10.2 Somatosensory Tracts 2 (Dennis)

Question 1

What types of sensation are carried in the anterolateral system?

Answer 1

Crude touch

Thermal sensation

Nociception

Itching

Question 2

What is the direct pathway of the ALS from primary neuron to thalamus?

Answer 2

Primary neurons (may travel up or down 3 vertebral levels) then synapse in lamina I.

They then decusate (forming the anterior white commisure) and travel up the anterolateral system.

They synapse again at the ventral posterolateral nucleus of the thalamus

then travels thru the posterior limb of the internal capsule on its way to S1

Question 3

What is the indirect pathway of the ALS from primary neuron to thalamus?

Answer 3

Primary neurons (may travel up or down 3 vertebral levels) then synapse in lamina II or III.

They then decusate (forming the anterior white commisure) and travel up the anterolateral system.

They then synapse on the “reticular system of the spinal cord / brain stem reticular formation”.

Neurons in the reticular formation then project to the medial thalamic nuclei.

third order neurons in RF will project to intralaminar nuclei and posterior group nuclei

Question 4

How are the fibers in the ALS somatotopically arranged?

Answer 4

Opposite to the PCML pathway. The lower levels are posterolateral, and the upper levels get added anteromedially.

(Upper levels are medial, lower levels are lateral)

Question 5

What are Aδ nerve fibers responsible for?

What are C fibers responsible for?

Answer 5

Aδ nerve fibers are responsible for acute “sharp” pain.

C fibers are for dull chronic pain.

Question 6

What is the function of the spinohypothalamic fibers?

Answer 6

Nocioceptive and somatosensory issues that associates with the hypothalamus for memory formation and interpretation of pain.

Question 7

Describe the central pathway from the body of cutaneous nociceptors and primary neurons

Answer 7

free nerve endings all over the body come into contact with a stimulus and via signal from ligand gated ion channels, GPCR, etc.

nerve ending axons enter spinal cord through lateral division of the posterior root entry zone

target primary afferent lamina of I, II and V of the posterior horn

then they move within the posterolatral fasciulus (Lissaur’s tract)

decussate and ascend or descend the spinal cord

can also initate reflexes if interacting with interneurons

Question 8

What is dissociated sensory loss, and what might cause it?

Answer 8

Dissociated sensory loss is where one afferent tract (say PCML, for example) is compressed, but another one (say, ALS) is not. This leads to damage to one modality, but not the other.

Question 9

What is Brown-Sequard?

Answer 9

Contralateral: loss of pain and thermal sensation over body two segments below lesion (ALS damage)

Ipsilateral: loss of discriminative, vibrator and position sense at level of lesion (PCMLS damage) and motor loss with paralysis of extremities depending on level

Question 10

What is the blood supply to the spinal trigeminal nucleus and tract within the medulla?

Answer 10

PICA and PSA

Question 11

Pathway of facial sensory input

Answer 11

pain/temp–>trigeminal ganglion–>nucleus of spinal tract of trigeminal–>project down to C3, lateral to nucleus ->Lissaur’s tract to target the VPM of the thalamus

Question 12

What is the pars caudalis?

Answer 12

it’s a division of the spinal trigeminal nucleus

it is basically the inversion of the anatomic distribution of the facial nerve with the V1 division being the most inferior and the V3 division being most superior

most caudally located from spinal cord C2-3 to obex

Question 13

What are the sx of a lesion of the Pars Caudalis?

Answer 13

onion peel sensory loss

more caudal lesion: larger area surrounding mouth that is SPARED from sensory loss

more rostral lesion (into brain stem): sensory loss that starts at back of head and goes toward mouth

Question 14

what is the pars oralis?

Answer 14

extends from level of entry (pons) to superior medulla

receives tactile info from central face

Question 15

What is the pars interpolaris?

Answer 15

extends from superior medulla to obex

receives info from peripheral face and projects via inferior cerebellar peduncle

relays tactile info the contralateral VPM

Question 16

What sx will you see in an occulusion of the arterial vasocorona (branch of sulcal branches from ASA)

what sx will you see with an anterolateral cordotomy?

Answer 16

patchy loss of nociceptive, thermal, and touch over contralateral side of body 2 segments below lesion

complete loss of the above sensations

Question 17

What is syringomyelia?

what sx will you see with this?

Answer 17

cystic cavitation of central regions of spinal gray matter, may impinge on anterior white commissure containing Decussating ALS fibers

bilateral loss of crude touch, pain, and thermal senses several segments below lesion

cape-like distribution over shoulders to nips

Question 18

What sx will you see with herpes zoster infection?

(you better know this by now, it comes back in every block)

Answer 18

shingles

viral, known for dermatomal distribution

causes dimished sensation/postherpetic neuralgia

Question 19

What is medullary syndrome?

Waht sx will you see?

Answer 19

vascular lesions or tumors in brain stem/medial portion of pons

contralateral loss of discriminative touch and vibratory sense

pain and thermal sensation intact

dissociated sensory loss

Question 20

Describe lateral medullary Syndrome AKA Wallenberg Syndrome

Answer 20

vascular lesion to PICA

contralateral loss of pain and temp over body with ipsilateral loss of pain and temp over face

Question 21

What sx will you see with unilateral lesion of the trigeminal nerve?

Answer 21

loss of sensation over dermatomes

loss of jaw-jerk reflex (may not if only one side is injured)

atrophy of MoM

loss of ipsilateral and consensual corneal reflex

Question 22

What causes alternating analgesia of trigeminal nerve

what sx will you seE?

Answer 22

brainstem lesions in upper medulla that destory spinal tract

ipsilateral hemianalgesia of face and contralateral hemianalgesia of body

Question 23

What sx will you see with alteranting trigeminal hemiplegia?

What causes it ?

Answer 23

ipsilateral trigeminal anasthesia and paralysis

contralateral spastic hemiplegia

unilateral destruction of trigeminal nerve and CST of pons

Question 24

What are teh sx of a lesion of the central nuclei of the trigeminal nerve?

Answer 24

ipsilateral loss of pain, thermal and tactile sensation on face, scalp, mouth, teeth

ipsilateral paralysis of MoM

possible ipsilateral loss of the corneal reflex (afferent limb) –>touching corneal on other side will ilicit blink in both eyes

may not cause loss of jaw jerk reflex if only one of the trigeminal nerves is damaged

Question 25

Describe the corneal reflex

Answer 25

afferent limb from pain/touch receptors in cornea

travel on V1 to trigeminal ganglion terminaing in ipsilateral spinal trigeminal nucleus

trigeminothalamic fibers send collaterals bilaterally into facial motor nucleus (eff. limb) of the reflex

the eye blinks as a response

Question 26

Describe the trigemino-reticulo-thalamic pathway

Answer 26

pain fibers project bilaterally to reticular formation as trigeminoreticular fibers

input facilitates the ascending reticular activating system in arousal and alertness

RF helps tor regulate sleep-awake transitions and the ARAS is part of the RF, mostly made of nuclei in the thalamus

Question 28

Paths of the spinothalamic and spinomesencephalic

Answer 28

spinothalamic: spine to thalamus to VPL nuclei and VPI nuclei
spinomesencephalic: Spine to midbrane RF and periaquaductal gray

Question 29

Paths of spinobulbar , spinohypothalamic, and spinoreticular

Answer 29

spinobulbar: spine to various nuclei of the brain
spinohypothalamic: spine to hypothalamus and other nuclei
spinoreticular: (indirect) spine to medulla, pons and midbrain (RF)