Neuropathy, Somatosensory Pathways Flashcards
describe the properties of the following types of afferent axons:
a. A-alpha
b. A-beta
c. A-delta
d. C
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
contrast the properties of efferent fibers of the motor vs autonomic system
motor efferents have large diameters, heavily myelinated, fast conducting
autonomic efferents are lightly myelinated or unmyelinated, slow conducting
what kind of symptoms would accompany disease process affecting mainly small nerve fibers vs those affecting mainly myelinated large fibers?
disease of small fibers —> pain, temp, and autonomic loss
disease of large myelinated fibers —> vibration, proprioception, and motor loss
neuropathic pain may present with allodynia and dysesthesias - describe these phenomena
allodynia = lowering of pain threshold
dysesthesias = abnormal sensations to sensory stimuli
contrast presentation of radiculopathy vs mononeuropathy
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]
describe Wallerian degeneration following axon damage
aka “dying forward” - distal axon degenerates, nucleolus expands and moves peripherally (“chromatolysis”), Nissle substance disintegrates
most nutritional, metabolic, and toxic etiologies of non-traumatic peripheral neuropathy present with what type of distribution? (pattern of nerve damage)
“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
What is the most common metabolic neuropathy (read: metabolic cause of neuropathy). What are some key clinical features?
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]
how does Guillain Barre present?
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!)
the 2 most important diagnostic tests for Guillain-Barre are…
[affects myelinated fibers —> ascending paralysis, loss of vibration/proprioception]
- CSF analysis: shows increased protein with normal leukocytes (“albuminocytological dissociation”)
- nerve conduction velocity studies: shows decreased conduction velocity
how does Charcot-Marie-Tooth Disease present?
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
order from superficial to deep in the dermis: Ruffini corpuscle, Merkel cells, Meissner corpuscle, Pacinian corpuscle
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
what is sensed by muscle spindles vs Golgi tendon organs?
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
fasciculus cuneatus vs fasciculus gracilis
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!]
name the neurons of the dorsal column-medial lemniscus pathway, include where decussation occurs
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)
