Lecture 15: Somatosensory

Question 1

Do cells in the DRG have dendrites? Peripheral process becomes…Centrally directed process becomes…

Answer 1

No; spinal nerve; dorsal root

Question 2

2 submodalities

Answer 2

Touch, pressure, vibration & position and movement; pain and temperature

Question 3

Accessory structures for touch, pressure, vibration (4)

Answer 3

Meissner (light pressure, sensitive), Pacinian (pick up high frequency vibration, sensitive) and Ruffin’s corpuscles (pressure), Merkel’s disks

Question 4

Accessory structures for position and movement

Answer 4

Muscle spindle (within skeletal muscle, encodes length) and Golgi tendon organ (encodes tension/stretch at tendon junction)

Question 5

Pain and temperature use…

Answer 5

Free nerve endings (no myelin); will encode some crude touch

Question 6

Four principles of encoding

Answer 6

1. Each neuron encodes one type of stimulus; 2. Increase in intensity w/ increase in frequency of APs and eventual increase in # axons recruited; 3. Variations in adaption (slowly vs rapidly adapting –> fires AP at onset and offset of stimulus); 4. Receptive fields and perception acuity

Question 7

About how much overlap in dermatome map vs sensor receptive fields. Same for pain?

Answer 7

50%; no, pain is less overlapped

Question 8

Order the muscle axons. Which is totally unmyelinated?

Answer 8

Group 1a (primary muscle spindle) –> 1b (Golgi tendon organ) –> II (secondary muscle spindle) –> III and IV (pain and temperature); Group IV

Question 9

Order the cutaneous axons. Which is totally unmyelinated?

Answer 9

AB (touch, pressure) –> Ad and C (pain, temperature, crude touch); C

Question 10

What are the two major ascending systems and what do they carry?

Answer 10

1. DC-ML (mechanosensation; touch, pressure, vibration = AB; position and movement = Group I, II); 2. Spinothalamic (anterolateral) system (pain and temperature, crude touch = Ad, C; Group III, IV)

Question 11

F. gracilis where? F. cuneatus where? Significance. What’s above the facilicus? What happens here? What’s the new tract? Where does it travel?

Answer 11

All levels; T6 and above; axons entering above T6 travel on f. cuneatus; the nucleus cuneatus/gracilis; SYNAPSE and then the “great sensory decussation” (arcuate fibers); medial leminscus; thalamus

Question 12

Do all neurons ascend?

Answer 12

No! Some synapse at the level of the spinal cord (reflexes, cerebellum)

Question 13

What size of fibers take the lateral route?

Answer 13

Smaller fibers

Question 14

Lissaurer’s tract and assoicated

Answer 14

Ipsilateral 2-5 segments of bifurcated tract in spinal cord (from Spinothalamic neurons) where synapsing b/t axons and dorsal horn neurons occurs

Question 15

Where is the first synapse in the Spinothalamic tract? Then what?

Answer 15

In the spinal cord; secondary or tertiary neuron crosses the ventral white commissure to form lateral spinothalamic tract

Question 16

What are the three distinctions?

Answer 16

1. First synapse; 2. Location of decussation

Question 17

Brown-Sequard Syndrome

Answer 17

Hemi-section of spinal cord. Above lesion: intact; Below lesion: DC-ML on ipsilateral side; Spinothalamic on contralateral side

Question 18

Syringomyelia can cause…

Answer 18

Bilateral, segmental loss of pain and temperature just at the level of the lesion

Question 19

Somatotropic organization of spinal cord, medulla, pons, midbrain

Answer 19

Spinal cord (medial –> lateral) organized leg, lower trunk, upper trunk, neck; Mid-medulla (dorsal –> ventral) N, A, T, L; Pons (medial –> lateral) F, N, A, T, L; Midbrain (ventral –> dorsal) L, T, A, N, F

Question 20

Second order axons from principal sensory nucleus do what?

Answer 20

Decussate and then join medial lemniscus via trigeminal thalamic tract

Question 21

Primary neurons headed to the spinal nucleus of V do what? Then what?

Answer 21

Travel downward via spinal tract of V; decussate and join SPINOTHALAMIC tract

Question 22

Mesencephalic nucleus recieves what kind of information? Where are the primary cell bodies? What happens?

Answer 22

Proprioception (Group I, II); within mesencephalic nucleus; Secondary process (still primary cell) sends projections to the motor nucleus of V (jaw jerk reflex)

Question 23

Trigeminal: medullary lesion

Answer 23

Ipsilateral pain and temperature deficits (mostly)

Question 24

Trigeminal: pontine lesion

Answer 24

Ipsilateral touch, pressure, proprioception (muscles of mastication); damage to Vm motorneurons (areflexia); deficits in pain and temperature (ipsilateral if V root affected, contralateral if broad lesion affecting spinothalamic)

Question 25

Trigeminal: above brainstem lesion

Answer 25

All sensory modalities contralateral; motor responses (Vm) not affected because bilaterally innervated

Question 26

Ventral posterior lateral (VPL)

Answer 26

Receives input coming from dorsal column nuclei (medial lemniscal axons)

Question 27

Ventral posterior medial (VPM)

Answer 27

Receives input coming from Principal trigeminal nucleus (face)

Question 28

Somatotropic orgaization of DC-ML system in thalamus

Answer 28

Lateral –> medial: L T A N F (with lips, fingers, toes down)

Question 29

VPL and VPM cell bodies are ipsi or contralateral?

Answer 29

CONTRALATERAL

Question 30

T/F: Do we find proportional or disproportional representations of sensation in the VPL/VPM nuclei?

Answer 30

Disproportional

Question 31

Is place/submodality retained at the level of the thalamus?

Answer 31

Yes for DC-ML but NOT for Spinothalamic (large, promiscuous receptive fields)

Question 32

Is the cortex ipsi or contralateral? How about proportional representation? How is the body arranged?

Answer 32

Contral; homunculus; (dorsal –> ventral) L T A F

Question 33

What structure do the axons from the VPM/VPL thalamus project through?

Answer 33

Internal capsule

Question 34

What are the four separate functional areas of S1? Information.

Answer 34

3a, 3b, 1, 2; proprioception (Group I, II) to 3a and 2; cutaneous (AB) to 3b and 1; pain and temperature to ALL fields of S1

Question 35

Discuss S1 lesions

Answer 35

3b: all tactile cutaneous information; 1/2: partial deficit in discrimination of texture, size, shape; 3a/2: proprioceptive

Question 36

Cortex structure relationship to leminscal/spinothalamic projections

Answer 36

Layer IV: lemniscal; Layer I: spinothalamic

Question 37

What information does S2 get? What is special/different about S2? What does destruction lead to?

Answer 37

Branched axons from thalamic relay neurons headed toward S1; only single representation of information; (anterior –> posterior) E A T L; representation is BILATERAL via fiber tracts from the CC; loss of interhemispheric transfer of info and permanent impairment in object discrimination on basis of size, texture