Muscle and Neuromuscular Junction Flashcards
what are three characteristics of lower motor neurone disorders?
weak
low tone
fasiculations
where do the cell bodies of motor neurones which innervate skeletal muscle arise?
ventral horn of spinal cord
what does the terminal portion of motor neurones give rise to?
very fine projections that run along muscle cell
what name is given to the synapses formed between motor neurones and muscle?
motor end plate
how many motor neurones can one muscle cell respond to?
one
how many muscle cells may a single motor neurone control?
many
what is the presynaptic process of NMJ?
action potential moves along nerve
voltage gated calcium channels open allowing influx of calcium
vesicles of acetyl choline released into synaptic cleft
what is the post synaptic process of NMJ?
acetyl choline diffuses across synaptic cleft
the acetyl choline receptor opens and renders membrane permeable to Na / K ions
the depolarisation starts an action potential at motor end plate
what is the role of acetylcholinesterase?
breaks down acetyl choline into acetate and choline
what happens to choline after it being formed from breakdown of acetylcholine?
sequestered into presynaptic vesicles
what is name given to substance which occupies same position on ACh receptor but does not open ion channel?
curare (d tubocurarine)
what is the fatal consequence of curare?
no muscle contraction so no respiration
in what circumstances is curare toxic?
when given IV / IM (1-15 mins)
how does novichok work?
inhibits cholinesterase
what is clostridium botulinum?
organism present in soil
food and wounds can become infected
IVDU - black tar heroin
how does botulinum toxin work?
cleaves presynaptic proteins involved in vesicle formation and blocks vesicle docking with presynaptic membrane
rapid onset weakness without sensory loss
what is lambert eaton myasthenic syndrome (LEMS)?
antibodies to presynaptic calcium channels leads to less vesicle release
what cancer is strongly associated to LEMS?
small cell carcinoma
how is LEMS treated?
3-4 diaminopyridine
what is myasthenia gravis?
autoimmune disorder where antibodies produced to acetyl choline receptors (AChR)
this causes reduced number of functioning receptors which leads to muscle weakness and fatiguability
what is the pathophysiology of MG?
reduced number of ACh receptors and flattening of endplate folds
even with normal ACh the transmission becomes inefficient
when do symptoms of MG begin?
when ACh receptors are reduced to 30% of normal
in what percentage of patients with MG are ACh antibodies found?
80-90%
as well as blocking binding of ACh, what else to the antibodies do in MG?
trigger inflammatory cascades that damage the folds of postsynaptic membrane
as thymus plays a role in MG, 75% patients also have what other conditions?
hyperplasia or thymoma
MG may occur at any age but what are the two peaks of incidence?
females in 3rd decade
males in 6th or 7th decade
what is the clinical feature of MG?
weakness fluctuating - worse through day
what is most common presentation of MG?
extraocular weakness
facial weakness
bulbar weakness
limb weakness is typically proximal or distal?
proximal