Somatosensory Pathways

Question 1

Posterior column-medial lemniscal system is sensory for what

Answer 1

Perception of mechanical stimuli:
Size, shape & texture discrimination, 3-D shape
Conscious awareness of body position and limb movement in space

Question 2

Features of PCMLS

Answer 2

Fast conduction velocities and precise somatotopic organization

Question 3

Two-point discrimination of PCMLS

Answer 3

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

Question 4

Cell bodies of primary sensory axons for PCMLS and ALS are located where

Answer 4

Dorsal root ganglia/spinal ganglia

Question 5

Large diameter fibers relay what and enter SC where

Answer 5

Discriminative touch, flutter vibration & proprioception

Medial division of the posterior root

Question 6

Gracile fasciculus

Answer 6

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

Question 7

Cuneate fasciculus

Answer 7

Fibers from T6 and above

Question 8

Gracile and cuneate nuclei

Answer 8

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

Question 9

Medial lemniscus

Answer 9

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

Question 10

Ventral posterior nuclei

Answer 10

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

Question 11

Third order neurons of PCMLS pathway

Answer 11

Located in ventral posterolateral nucleus

Traverse posterior limb of the internal capsule and travel to primary and secondary somatosensory cortices

Question 12

Primary somatosensory cortex (SI)

Answer 12

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

Question 13

Somatosensory homonculus

Answer 13

“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

Question 14

Secondary somatosensory cortex (SII)

Answer 14

Inner face of the upper bank of lateral surface

Inputs from ipsilateral SI cortex and ventral posterior inferior nucleus of the thalamus

Question 15

Parietal cortical association lesion

Answer 15

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

Question 16

Trigeminal nerve primary afferents

Answer 16

Largely distribute from trigeminal ganglion but also from the geniculate ganglion and superior ganglion
Pain, temperature, discriminative and non-discriminative touch

Question 17

Trigeminal nuclei- different nuclei within

Answer 17

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)

Question 18

Principal sensory nucleus

Answer 18

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

Question 19

Mesencephalic nucleus

Answer 19

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

Question 20

Jaw-jerk reflex

Answer 20

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

Question 21

Mesencephalic nucleus and tract blood supply in pons

Answer 21

Long circumferential branches of basilar artery and branches of superior cerebellar artery

Question 22

Trigeminal motor and principal sensory nuclei blood supply in pons

Answer 22

Long circumferential branches of basilar artery and branches of anterior inferior cerebellar artery

Question 23

Effects of damage to primary afferent sensory nerves (side, strength, reflexes, sensation

Answer 23

Side: ipsilateral
Strength: no change
Reflexes: decreased/loss of function
Sensation: decreased/loss of function

Question 24

Effects of damage to sensory pathways in brainstem/second order neurons (side, strength, reflexes, sensation)

Answer 24

Side: contralateral
Strength: no change
Reflexes: no change
Sensation: loss of function

Question 25

Effects of damage to primary somatosensory (SI) cortex (side, strength, reflexes, sensation)

Answer 25

Side: contralateral Strength: no change Reflexes: no change Sensation: loss of function

Question 26

ALS functions

Answer 26

Nociception Thermal sensation Non discriminative touch from body/face Itching sensations

Question 27

ALS tract fiber systems

Answer 27

``` Spinothalamic Spinohypothalamic Spinomesencephalic Spinoreticular Spinobulbar ```

Question 28

Spinothalamic route

Answer 28

Spine--> Thalamus--> Ventral posterolateral nuclei & ventral posterior inferior nuclei

Question 29

Spinomesencephalic route

Answer 29

Spine--> | Midbrain reticular formation & periaqueductal gray

Question 30

Spinobulbar route

Answer 30

Spine--> | Various nuclei of the brain

Question 31

Spinoreticular route

Answer 31

Spine--> | Medulla, pons and midbrain reticular formation

Question 32

Nerve fibers from free nerve endings (such as the ones that pick up thermal) enter spinal cord through

Answer 32

Lateral division of the posterior root entry zone | Central targets include laminae I, II and V

Question 33

Fibers of ALS ascend/descend in the

Answer 33

Lassauer tract aka posterolateral fasciculus | Or stay in same spinal level and terminate on interneurons for reflexes

Question 34

Direct spinothalamic pathway

Answer 34

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

Question 35

Indirect spinothalmic pathways

Answer 35

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

Question 36

ALS somatotopic?

Answer 36

Yes, lower levels are located laterally while upper levels are medial

Question 37

Course of facial sensory distribution

Answer 37

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

Question 38

Blood supply to trigeminal structures in medulla

Answer 38

PICA and posterior spinal artery

Question 39

Locations of V1-3 in spinal trigeminal nucleus

Answer 39

Face is inverted- ophthalmic represented inferiorly, mandibular represented superiorly

Question 40

Pars caudalis

Answer 40

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

Question 41

Pars oralis

Answer 41

Extends from level of entry (pons) to superior medulla | Receives tactile info from central face

Question 42

Pars interpolaris

Answer 42

Extends from superior medulla to obex Receives info from peripheral face Projects to cerebellum via inferior cerebellar peduncle Relays tactile info to contralateral VPM

Question 43

Trigemino-reticulo-thalamic pathway

Answer 43

Pain fibers project bilaterally to reticular formation as trigeminoreticular fibers Input facilitates the ascending reticular activating system in arousal

Question 44

Blood supply to ALS

Answer 44

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

Question 45

Hemisection of spinal cord (brown-sequard)

Answer 45

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

Question 46

Syringomyelia

Answer 46

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

Question 47

Herpes Zoster infection aka shingles

Answer 47

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

Question 48

Medullary syndrome

Answer 48

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

Question 49

Lateral medullary (Wallenberg) syndrome

Answer 49

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

Question 50

Unilateral lesion of trigeminal nerve

Answer 50

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

Question 51

Alternating analgesia trigeminal

Answer 51

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

Question 52

Alternating trigeminal hemiplegia

Answer 52

Unilateral destruction of the trigeminal nerve & CST in the pons Ipsilateral trigeminal anesthesia & paralysis, & contralateral spastic hemiplegia

Question 53

Primary deficits in trigeminal nerve/nuclei lesions

Answer 53

Ipsilateral loss of pain, thermal and tactile sensations of face, scalp, oral cavity and teeth Ipsilateral paralysis of masticatory muscles

Question 54

Corneal reflex

Answer 54

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