Neuromuscular Junction + Disorders Flashcards

1
Q

Motor Unit

A

A single axon and all of the muscle fibers innervated by that muscle fiber; size varies from a few to thousands of muscle fibers per motor unit

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2
Q

Miniature End Plate Potential (MEPP)

A

~1mV depolarizations of skeletal muscle representing spontaneous release of single vesicles of ACh, each containing 5,000-10,000 ACh molecules

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3
Q

Safety factor for synaptic transmission

A

Refers to the fact that the motor nerve terminal secretes the contents of a few times more than the minimum number of synaptic vesicles needed to initiate a muscular action potential

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4
Q

Synaptic Facilitation

A

During repetitive, high frequency stimulation, Ca2+ accumulates in the nerve terminal due to the inability of the Ca2+ pumps to keep pace with Ca2+ influx; residual calcium increases the number of quanta released with subsequent stimulation

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5
Q

Synaptic depression

A

Caused by depletion of synaptic vesicles during repetitive stimulation; after a single action potential, quantal release is reduced by about 10% and recovers over 5 seconds

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6
Q

Myasthenia Gravis

A

An autoimmune diseases caused by inappropriate antibody production to the acetylcholine receptor; antibodies block the receptors causing a reduction in EPP amplitude

Greater quantal release is needed to reach the same EPP amplitude; therefore these patients are prone to synaptic depression as vesicles are depleted

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7
Q

Myasthenic Syndrome

A

Autoantibody production to the pre-synaptic Ca2+ channel; antibodies block the Ca2+ channel so that fewer quanta are released and less ACh is released

Synaptic facilitation during exercise increases Ca2+ within the nerve terminal and is beneficial

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8
Q

Clinical features of ALS

A

Asymmetric, progressive weakening and wasting of the limbs, often with fasciculations and foot drop; upper motor neuron signs (hyperreflexia, spasticity, Babinski sign) as well as lower motor neuron sign (flaccid paralysis) are seen

Cognitive defects seen in up to 30%

Diaphragm weakness and impaired swallowing increase risk of fatal aspiration pneumonia; mean mortality is 3.5 years

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9
Q

CMT Type 1

A

Demyelinating

NCV < 38m/s

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10
Q

CMT Type 2

A

Axonal degeneration (NCV > 38m/s)

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11
Q

Clinical features of diabetic neuropathy

A

Distal sensory/sensorimotor polyneuropathy seen in 50-60% of diabetics; characterized by “stocking glove” distribution of numbness and burning in feet and hands

May also present with weakness of dorsiflexor muscles causing slapping foot drop gait, diminished grip strength and fine hand dexterity

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12
Q

2 patterns of diabetic neuropathy

A

Large fiber - loss of position, vibration, light touch sensation, and decreased reflexes

Small fiber - loss of pain and temperature

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13
Q

Clinical features of Myasthenia Gravis

A

Rapidly developing weakness often affecting cranial muscles; presents as droopy eyelids (ptosis), double vision, slurred speech, difficulty swallowing

Difficulty swallowing and respiratory muscle weakness increases risk of choking and aspiration

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14
Q

Clinical features of DMD

A

Clumsy, waddling gate
Pseudohypertrophy of the calves with tendency for toe walking
Difficulty rising from the floor (Gower’s maneuver)

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15
Q

Pathologic changes of ALS

A

Degeneration of UMNs, brainstem, and spinal cord LMNs

Vast majority of cases are sporadic; only 5-10% are familial

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16
Q

Pathologic changes of CMT - Types I and II

A

CMT I - Slow nerve conduction velocity (demyelinating)

CMT II - Normal nerve conduction velocity (axonal degeneration)

17
Q

Pathologic changes of MG

A

Thymic hyperplasia; autoimmune

18
Q

Pathologic changes of DMD

A

Dystrophin mutation

19
Q

Genetic defect of CMT1A

A

Duplication of PMP22 gene on chromosome 17

20
Q

Genetic defect of DMD

A

X-linked mutation of dystrophin, which links actin to dystroglycan in the sarcolemma

21
Q

Immune-pathogenesis of MG

A

Production of auto-antibodies against the AChR; diagnosis is confirmed by the presence of AChR antibodies in the serum and by repetitive nerve stimulation, which produces a decremental response in the train of muscle twitches that can be restored with edrophonium

22
Q

Treatment - MG

A

Oral cholinesterase inhibitors (pyridostimine)
Corticosteroids (prednisone)
Thymectomy
Plasma exchange

23
Q

Treatment of ALS

A

Riluzole extends life 3-6 months

24
Q

Pseudobulbar affect in ALS

A

Inappropriate laughing and/or crying; caused by demyelination of fibers that project from the cortex to the brainstem

25
Q

Genetic defect of hereditary neuropathy with pressure palsy (HNPP)

A

Deletion of PMP22 gene on chromosome 17

26
Q

Upper motor neuron signs

A

Spasticity
Hyperactive tendon reflexes
Pathological reflexes (Babinski)
Emotional lability

27
Q

Lower motor neuron sign

A

Muscle atrophy
Fasciculations
Diminished tone
Reduced/absent reflexes