Somatosensory Pathways Flashcards
Posterior column-medial lemniscal system is sensory for what
Perception of mechanical stimuli:
Size, shape & texture discrimination, 3-D shape
Conscious awareness of body position and limb movement in space
Features of PCMLS
Fast conduction velocities and precise somatotopic organization
Two-point discrimination of PCMLS
Ability to discriminate b/w two stimuli simultaneously
-Varies widely over different parts of the body and is related to density of peripheral nerve endings
Digits & perioral region have high density receptors & small receptive fields, back has low density and large receptive fields
Cell bodies of primary sensory axons for PCMLS and ALS are located where
Dorsal root ganglia/spinal ganglia
Large diameter fibers relay what and enter SC where
Discriminative touch, flutter vibration & proprioception
Medial division of the posterior root
Gracile fasciculus
Located medial to cuneate fasciculus
Contains fibers from T6 and below
Lesion in this fasciculus would cause ipsilateral loss of discriminative, positional and vibratory tactical sensations at and below level of injury
Patient may have sensory ataxia, loss of tendon reflex and may have wide based stance and place feet down with force to try to create the missing sensation
Cuneate fasciculus
Fibers from T6 and above
Gracile and cuneate nuclei
Located in posterior medulla
Contain second order neurons of PCLMS
Receive input from first order neurons from ipsilateral dorsal root ganglion
Send axons to contralateral thalamus
Cross midline of medulla as internal arcuate fibers, ascend as medial lemniscus on opposite side
Medial lemniscus
Rotates laterally in the pons- UE fibers are medial and LE fibers are lateral
Shifts even more laterally in the midbrain
Terminates in ventral posterolateral nucleus of the thalamus
Ventral posterior nuclei
Located in caudal thalamus, contains posterolateral and posteromedial portions
Somatotopic arrangement is maintained
Supplied by thalamogeniculate branches of the PCA
Lesions result in loss of all tactile sensation over the contralateral body and head
Third order neurons of PCMLS pathway
Located in ventral posterolateral nucleus
Traverse posterior limb of the internal capsule and travel to primary and secondary somatosensory cortices
Primary somatosensory cortex (SI)
Comprises postcentral gyrus and posterior paracentral gyrus
Blood supply provided by anterior and middle cerebral arteries
MCA lesion produces tactile loss over contralateral upper body/face
ACA lesion affects the contralateral lower limbs
Somatosensory homonculus
“Foot to tongue” pattern along medial–> lateral axis
Regions with higher receptor density have larger amount of dedicated cortical tissue than those with low density
Secondary somatosensory cortex (SII)
Inner face of the upper bank of lateral surface
Inputs from ipsilateral SI cortex and ventral posterior inferior nucleus of the thalamus
Parietal cortical association lesion
Association regions also receive tactile inputs
This region is located underneath the fold above the lateral sulcus
Lesions in parietal association area can produce agnosia
Limb is not recognized as part of patients own body
Sensation is not radically altered
Trigeminal nerve primary afferents
Largely distribute from trigeminal ganglion but also from the geniculate ganglion and superior ganglion
Pain, temperature, discriminative and non-discriminative touch
Trigeminal nuclei- different nuclei within
Long, continuous column extending from rostral midbrain to spinomedullary junction
Mesencephalic nucleus -midbrain
Principal sensory nucleus-
Trigeminal motor nucleus-mid pons
Spinal nucleus-obex–>C2/3 (pain temperature, non discriminative touch)
Principal sensory nucleus
Located in mid pons, contains second order neurons- discriminative touch, pressure, proprioception
Dorsomedial division- oral cavity- project ipsilaterally to VPM via posterior trigeminothalamic tract
Ventrolateral division- V1, V2, V3- project contralaterally to VPM via anterior trigeminothalamic tract
Mesencephalic nucleus
Contains primary afferents conveying proprioceptive and pressure info for TMJ, masticatory and extraocular muscles
Only ‘nucleus’ within the CNS
Projects to the principal sensory nucleus and spinal nucleus
Some axons terminate in trigeminal motor nucleus forming circuit for jaw-jerk reflex
Jaw-jerk reflex
Stretching the masseter (downward tap on chin), causes it to contract bilaterally
Afferent limb- mesencephalic trigeminal neuron whose peripheral process innervates a masseter muscle spindle and whose central process synapses on a trigeminal motor neuron (efferent limb)
Amplitude of reflex is typically minor but is enhanced after UMN damage
Mesencephalic nucleus and tract blood supply in pons
Long circumferential branches of basilar artery and branches of superior cerebellar artery
Trigeminal motor and principal sensory nuclei blood supply in pons
Long circumferential branches of basilar artery and branches of anterior inferior cerebellar artery
Effects of damage to primary afferent sensory nerves (side, strength, reflexes, sensation
Side: ipsilateral
Strength: no change
Reflexes: decreased/loss of function
Sensation: decreased/loss of function
Effects of damage to sensory pathways in brainstem/second order neurons (side, strength, reflexes, sensation)
Side: contralateral
Strength: no change
Reflexes: no change
Sensation: loss of function
Effects of damage to primary somatosensory (SI) cortex (side, strength, reflexes, sensation)
Side: contralateral
Strength: no change
Reflexes: no change
Sensation: loss of function
ALS functions
Nociception
Thermal sensation
Non discriminative touch from body/face
Itching sensations
ALS tract fiber systems
Spinothalamic Spinohypothalamic Spinomesencephalic Spinoreticular Spinobulbar
Spinothalamic route
Spine–>
Thalamus–>
Ventral posterolateral nuclei & ventral posterior inferior nuclei
Spinomesencephalic route
Spine–>
Midbrain reticular formation & periaqueductal gray
Spinobulbar route
Spine–>
Various nuclei of the brain
Spinoreticular route
Spine–>
Medulla, pons and midbrain reticular formation
Nerve fibers from free nerve endings (such as the ones that pick up thermal) enter spinal cord through
Lateral division of the posterior root entry zone
Central targets include laminae I, II and V
Fibers of ALS ascend/descend in the
Lassauer tract aka posterolateral fasciculus
Or stay in same spinal level and terminate on interneurons for reflexes
Direct spinothalamic pathway
Enters posterolateral fasciculus (lissauers tract) and bifurcates
Ascending branches terminate on 2nd order neurons of posterior horn, which eventually project to lateral thalamus
Axons cross midline via anterior white commissure
Ascend in contralateral ALS, few will ascend in ipsilateral ALS (not clinically relevant)
Thalamic/3rd order neurons are located in VPL and travel to sensory cortex
Descending branches terminate on interneurons within gray matter and may contribute to segmental spinal reflexes
Indirect spinothalmic pathways
These are spinoreticular fibers which terminate in the RF
Relay noxious and innocuous mechanical/thermal info
Signals that your body is aware of, but not as directly
Fibers ascend/descend in posterolateral fasciculus
Synapse in laminae II and III, also influence cells in V-VIII
Send axons that cross to join contralateral ALS
Third order neurons in RF project to intralaminar nuclei and posterior group nuclei
ALS somatotopic?
Yes, lower levels are located laterally while upper levels are medial
Course of facial sensory distribution
Second order axons from caudal nucleus decussate and ascend in anterior trigeminothalamic tract
Terminates in the contralateral VPM of thalamus
Tertiary axons extend in posterior limb of internal capsule and travel to primary somatosensory cortex
Blood supply to trigeminal structures in medulla
PICA and posterior spinal artery
Locations of V1-3 in spinal trigeminal nucleus
Face is inverted- ophthalmic represented inferiorly, mandibular represented superiorly
Pars caudalis
Most caudal part of trigeminal nucleus, extending from spinal cord C2-3 to the obex
Circumoral and intraoral fibers terminate near obex
Caudal and lateral fibers terminate in more caudal regions of spinal cord
Pars oralis
Extends from level of entry (pons) to superior medulla
Receives tactile info from central face
Pars interpolaris
Extends from superior medulla to obex
Receives info from peripheral face
Projects to cerebellum via inferior cerebellar peduncle
Relays tactile info to contralateral VPM
Trigemino-reticulo-thalamic pathway
Pain fibers project bilaterally to reticular formation as trigeminoreticular fibers
Input facilitates the ascending reticular activating system in arousal
Blood supply to ALS
Originates from arterial vasocorona & via sulcal branches of anterior spinal artery
Occlusion results in patchy loss of nociceptive, thermal and touch over contralateral body beginning about 2 segments below lesion
Anterolateral cordotomy results in complete loss of these sensations
Hemisection of spinal cord (brown-sequard)
Contralateral loss of nociceptive and thermal sensations over body beginning two segments below
Ipsilateral loss of discriminative tactile, vibratory and position sense over body at and below level of lesion
Motor loss with paralysis of extremities
Syringomyelia
Cystic cavitation of central regions of spinal gray matter
May impinge on anterior white commissure that contains decussating ALS fibers
When this occurs at C4-C5:
Bilateral loss of nondiscriminative tactile, nociceptive and thermal sensations
Starts about 2 segments below lesion
Cape-like distribution of loss- over the shoulders down to nipple area and down the arms
Herpes Zoster infection aka shingles
Viral DNA makes home in nervous system
Can reactivate during stress and travel down neurons to produce skin irritation over the dermatome
Diminished sensibility over those dermatomes
Postherpetic neuralgia
Medullary syndrome
In the medulla, ALS fibers are near anterolateral surface
They remain separate from PCMLS through medulla and pons
Vascular lesions or tumors in brainstem can affect discriminative touch and nociception differentially
Ex: lesion at medial portion of medulla:
Contralateral loss of discriminative touch and vibration
Pain and thermal sensation remain intact
Lateral medullary (Wallenberg) syndrome
Vascular lesion to the PICA (supplies the territory of the ALS and the spinal trigeminal nucleus/tract
Contralateral loss of pain and temp over body with ipsilateral loss of pain and temp over the face
Unilateral lesion of trigeminal nerve
Anesthesia and loss of general sensations in trigeminal dermatomes
Loss of jaw-jerk reflex
Atrophy of muscles of mastication
Loss of ipsilateral and consensual corneal reflex
Alternating analgesia trigeminal
Brainstem lesions in the upper medulla may destroy the primary fibers in the spinal trigeminal tract & secondary fibers in the spinal lemniscus
Patients demonstrate ipsilateral hemianalgesia of the face and contralateral hemianalgesia of the body
Alternating trigeminal hemiplegia
Unilateral destruction of the trigeminal nerve & CST in the pons
Ipsilateral trigeminal anesthesia & paralysis, & contralateral spastic hemiplegia
Primary deficits in trigeminal nerve/nuclei lesions
Ipsilateral loss of pain, thermal and tactile sensations of face, scalp, oral cavity and teeth
Ipsilateral paralysis of masticatory muscles
Corneal reflex
Afferent limb originates from pain/touch receptors in cornea
Fibers travel on V1 and have cell bodies in trigeminal ganglion- terminate in ipsilateral trigeminal nucleus
Trigeminothalamic fibers send collaterals bilaterally into the facial motor nucleus- both eyes will blink
Touching the cornea on contralateral side of a trigeminal lesion will result in a blink from that eye as well as the contralateral eye
