Neuropathy, Somatosensory Pathways

Question 1

describe the properties of the following types of afferent axons:
a. A-alpha
b. A-beta
c. A-delta
d. C

Answer 1

A-alpha and A-beta: large diameter, heavily myelinated (fast) - sense pressure, sense, vibration

A-delta and C: small diameter, little/no myelin (slow) - pain and temperature

Question 2

contrast the properties of efferent fibers of the motor vs autonomic system

Answer 2

motor efferents have large diameters, heavily myelinated, fast conducting

autonomic efferents are lightly myelinated or unmyelinated, slow conducting

Question 3

what kind of symptoms would accompany disease process affecting mainly small nerve fibers vs those affecting mainly myelinated large fibers?

Answer 3

disease of small fibers —> pain, temp, and autonomic loss

disease of large myelinated fibers —> vibration, proprioception, and motor loss

Question 4

neuropathic pain may present with allodynia and dysesthesias - describe these phenomena

Answer 4

allodynia = lowering of pain threshold

dysesthesias = abnormal sensations to sensory stimuli

Question 5

contrast presentation of radiculopathy vs mononeuropathy

Answer 5

radiculopathy symptoms follow nerve root pattern, often caused by compression of nerve roots from protruding discs

mononeuropathy symptoms follow peripheral nerve, often caused by injuries

[and polyneuropathy is a generalized process affecting peripheral nerves]

Question 6

describe Wallerian degeneration following axon damage

Answer 6

aka “dying forward” - distal axon degenerates, nucleolus expands and moves peripherally (“chromatolysis”), Nissle substance disintegrates

Question 7

most nutritional, metabolic, and toxic etiologies of non-traumatic peripheral neuropathy present with what type of distribution? (pattern of nerve damage)

Answer 7

“glove and stocking”, aka length-dependent polyneuropathy - symptoms begin in feet, moves more proximally up legs and distal upper arms

ex: diabetic neuropathy, vitamin deficiency (B6, B12, thiamine), uremia, isoniazid, colchicine

Question 8

What is the most common metabolic neuropathy (read: metabolic cause of neuropathy). What are some key clinical features?

Answer 8

vitamin B12 deficiency - may affect peripheral nerves, optic nerves, spinal cord, and brain (required for myelin production)

symptoms begin at distal limbs, more commonly in upper limbs

loss of vibration is most common feature

cause of subacute combined degeneration (lateral and dorsal columns of spinal cord)

[also recall B12 deficiency can cause pernicious anemia]

Question 9

how does Guillain Barre present?

Answer 9

aka acute inflammatory demyelinating polyneuropathy - most common cause of acute paralysis, most rapidly progressive form of neuropathy (autoimmune, usually follows infection or vaccination)

primarily motor - ascending paralysis, may begin with paresthesias in toes/fingers, sensory loss is mainly vibration/proprioception (carried by myelinated fibers!)

Question 10

the 2 most important diagnostic tests for Guillain-Barre are…

Answer 10

[affects myelinated fibers —> ascending paralysis, loss of vibration/proprioception]

1. CSF analysis: shows increased protein with normal leukocytes (“albuminocytological dissociation”)
2. nerve conduction velocity studies: shows decreased conduction velocity

Question 11

how does Charcot-Marie-Tooth Disease present?

Answer 11

aka “hereditary motor and sensory neuropathy”: affects either myelin (CMT1) or axons (CMT2) directly

CMT1 most common —> combined motor sensory neuropathy: primarily distal muscle + vibration/proprioception (myelinated)

onset in late childhood, slowly progresses to affect other nerves

Question 12

order from superficial to deep in the dermis: Ruffini corpuscle, Merkel cells, Meissner corpuscle, Pacinian corpuscle

Answer 12

Meissner corpuscle: just below hairless skin - surface, motion

Merkel cell: floor of epidermal ridges - edges, indentations

Ruffini corpuscle: parallel to stretch lines (dermis) - skin stretch

Pacinian corpuscle: deep and onion-like with layers (subcutaneous) - vibration sense

Question 13

what is sensed by muscle spindles vs Golgi tendon organs?

Answer 13

muscle spindles: arranged in parallel with extrafusal muscle fibers, detects muscle length

Golgi tendon organs: arranged in series with extrafusal muscle fibers (in between end of muscle and beginning of tendon), detects muscle tension

Question 14

fasciculus cuneatus vs fasciculus gracilis

Answer 14

portions of dorsal spinal columns

upper limbs, trunk, neck send fibers through lateral fasciculus cuneatus

lower body sends fibers through medial fasciculus gracilis [the GRACILIS muscle is in your leg!]

Question 15

name the neurons of the dorsal column-medial lemniscus pathway, include where decussation occurs

Answer 15

1st order: large myelinated fibers of DRG - sense light touch, vibration, proprioception

2nd order: dorsal column nuclei (fasciculus gracilis = lower body, fasciculus cuneatus = upper body)

decussation in caudal medulla (internal arcuate fibers)

3rd order: ventral posterolateral (VPL) nucleus of thalamus

terminates in postcentral gyrus (primary somatosensory cortex)

Question 16

cause and presentation of Tabes Dorsalis

Answer 16

Tabes Dorsalis: caused by tertiary syphilis infection. Degeneration of dorsal columns causing impaired sensation and proprioception and progressive sensory ataxia.

Question 17

cause and presentation of subacute combined degeneration

Answer 17

Subacute Combined Degeneration caused by Vitamin B12 or E deficiency. Demyelination of dorsal columns, lateral corticospinal tracts, and spinocerebellar tracts causing ataxic gait, paresthesia, impaired position and vibration sense

Question 18

what is the somatotopic organization of the spinothalamic tract in the spinal cord?

Answer 18

senses pain and temperature (via free nerve endings)

Upper body is represented more medially and lower body more laterally for pain and temperature

Question 19

Contralateral loss of pain and temperature, ipsilateral loss of joint position sense, ipsilateral UMN signs =

Answer 19

Brown-Sequard Syndrome: hemisection of spinal cord —> affects Spinothalamic tract, Dorsal Column, Corticospinal Tract

Question 20

Bilateral loss of vibration and joint position sense,
bilateral UMN signs and ataxia =

Answer 20

Sub Acute Combined Degeneration: Vitamin B12 or E deficiency —> affects Dorsal Column, Corticospinal tract, Spinocerebellar tract