Major subsystems

Ascending reticular activating system

Ascending reticular activating system. Reticular formation labeled near center.

The ascending reticular activating system (ARAS), also known as the extrathalamic control modulatory system or simply the reticular activating system (RAS), is a set of connected nuclei in the brains of vertebrates that is responsible for regulating wakefulness and sleep-wake transitions. The ARAS is a part of the reticular formation and is mostly composed of various nuclei in the thalamus and a number of dopaminergic, noradrenergic, serotonergic, histaminergic, cholinergic, and glutamatergic brain nuclei.

Structure of the ARAS

The ARAS is composed of several neural circuits connecting the dorsal part of the posterior midbrain and anterior pons to the cerebral cortex via distinct pathways that project through the thalamus and hypothalamus. The ARAS is a collection of different nuclei – more than 20 on each side in the upper brainstem, the pons, medulla, and posterior hypothalamus. The neurotransmitters that these neurons release include dopamine, norepinephrine, serotonin, histamine, acetylcholine, and glutamate. They exert cortical influence through direct axonal projections and indirect projections through thalamic relays.

The thalamic pathway consists primarily of cholinergic neurons in the pontine tegmentum, whereas the hypothalamic pathway is composed primarily of neurons that release monoamine neurotransmitters, namely dopamine, norepinephrine, serotonin, and histamine. The glutamate-releasing neurons in the ARAS were identified much more recently relative to the monoaminergic and cholinergic nuclei; the glutamatergic component of the ARAS includes one nucleus in the hypothalamus and various brainstem nuclei. The orexin neurons of the lateral hypothalamus innervate every component of the ascending reticular activating system and coordinate activity within the entire system.

Key components of the ascending reticular activating system

Nucleus type	Corresponding nuclei that mediate arousal
Dopaminergic nuclei	Ventral tegmental area Substantia nigra pars compacta
Noradrenergic nuclei	Locus coeruleus Related noradrenergic brainstem nuclei
Serotonergic nuclei	Dorsal raphe nucleus Median raphe nucleus
Histaminergic nuclei	Tuberomammillary nucleus
Cholinergic nuclei	Forebrain cholinergic nuclei Pontine tegmental nuclei: laterodorsal and pedunculopontine tegmental nucleus
Glutamatergic nuclei	Brainstem nuclei: parabrachial nucleus, precoeruleus, and pedunculopontine tegmental nucleus Hypothalamic nuclei: supramammillary nucleus
Thalamic nuclei	Thalamic reticular nucleus Intralaminar nucleus, including the centromedian nucleus

The ARAS consists of evolutionarily ancient areas of the brain, which are crucial to the animal's survival and protected during adverse periods, such as during inhibitory periods of Totsellreflex, aka, "animal hypnosis". The ascending reticular activating system which sends neuromodulatory projections to the cortex - mainly connects to the prefrontal cortex. There seems to be low connectivity to the motor areas of the cortex.

Functions of the ARAS

Consciousness

The ascending reticular activating system is an important enabling factor for the state of consciousness. The ascending system is seen to contribute to wakefulness as characterised by cortical and behavioural arousal.

Regulating sleep-wake transitions

Further information: Neuroscience of sleep

The main function of the ARAS is to modify and potentiate thalamic and cortical function such that electroencephalogram (EEG) desynchronization ensues. There are distinct differences in the brain's electrical activity during periods of wakefulness and sleep: Low voltage fast burst brain waves (EEG desynchronization) are associated with wakefulness and REM sleep (which are electrophysiologically similar); high voltage slow waves are found during non-REM sleep. Generally speaking, when thalamic relay neurons are in burst mode the EEG is synchronized and when they are in tonic mode it is desynchronized. Stimulation of the ARAS produces EEG desynchronization by suppressing slow cortical waves (0.3–1 Hz), delta waves (1–4 Hz), and spindle wave oscillations (11–14 Hz) and by promoting gamma band (20 – 40 Hz) oscillations.

The physiological change from a state of deep sleep to wakefulness is reversible and mediated by the ARAS. The ventrolateral preoptic nucleus (VLPO) of the hypothalamus inhibits the neural circuits responsible for the awake state, and VLPO activation contributes to the sleep onset. During sleep, neurons in the ARAS will have a much lower firing rate; conversely, they will have a higher activity level during the waking state. In order that the brain may sleep, there must be a reduction in ascending afferent activity reaching the cortex by suppression of the ARAS.

Attention

The ARAS also helps mediate transitions from relaxed wakefulness to periods of high attention. There is increased regional blood flow (presumably indicating an increased measure of neuronal activity) in the midbrain reticular formation (MRF) and thalamic intralaminar nuclei during tasks requiring increased alertness and attention.

Clinical significance of the ARAS

Mass lesions in brainstem ARAS nuclei can cause severe alterations in level of consciousness (e.g., coma). Bilateral damage to the reticular formation of the midbrain may lead to coma or death.

Direct electrical stimulation of the ARAS produces pain responses in cats and elicits verbal reports of pain in humans. Ascending reticular activation in cats can produce mydriasis, which can result from prolonged pain. These results suggest some relationship between ARAS circuits and physiological pain pathways.

Pathologies

Some pathologies of the ARAS may be attributed to age, as there appears to be a general decline in reactivity of the ARAS with advancing years. Changes in electrical coupling have been suggested to account for some changes in ARAS activity: if coupling were down-regulated, there would be a corresponding decrease in higher-frequency synchronization (gamma band). Conversely, up-regulated electrical coupling would increase synchronization of fast rhythms that could lead to increased arousal and REM sleep drive. Specifically, disruption of the ARAS has been implicated in the following disorders:

• Narcolepsy: Lesions along the pedunculopontine (PPT/PPN) / laterodorsal tegmental (LDT) nuclei are associated with narcolepsy. There is a significant down-regulation of PPN output and a loss of orexin peptides, promoting the excessive daytime sleepiness that is characteristic of this disorder.
• Progressive supranuclear palsy (PSP) : Dysfunction of nitrous oxide signaling has been implicated in the development of PSP.
• Parkinson's disease: REM sleep disturbances are common in Parkinson's. It is mainly a dopaminergic disease, but cholinergic nuclei are depleted as well. Degeneration in the ARAS begins early in the disease process.

Developmental influences

There are several potential factors that may adversely influence the development of the ascending reticular activating system:

• Preterm birth: Regardless of birth weight or weeks of gestation, premature birth induces persistent deleterious effects on pre-attentional (arousal and sleep-wake abnormalities), attentional (reaction time and sensory gating), and cortical mechanisms throughout development.
• Smoking during pregnancy: Prenatal exposure to cigarette smoke is known to produce lasting arousal, attentional and cognitive deficits in humans. This exposure can induce up-regulation of α4β2 nicotinic receptors on cells of the pedunculopontine nucleus (PPN), resulting in increased tonic activity, resting membrane potential, and hyperpolarization-activated cation current. These major disturbances of the intrinsic membrane properties of PPN neurons result in increased levels of arousal and sensory gating, deficits (demonstrated by a diminished amount of habituation to repeated auditory stimuli). It is hypothesized that these physiological changes may intensify attentional dysregulation later in life.

Descending reticulospinal tracts

Spinal cord tracts - reticulospinal tract labeled in red, near-center at left in figure

The reticulospinal tracts, also known as the descending or anterior reticulospinal tracts, are extrapyramidal motor tracts that descend from the reticular formation in two tracts to act on the motor neurons supplying the trunk and proximal limb flexors and extensors. The reticulospinal tracts are involved mainly in locomotion and postural control, although they do have other functions as well. The descending reticulospinal tracts are one of four major cortical pathways to the spinal cord for musculoskeletal activity. The reticulospinal tracts works with the other three pathways to give a coordinated control of movement, including delicate manipulations. The four pathways can be grouped into two main system pathways – a medial system and a lateral system. The medial system includes the reticulospinal pathway and the vestibulospinal pathway, and this system provides control of posture. The corticospinal and the rubrospinal tract pathways belong to the lateral system which provides fine control of movement.

Components of the reticulospinal tracts

This descending tract is divided into two parts, the medial (or pontine) and lateral (or medullary) reticulospinal tracts (MRST and LRST).

• The MRST is responsible for exciting anti-gravity, extensor muscles. The fibers of this tract arise from the caudal pontine reticular nucleus and the oral pontine reticular nucleus and project to lamina VII and lamina VIII of the spinal cord.
• The LRST is responsible for inhibiting excitatory axial extensor muscles of movement. It is also responsible for automatic breathing. The fibers of this tract arise from the medullary reticular formation, mostly from the gigantocellular nucleus, and descend the length of the spinal cord in the anterior part of the lateral column. The tract terminates in lamina VII mostly with some fibers terminating in lamina IX of the spinal cord.

The ascending sensory tract conveying information in the opposite direction is known as the spinoreticular tract.

Functions of the reticulospinal tracts

1. Integrates information from the motor systems to coordinate automatic movements of locomotion and posture
2. Facilitates and inhibits voluntary movement; influences muscle tone
3. Mediates autonomic functions
4. Modulates pain impulses
5. Influences blood flow to lateral geniculate nucleus of the thalamus.

Clinical significance of the reticulospinal tracts

The reticulospinal tracts provide a pathway by which the hypothalamus can control sympathetic thoracolumbar outflow and parasympathetic sacral outflow.

Two major descending systems carrying signals from the brainstem and cerebellum to the spinal cord can trigger automatic postural response for balance and orientation: vestibulospinal tracts from the vestibular nuclei and reticulospinal tracts from the pons and medulla. Lesions of these tracts result in profound ataxia and postural instability.

Physical or vascular damage to the brainstem disconnecting the red nucleus (midbrain) and the vestibular nuclei (pons) may cause decerebrate rigidity, which has the neurological sign of increased muscle tone and hyperactive stretch reflexes. Responding to a startling or painful stimulus, both arms and legs extend and turn internally. The cause is the tonic activity of lateral vestibulospinal and reticulospinal tracts stimulating extensor motoneurons without the inhibitions from rubrospinal tract.

Brainstem damage above the red nucleus level may cause decorticate rigidity. Responding to a startling or painful stimulus, the arms flex and the legs extend. The cause is the red nucleus, via the rubrospinal tract, counteracting the extensor motorneuron's excitation from the lateral vestibulospinal and reticulospinal tracts. Because the rubrospinal tract only extends to the cervical spinal cord, it mostly acts on the arms by exciting the flexor muscles and inhibiting the extensors, rather than the legs.

Damage to the medulla below the vestibular nuclei may cause flaccid paralysis, hypotonia, loss of respiratory drive, and quadriplegia. There are no reflexes resembling early stages of spinal shock because of complete loss of activity in the motorneurons, as there is no longer any tonic activity arising from the lateral vestibulospinal and reticulospinal tracts.

History

The term "reticular formation" was coined in the late 19th century by Otto Deiters, coinciding with Ramon y Cajal's neuron doctrine. Allan Hobson states in his book The Reticular Formation Revisited that the name is an etymological vestige from the fallen era of the aggregate field theory in the neural sciences. The term "reticulum" means "netlike structure", which is what the reticular formation resembles at first glance. It has been described as being either too complex to study or an undifferentiated part of the brain with no organization at all. Eric Kandel describes the reticular formation as being organized in a similar manner to the intermediate gray matter of the spinal cord. This chaotic, loose, and intricate form of organization is what has turned off many researchers from looking farther into this particular area of the brain. The cells lack clear ganglionic boundaries, but do have clear functional organization and distinct cell types. The term "reticular formation" is seldom used anymore except to speak in generalities. Modern scientists usually refer to the individual nuclei that compose the reticular formation.

Moruzzi and Magoun first investigated the neural components regulating the brain's sleep-wake mechanisms in 1949. Physiologists had proposed that some structure deep within the brain controlled mental wakefulness and alertness. It had been thought that wakefulness depended only on the direct reception of afferent (sensory) stimuli at the cerebral cortex.

As direct electrical stimulation of the brain could simulate electrocortical relays, Magoun used this principle to demonstrate, on two separate areas of the brainstem of a cat, how to produce wakefulness from sleep. He first stimulated the ascending somatic and auditory paths; second, a series of "ascending relays from the reticular formation of the lower brain stem through the midbrain tegmentum, subthalamus and hypothalamus to the internal capsule." The latter was of particular interest, as this series of relays did not correspond to any known anatomical pathways for the wakefulness signal transduction and was coined the ascending reticular activating system (ARAS).

Next, the significance of this newly identified relay system was evaluated by placing lesions in the medial and lateral portions of the front of the midbrain. Cats with mesencephalic interruptions to the ARAS entered into a deep sleep and displayed corresponding brain waves. In alternative fashion, cats with similarly placed interruptions to ascending auditory and somatic pathways exhibited normal sleeping and wakefulness, and could be awakened with physical stimuli. Because these external stimuli would be blocked on their way to the cortex by the interruptions, this indicated that the ascending transmission must travel through the newly discovered ARAS.

Finally, Magoun recorded potentials within the medial portion of the brain stem and discovered that auditory stimuli directly fired portions of the reticular activating system. Furthermore, single-shock stimulation of the sciatic nerve also activated the medial reticular formation, hypothalamus, and thalamus. Excitation of the ARAS did not depend on further signal propagation through the cerebellar circuits, as the same results were obtained following decerebellation and decortication. The researchers proposed that a column of cells surrounding the midbrain reticular formation received input from all the ascending tracts of the brain stem and relayed these afferents to the cortex and therefore regulated wakefulness.

Commutator (electric)

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Commutator_(electric) 

Commutator in a universal motor from a vacuum cleaner. Parts: (A) commutator, (B) brush, (C) rotor (armature) windings, (D) stator (field) windings, (E) brush guides, (F) electrical connections.

A commutator is a rotary electrical switch in certain types of electric motors and electrical generators that periodically reverses the current direction between the rotor and the external circuit. It consists of a cylinder composed of multiple metal contact segments on the rotating armature of the machine. Two or more electrical contacts called "brushes" made of a soft conductive material like carbon press against the commutator, making sliding contact with successive segments of the commutator as it rotates. The windings (coils of wire) on the armature are connected to the commutator segments.

Commutators are used in direct current (DC) machines: dynamos (DC generators) and many DC motors as well as universal motors. In a motor the commutator applies electric current to the windings. By reversing the current direction in the rotating windings each half turn, a steady rotating force (torque) is produced. In a generator the commutator picks off the current generated in the windings, reversing the direction of the current with each half turn, serving as a mechanical rectifier to convert the alternating current from the windings to unidirectional direct current in the external load circuit. The first direct current commutator-type machine, the dynamo, was built by Hippolyte Pixii in 1832, based on a suggestion by André-Marie Ampère.

Commutators are relatively inefficient, and also require periodic maintenance such as brush replacement. Therefore, commutated machines are declining in use, being replaced by alternating current (AC) machines, and in recent years by brushless DC motors which use semiconductor switches.

Principle of operation

A commutator consists of a set of contact bars fixed to the rotating shaft of a machine, and connected to the armature windings. As the shaft rotates, the commutator reverses the flow of current in a winding. For a single armature winding, when the shaft has made one-half complete turn, the winding is now connected so that current flows through it in the opposite of the initial direction. In a motor, the armature current causes the fixed magnetic field to exert a rotational force, or a torque, on the winding to make it turn. In a generator, the mechanical torque applied to the shaft maintains the motion of the armature winding through the stationary magnetic field, inducing a current in the winding. In both the motor and generator case, the commutator periodically reverses the direction of current flow through the winding so that current flow in the circuit external to the machine continues in only one direction.

Simplest practical commutator

Practical commutators have at least three contact segments, to prevent a "dead" spot where two brushes simultaneously bridge only two commutator segments. Brushes are made wider than the insulated gap, to ensure that brushes are always in contact with an armature coil. For commutators with at least three segments, although the rotor can potentially stop in a position where two commutator segments touch one brush, this only de-energizes one of the rotor arms while the others will still function correctly. With the remaining rotor arms, a motor can produce sufficient torque to begin spinning the rotor, and a generator can provide useful power to an external circuit.

Ring/segment construction

Cross-section of a commutator that can be disassembled for repair.

A commutator consists of a set of copper segments, fixed around the part of the circumference of the rotating machine, or the rotor, and a set of spring-loaded brushes fixed to the stationary frame of the machine. Two or more fixed brushes connect to the external circuit, either a source of current for a motor or a load for a generator.

Commutator segments are connected to the coils of the armature, with the number of coils (and commutator segments) depending on the speed and voltage of the machine. Large motors may have hundreds of segments. Each conducting segment of the commutator is insulated from adjacent segments. Mica was used on early machines and is still used on large machines. Many other insulating materials are used to insulate smaller machines; plastics allow quick manufacture of an insulator, for example. The segments are held onto the shaft using a dovetail shape on the edges or underside of each segment. Insulating wedges around the perimeter of each segment are pressed so that the commutator maintains its mechanical stability throughout its normal operating range.

In small appliance and tool motors the segments are typically crimped permanently in place and cannot be removed. When the motor fails it is discarded and replaced. On large industrial machines (say, from several kilowatts to thousands of kilowatts in rating) it is economical to replace individual damaged segments, and so the end-wedge can be unscrewed and individual segments removed and replaced.

Replacing the copper and mica segments is commonly referred to as "refilling". Refillable dovetailed commutators are the most common construction of larger industrial type commutators, but refillable commutators may also be constructed using external bands made of fiberglass (glass banded construction) or forged steel rings (external steel shrink ring type construction and internal steel shrink ring type construction).

Disposable, molded type commutators commonly found in smaller DC motors are becoming increasingly more common in larger electric motors. Molded type commutators are not repairable and must be replaced if damaged.

In addition to the commonly used heat, torque, and tonnage methods of seasoning commutators, some high performance commutator applications require a more expensive, specific "spin seasoning" process or over-speed spin-testing to guarantee stability of the individual segments and prevent premature wear of the carbon brushes. Such requirements are common with traction, military, aerospace, nuclear, mining, and high speed applications where clamping failure and segment or insulation protrusion can lead to serious negative consequences.

Friction between the segments and the brushes eventually causes wear to both surfaces. Carbon brushes, being made of a softer material, wear faster and may be designed to be replaced easily without dismantling the machine. Older copper brushes caused more wear to the commutator, causing deep grooving and notching of the surface over time.

The commutator on small motors (say, less than a kilowatt rating) is not designed to be repaired through the life of the device. On large industrial equipment, the commutator may be re-surfaced with abrasives, or the rotor may be removed from the frame, mounted in a large metal lathe, and the commutator resurfaced by cutting it down to a smaller diameter. The largest of equipment can include a lathe turning attachment directly over the commutator.

A tiny 5-segment commutator less than 2 mm in diameter, on a direct-current motor in a toy radio control ZipZaps car.

Brush construction

Various types of copper and carbon brushes.

Early machines used brushes made from strands of copper wire to contact the surface of the commutator. However, these hard metal brushes tended to scratch and groove the smooth commutator segments, eventually requiring resurfacing of the commutator. As the copper brushes wore away, the dust and pieces of the brush could wedge between commutator segments, shorting them and reducing the efficiency of the device. Fine copper wire mesh or gauze provided better surface contact with less segment wear, but gauze brushes were more expensive than strip or wire copper brushes.

Modern rotating machines with commutators almost exclusively use carbon brushes, which may have copper powder mixed in to improve conductivity. Metallic copper brushes can be found in toy or very small motors, such as the one illustrated above, and some motors which only operate very intermittently, such as automotive starter motors.

Motors and generators suffer from a phenomenon known as 'armature reaction', one of the effects of which is to change the position at which the current reversal through the windings should ideally take place as the loading varies. Early machines had the brushes mounted on a ring that was provided with a handle. During operation, it was necessary to adjust the position of the brush ring to adjust the commutation to minimise the sparking at the brushes. This process was known as 'rocking the brushes'.

Various developments took place to automate the process of adjusting the commutation and minimizing the sparking at the brushes. One of these was the development of 'high resistance brushes', or brushes made from a mixture of copper powder and carbon. Although described as high resistance brushes, the resistance of such a brush was of the order of milliohms, the exact value dependent on the size and function of the machine. Also, the high resistance brush was not constructed like a brush but in the form of a carbon block with a curved face to match the shape of the commutator.

The high resistance or carbon brush is made large enough that it is significantly wider than the insulating segment that it spans (and on large machines may often span two insulating segments). The result of this is that as the commutator segment passes from under the brush, the current passing to it ramps down more smoothly than had been the case with pure copper brushes where the contact broke suddenly. Similarly the segment coming into contact with the brush has a similar ramping up of the current. Thus, although the current passing through the brush was more or less constant, the instantaneous current passing to the two commutator segments was proportional to the relative area in contact with the brush.

The introduction of the carbon brush had convenient side effects. Carbon brushes tend to wear more evenly than copper brushes, and the soft carbon causes far less damage to the commutator segments. There is less sparking with carbon as compared to copper, and as the carbon wears away, the higher resistance of carbon results in fewer problems from the dust collecting on the commutator segments.

The ratio of copper to carbon can be changed for a particular purpose. Brushes with higher copper content perform better with very low voltages and high current, while brushes with a higher carbon content are better for high voltage and low current. High copper content brushes typically carry 150 to 200 amperes per square inch of contact surface, while higher carbon content only carries 40 to 70 amperes per square inch. The higher resistance of carbon also results in a greater voltage drop of 0.8 to 1.0 volts per contact, or 1.6 to 2.0 volts across the commutator.

Brush holders

Compound carbon brush holder, with individual clamps and tension adjustments for each block of carbon.

A spring is typically used with the brush, to maintain constant contact with the commutator. As the brush and commutator wear down, the spring steadily pushes the brush downwards towards the commutator. Eventually the brush wears small and thin enough that steady contact is no longer possible or it is no longer securely held in the brush holder, and so the brush must be replaced.

It is common for a flexible power cable to be directly attached to the brush, because current flowing through the support spring would cause heating, which may lead to a loss of metal temper and a loss of the spring tension.

When a commutated motor or generator uses more power than a single brush is capable of conducting, an assembly of several brush holders is mounted in parallel across the surface of the very large commutator. This parallel holder distributes current evenly across all the brushes, and permits a careful operator to remove a bad brush and replace it with a new one, even as the machine continues to spin fully powered and under load.

High power, high current commutated equipment is now uncommon, due to the less complex design of alternating current generators that permits a low current, high voltage spinning field coil to energize high current fixed-position stator coils. This permits the use of very small singular brushes in the alternator design. In this instance, the rotating contacts are continuous rings, called slip rings, and no switching happens.

Modern devices using carbon brushes usually have a maintenance-free design that requires no adjustment throughout the life of the device, using a fixed-position brush holder slot and a combined brush-spring-cable assembly that fits into the slot. The worn brush is pulled out and a new brush inserted.

Brush contact angle

Different types of brushes have different brush contact angles

 

Commutator and brush assembly of a traction motor; the copper bars can be seen with lighter insulation strips between the bars. Each dark grey carbon brush has a short flexible copper jumper lead attached. Parts of the motor field winding, in red, can be seen to the right of the commutator.

The different brush types make contact with the commutator in different ways. Because copper brushes have the same hardness as the commutator segments, the rotor cannot be spun backwards against the ends of copper brushes without the copper digging into the segments and causing severe damage. Consequently, strip/laminate copper brushes only make tangential contact with the commutator, while copper mesh and wire brushes use an inclined contact angle touching their edge across the segments of a commutator that can spin in only one direction.

The softness of carbon brushes permits direct radial end-contact with the commutator without damage to the segments, permitting easy reversal of rotor direction, without the need to reorient the brush holders for operation in the opposite direction. Although never reversed, common appliance motors that use wound rotors, commutators and brushes have radial-contact brushes. In the case of a reaction-type carbon brush holder, carbon brushes may be reversely inclined with the commutator so that the commutator tends to push against the carbon for firm contact.

The commutating plane

Commutating plane definitions.

The contact point where a brush touches the commutator is referred to as the commutating plane. To conduct sufficient current to or from the commutator, the brush contact area is not a thin line but instead a rectangular patch across the segments. Typically the brush is wide enough to span 2.5 commutator segments. This means that two adjacent segments are electrically connected by the brush when it contacts both.

Rotation of brushes for stator field distortion

Centered position of the commutating plane if there were no field distortion effects.

Most introductions to motor and generator design start with a simple two-pole device with the brushes arranged at a perfect 90-degree angle from the field. This ideal is useful as a starting point for understanding how the fields interact but it is not how a motor or generator functions in actual practice.

On the left is an exaggerated example of how the field is distorted by the rotor. On the right, iron filings show the distorted field across the rotor.

In a real motor or generator, the field around the rotor is never perfectly uniform. Instead, the rotation of the rotor induces field effects which drag and distort the magnetic lines of the outer non-rotating stator.

Actual position of the commutating plane to compensate for field distortion.

The faster the rotor spins, the further this degree of field distortion. Because a motor or generator operates most efficiently with the rotor field at right angles to the stator field, it is necessary to either retard or advance the brush position to put the rotor's field into the correct position to be at a right angle to the distorted field.

These field effects are reversed when the direction of spin is reversed. It is therefore difficult to build an efficient reversible commutated dynamo, since for highest field strength it is necessary to move the brushes to the opposite side of the normal neutral plane. These effects can be mitigated by a compensation winding in the face of the field pole that carries armature current.

The effect can be considered to be analogous to timing advance in an internal combustion engine. Generally a dynamo that has been designed to run at a certain fixed speed will have its brushes permanently fixed to align the field for highest efficiency at that speed.

Further compensation for self-induction

Brush advance for Self-Induction.

Self-induction – The magnetic fields in each coil of wire join and compound together to create a magnetic field that resists changes in the current, which can be likened to the current having inertia.

In the coils of the rotor, even after the brush has been reached, currents tend to continue to flow for a brief moment, resulting in a wasted energy as heat due to the brush spanning across several commutator segments and the current short-circuiting across the segments.

Spurious resistance is an apparent increase in the resistance in the armature winding, which is proportional to the speed of the armature, and is due to the lagging of the current.

To minimize sparking at the brushes due to this short-circuiting, the brushes are advanced a few degrees further yet, beyond the advance for field distortions. This moves the rotor winding undergoing commutation slightly forward into the stator field which has magnetic lines in the opposite direction and which oppose the field in the stator. This opposing field helps to reverse the lagging self-inducting current in the stator.

So even for a rotor which is at rest and initially requires no compensation for spinning field distortions, the brushes should still be advanced beyond the perfect 90-degree angle as taught in so many beginners textbooks, to compensate for self-induction.

Use of interpoles to correct field distortions

Modern motor and generator devices with commutators are able to counteract armature reaction through the use of interpoles, which are small field coils and pole pieces positioned approximately halfway between the primary poles of the stator.

By applying a dynamic varying field to the interpoles as the load, RPM, or direction of rotation of the device changes, it is possible to balance out field distortions from armature reaction so that the brush position can remain fixed and sparking across the segments is minimized.

Limitations and alternatives

Low voltage dynamo from late 1800s for electroplating. The resistance of the commutator contacts causes inefficiency in low voltage, high current machines like this, requiring a huge elaborate commutator. This machine generated 7 volts at 310 amps.

Although direct current motors and dynamos once dominated industry, the disadvantages of the commutator have caused a decline in the use of commutated machines in the last century. These disadvantages are:

• The sliding friction between the brushes and commutator consumes power, which can be significant in a low power machine.
• Due to friction, the brushes and copper commutator segments wear down, creating dust. In small consumer products such as power tools and appliances the brushes may last as long as the product, but larger machines require regular replacement of brushes and occasional resurfacing of the commutator. So commutated machines are not used in low particulate or sealed applications or in equipment that must operate for long periods without maintenance.
• The resistance of the sliding contact between brush and commutator causes a voltage drop called the "brush drop". This may be several volts, so it can cause large power losses in low voltage, high current machines. Alternating current motors, which do not use commutators, are much more efficient.
• There is a limit to the maximum current density and voltage which can be switched with a commutator. Very large direct current machines, say, more than several megawatts rating, cannot be built with commutators. The largest motors and generators are all alternating-current machines.
• The switching action of the commutator causes sparking at the contacts, posing a fire hazard in explosive atmospheres, and generating electromagnetic interference.

With the wide availability of alternating current, DC motors have been replaced by more efficient AC synchronous or induction motors. In recent years, with the widespread availability of power semiconductors, in many remaining applications commutated DC motors have been replaced with "brushless direct current motors". These don't have a commutator; instead the direction of the current is switched electronically. A sensor keeps track of the rotor position and semiconductor switches such as transistors reverse the current. Operating life of these machines is much longer, limited mainly by bearing wear.

Repulsion induction motors

See also: Repulsion motor

These are single-phase AC-only motors with higher starting torque than could be obtained with split-phase starting windings, before high-capacitance (non-polar, relatively high-current electrolytic) starting capacitors became practical. They have a conventional wound stator as with any induction motor, but the wire-wound rotor is much like that with a conventional commutator. Brushes opposite each other are connected to each other (not to an external circuit), and transformer action induces currents into the rotor that develop torque by repulsion.

One variety, notable for having an adjustable speed, runs continuously with brushes in contact, while another uses repulsion only for high starting torque and in some cases lifts the brushes once the motor is running fast enough. In the latter case, all commutator segments are connected together as well, before the motor attains running speed.

Once at speed, the rotor windings become functionally equivalent to the squirrel-cage structure of a conventional induction motor, and the motor runs as such.

Laboratory commutators

Commutators were used as simple forward-off-reverse switches for electrical experiments in physics laboratories. There are two well-known historical types:

Ruhmkorff commutator

This is similar in design to the commutators used in motors and dynamos. It was usually constructed of brass and ivory (later ebonite).

Pohl commutator

This consisted of a block of wood or ebonite with four wells, containing mercury, which were cross-connected by copper wires. The output was taken from a pair of curved copper wires which were moved to dip into one or other pair of mercury wells. Instead of mercury, ionic liquids or other liquid metals could be used.

Environmental politics

From Wikipedia, the free encyclopedia

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

Environmental politics designate both the politics about the environment (see also environmental policy) and an academic field of study focused on three core components:

• The study of political theories and ideas related to the environment;
• The examination of the environmental stances of both mainstream political parties and environmental social movements; and
• The analysis of public policymaking and implementation affecting the environment, at multiple geo-political levels.

Neil Carter, in his foundational text Politics of the Environment (2009), suggests that environmental politics is distinct in at least two ways: first, "it has a primary concern with the relationship between human society and the natural world" (page 3); and second, "unlike most other single issues, it comes replete with its own ideology and political movement" (page 5, drawing on Michael Jacobs, ed., Greening the Millenium?, 1997).

Further, he distinguishes between modern and earlier forms of environmental politics, in particular conservationism and preservationism. Contemporary environmental politics "was driven by the idea of a global ecological crisis that threatened the very existence of humanity." And "modern environmentalism was a political and activist mass movement which demanded a radical transformation in the values and structures of society."

Environmental concerns were rooted in the vast social changes that took place in the United States after World War II. Although environmentalism can be identified in earlier years, only after the war did it become widely shared social priority. This began with outdoor recreation in the 1950s, extended into the wider field of the protection of natural environments, and then became infused with attempts to cope with air and water pollution and still later with toxic chemical pollutants. After World War II, environmental politics became a major public concern. The development of environmentalism in the United Kingdom emerged in this period following the great London smog of 1952 and the Torrey Canyon oil spill of 1967. This is reflected by the emergence of Green politics in the Western world beginning in the 1970s.

Democratic challenges

The roles of democracy and democratic institutions in advancing environmental policy and, in particular, climate policy are mixed, as evidenced by the variation in the environmental progress of different democratic governments. From a theoretical perspective, democratic procedures can effect meaningful reform if public support for these reforms exists, especially when compared with autocratic regimes, as the set of incentives for policymakers to legislate toward these ends in a system deriving legitimacy from the consent of the governed is substantive; for instance, given political responsiveness as a result of electoral accountability, policymakers in democratic governments have reason to consider a wide view of the public interest that incorporates the varied positions of their constituents and work to efficiently create change. On such a view, democracies will likely consider the consequential impacts to most, if not all constituents, caused by climate change. Factors like regime stability and ruler or governing official interests, too, seem better aligned for progress in a democracy; civil unrest is less likely in a state perceived as legitimate, as is graft, both of which appear likely to inhibit climate action.

In contrast, empirical evidence does show inconsistencies in the ways in which democracies address environmental problems. Though the reason for this variation is largely unclear, a number of features of democratic state organization appear to contribute to observed failures to act on climate change, among other environmental issues. Leaders may, in practice, not be motivated by a theoretical public good, but instead expend resources on resolving those policy challenges which are most visible to their electorate. Given the largely intangible nature of climate change as a problem – one that is gradual, invisible, and global – the political opportunity cost of focusing on this challenge or other less visible environmental issues may be high for electorally accountable democratic leaders.

Economic interests and outside influences may also limit the ability of democratic actors to drive meaningful environmental change. In developed democracies, businesses and other groups with economic motivations often hold considerable lobbying power and, therefore, have the ability to forestall climate or environmental progress, which are often unaligned with these groups' financial interests. In developing democracies, environmental reforms are often seen as lesser priorities, given the need for addressing more proximate public concerns, including poverty, infrastructure, and general economic development. Financial incentive can also play a role in preventing the passage of environmental policy outside of the legal realm; some evidence suggests that corruption, present in some form in a number of democratic institutions globally, erodes regulatory ability and public trust in state institutions, reducing the ability of democracies to effectively mitigate carbon emissions and other sources of pollution.

In addition, the problem of popular disinterest in advancing environmental policy presents challenges for the prospects of democratic institutions' ability to drive environmental progress. Despite growing public understanding of the threat posed by climate change, the last decade has seen considerable opposition to pro-environmental policies across broad coalitions and around the globe. Populist movements in Western democracies over the last several years, in particular, have taken positions that actively oppose such policies, and analyses of deliberative modes of participatory democracy have shown results that mirror the interests of those participating and do not necessarily tend towards a more favorable view of environmental or climate action. As redress to these potential shortcomings, means of reforming democratic processes, both theoretical and pragmatic, to correct for what may be short-sighted political interests have been suggested, though these reforms may reduce democratic choice or participation.

Questions of environmental justice, too, may be unanswered by democratic decision-making processes. Not only are those minority groups without meaningful representation in either single-member districts or majority-rule electorates disadvantaged in the realm of political interests, but these same groups are often those most impacted by the effects of climate change and other environmental problems. In addition, recent literature around non-human representation has investigated the ways in which the interests of affected conscious agents, which are definitionally uninvolved in the political decisions of human society, are consistently underrepresented; solutions accounting for this disparity often appeal to reforms that would reduce democratic choice from a traditional perspective, including by giving biological experts greater say in policymaking, though even their ability to determine the interests of non-humans is uncertain. On a global scale, those most impacted by the effects of climate change may have little say in determining policies that would curb emissions or otherwise work to adapt to climate outcomes. Not only do individuals only have the ability to determine climate policy in their own state, but those states that emit the least atmospheric carbon are often most vulnerable to the impacts of climate change, while those that emit the most are often least vulnerable, a discrepancy unaccounted for by democratic processes.

Climate change is slow relative to political cycles of leadership in electoral democracies, which impedes responses by politicians who are elected and re-elected on much shorter timescales.

In the United States, although "environmentalism" was once considered a White phenomenon, scholars have identified "pro-environment positions among Latino, African-American, and non-Hispanic white respondents," with growing environmental concern especially among Latinos. Other scholars have similarly noted that Asian Americans are strongly pro-environmental, with some variation among ethnic subgroups.

Effectively responding to global warming necessitates some form of international environmental governance to achieve shared targets related to energy consumption and environmental usage. Climate change complicates political ideology and practice, affecting conceptions of responsibility for future societies as well as economic systems. Material inequality between nations make technological solutions insufficient for climate change mitigation. Rather, political solutions can navigate the particularities of various facets of environmental crisis. Climate change mitigation strategies can be at odds with democratic priorities of prosperity, progress, and state sovereignty, and instead underscore a collective relationship with the environment.

The international political community is presently based on liberal principles that prioritize individual freedoms and capitalist systems that make quick and ambitious climate responses difficult. Interest-group liberalism is guided by individual human priorities. Groups unable to voice their self-interest, such as minorities without suffrage, or non-humans, are not included in the political compromise. Addressing environmental crises can be impeded when citizens of liberal democracies do not see environmental problems as impacting their lives, or when they lack the education to evaluate the importance of the problem. The human benefits from environmental exploitation and protection compete. Considering the implications of ecological degradation for future human generations can give environmental concerns a basis in anthropocentric liberal democratic politics.

William Ophuls posits that liberal democracies are unfit to address environmental problems, and that the prioritization of these challenges would involve a transition to more authoritarian forms of government. Others counter this by pointing to the past successes of environmental reform movements to improve water and air quality in liberal societies. In practice, environmentalism can improve democracy rather than necessitate its end, by expanding democratic participation and promoting political innovations.

The tensions between liberal democracy and environmental goals raise questions about the possible limitations of democracy (or at least democracy as we know it): in its responsiveness to subtle but large-scale problems, its ability to work from a holistic societal perspective, its aptness in coping with environmental crisis relative to other forms of government. Democracies do not have the provisions to make environmental reforms that are not mandated by voters, and many voters lack incentives or desire to demand policies that could compromise immediate prosperity. The question arises as to whether the foundation of politics is morality or practicality. A scheme that conceives of and values the environment beyond its human utility, an environmental ethics, could be crucial for democracies to respond to climate change.

Alternative forms of democracy for environmental policy

In political theory, deliberative democracy has been discussed as a political model more compatible with environmental goals. Deliberative democracy is a system in which informed political equals weigh values, information, and expertise, and debate priorities to make decisions, as opposed to a democracy based on interest aggregation. This definition of democracy emphasizes informed discussion among citizens in the decision making process, and encourages decisions to benefit the common good rather than individual interests. Amy Gutmann and Dennis Thompson claimed that reason prevails over self-interest in deliberative democracy, making it a more just system. The broad perspective that this discursive model encourages could lead to a stronger engagement with environmental concerns. When compared to non-democracies, democracies are in fact more cooperative in climate change policy creation, but not necessarily on the outcome and effects of these policies.

This can be explained more exhaustively with the concept of grass-roots democracy. Grass-roots democracy is an approach in which ordinary citizens are in charge of politics, in opposition to ‘larger organizations and wealthy individuals with concentrated vested interests in particular policies’. Green parties were once dedicated to offer a project valuing the ideology of grass-roots democracy. However, according to Ostrogorski and Michels, all parties follow inevitably a similar path towards concentration of power and oligarchy. Green parties thus follow different principles nowadays.

In political theory, the lottery system is a democratic design that allows governments to address problems with future, rather than immediate, impacts. Deliberative bodies composed of randomly selected representatives can draft environmental policies that have short-term costs without considering the political consequences for re-election.

New materialism and environmental justice

New materialism is a strain of thought in philosophy and the social sciences that conceives of all material as having life or agency. It criticizes frameworks of justice that center on human attributes like consciousness as insufficient for modern ethical problems that concern the natural environment. It is a post-humanist consideration of all matter that rejects arguments of utility that privilege humans. This politically relevant social theory combats inequality beyond the interpersonal plane. People are ethically responsible for one another, and for the physical spaces they navigate, including animal and plant life, and the inanimate matter that sustains it, like soil. New materialism encourages political action according to this world vision, even if it is incompatible with economic growth.

Jane Bennett uses the term "vital materialism" in her book Vibrant Matter: A Political Ecology of Things. She develops the concept of materialism with the aim of providing a stronger basis in political theory for environmental politics.

New materialists have invoked Derrida and other historical thinkers to trace the emergence of their philosophy and to justify their environmental claims:

"No justice ... seems possible or thinkable without the principle of some responsibility, beyond all living present, within that which disjoins the living present, before the ghosts of those who are not yet born or who are already dead [...]. Without this non-contemporaneity with itself of the living present ... without this responsibility and this respect for justice concerning those who are not there, of those who are no longer or who are not yet present and living, what sense would there be to ask the question 'where?' 'where tomorrow?' 'whither?'"

All material, living and dead, is interrelated in "the mesh" as described by Timothy Morton. As all matter is interdependent, humans have obligations to all parts of the material world, including those that are unfamiliar.

New materialism is related to a shift from the view of the environment as a form of capital to a form of labor (see Ecosystem services).

Emerging nations

Brazil, Russia, India, and China (known as the "BRIC" nations) are rapidly industrializing, and are increasingly responsible for global carbon emissions and the associated climate change. Other forms of environmental degradation have also accompanied the economic growth in these nations. Environmental degradation tends to motivate action more than the threat of global warming does, since air and water pollution cause immediate health problems, and because pollutants can damage natural resources, hampering economic potential.

The Kuznets curve is a hypothetical curve representing the trajectory of environmental degradation in developing nations as a function of per capita income.

With rising incomes, environmental degradation tends to decrease in industrializing nations, as depicted in the Environmental Kuznets Curve (described in a section of the Kuznets Curve article). Citizens demand better air and water quality, and technology becomes more efficient and clean when incomes increase. The level of income per capita needed to reverse the trend of environmental degradation in industrializing nations varies with the environmental impact indicator. More developed nations can facilitate eco-friendly transitions in emerging economies by investing in the development of clean technologies.

Laws implemented in response to environmental concerns vary by nation (see List of environmental laws by country).

China

China's environmental ills include acid rain, severe smog, and a reliance on coal-burning for energy. China has instated environmental policies since the 1970s, and has one of the most extensive environmental conservation programs on paper. However, regulation and enforcement by the central government in Beijing are weak, so solutions are decentralized. Wealthier provinces are far more effective in their preservation and sustainable development efforts than poorer regions. China therefore provides an example of the consequences of environmental damage falling disproportionately on the poor. NGOs, the media, and the international community have all contributed to China's response to environmental problems.

For history, laws, and policies, see Environmental policy in China.

India

In 1976, the Constitution of India was amended to reflect environmental priorities, motivated in part by the potential threat of natural resource depletion to economic growth:

"The State shall endeavour to protect and improve the environment and to safeguard the forests and wildlife." (Art. 48A)

"It shall be the duty of every citizen of India [...] to protect and improve the natural environment including forests, lakes, rivers and wildlife, and to have compassion for living creatures." (Art. 51A)

However, in India, as in China, the implementation of written environmental policies, laws, and amendments has proven challenging. Official legislation by the central government (see a partial list at Environmental policy of the Government of India) is often more symbolic than practical. The Ministry of Environment and Forests was established in 1985, but corruption within bureaucratic agencies, namely the influence of wealthy industry leaders, limited any attempts at enforcement of the policies put in place.

Journals

Scholarly journals representing this field of study include:

• Environmental Politics
• Global Environmental Politics
• International Environmental Agreements