Peripheral Nervous System Disorders Flashcards
contrast negative vs positive PNS lesion
- negative: loss of normal function
- muscle weakness
- loss of tendon reflexes
- autonomic deficits (anhydrosis)
- impaired sensation (anesthesia)
- positive: emergent or exaggerated phenomena
- paresthesias (numbness, pins and needles, tingling) possibly reflecting ephatic transmission between adjacent damaged sensory nerve fibers that have become hyperexcitable
- pain cuased by compression of nerves leading to hyperexcitability (carpal tunnel)
what is the equation for conduction velocity?
= D2 /(T2-T1)
describe how lesions affect mean conduction velocities
-
lesion in motor neuron soma: slight or no change in motor
- no change in sensory
- demyelination of peripheral nerve: marked reduction in motor + sens. CV
- compression of periph. nerve: slowing of motor + sens. at site
- axonal degeneration of periph. n.: no or slight reduction
- NMJ: no change in motor + sensory
- muscle: no change in motor + sensory
the ____ is the most vulnerable part of the peripheral nerve, whereas the tough _____ is the most damage resistant
the axon is the most vulnerable part of the peripheral nerve, whereas the tough epineurium is the most damage resistant
describe chromatolysis
dilution of somatic organelles with enlargement and displacement of the nucleus/nucleolus (often precedes apoptosis)
describe Wallerian degeneration
degeneraton of the distal part of the transected axon
describe anterograde transneural degeneration
degeneration of downstream nerve cells
describe retrograde transneural degeneration
degeneration of upstream nerve cells
describe the degeneration seen here
describe the degeneration seen here
describe the degeneration seen here
describe the regeneration and regrowth in the PNS
- after transection of a peripheral nerve, axonal sprouting may occur
- the sprouts use the lingering Schwann cells of the Wallerian-degenerated axons as guide tubes for regrowth (~1mm/day)
- functional axonal regrowth is facilitated by “nerve growth factor” (which is released by the Schwann cells), laminins and adhesion molecules
- later, Schwann cells may remyelinate axons, restoring conduction velocities
describe why regeneration doesn’t occur in the CNS
- oligodendrocytes neither release nerve growth factor nor guide regenerating axonal sprouts (unlike Schwann cells)
- gliosis occurs, where astrocytes proliferate in traumatized regions and form glial scars that act as mechanical barriers to sprouting axons
- an inhibitory chemical messenger accumulates within the CNS to oppose axonal regeneratio in adults
describe reinnervation of denervated skeletal muscle
- the regenerating axons differentiate into nerve terminals when they contact the basal lamina
- certain components of the basal lamina facilitate reinnervation of synaptic sites and trigger the differentiation of axonal grwoth cones into presynaptic nerve terminals
describe what is seen
- when the muscle is injured (including the basal lamina), there is an absence/decreased amount of receptors, so function is difficult to regain in those patients where basal lamina is not injured
describe the different laminin isoforms
different laminin isoforms exist in synaptic and extrasynaptic basal lamina
- laminin 11 is present int he synaptic basal lamina
- laminin 2 arises in the extrasynaptic basal lamina
describe Guillain-Barre syndrome
refulects progressive (often ascending) muscle weakness and paralysis that may follow an infectious illness (often respiratory or GI)
- causes: acute inflammation damages the myelin sheath
- as a result, the conduction of impulses through the nerve slow or ceases
describe what is seen in the tests of Guillian-Barre patients
- lumbar puncture: elevated protein in CSF (100-300 mg/dL)
- nerve conduction velocity decreased
- EMG: diminished nervous input
describe treatment of Guillain-Barre
- intravenous immune globulin
- plasmapheresis (plasma exchange)
- ventilatory support
describe leprosy (Hansen disease)
chornic infection that affects the skin and the peripheral nerves; one of the commonest treatable neuropathy
- etiology: infection of skin and peripheral nerves by myobacterium leprae, usually after a long incubation or slow multiplication
- bacterial multiplication causes compression and ischemia of peripheral axons which result in profound sensory losses (pain and temp.)
- bacterial entry proceeds through a cutaneous lesion, often following contact with nasal secretions from infected persons
describe lead poisoning
- children younger than 6 years are vulnerable to lead toxicity
- relatively low blood levels of lead may lead to encephalopathy with diminished IQ, attention problems, and learning disabilities
- in adults, lead poisoning may cause memory and concetration problems and peripheral motor neuropathy, often preferentially targeting extensor muscles (wrist drop is seen)
describe alcoholic peripheral neuropathy
- reflects the neurotoxic effects of alcohol or associated nutritional (thiamin) deficiencies in persons consuming more than 80-100 g alc/day
- sensory and motor losses, both symmetrical
- starts with sensory neuropathy from distal (foot, leg, etc)
- motor losses follow
- nerve conduction velocity normal in most cases
describe diabetes mellitus (in peripheral neuropathy)
- many diabetics (type I and type II) eventually develop polyneuropathy
- types: sensory (usually symmetric in the distal extremities), motor (usually asymmetric) and autonomic neuropathy
- begins in the extremities (usually legs) with abnormalities of unmyelinated axons carrying pain and temp. in a “stocking and glove” distribution
- may originate in failure of the cell body to supply its distal parts with nutrients and essential cytoskeletal proteins
- accompanying vasculopathy may also be implicated
