A cat attempting to use its left foreleg to scoop litter several months after it has been amputated
A phantom limb is the sensation that an amputated or missing limb is still attached. Approximately 80 to 100% of individuals with an amputation
experience sensations in their amputated limb. However, only a small
percentage will experience painful phantom limb sensation. These
sensations are relatively common in amputees and usually resolve within
two to three years without treatment. Research continues to explore the
underlying mechanisms of phantom limb pain (PLP) and effective treatment
options.
Signs and symptoms
Most (80% to 100%) amputees experience a phantom with some non-painful sensations. The amputee may feel very strongly that the phantom limb is still part of the body.
People will sometimes feel as if they are gesturing, feel itches,
twitch, or even try to pick things up. The missing limb often feels
shorter and may feel as if it is in a distorted and painful position.
Occasionally, the pain can be made worse by stress, anxiety and weather changes.
Exposure to extreme weather conditions, especially below freezing
temperatures, can cause increased sensitivity to the sensation. Phantom
limb pain is usually intermittent, but can be continuous in some cases.
The frequency and intensity of attacks usually declines with time.
Repressed memories in phantom limbs could potentially explain the
reason for existing sensations after amputation. Specifically, there
have been several reports from patients of painful clenching spasms in
the phantom hand with the feeling of their nails digging into their
palms. The motor output is amplified due to the missing limb; therefore,
the patient may experience the overflow of information as pain. The
patient contains repressed memories from previous motor commands of
clenching the hand and sensory information from digging their nails into
their palm. These memories remain due to previous neural connections in
the brain.
Phantom limb syndrome
The term "phantom limb" was coined by physician Silas Weir Mitchell in 1871.
For many years, the dominant hypothesis for the cause of phantom limbs
was irritation in the peripheral nervous system at the amputation site (neuroma). By the late 1980s, Ronald Melzack
had recognized that the peripheral neuroma account could not be
correct, because many people born without limbs also experienced phantom
limbs.
According to Melzack the experience of the body is created by a wide
network of interconnecting neural structures, which he called the
"neuromatrix".
Pons and colleagues (1991) at the National Institutes of Health
(NIH) showed that the primary somatosensory cortex in macaque monkeys
undergoes substantial reorganization after the loss of sensory input.
Hearing about these results, Vilayanur S. Ramachandran
hypothesized that phantom limb sensations in humans could be due to
reorganization in the human brain's somatosensory cortex. Ramachandran
and colleagues illustrated this hypothesis by showing that stroking
different parts of the face led to perceptions of being touched on
different parts of the missing limb. Later brain scans of amputees
showed the same kind of cortical reorganization that Pons had observed
in monkeys.
Maladaptive changes in the cortex may account for some but not
all phantom limb pain. Pain researchers such as Tamar Makin (Oxford) and
Marshall Devor (Hebrew University, Jerusalem) argue that phantom limb
pain is primarily the result of "junk" inputs from the peripheral
nervous system.
Despite a great deal of research on the underlying neural
mechanisms of phantom limb pain there is still no clear consensus as to
its cause. Both the brain and the peripheral nervous system may be
involved.
Research continues into more precise mechanisms and explanations.
In phantom limb syndrome, there is sensory input indicating pain
from a part of the body that is no longer existent. This phenomenon is
still not fully understood, but it is hypothesized that it is caused by
activation of the somatosensory cortex.
Treatment
Most
approaches to treatment over the past two decades have not shown
consistent symptom improvement. Treatment approaches have included medication such as antidepressants, spinal cord stimulation, vibrationtherapy, acupuncture, hypnosis, and biofeedback. Reliable evidence is lacking on whether any treatment is more effective than the others.
A mirror box used for treating phantom limbs, developed by V.S. Ramachandran
Most treatments are not very effective. Ketamine or morphine may be useful around the time of surgery. Morphine may be helpful for longer periods of time. Evidence for gabapentin is mixed. Perineural catheters that provide local anesthetic agents have poor evidence of success when placed after surgery in an effort to prevent phantom limb pain.
One approach that has received public interest is the use of a mirror box.
The mirror box provides a reflection of the intact hand or limb that
allows the patient to "move" the phantom limb, and to unclench it from
potentially painful positions.
Although mirror therapy was introduced by VS Ramachandran in the
early 1990s, little research was done on it before 2009, and much of the
subsequent research has been of poor quality, according to a 2016
review.
A 2018 review, which also criticized the scientific quality of many
reports on mirror therapy (MT), found 15 good-quality studies conducted
between 2012 and 2017 (out of a pool of 115 publications), and
concluded that "MT seems to be effective in relieving PLP, reducing the
intensity and duration of daily pain episodes. It is a valid, simple,
and inexpensive treatment for PLP."
Other phantom sensations
Phantom sensations may also occur after the removal of body parts other than the limbs, e.g. after amputation of the breast, extraction of a tooth (phantom tooth pain) or removal of an eye (phantom eye syndrome).
Some people who have undergone gender reassignment surgery have
reported the sensation of phantom genitals. The reports were less common
among post-operativetransgender women, but did occur in transgender men. Similarly, subjects who had undergone mastectomy reported experiencing phantom breasts; these reports were substantially less common among post-operative transgender men.
Proprioception (/ˌproʊprioʊˈsɛpʃən,-priə-/PROH-pree-o-SEP-shən), also referred to as kinaesthesia (or kinesthesia), is the sense of self-movement and body position. It is sometimes described as the "sixth sense".
Proprioception is mediated by proprioceptors, mechanosensory neurons located within muscles, tendons, and joints.
There are multiple types of proprioceptors which are activated during
distinct behaviors and encode distinct types of information: limb
velocity and movement, load on a limb, and limb limits. Vertebrates and invertebrates have distinct but similar modes of encoding this information.
More recently proprioception has also been described in flowering land plants (angiosperms).
System overview
In vertebrates, limb velocity and movement (muscle length and the rate of change) are encoded by one group of sensory neurons (Type Ia sensory fiber) and another type encode static muscle length (Group II neurons).
These two types of sensory neurons compose muscle spindles. There is a
similar division of encoding in invertebrates; different subgroups of
neurons of the Chordotonal organ encode limb position and velocity.
To determine the load on a limb, vertebrates use sensory neurons in the Golgi tendon organs:
type Ib afferents. These proprioceptors are activated at given muscle
forces, which indicate the resistance that muscle is experiencing.
Similarly, invertebrates have a mechanism to determine limb load: the Campaniform sensilla. These proprioceptors are active when a limb experiences resistance.
A third role for proprioceptors is to determine when a joint is at a specific position. In vertebrates, this is accomplished by Ruffini endings and Pacinian corpuscles.
These proprioceptors are activated when the joint is at a threshold,
usually at the extremes of joint position. Invertebrates use hair plates to accomplish this; a row of bristles located along joints detect when the limb moves.
Reflexes
The
sense of proprioception is ubiquitous across mobile animals and is
essential for the motor coordination of the body. Proprioceptors can
form reflex circuits with motor neurons to provide rapid feedback about
body and limb position. These mechanosensory circuits are important for
flexibly maintaining posture and balance, especially during locomotion.
For example, consider the stretch reflex, in which stretch across a muscle is detected by a sensory receptor (e.g., muscle spindle, chordotonal neurons),
which activates a motor neuron to induce muscle contraction and oppose
the stretch. During locomotion, sensory neurons can reverse their
activity when stretched, to promote rather than oppose movement.
Conscious and non-conscious
In humans, a distinction is made between conscious proprioception and non-conscious proprioception:
A non-conscious reaction is seen in the human proprioceptive reflex, or righting reflex—in the event that the body tilts in any direction, the person will cock their head back to level the eyes against the horizon. This is seen even in infants as soon as they gain control of their neck muscles. This control comes from the cerebellum, the part of the brain affecting balance.
Mechanisms
Proprioception is mediated by mechanically sensitive proprioceptor neurons distributed throughout an animal's body. Most vertebrates possess three basic types of proprioceptors: muscle spindles, which are embedded in skeletal muscle fibers, Golgi tendon organs, which lie at the interface of muscles and tendons, and joint receptors, which are low-threshold mechanoreceptors embedded in joint capsules. Many invertebrates, such as insects, also possess three basic proprioceptor types with analogous functional properties: chordotonal neurons, campaniform sensilla, and hair plates.
The initiation of proprioception is the activation of a proprioceptor in the periphery. The proprioceptive sense is believed to be composed of information from sensory neurons located in the inner ear (motion and orientation) and in the stretch receptors located in the muscles
and the joint-supporting ligaments (stance). There are specific nerve
receptors for this form of perception termed "proprioceptors", just as
there are specific receptors for pressure, light, temperature, sound,
and other sensory experiences. Proprioceptors are sometimes known as adequate stimuli receptors.
Members of the transient receptor potential family of ion channels have been found to be important for proprioception in fruit flies, nematode worms, African clawed frogs, and zebrafish. PIEZO2, a nonselective cation channel, has been shown to underlie the mechanosensitivity of proprioceptors in mice. Humans with loss-of-function mutations in the PIEZO2 gene exhibit specific deficits in joint proprioception, as well as vibration and touch discrimination, suggesting that the PIEZO2 channel is essential for mechanosensitivity in some proprioceptors and low-threshold mechanoreceptors.
Although it was known that finger kinesthesia relies on skin sensation, recent research has found that kinesthesia-based haptic perception relies strongly on the forces experienced during touch. This research allows the creation of "virtual", illusory haptic shapes with different perceived qualities.
An
important role for proprioception is to allow an animal to stabilize
itself against perturbations. For instance, for a person to walk or
stand upright, they must continuously monitor their posture and adjust
muscle activity as needed to provide balance. Similarly, when walking on
unfamiliar terrain or even tripping, the person must adjust the output
of their muscles quickly based on estimated limb position and velocity. Proprioceptor reflex circuits
are thought to play an important role to allow fast and unconscious
execution of these behaviors, To make control of these behaviors
efficient, proprioceptors are also thought to regulate reciprocal
inhibition in muscles, leading to agonist-antagonist muscle pairs.
Planning and refining movements
When planning complex movements such as reaching or grooming,
animals must consider the current position and velocity of their limb
and use it to adjust dynamics to target a final position. If the
animal's estimate of their limb's initial position is wrong, this can
lead to a deficiency in the movement. Furthermore, proprioception is
crucial in refining the movement if it deviates from the trajectory.
Development
In adult Drosophila, each proprioceptor class arises from a specific cell lineage
(i.e. each chordotonal neuron is from the chordotonal neuron lineage,
although multiple lineages give rise to sensory bristles). After the
last cell division, proprioceptors send out axons toward the central
nervous system and are guided by hormonal gradients to reach stereotyped
synapses.
The mechanisms underlying axon guidance are similar across invertebrates and vertebrates.
In mammals with longer gestation periods, muscle spindles are fully formed at birth. Muscle spindles continue to grow throughout post-natal development as muscles grow.
Mathematical models
Proprioceptors
transfer the mechanical state of the body into patterns of neural
activity. This transfer can be modeled mathematically, for example to
better understand the internal workings of a proprioceptor or to provide more realistic feedback in neuromechanical simulations.
A number of different proprioceptor models of varying degrees of
complexity have been developed. They range from simple phenomenological
models to complex structural models, in which the mathematical elements
correspond to anatomical features of the proprioceptor. The focus has
been on muscle spindles, but Golgi tendon organs and insects hair plates have been modeled too.
Muscle spindles
Poppelle and Bowman used linear system theory
to model mammalian muscle spindles Ia and II afferents. They obtained a
set of de-efferented muscle spindles, measured their response to a
series of sinusoidal and step function stretches, and fit a transfer
function to the spike rate. They found that the following Laplace transfer function describes the firing rate responses of the primary sensory fibers for a change in length:
More recently, Blum et al showed that the muscle spindle firing rate is modeled better as
tracking the force of the muscle, rather than the length. Furthermore,
muscle spindle firing rates show history dependence which cannot be
modeled by a linear time-invariant system model.
Golgi tendon organs
Houk and Simon provided one of the first mathematical models of a Golgi tendon organ
receptor, modeling the firing rate of the receptor as a function of the
muscle tension force. Just as for muscle spindles, they find that, as
the receptors respond linearly to sine waves of different frequencies
and has little variance in response over time to the same stimulus,
Golgi tendon organ receptors may be modeled as linear time-invariant
systems. Specifically, they find that the firing rate of a Golgi tendon
organ receptor may be modeled as a sum of 3 decaying exponentials:
where is the firing rate and is a step function of force.
The corresponding Laplace transfer function for this system is:
For a soleus receptor, Houk and Simon obtain average values of K=57 pulses/sec/kg, A=0.31, a=0.22 sec−1, B=0.4, b=2.17 sec−1, C=2.5, c=36 sec−1 .
When modeling a stretch reflex, Lin and Crago improved upon this model by adding a logarithmic nonlinearity before the Houk and Simon model and a threshold nonlinearity after.
Clinical relevance
Impairment
Temporary
loss or impairment of proprioception may happen periodically during
growth, mostly during adolescence. Growth that might also influence this
would be large increases or drops in bodyweight/size due to
fluctuations of fat (liposuction, rapid fat loss or gain) and/or muscle content (bodybuilding, anabolic steroids, catabolisis/starvation). It can also occur in those that gain new levels of flexibility, stretching, and contortion.
A limb's being in a new range of motion never experienced (or at least,
not for a long time since youth perhaps) can disrupt one's sense of
location of that limb. Possible experiences include suddenly feeling
that feet or legs are missing from one's mental self-image; needing to
look down at one's limbs to be sure they are still there; and falling
down while walking, especially when attention is focused upon something
other than the act of walking.
Proprioception is occasionally impaired spontaneously, especially when one is tired. Similar effects can be felt during the hypnagogic state of consciousness,
during the onset of sleep. One's body may feel too large or too small,
or parts of the body may feel distorted in size. Similar effects can
sometimes occur during epilepsy or migraineauras. These effects are presumed to arise from abnormal stimulation of the part of the parietal cortex of the brain involved with integrating information from different parts of the body.
Proprioceptive illusions can also be induced, such as the Pinocchio illusion.
The proprioceptive sense is often unnoticed because humans will adapt to a continuously present stimulus; this is called habituation, desensitization, or adaptation.
The effect is that proprioceptive sensory impressions disappear, just
as a scent can disappear over time. One practical advantage of this is
that unnoticed actions or sensation continue in the background while an
individual's attention can move to another concern.
People who have a limb amputated may still have a confused sense of that limb's existence on their body, known as phantom limb syndrome.
Phantom sensations can occur as passive proprioceptive sensations of
the limb's presence, or more active sensations such as perceived
movement, pressure, pain, itching, or temperature. There are a variety
of theories concerning the etiology of phantom limb
sensations and experience. One is the concept of "proprioceptive
memory", which argues that the brain retains a memory of specific limb
positions and that after amputation there is a conflict between the
visual system, which actually sees that the limb is missing, and the
memory system which remembers the limb as a functioning part of the
body.
Phantom sensations and phantom pain may also occur after the removal of
body parts other than the limbs, such as after amputation of the
breast, extraction of a tooth (phantom tooth pain), or removal of an eye
(phantom eye syndrome).
Temporary impairment of proprioception has also been known to occur from an overdose of vitamin B6
(pyridoxine and pyridoxamine). Most of the impaired function returns to
normal shortly after the amount of the vitamin in the body returns to a
level that is closer to that of the physiological norm. Impairment can
also be caused by cytotoxic factors such as chemotherapy.
It has been proposed that even common tinnitus
and the attendant hearing frequency-gaps masked by the perceived sounds
may cause erroneous proprioceptive information to the balance and
comprehension centers of the brain, precipitating mild confusion.
Proprioception is permanently impaired in patients that suffer from joint hypermobility or Ehlers-Danlos syndrome (a genetic condition that results in weak connective tissue throughout the body).
It can also be permanently impaired from viral infections as reported
by Sacks. The catastrophic effect of major proprioceptive loss is
reviewed by Robles-De-La-Torre (2006).
Proprioception is also permanently impaired in physiological aging (presbypropria).
Parkinson's disease
is characterized by a decline in motor function as a result of
neurodegeneration. It is likely that some of the symptoms of
Parkinson's disease are in part related to disrupted proprioception.
Whether this symptom is caused by degeneration of proprioceptors in
the periphery or disrupted signaling in the brain or spinal cord is an
open question.
Diagnosis
"Joint
position matching" is an established protocol for measuring
proprioception, and joint position sense specifically, without the aid
of visual or vestibular information.
During such tasks, individuals are blindfolded while a joint is moved
to a specific angle for a given period of time, returned to neutral, and
the subjects are asked to replicate the specified angle. Measured by
constant and absolute errors, ability to accurately identify joint
angles over a series of conditions is the most accurate means of
determining proprioceptive acuity in isolation to date.
Recent investigations have shown that hand dominance, participant
age, active versus passive matching, and presentation time of the angle
can all affect performance on joint position matching tasks. Joint position matching has been used in clinical settings in both the upper and lower extremities.
Proprioception is tested by American police officers using the field sobriety testing to check for alcohol intoxication.
The subject is required to touch his or her nose with eyes closed;
people with normal proprioception may make an error of no more than
20 mm (0.79 in), while people suffering from impaired proprioception (a
symptom of moderate to severe alcohol intoxication) fail this test due
to difficulty locating their limbs in space relative to their noses.
Training
Proprioception
is what allows someone to learn to walk in complete darkness without
losing balance. During the learning of any new skill, sport, or art, it
is usually necessary to become familiar with some proprioceptive tasks
specific to that activity. Without the appropriate integration of
proprioceptive input, an artist would not be able to brush paint onto a
canvas without looking at the hand as it moved the brush over the
canvas; it would be impossible to drive an automobile because a motorist
would not be able to steer or use the pedals while looking at the road
ahead; a person could not touch type or perform ballet; and people would not even be able to walk without watching where they put their feet.
Oliver Sacks reported the case of a young woman who lost her proprioception due to a viral infection of her spinal cord.
At first she could not move properly at all or even control her tone of
voice (as voice modulation is primarily proprioceptive). Later she
relearned by using her sight (watching her feet) and inner ear
only for movement while using hearing to judge voice modulation. She
eventually acquired a stiff and slow movement and nearly normal speech,
which is believed to be the best possible in the absence of this sense.
She could not judge effort involved in picking up objects and would grip
them painfully to be sure she did not drop them.
The proprioceptive sense can be sharpened through study of many disciplines. Juggling trains reaction time, spatial location, and efficient movement.[citation needed] Standing on a wobble board or balance board is often used to retrain or increase proprioception abilities, particularly as physical therapy for ankle or knee injuries. Slacklining is another method to increase proprioception.
Standing on one leg (stork standing) and various other body-position challenges are also used in such disciplines as yoga, Wing Chun and tai chi. The vestibular system of the inner ear, vision and proprioception are the main three requirements for balance. Moreover, there are specific devices designed for proprioception training, such as the exercise ball, which works on balancing the abdominal and back muscles.
History of study
The position-movement sensation was originally described in 1557 by Julius Caesar Scaliger as a "sense of locomotion". Much later, in 1826, Charles Bell expounded the idea of a "muscle sense", which is credited as one of the first descriptions of physiologic feedback mechanisms.
Bell's idea was that commands are carried from the brain to the
muscles, and that reports on the muscle's condition would be sent in the
reverse direction. In 1847 the London neurologist Robert Todd highlighted important differences in the anterolateral and posterior columns of the spinal cord, and suggested that the latter were involved in the coordination of movement and balance.
At around the same time, Moritz Heinrich Romberg, a Berlin neurologist, was describing unsteadiness made worse by eye closure or darkness, now known as the eponymous Romberg's sign, once synonymous with tabes dorsalis, that became recognised as common to all proprioceptive disorders of the legs. Later, in 1880, Henry Charlton Bastian suggested "kinaesthesia" instead of "muscle sense" on the basis that some of the afferent information (back to the brain) comes from other structures, including tendons, joints, and skin. In 1889, Alfred Goldscheider suggested a classification of kinaesthesia into three types: muscle, tendon, and articular sensitivity.
In 1906, Charles Scott Sherrington published a landmark work that introduced the terms "proprioception", "interoception", and "exteroception".
The "exteroceptors" are the organs that provide information originating
outside the body, such as the eyes, ears, mouth, and skin. The interoceptors
provide information about the internal organs, and the "proprioceptors"
provide information about movement derived from muscular, tendon, and
articular sources. Using Sherrington's system, physiologists and
anatomists search for specialised nerve endings that transmit mechanical
data on joint capsule, tendon and muscle tension (such as Golgi tendon organs and muscle spindles), which play a large role in proprioception.
Primary endings of muscle spindles "respond to the size of a
muscle length change and its speed" and "contribute both to the sense of
limb position and movement".
Secondary endings of muscle spindles detect changes in muscle length,
and thus supply information regarding only the sense of position. Essentially, muscle spindles are stretch receptors.
It has been accepted that cutaneous receptors also contribute directly
to proprioception by providing "accurate perceptual information about
joint position and movement", and this knowledge is combined with
information from the muscle spindles.
Etymology
Proprioception is from Latinproprius, meaning "one's own", "individual", and capio, capere,
to take or grasp. Thus to grasp one's own position in space, including
the position of the limbs in relation to each other and the body as a
whole.
The word kinesthesia or kinæsthesia (kinesthetic sense)
refers to movement sense, but has been used inconsistently to refer
either to proprioception alone or to the brain's integration of
proprioceptive and vestibular inputs. Kinesthesia is a modern medical
term composed of elements from Greek; kinein "to set in motion; to move" (from PIE root *keie- "to set in motion") + aisthesis "perception, feeling" (from PIE root *au- "to perceive") + Greek abstract noun ending -ia (corresponds to English -hood e.g. motherhood).
Plants
Terrestrial plants control the orientation of their primary growth through the sensing of several vectorial stimuli such as the light gradient or the gravitational acceleration. This control has been called tropism. However, a quantitative study of shootgravitropism
demonstrated that, when a plant is tilted, it cannot recover a steady
erected posture under the sole driving of the sensing of its angular
deflection versus gravity. An additional control through the continuous
sensing of its curvature by the organ and the subsequent driving an active straightening process are required.
Being a sensing by the plant of the relative configuration of its
parts, it has been called proprioception. This dual sensing and control
by gravisensing and proprioception has been formalized into a unifying mathematical model simulating the complete driving of the gravitropic movement. This model has been validated on 11 species sampling the phylogeny of land angiosperms, and on organs of very contrasted sizes, ranging from the small germination of wheat (coleoptile) to the trunk of poplar trees. This model also shows that the entire gravitropic dynamics is controlled by a single dimensionless number
called the "Balance Number", and defined as the ratio between the
sensitivity to the inclination angle versus gravity and the
proprioceptive sensitivity. This model has been extended to account for
the effects of the passive bending of the organ under its self-weight,
suggesting that proprioception is active even in very compliant stems,
although they may not be able to efficiently straighten depending on
their elastic deformation under the gravitational pull.
Further studies have shown that the cellular mechanism of proprioception in plants involves myosin and actin, and seems to occur in specialized cells. Proprioception was then found to be involved in other tropisms and to be central also to the control of nutation.
These results change the view we have on plant sensitivity. They are also providing concepts and tools for the breeding of crops that are resilient to lodging, and of trees with straight trunks and homogeneous wood quality.
The discovery of proprioception in plants has generated an interest in the popular science and generalist media. This is because this discovery questions a long-lasting a priori that we have on plants. In some cases this has led to a shift between proprioception and self-awareness or self-consciousness.
There is no scientific ground for such a semantic shift. Indeed, even
in animals, proprioception can be unconscious; so it is thought to be
in plants.
The parts of the brainstem are the midbrain, the pons, and the medulla oblongata, and sometimes the diencephalon.
Midbrain
Diagram showing the position of the colliculi. Superior shown in green and inferior in orange.
View of midbrain showing covering tectum and tegmental floor
The midbrain is further subdivided into three parts: tectum, tegmentum, and the ventral tegmental area. The tectum forms the ceiling. The tectum comprises the paired structure of the superior and inferior colliculi and is the dorsal covering of the cerebral aqueduct. The inferior colliculus
is the principal midbrain nucleus of the auditory pathway and receives
input from several peripheral brainstem nuclei, as well as inputs from
the auditory cortex. Its inferior brachium (arm-like process) reaches to
the medial geniculate nucleus of the diencephalon. The superior colliculus
is positioned above the inferior colliculus, and marks the rostral
midbrain. It is involved in the special sense of vision and sends its
superior brachium to the lateral geniculate body of the diencephalon.
Substantia nigra pars compacta: This is a concentration of neurons in the ventral portion of the midbrain that uses dopamine as its neurotransmitter and is involved in both motor function and emotion. Its dysfunction is implicated in Parkinson's disease.
Reticular formation: This is a large area in the midbrain
that is involved in various important functions of the midbrain. In
particular, it contains lower motor neurons, is involved in the pain
desensitization pathway, is involved in the arousal and consciousness
systems, and contains the locus coeruleus, which is involved in intensive alertness modulation and in autonomic reflexes.
Central tegmental tract: Directly anterior to the floor of the fourth ventricle, this is a pathway by which many tracts project up to the cortex and down to the spinal cord.
The appearance of a cadaveric brainstem from the front, with major parts labelled
In the medial part of the medulla is the anterior median fissure. Moving laterally on each side are the medullary pyramids. The pyramids contain the fibers of the corticospinal
tract (also called the pyramidal tract), or the upper motor neuronal
axons as they head inferiorly to synapse on lower motor neuronal cell
bodies within the anterior grey column of the spinal cord.
The anterolateral sulcus is lateral to the pyramids. Emerging from the anterolateral sulci are the CN XII (hypoglossal nerve) rootlets. Lateral to these rootlets and the anterolateral sulci are the olives.
The olives are swellings in the medulla containing underlying inferior
nucleary nuclei (containing various nuclei and afferent fibers).
Lateral (and dorsal) to the olives are the rootlets for CN IX (glossopharyngeal), CN X (vagus) and CN XI (accessory nerve). The pyramids end at the pontinemedulla junction, noted most obviously by the large basal pons. From this junction, CN VI (abducens nerve), CN VII (facial nerve) and CN VIII (vestibulocochlear nerve) emerge. At the level of the midpons, CN V (the trigeminal nerve) emerges. Cranial nerve III (the oculomotor nerve) emerges ventrally from the midbrain, while the CN IV (the trochlear nerve) emerges out from the dorsal aspect of the midbrain.
Between the two pyramids can be seen a decussation
of fibers which marks the transition from the medulla to the spinal
cord. The medulla is above the decussation and the spinal cord below.
From behind
The appearance of a cadaveric brainstem from behind, with major parts labelled
The most medial part of the medulla is the posterior median sulcus. Moving laterally on each side is the gracile fasciculus, and lateral to that is the cuneate fasciculus. Superior to each of these, and directly inferior to the obex,
are the gracile and cuneate tubercles, respectively. Underlying these
are their respective nuclei. The obex marks the end of the fourth ventricle and the beginning of the central canal.
The posterior intermediate sulcus separates the gracile fasciculus
from the cuneate fasciculus. Lateral to the cuneate fasciculus is the lateral funiculus.
Superior to the obex is the floor of the fourth ventricle. In
the floor of the fourth ventricle, various nuclei can be visualized by
the small bumps that they make in the overlying tissue. In the midline
and directly superior to the obex is the vagal trigone and superior to that it the hypoglossal trigone.
Underlying each of these are motor nuclei for the respective cranial
nerves. Superior to these trigones are fibers running laterally in both
directions. These fibers are known collectively as the striae medullares. Continuing in a rostral direction, the large bumps are called the facial colliculi. Each facial colliculus,
contrary to their names, do not contain the facial nerve nuclei.
Instead, they have facial nerve axons traversing superficial to
underlying abducens (CN VI) nuclei. Lateral to all these bumps
previously discussed is an indented line, or sulcus that runs rostrally, and is known as the sulcus limitans. This separates the medial motor neurons from the lateral sensory neurons. Lateral to the sulcus limitans is the area of the vestibular system,
which is involved in special sensation. Moving rostrally, the
inferior, middle, and superior cerebellar peduncles are found connecting
the midbrain to the cerebellum. Directly rostral to the superior
cerebellar peduncle, there is the superior medullary velum and then the
two trochlear nerves. This marks the end of the pons as the inferior colliculus
is directly rostral and marks the caudal midbrain. Middle cerebellar
peduncle is located inferior and lateral to the superior cerebellar
peduncle, connecting pons to the cerebellum. Likewise, inferior
cerebellar peduncle is found connecting the medulla oblongata to the
cerebellum.
The human brainstem emerges from two of the three primary brain vesicles formed of the neural tube. The mesencephalon is the second of the three primary vesicles, and does not further differentiate into a secondary brain vesicle. This will become the midbrain. The third primary vesicle, the rhombencephalon (hindbrain) will further differentiate into two secondary vesicles, the metencephalon and the myelencephalon. The metencephalon will become the cerebellum and the pons. The more caudal myelencephalon will become the medulla.
Function
There are three main functions of the brainstem:
The brainstem plays a role in conduction. That is, all
information relayed from the body to the cerebrum and cerebellum and
vice versa must traverse the brainstem. The ascending pathways coming
from the body to the brain are the sensory pathways and include the spinothalamic tract for pain and temperature sensation and the dorsal column-medial lemniscus pathway (DCML) including the gracile fasciculus and the cuneate fasciculus for touch, proprioception,
and pressure sensation. The facial sensations have similar pathways and
will travel in the spinothalamic tract and the DCML. Descending tracts
are the axons of upper motor neurons destined to synapse on lower motor
neurons in the ventral horn and posterior horn.
In addition, there are upper motor neurons that originate in the
brainstem's vestibular, red, tectal, and reticular nuclei, which also
descend and synapse in the spinal cord.
The cranial nerves III-XII emerge from the brainstem. These cranial nerves supply the face, head, and viscera. (The first two pairs of cranial nerves arise from the cerebrum).
The brainstem has integrative functions being involved in
cardiovascular system control, respiratory control, pain sensitivity
control, alertness, awareness, and consciousness. Thus, brainstem
damage is a very serious and often life-threatening problem.
Cranial nerves
A cross-section of the brainstem showing the multiple nuclei of the ten pairs of cranial nerves that emerge from it
Ten of the twelve pairs of cranial nerves either target or are sourced from the brainstem nuclei.
The nuclei of the oculomotor nerve (III) and trochlear nerve (IV) are
located in the midbrain. The nuclei of the trigeminal nerve (V),
abducens nerve (VI), facial nerve (VII) and vestibulocochlear nerve
(VIII) are located in the pons. The nuclei of the glossopharyngeal nerve
(IX), vagus nerve (X), accessory nerve (XI) and hypoglossal nerve (XII)
are located in the medulla. The fibers of these cranial nerves exit the
brainstem from these nuclei.
Clinical significance
Diseases
of the brainstem can result in abnormalities in the function of cranial
nerves that may lead to visual disturbances, pupil abnormalities,
changes in sensation, muscle weakness, hearing problems, vertigo,
swallowing and speech difficulty, voice change, and co-ordination
problems. Localizing neurological lesions in the brainstem may be very
precise, although it relies on a clear understanding on the functions of
brainstem anatomical structures and how to test them.
Duret haemorrhages are areas of bleeding in the midbrain and upper pons due to a downward traumatic displacement of the brainstem.
Cysts known as syrinxes can affect the brainstem, in a condition, called syringobulbia. These fluid-filled cavities can be congenital, acquired or the result of a tumor.
Criteria for claiming brainstem death
in the UK have developed in order to make the decision of when to stop
ventilation of somebody who could not otherwise sustain life. These
determining factors are that the patient is irreversibly unconscious and
incapable of breathing unaided. All other possible causes must be ruled
out that might otherwise indicate a temporary condition. The state of
irreversible brain damage has to be unequivocal. There are brainstem
reflexes that are checked for by two senior doctors so that imaging technology is unnecessary. The absence of the cough and gag reflexes, of the corneal reflex and the vestibulo–ocular reflex
need to be established; the pupils of the eyes must be fixed and
dilated; there must be an absence of motor response to stimulation and
an absence of breathing marked by concentrations of carbon dioxide in
the arterial blood. All of these tests must be repeated after a certain
time before death can be declared.
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