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Somatosensory System
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General Organization of Somatosensory System
The somatosensory system is composed of the neurons that make sensing touch, temperature,
and position in space possible.
1.
fig. 1 shows a dorsal root ganglion
Sensory nerves of a dorsal root ganglion are depicted entering the spinal cord.
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Our somatosensory system consists of primary, secondary, and tertiary neurons.
Sensory receptors housed in dorsal root ganglia project to secondary neurons of the spinal
cord that decussate and project to the thalamus or cerebellum.
Tertiary neurons project to the postcentral gyrus of the parietal lobe, forming a sensory
humunculus.
A sensory homunculus maps sub-regions of the cortical poscentral gyrus to certain parts of
the body.
Decussate: Where nerve fibers obliquely cross from one lateral part of the body to the other.
Postcentral Gyrus: A prominent structure in the parietal lobe of the human brain and an
important landmark that is the location of the primary somatosensory cortex, the main
sensory receptive area for the sense of touch.
Thalamus: Either of two large, ovoid structures of gray matter within the forebrain that relay
sensory impulses to the cerebral cortex.
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2.
fig. 2 shows the sensory homunculus of the human brain
3.
fig. 3 shows the sagittal MRI of the human brain
The thalamus is marked by a red arrow in this MRI cross-section.
Our somatosensory system is distributed throughout all major parts of our body. It is responsible
for sensing touch, temperature, posture, limb position, and more. It includes both sensory
receptor neurons in the periphery - skin, muscle and organs, for example - and deeper neurons
within the central nervous system.
Structure:
A somatosensory pathway will typically consist of three neurons: primary, secondary, and
tertiary. In the periphery, the primary neuron is the sensory receptor that detects sensory stimuli
like touch or temperature. The cell body of the primary neuron is housed in
the dorsal root ganglion of a spinal nerve Figure 0 or, if sensation is in head or neck, ganglia of
the trigeminal or cranial nerves. The secondary neuron acts as a relay and is located in either
the spinal cord or the brain stem. This neuron's ascending axons will cross, or decussate, to the
opposite side of the spinal cord or brainstem and travel up the spinal cord to the brain where
most will terminate in either the thalamus Figure 2 or the cerebellum. In the case of touch, the
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tertiary neuron has its cell body in the thalamus and projects to the parietal lobe of the brain. In
the case of the maintenance of posture, the tertiary neuron is located in the cerebellum.
Processing:
The primary somatosensory area of the human cortex is located in the postcentral gyrus of
the parietal lobe. The postcentral gyrus is the location of the primary somatosensory area, the
area of the cortex dedicated to the processing of touch information. Like other sensory areas, at
this location there is a map of sensory space referred to as a sensory homunculus: sub-regions of
the poscentral gyrus map to certain parts of the body as illustrated in Figure 1, and the surface
area of cortex dedicated to a body part correlates with the amount of somatosensory input from
that area. For example, there is a large area of cortex devoted to sensation in our hands, while our
back claims a much smaller area. Somatosensory information involved with proprioception and
posture is processed in a different part of the brain: the cerebellum.
Tactile Sensation
Touch is sensed by mechanoreceptive neurons that respond to pressure in various ways.
4.
fig. 4 shows an illustration of a pacinian corpuscle, with its system of capsules and central cavity.
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Our sense of touch, or tactile sensation, is mediated by cutaneous mechanoreceptors located
in our skin.
There are four main types of cutaneous mechanoreceptors: Pacinian corpuscles, Meissner's
corpuscles, Merkel's discs, and Ruffini endings.
Cutaneous mechanoreceptors can be categorized by morphology, by what kind of
sensation they perceive and by the rate of adaptation. Furthermore, each has a
different receptive field.
Pacinian Corpuscle: Nerve endings in the skin responsible for sensitivity to vibration and
pressure.
Ruffini Ending: A class of slowly adapting mechanoreceptor thought to exist only in the
glabrousdermis and subcutaneous tissue of humans.
Merkel's Disc: Mechanoreceptors found in the skin and mucosa of vertebrates that provide
touch information regarding pressure and texture to the brain.
5.
fig. 5 shows an illustration of a Ruffini nerve ending which is a touch and temperature sensitive
receptor in our skin.
A mechanoreceptor is a sensory receptor that responds to mechanical pressure or distortion.
Normally there are four main types in glabrous skin: Pacinian corpuscles, Meissner's corpuscles,
Merkel's discs, and Ruffini endings. There are also mechanoreceptors in hairy skin. The hair
cells in the cochlea are the most sensitive mechanoreceptors, transducing air pressure waves
into nerve signals sent to the brain. In the periodontal ligament, there are some
mechanoreceptors, which allow the jaw to relax when biting down on hard objects; the
mesencephalic nucleus is responsible for this reflex.
Cutaneous mechanoreceptors are located in the skin, like other cutaneous receptors. They are all
innervated by Aβ fibers, except the mechanorecepting freenerve endings, which are innervated
by Aδ fibers. They can be categorized by morphology, by what kind of sensation they perceive
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and by the rate of adaptation. Furthermore, each has a different receptive field.
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Ruffini's end organs detect tension deep in the skin Figure 1.
Meissner's corpuscles detect changes in texture (vibrations around 50 Hz) and adapt
rapidly.
Pacinian corpuscles detect rapid vibrations (about 200–300 Hz) Figure 0.
Merkel's discs detect sustained touch and pressure.
Mechanoreceiving free nerve endings detect touch, pressure and stretching
Hair follicle receptors are located in hair follicles and sense position changes of hairs.
Cutaneous mechanoreceptors provide the senses of touch, pressure, vibration,
proprioception, and others.
Lamellar corpuscles or Pacinian corpuscles are one of the four major types of mechanoreceptors.
They are nerve endings in the skin, responsible for sensitivity to vibration and pressure.
Vibrational role may be used to detect surface, e.g., rough vs. smooth.
The Bulbous corpuscle or Ruffini ending or Ruffini corpuscle is a class of slowly adapting
mechanoreceptors thought to exist only in the glabrous dermis and subcutaneous tissue of
humans. It is named after Angelo Ruffini.
This spindle-shaped receptor is sensitive to skin stretch, and contributes to the kinesthetic sense
of and control of finger position and movement. It is believed to be useful for monitoring
slippage of objects along the surface of the skin, allowing modulation of grip on an object.
Ruffini endings are located in the deep layers of the skin, and register mechanical deformation
within joints, more specifically angle change, with a specificity of up to two degrees, as well as
continuous pressure states. They also act as thermoreceptors that respond for a long time, so in
case of deep burn there will be no pain as these receptors will be burned off.
Meissner's corpuscles (or tactile corpuscles) are a type of mechanoreceptor. They are a type
of nerve ending in the skin that is responsible for sensitivity to light touch. In particular, they
have highest sensitivity (lowest threshold) when sensing vibrations lower than 50 hertz. They are
rapidly adaptive receptors.
Merkel nerve endings are mechanoreceptors found in the skin and mucosa of vertebrates that
provide touch information to the brain. The information they provide are those regarding
pressure and texture. Each ending consists of a Merkel cell in close apposition with an
enlarged nerve terminal. This is sometimes referred to as a Merkel cell–neurite complex, or a
Merkel disk receptor. A single afferent nerve fiber branches to innervate up to 90 such endings.
They are classified as slowly adapting type I mechanoreceptors.
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Proprioceptive Sensations
Proprioception refers to the sense of knowing how one’s body is positioned in three-dimensional
space.
6.
fig. 6 shows a human brain, with the cerebellum colored in purple. The cerebellum is largely
responsible for coordinating the unconscious aspects of proprioception.
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Proprioception is the sense of the position of parts of our body and force being generated
during movement.
Proprioception relies on two, primary stretch receptors: Golgi tendon organs
andmuscle spindles.
Muscle spindles are sensory receptors within the belly of a muscle, which primarily detect
changes in the length of this muscle. They convey length information to the central nervous
system via sensory neurons. This information can be processed by the brain to determine the
position of body parts.
The Golgi organ (also called Golgi tendon organ, tendon organ, neurotendinous organ, or
neurotendinous spindle), is a proprioceptive sensory receptor organ that is located at
the insertion of skeletal muscle fibers into the tendons of skeletalmuscle.
Muscle Spindle: Sensory receptors within the belly of a muscle, which primarily detect
changes in the length of this muscle.
Golgi Tendon Organ: A proprioceptive sensory receptor organ that is located at
the insertion of skeletalmuscle fibers into the tendons of skeletal muscle.
Proprioception: The sense of the position of parts of the body, relative to other neighboring
parts of the body.
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7.
fig. 7 shows a muscle spindle
Mammalian muscle spindle showing typical position in a muscle (left), neuronal connections in
spinal cord (middle), and expanded schematic (right). The spindle is a stretch receptor with its
own motor supply consisting of several intrafusal muscle fibers. The sensory endings of a
primary (group Ia) afferent and a secondary (group II) afferent coil around the non-contractile
central portions of the intrafusal fibers.
8.
fig. 8 shows the Golgi tendon organ contributes to the Golgie tendon reflex and provides
proprioceptive information about joint position.
Proprioception, is the sense of the relative position of neighboring parts of the body and strength
of effort being employed in movement. It is distinguished from exteroception, by which we
perceive the outside world, and interoception, by which we perceive pain, hunger, etc., and the
movement of internal organs.
The initiation of proprioception is the activation of a proprioreceptor in the periphery. The
proprioceptive sense is believed to be composed of information from sensory neurons located in
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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 "proprioreceptors," just as there are specific receptors for pressure, light,
temperature, sound, and other sensory experiences.
Conscious proprioception is communicated by the posterior column-medial lemniscus pathway
to the cerebrum. Unconscious proprioception is communicated primarily via the dorsal
spinocerebellar tract, to the cerebellum. An unconscious reaction is seen in the human
proprioceptive reflex, or Law of Righting – 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.
Proprioception is occasionally impaired spontaneously, especially when one is tired. One's body
may appear too large or too small, or parts of the body may appear distorted in size. Similar
effects can sometimes occur during epilepsy or migraine auras. 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.
Stretch receptors are mechanoreceptors responsive to distention of various organs and muscles.
They are neurologically linked to the medulla in the brain stem via afferent nerve fibers.
Examples include stretch receptors in the arm and leg muscles and tendons, in the heart, in
the colon wall, and in the lungs. Stretch receptors are also found around the carotid artery, where
they monitor bloodpressure and stimulate the release of ADH from posterior pituitary gland.
Muscle spindles are sensory receptors within the belly of a muscle, which primarily detect
changes in the length of this muscle Figure 1. They convey length information to the
central nervous system via sensory neurons. This information can be processed by the brain to
determine the position of body parts. The responses of muscle spindles to changes in length also
play an important role in regulating the contraction of muscles, by activating motoneurons via
the stretch reflex to resist muscle stretch.
The Golgi organ (also called Golgi tendon organ, tendon organ, neurotendinous organ or
neurotendinous spindle) Figure 2, is a proprioceptive sensory receptororgan that is located at
the insertion of skeletal muscle fibers into the tendons of skeletal muscle. It provides the sensory
component of the Golgi tendon reflex. The Golgi organ should not be confused with the Golgi
Apparatus, which is an organellein the eukaryotic cell, or the Golgi stain, which is a histologic
stain for neuron cell bodies.
The Golgi tendon reflex is a normal component of the reflex arc of the peripheral nervous
system. In a Golgi tendon reflex, skeletal muscle contraction causes theagonist muscle to
simultaneously lengthen and relax. This reflex is also called the inverse myotatic reflex, because
it is the inverse of the stretch reflex. Although muscle tension is increasing during the
contraction, alpha motor neurons in the spinal cord supplying the muscle are inhibited. However,
antagonistic muscles are activated.
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Somatic Sensory Pathways
The somatosensory pathway is composed of three neurons located in the dorsal root ganglion,
the spinal cord, and the thalamus.
9.
fig. 9 shows a pictorial representation of the anatomical divisions of the primary sensory cortex.
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A somatosensory pathway will typically have three neurons: primary, secondary, and tertiary.
The cell bodies of the three neurons in a typical somatosensory pathway are located in
the dorsal root ganglion, the spinal cord, and the thalamus, respectively.
A major target of somatosensory pathways is the postcentral gyrus in the parietal lobe of
the cerebral cortex.
A major somatosensory pathway is the Dorsal Column Medial Lemniscal pathway.
The postcentral gyrus is the location of the primary somatosensory area which takes the form
of a map called the sensory homunculus.
Postcentral Gyrus: A prominent structure in the parietal lobe of the human brain and an
important landmark that is the location of the primary somatosensory cortex, the main
sensory receptive area for the sense of touch.
Parietal Lobe: A part of the brain positioned superior to the occipital lobe and posterior to
the frontal lobe, that integrates sensory information from different modalities, particularly
determining spatial sense and navigation.
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Thalamus: Either of two large, ovoid structures of gray matter within the forebrain that relay
sensory impulses to the cerebral cortex.
10.
fig. 10 shows a spinal nerve with its anterior and posterior roots. The dorsal root ganglion is the
"spinal ganglion", following the posterior/dorsal root.
A somatosensory pathway will typically have three long neurons: primary, secondary and
tertiary. The first always has its cell body in the dorsal root ganglion of the spinal nerve. The
second has its cell body either in the spinal cord or in the brainstem; this neuron's ascending
axons will cross to the opposite side either in the spinal cord or in the brainstem. The axons of
many of these neurons terminate in the thalamus, others terminate in the reticular system or the
cerebellum. In the case of touch and certain types of pain, the third neuron has its cell body in
the ventral posterior nucleus of the thalamus and ends in the postcentral gyrus of the parietal
lobe.
In the periphery, the somatosensory system detects various stimuli by sensory receptors, e.g., by
mechanoreceptors for tactile sensation and nociceptors for painsensation. The sensory
information (touch, pain, temperature, etc.,) is then conveyed to the central nervous
system by afferent neurons, of which there are a number of different types which vary in their
size, structure and properties. Generally, there is a correlation between the type of sensory
modality detected and the type of afferent neuron involved. For example, slow, thin, unmyelinated neurons conduct pain whereas faster, thicker, myelinated neurons conduct casual
touch.
Ascending Pathways:
In the spinal cord, the somatosensory system includes ascending pathways from the body to
the brain (Figure 2). One major target within the brain is thepostcentral gyrus in the cerebral
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cortex. This is the target for neurons of the Dorsal Column Medial Lemniscal pathway and the
Ventral Spinothalamic pathway. Note that many ascending somatosensory pathways include
synapses in either the thalamus or the reticular formation before they reach the cortex. Other
ascending pathways, particularly those involved with control of posture, are projected to the
cerebellum, including the ventral and dorsal spinocerebellar tracts. Another important target
for afferent somatosensory neurons which enter the spinal cord are those neurons involved with
local segmental reflexes.
Parietal Lobe: Primary Somatosensory Area:
The primary somatosensory area in the human cortex is located in the postcentral gyrus of
the parietal lobe. This is the main sensory receptive area for the sense of touch. Like other
sensory areas, there is a map of sensory space called a homunculus at this location. For
the primary somatosensory cortex, this is called the sensory homunculus. Areas of this part of the
human brain map to certain areas of the body, dependent on the amount or importance of
somatosensory input from that area. For example, there is a large area of cortex devoted
tosensation in the hands, while the back has a much smaller area. Somatosensory information
involved with proprioception and posture also targets an entirely different part of the brain, the
cerebellum.
Cortical Homunculus:
This is a pictorial representation of the anatomical divisions of the primary motor cortex and
the primary somatosensory cortex, i.e., the portion of the human braindirectly responsible for the
movement and exchange of sensory and motor information of the body.
The cortical homunculus is a visual representation of the concept of "the body within the brain" - that one's hand or face exists as much as a series of nervestructures or a "neuron concept" as it
does in a physical form. This concept relates to many neuro-biological phenomena including
"phantom limb" and "body integrity identity disorder".
Thalmus:
The thalamus is a midline symmetrical structure within the brain of vertebrates including
humans, situated between the cerebral cortex and midbrain (Figure 0). Its function includes
relaying sensory and motor signals to the cerebral cortex, along with the regulation of
consciousness, sleep, and alertness.
The thalamus surrounds the third ventricle. It is the main product of the embryonic diencephalon.
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Mapping Primary Somatosensory Area
The cortical sensory homuculus is a representation of the body by the brain located on the postcentral gyrus.
11.
fig. 11 shows the sensory homunculus
The idea of the cortical homunculus was created by Wilder Penfield and serves as a rough map
of the receptive fields for regions of primary somatosensory cortex.
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A sensory homunculus is a pictorial representation of the primary somatosensory cortex.
Somatotopy is the correspondence of an area of the body to a specific point in the brain.
Wilder Penfield was a researcher and surgeon who created maps of the somatosensory
cortex.
Sensory Homunculus: A pictorial representation of the anatomical divisions of the primary
somatosensory cortex.
Postcentral Gyrus: A prominent structure in the parietal lobe of the human brain and an
important landmark that is the location of the primary somatosensory cortex, the main
sensory receptive area for the sense of touch.
Somatotopy: The correspondence between the position of a receptor in part of the body and
the corresponding area of the cerebral cortex that is activated by it.
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2.
fig. 2 shows the postcentral gyrus
Positioned in the parietal lobe of the human cortex and the location of the primary
somatosensory cortex.
A cortical homunculus is a pictorial representation of the anatomical divisions of the primary
motor cortex and the primary somatosensory cortex, i.e., the portion of the human brain directly
responsible for the movement and exchange of sensory and motor information of the body. It is a
visual representation of the concept of "the body within the brain" -- that one's hand or face
exists as much as a series of nerve structures or a "neuron concept" as it does in a physical form.
There are two types of homunculus: sensory and motor. Each one shows a representation of how
much of its respective cortex innervates certain body parts.
The primary somesthetic cortex (sensory) pertains to the signals within thepostcentral
gyrus coming from the thalamus, and the primary motor cortexpertains to signals within the
precentral gyrus coming from the premotor area of the frontal lobes. These are then transmitted
from the gyri to the brain stem and spinal cord via corresponding sensory or motor nerves.
The reason for the distorted appearance of the homunculus is that the amount of cerebral tissue
or cortex devoted to a given body region is proportional to how richly innervated that region is,
not to its size. The homunculus is like an upside-down sensory or motor map of
the contralateral side of the body. This can be seen, since the upper extremities such as the facial
body parts and hands are closer to the lateral sulcus than lower extremities such as the leg and
toes.
The resulting image is a grotesquely disfigured human with disproportionately huge hands, lips,
and face in comparison to the rest of the body. Because of the fine motor skills and sense nerves
found in these particular parts of the body, they are represented as being larger on the
homunculus. A part of the body with fewer sensory and/or motor connections to the brain is
represented to appear smaller.
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Somatotopy:
This is the point-for-point correspondence of an area of the body to a specific point on the
central nervous system. Typically, the area of the body corresponds to a point on the primary
somatosensory cortex (postcentral gyrus).
This cortex is typically represented as a sensory homunculus which orients the specific body
parts and their respective locations upon the homunculus. Areas such as the appendages, digits,
and face can draw their sensory locations upon the somatosensory cortex. The areas which are
finely controlled (i.e., the digits) have larger portions of the somatosensory cortex whereas areas
which are coarsely controlled (i.e., the trunk) have smaller portions. Areas such as the viscera do
not have sensory locations on the post central gyrus.
Montreal Procedure:
Penfield was a groundbreaking researcher and highly original surgeon. With his colleague,
Herbert Jasper, he invented the Montreal procedure, in which he treated patients with severe
epilepsy by destroying nerve cells in the brain where the seizures originated. Before operating,
he stimulated the brain with electrical probes while the patients were conscious on the operating
table (under only local anesthesia), and observed their responses. In this way he could more
accurately target the areas of the brain responsible, reducing the side-effects of the surgery.
This technique also allowed him to create maps of the sensory and motor cortices of the brain
(see cortical homunculus) showing their connections to the various limbs and organs of the body.
These maps are still used today, practically unaltered.
Along with Herbert Jasper, he published this work in 1951 (2nd ed., 1954) as the
landmark Epilepsy and the Functional Anatomy of the Human Brain. This work contributed a
great deal to understanding the lateralization of brain function. Penfield's maps showed
considerable overlap between regions (i.e., the motor region controlling muscles in the hand
sometimes also controlled muscles in the upper arm and shoulder), a feature which he put down
to individual variation in brain size and localization; we now know that this is due to the
fractured somatotropy of the motor cortex.
Somatic Sensory Pathways to Cerebellum
The ventral spinocerebellar tract conveys proprioceptive information from the body to the
cerebellum.
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The main somatosensory pathways that communicate with the cerebellum are the ventral (or
anterior) and dorsal (or posterior) spinocerebellar tracts.
The ventral spinocerebellar tract will cross to the opposite side of the body then cross again
to end in the cerebellum (referred to as a "double cross"), as compared to
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the dorsal spinocerebellar tract, which does not decussate, or cross sides, at all through its
path.
The dorsal spinocerebellar tract (posterior spinocerebellar tract, Flechsig's fasciculus,
Flechsig's tract) conveys inconscient proprioceptive information from the body to
the cerebellum.
Dorsal Spinocerebellar Tract: A neuronal pathway that conveys subconscious
proprioceptive information from the body to the cerebellum.
Ventral Spinocerebellar Tract: A neuronal pathway that conveys touch and proprioceptive
information the the cerebellum. Unlike the dorsal spinocerebellar tract, the ventral tract will
cross (or decussate) twice from one side of the spinal cord to the other prior to reaching the
cerebellum.
A sensory system is a part of the nervous system responsible for processing sensory information.
This system consists of sensory receptors, neural pathways, and parts of the brain involved in
sensory perception. Commonly recognized sensory systems are those for vision,
hearing, somatic sensation (touch), taste, and olfaction (smell). In short, senses are transducers
from the physical world to the realm of the mind where we interpret the information, creating
our perception of the world around us.
The ventral spinocerebellar tract conveys proprioceptive information from the body to
the cerebellum Figure 0. It is part of the somatosensory system and runs in parallel with
the dorsal spinocerebellar tract. Both these tracts involve two neurons.
The ventral spinocerebellar tract will cross to the opposite side of the body then cross again to
end in the cerebellum (referred to as a "double cross"), as compared to the dorsal spinocerebellar
tract, which does not decussate, or cross sides, at all through its path.
The ventral tract (under L2/L3) gets its proprioceptive/fine touch/vibration information from a
first order neuron, with its cell body in a dorsal ganglion. The axon runs via the fila radicularia to
the dorsal horn of the gray matter. There it makes a synapse with the dendrites of two neurons:
they send their axons bilaterally to the ventral border of the lateral funiculi.
The ventral spinocerebellar tract then enters the cerebellum via the superior cerebellar peduncle.
This is in contrast with the dorsal spinocerebellar tract (C8 - L2/L3), which only has one
unilateral axon that has its cell body in the Clarke's nuclei (only at the level of C8 - L2/L3). The
fibers of the ventral spinocerebellar tract then eventually enter the cerebellum via the superior
cerebellar peduncle. This is one of the few afferenttracts through the superior cerebellar
peduncle. Axons first cross midline in the spinal cord and run in the ventral border of the lateral
funiculi. These axons ascend to the pons where they join the superior cerebellar peduncle to enter
the cerebellum. Once in the deep white matter of the cerebellum, the axons recross the midline,
give off collaterals to the globose and emboliform nuclei, and terminate in the cortex of the
anterior lobe and vermis of the posterior lobe.
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The dorsal spinocerebellar tract (posterior spinocerebellar tract, Flechsig's fasciculus,
(Flechsig’s tract) conveys inconscient proprioceptive information from the body to the
cerebellum. It is part of the somatosensory system and runs in parallel with the ventral
spinocerebellar tract. Proprioceptive information is taken to the spinal cord via central processes
of dorsal root ganglia (first order neurons). These central processes travel through the dorsal horn
where they synapse with second order neurons of Clarke's nucleus. Axon fibers from Clarke's
Nucleus convey this proprioceptive information in the spinal cord in the peripheral region of the
posteriolateral funiculus ipsilaterally until it reaches the cerebellum, where unconscious
proprioceptive information is processed. This tract involves two neurons and ends up on the
same side of the body.
Source URL: https://www.boundless.com/physiology/peripheral-nervous-system-pns/somatosensory-system/
Saylor URL: http://www.saylor.org/courses/psych402/
Attributed to: [Boundless]
www.saylor.org
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