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Somatosensory System Boundless

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.

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.

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.

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.

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, un-

myelinated 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 post-

central 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.

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.

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.