Somatic sensory system

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(LABS 4 through 9 by Lao Saal, Tim Shepard, Mike Khouri, and Kevin Daly)


Somatic Sensory System

Dorsal Root Ganglion Cell

Location: Cell body in dorsal root ganglion in vertebral foramina. Pseudounipolar neuron – ie single axon that bifurcates, peripheral branch terminates in sensory receptor, central branch to CNS. Function: Transduce sensory information into neural signals. Transmit these signals to CNS. Ascend ipsilaterally, synapse on 2nd order neuron in medulla. (Lesion: extensive dermatome overlap, normally no anesthetic area. Injury commonly produces radicular pain localized to dermatome.) (Ref: NA 62,126,132)

Lissauer’s Tract

Location: White matter region that caps dorsal horn of spinal cord. Function: Location of entry of small diameter fibers (pain & temp). Branches ascend and descend and terminate in laminae 1 & 2 to synapse on 2nd order fibers which decussate and ascend in the contralateral anterolateral tract. (Lesion: Local loss of ipsilateral pain) (Ref: NA 136,138)

Dorsal Column

Location: White matter between dorsal horns. Ascending and descending branches of large diameter dorsal root ganglion neurons. Synapse on 2nd order neurons in medulla. Function: Transmit sensory information from ipsilateral side. (Lesion: Ipsilateral sensory loss caudal to lesion) (Ref: NA 65,130,128,138)

Lateral Column

Location: White matter between dorsal and ventral horns. Contains corticospinal tract. Cell bodies are in contralateral primary motor cortex. Decussation at pyramidal (motor) decussation. (Lesion: Ipsilateral paralysis below the lesion) (Ref: NA 65,66,136)

Ventral Column

Location: White matter between ventral horns. Contains ascending and descending motor pathways (ventral corticospinal tract, pontine reticulospinal tract, medial vestibulospinal tract, tectospinal tract) (Ref: NA 65,499-507)

Spinoreticular Tract

Location: Lateral & ventral spinal cord. Ascending tract of anterolateral system. Terminates in ipsilateral pontomedullary reticular formation. Some fibers to intralaminar nuclei of thalamus. Function: Regulation of arousal and the emotional aspects of pain. (Lesion: will effect contralateral side below lesion) (Ref: NA 127,129,131,132,147,148)

Spinothalamic Tract

Location: Lateral & ventral spinal cord. Ascending tract of anterolateral system. Terminates in ipsilateral VPL. Function: Carries info about painful stimuli to VPL. Mediates discrimination of pain: pin-prick localization. (Lesion will affect contralateral side below. (Ref: NA 127,129,131,148)

Spinotectal Tract

Location: Lateral & ventral spinal cord. Ascending tract of anterolateral system. Terminates in ipsilateral midbrain tectum (superior colliculus) and periaqueductal grey. Function: orientation of head and body and feedback regulation of pain. (Lesion will effect contralateral side below) (Ref: NA 127,129,131,132,148)

Marginal Zone (lamina I) and termination of lamina I projection neurons

Location: Very dorsal tip of dorsal horn of spinal cord. Function: Synapse between small diameter myelinated & unmyelinated axons mediating pain & temp and 2nd order neurons. These decussate and ascend in the contralateral anterolateral system. (Lesion: possibly ipsilateral local anesthesia of the affected dermatomes) (Ref: NA 136,137,138)

Substantia Gelatinosa (lamina II)

Location: Directly ventral to marginal zone. Function: same as marginal zone. (Ref: NA 136,137,138)

Nucleus Proprius (Laminae III, IV)

Location: Ventral to substantia gelatinosa, mid portion of dorsal horn. Function: Synapse between large diameter fibers (touch and limb position) and 2nd order neurons which ascend ipsilaterally. (Lesion: possibly ipsilateral local anesthesia depending on extent of lesion) (Ref: NA 136,137,138)

Gracile Fascicle

Location: Most medial dorsal column. Entire dorsal column below 6th thoracic segment. Function: Carries sensory information from ipsilateral lower limb and lower trunk. (Lesion: ipsilateral sensory loss (touch and limb position) below lesion) (Ref: NA 128,138,139,144,145)

Cuneate Fascicle

Location: Begins at 6th thoracic segment. Lateral to gracile fascicle. Separated by dorsal intermediate septum. Function: Carries sensory information from ipsilateral rostral trunk, upper limb, neck, occiput. (Lesion: ipsilateral sensory loss (touch and limb position) below lesion) (Ref: NA 128,138,139,144,145)

Gracile Nucleus

Location: Begins in dorsal column at level of pyramidal decussation. Very obvious at level of pyramid (lobster). Function: Synapse between 1st and 2nd order neurons for ipsilateral lower limb and caudal trunk. 2nd order neurons decussate here. (Blood supply: Posterior spinal artery) (Lesion: ipsilateral sensory loss (touch and limb position) from lower limb and caudal trunk) (Ref: NA 99,128,144)

Cuneate Nucleus

Location: same as above. Function: Synapse between 1st and 2nd order neurons for ipsilateral upper trunk, upperlimb, neck, and occiput. 2nd order neurons decussate. (Blood supply: Posterior spinal artery) (Lesion: ipsilateral sensory loss (touch and limb position) from upper trunk, upper limb, neck, occiput) (Ref: NA 99,128,144)

Internal Arcuate Fibers

Location: At level of pyramid, crossing fibers between dorsal column and medial lemniscus. Function: Decussation of 2nd order neurons mediating touch and limb position sense. (Blood supply: mostly vertebral artery) (Lesion: Ipsilateral touch and limb position sensory loss from whole body) (Ref: NA 99,144)

Periaqueductal Grey

Location: Midbrain, grey matter surrounding cerebral aqueduct. Fibers from spinomesencephalic tract terminate here. Function: Descending pain inhibition system. Neurons project to raphe nuclei. (Blood Supply: Superior cerebellar and posterior cerebral arteries) (Ref: NA 99,148)

Raphe Nuclei

Location: Numerous distinct groups of neurons in dorsal brainstem medial to medial lemniscus. Function: Use serotonin as neurotransmitter, project to dorsal horn of spinal cord. 1. Directly inhibit ascending projection neurons 2. excite inhibitory interneurons in dorsal horn which use enkephalin. (Ref: NA 87,88,147,148,515,521,523)

Midbrain Tectum

Location: Part of midbrain dorsal to cerebral aqueduct. Function: Includes superior and inferior colliculi. Integrates visual, auditory and somatic sensory information for orienting head and body. (Blood supply: Superior cerebellar, posterior cerebral arteries)

Thalamic Nuclei

Thalamic Blood Supply

Thalamic blood supply is from the posterior communicating, posterior choroidal, and posterior cerebral arteries. NA 127, 132, 149-151

Ventral posterior lateral nucleus

Processes contralateral somatic sensory and nociceptive input from the limbs and trunk, transmitted to the thalamus principally by the medial lemniscus and spinothalamic tract. Medial lemniscal projections terminate in rodshaped functional units which process a single somesthetic modality from a single body location and send projections to cortical columns. Component of spinothalamic tract which projects to VPL is known as the neospinothalamic tract. Sends projections to the more medial portions of the primary somatic sensory cortex in the postcentral gyrus. Important in discriminative aspects of somatic sensations (tap for dorsal column system, pin prick for spinothalamic tract).

Ventral posterior medial nucleus

Receives input principally from the face via the trigeminal nerve. Mediates somatic sensations of the face and perioral structures. See Trigeminal lab for more information.

Intralaminar nuclei

Medial nucleus of the thalamus. Receives projections from spinothalamic tract (known as the paleospinothalamic tract) and from the spinoreticular tract (minor projection) via neurons originating in the reticular formation of the pons and medulla. Sends diffuse projections to the cortex including projections to the somatic sensory areas of the parietal lobe, motor areas, and limbic association cortex. Important in mediating the emotional/affective aspects of pain and arousal (with reticular formation).

Cell Types

Pyramidal cells

Output cells of primary somatic sensory cortex. Have a recurrent branch which excites local neurons as well as neurons in the primary projection location. Information on projections is found in the primary somatosensory cortex description. Excitatory neurons which use glutamate or aspartate as their neurotransmitters. NS 3rd ed 364-365.

Stellate cells

Oriented vertically in the plane of the cortical columns. Receive input from thalamic neurons and convey information to other interneurons or to pyramidal cells in the same column. NS 3rd ed 779.


Primary somatosensory cortex

Located in the postcentral gyrus of the parietal lobe, receives projections from VPL and VPM via the posterior limb of the internal capsule. Organized into columns which contain information from the same peripheral location on the body and same class of peripheral sensory receptor. Axons from VP nuclei synapse in deep portion of layer 3 and throughout 4. Cortex corresponds to contralateral side of the body. The cortex is somatotopically organized with disproportionate representation of some parts of the body. Does not play an important role in processing painful stimuli. Corticocortical association neurons arise in layers 2 and 3 and project to higher order sensory cortical areas and primary motor cortex. Callosal neurons arise in layers 2 and 3 and project to the contralateral somatic sensory cortex via the body of the corpus callosum. Descending projection neurons descend to the striatum, VP nuclei, brain stem (esp. dorsal column nuclei), and the dorsal horn of the spinal cord. Thalamic projections arise in layer 6, all others arise in 5. These neurons regulate the quantity of somatic sensory information that reaches cortex. Occlusion of the anterior cerebral artery produces defects in genital and lower extremity sensation. NA 151-154.

Brodmann’s areas 1, 2, and 3

Areas 3a and 2 receive less detailed “proprioceptive” input and are important in limb position sense and shape discrimination of grasped objects. Areas 3b and 1 receive highly detailed input from mechanoreceptors and are important in touch perception.

Important Somatic Sensory System Neurotransmitters

Serotonin (5-HT)


Used by neurons in the raphe nuclei in the medulla that project to the dorsal horn of the spinal chord. 5-HT suppresses pain transmission in the dorsal horn by inhibiting ascending projection neurons and exciting inhibitory interneurons in the dorsal horn that use enkephalin. NA 148.


Used by pyramidal cells originating in the somatic sensory cortex. Probably the neurotransmitter released by A and C nociceptive fibers. NS 3rd ed 365, 390.


Trigeminal System Taken from NA Chapter 12

Trigeminal ganglion

Also called the semilunar ganglion. Contains the cell bodies of the primary afferent nerve fibers.

Spinal trigeminal tract

Small diameter afferent fibers and the descending branches of large-diameter afferent fibers travel in the STT. Extension of Lissauer's tract. Transecting this tract disrupts facial pain and temperature sense with little effect on light touch. Arranged somatotopically so that fibers originating closest to the nose enter more rostral and those farther away enter more caudal. Supplied by PICA and hence ipsilateral facial pain and temperature sense is disrupted in PICA or vertebral artery occlusion.

Trigeminal lemniscus

Ascending second order sensory neurons that synapse in VPM. The lemniscus originates in the main (or principal) trigeminal nucleus, decussates in the pons and ascends dorsomedial to the medial lemniscus.

Ventral posterior medial nucleus

Lateral division of VPM contains secondary sensory fibers from the trigeminal nuclei that synapse on third order sensory neurons. These neurons ascend in the posterior limb of the internal capsule to lateral part of primary somatic sensory cortex. Medial division of VPM (parvocellular) mediates taste and is pale in appearance on myelin stained sections.

Trigeminothalamic tract

Originates in caudal and interpolar nuclei, is predominantly crossed and ascends with anterolateral system. Terminates in VPM (involved in localization) and in the intralaminar nuclei (involved in affective and motivational aspects).

Primary somatic sensory cortex

3rd order neurons from VPM project to the lateral portions of the primary somatic sensory cortex on the postcentral gyrus. In humans, the fingers, tongue, and perioral region have larger cortical representation than other body parts. Projects to secondary somatic sensory cortex and posterior parietal cortex.

Reticular formation

Continuation of the intermediate zone of the spinal cord in the brainstem.

Sulcus limitans

Derived from the alar and basal plates, the sulcus limitans separates the efferent cranial nuclei from the afferent cranial nuclei in the pons and medulla.

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