Flashcards in Muscle and Neuromuscular Junction Deck (73):
what are three characteristics of lower motor neurone disorders?
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?
how many muscle cells may a single motor neurone control?
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?
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?
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?
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?
limb weakness is typically proximal or distal?
what is the acute treatment of MG?
acetylcholinesterase inhibitor - pyridostigmine
what is long term treatment of MG?
steroid sparing agents - azathioprine / mycophenolate
what is the emergency treatment of MG?
plasma exchange or immunoglobulin
what is biggest drug to avoid in MG?
what is mortality of MG?
what is most common cause of death in MG?
respiratory failure and aspiration pneumonia
what is the smallest contractile unit of skeletal muscle?
what is characteristics of muscle fibre?
long, cylindrical structure containing nucleii, mitochondria and sacromeres
what surrounds each muscle fibre?
thin layer endomysium
20-80 muscle fibres are grouped to form what?
what encapsulates these groups?
fascicle encapsulated by perimysium
what surrounds skeletal muscle as a whole?
what is type I muscle fibre also known as?
what is characteristics of type I muscle fibres?
dense capillary network
what is type IIa muscle fibres also known as?
what is characteristic of type IIa muscle fibres?
what is type IIb muscle fibres also known as?
what is characteristic of type IIb muscle fibres?
what is fasciculations?
visible, fast, fine, spontaneous twitch
can fasciculations occur in healthy muscle?
yes - precipitated by stress, caffeine and fatigue
in which abnormal muscle can fasciculations occur?
denervated muscle which become hyperexcitable
fasciculations are usually a sign of disease where?
motor neurone, not muscle
what is myotonia?
failure of muscle relaxation after use
what channel is involved in myotonia?
what are the 4 main signs / symptoms of muscle disease?
what are examples of immune mediated muscle disease?
what are examples of inherited muscle disease?
limb girdle muscular dystrophies
what are examples of congenital muscle disease?
congenital myasthenic syndromes
what is the clinical feature of polymyositis?
symmetrical, progressive proximal weakness developing over weeks to months
how is polymyositis treated?
responds to steroids
what is the clinical feature of dermatomyositis?
similar to polymyositis but also has skin lesions - heliotrope rash on face
up to 50% of patients with dermatomyositis have underlying what?
what is clinical sign of degenerative muscle disease?
typically slowly progressive weakness in 6th decade with characteristic thumb sparing
what is the most common muscular dystrophy?
what pattern of inheritance is myotonic dystrophy?
what is symptoms of myotonic dystrophy?
what type of disorder is myotonic dystrophy?
trinucleotide repeat disorder with anticipation
what are other 2 common muscular dystrophies?
duchenne and becker
what are infective causes of muscle disease?
viral - coxsacchie
cistercercosis - undercooked pork
what are toxic causes of muscle disease?
what causes rhabdomyolysis?
many things eg crush injuries, toxins, post convulsions and extreme exercise
what is the triad of clinical features of rhabdomyolysis?
what are 2 complications of rhabdomyolysis?
acute renal failure
disseminated intravascular coagulation
what are the 5 grades of MRC muscle power?
0 = no movement
1 = flicker of movement when attempting to contract muscle
2 = some movement if gravity removed but none against gravity
3 = movement against gravity but not against resistance
4 = movement against resistance but not full strength
5 = normal strength