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Neurology SB > Motor Neurone Physiology > Flashcards

Motor Neurone Physiology Flashcards

1
Q

where are UMN and LMN found

A

UMN - within the brain

LMN - brain stem and ventral horn of the spinal cord

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2
Q

what are LMNs comprised of

A

alpha motor neurones

gamma motor neurones

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3
Q

what do the alpha motor neurones in LMN do

A

innervate the bulk of fibres within a muscle that generate force

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4
Q

what do the gamma motor neurones in LMN do

A

innervate a sensory organ within the muscle known as a muscle spindle

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5
Q

what is meant by synergistic muscles and what are examples of these

A

groups of muscles that contract together to accomplish the same body movement

biceps brachii and brachialis

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6
Q

how do axons of LMN exit the spinal cord

A

via the ventral roots (or cranial nerves)

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7
Q

what does each ventral (anterior) root join with and what does this form

A 
a dorsal (posterior) root 
to form a mixed spinal nerve (sensory and motor fibres)
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8
Q

what is a motor unit

A

an alpha-motor neurone and all of the skeletal muscle fibres that it innervates

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9
Q

what is the smallest functional component of the motor system

A

a motor unit

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10
Q

what is the collection of alpha motor neurones that innervate a single muscle called

A

a motor neurone pool

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11
Q

how is the force of muscle contraction graded by an alpha-MN

A
  • freq of AP discharge of the alpha-MN

- recruitment of additional, synergistic motor units

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12
Q

what are the 3 sources of input to an alpha-MN that regulate its activity

A
  • spinal interneurones
  • UMNs
  • dorsal root ganglion cells
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13
Q

what does muscle strength depend on

A

neuromuscular activation
- firing rate of LMNs, no. of LMNs, co-ordination of the movement

force production by innervated muscle fibres

  • fibre size (hypertrophy)
  • fibre phenotype (‘fast’ or ‘slow’ contracting muscle
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14
Q

what would a single AP in an alpha-MN cause

A

muscle fibre to twitch

need summation of twitches to cause decent movement

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15
Q

where on a muscle fibre is it innervated by a motor axon

A

endplate - neuromuscular junction

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16
Q

what are the major types of skeletal muscle fibres

A

Type I - slow-oxidative fibres

Type II - fast fibres
– Type IIa and IIx

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17
Q

what are the features of Type I fibres

A

ATP derived from oxidative phosphorylation.

Slow contraction and relaxation.

Fatigue resistant.

Red fibres due to high myoglobin content.

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18
Q

what are the features of Type IIa fibres

A

ATP derived from oxidative phosphorylation.

Fast contraction and relaxation.

Fatigue resistant.

Red fibres and reasonably well vascularised.

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19
Q

what are the features of Type IIx fibres

A

ATP derived from glycolysis.

Fast contraction but NOT fatigue resistant

Pale in colour and poorly vascularised. “white meat”

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20
Q

what causes the differences in Type I and II fibres

A

how fast myosin ATPase splits ATP to provide energy for cross bridge cycling

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21
Q

in what type of fibres is there bigger motor units

A

Fast Fatiguing

i.e. Type IIx fibres

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22
Q

which size of motor unit has a smaller threshold

A

smaller ones

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23
Q

what is the myotatic reflex

A

when a skeletal muscle is pulled, it pulls back

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24
Q

what does the muscle spindle do

A

registers the change in length and rate of change in a muscle

contributes to non-conscious proprioception

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25
Q

what happens during the myotatic reflex

A 
1 - stretch of muscle spindle 
2 - activation of Ia afferent 
3 - excitatory synaptic tranmission in spinal cord
4 - activation of alpha-MN
5 - contraction of homonymous muscle

stage 3 mediated by release of glutamate

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26
Q

what are the spinal levels of the main deep tendon reflexes

A 
supinator - C5-C6
biceps - C5-C6
triceps - C7
quadriceps - L3-L4
gastrocnemius - S1
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27
Q

what fibres of muscles spindles are innervated by gamma motor neurones

A

intrafusal fibres

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28
Q

how to intrafusal fibres contract in relation to extrafusal

A

in parallel

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29
Q

what is the sensory function of intrafusal fibres

A

serve as proprioceptors that detect the amount and rate of change in length of a muscle

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30
Q

where are golgi tendon organs located

A

at the junction of muscle and tendon

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31
Q

what is the function of golgi tendon organs

A

monitor and regulate changes in muscle tension

are in series to extrafusal fibres

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32
Q

why are golgi tendon organs needed to regulate muscle tension

A
  • protect muscle from overload

- regulate muscle tension to an optimal range

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33
Q

where do spinal interneurones receive input from

A
  • primary sensory axons
  • descending axons from the brain
  • collateral branches of LMN
  • other interneurones
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34
Q

what do inhibitory interneurones mediate

A
  • the inverse myotatic response

- reciprocal inhibition between extensor and flexors muscles

35
Q

what is the inverse myotatic response (a.k.a golgi tendon reflex)

A

skeletal muscle contraction causes the agonist muscle to simultaneously lengthen and relax

36
Q

what is reciprocal inhibition

A
  • voluntary contraction of an extensor will stretch an antagonist flexor, initiating the myotatic reflex
  • However, descending pathways that activate the α-MN controlling the extensor muscles also, via inhibitory interneurons, inhibit the α-MNs supplying the antagonist muscles
  • unopposed extension
37
Q

what do excitatory interneurones mediate

A

flexor reflex
crossed extensor reflex

  • allows for co-ordinated control of flexors and extensors
38
Q

what is the flexor reflex

A

stimulus causes limb to flex by:

  • contraction of flexor muscles via excitatory interneurones
  • relaxation of extensor muscles via excitatory and inhibitory interneurones
39
Q

what is the crossed extensor reflex

A

stimulus causes limb to extend by:

  • contraction of extensor muscles via excitatory interneurones
  • relaxation of flexor muscles via excitatory and inhibitory interneurones
40
Q

what is the motor control hierarchy in the brain

A

Somatomotor cortex/Basal ganglia&raquo_space; Motor cortex/Cerebellum&raquo_space; Brainstem/spinal cord

41
Q

where do motor neurones cell bodies arise

A

in the ventral horn of the spinal corn

42
Q

what are the synopsis formed between motor neurones and muscles called

A

the motor end plate

43
Q

how does the relationship between a single motor neurone work

A

A single motor neuron may control many muscle cells but:

Each muscle cell responds to only one motor neurone

44
Q

what is the overview of muscle movement

A
  • AP moves along nerve
  • Voltage gated calcium open allowing influx of calcium
  • Vesicles of acetyl choline released into synaptic cleft
  • Acetyl choline diffuses across the synaptic cleft
  • Acetlcholine receptor opens and renders the membrane permeable to Na / K ions
  • Depolarisation starts an action potential at the motor end plate
45
Q

what is acetyl choline derived from

A

Acetylcholinesterase

46
Q

what drug can cause neuromuscular junction dysfunction

A
  • curare
  • occupies same position on ACh receptor but does not open ion channel
  • no muscle contraction = no respiration
47
Q

what causes botulism

A

bacteria - clostridium botulinum

found in soil

48
Q

who commonly gets botulism

A

IV drug users - black tar heroin

49
Q

what does the botulinum toxin do

A

cleave presynaptic proteins involved in vesicle formation and block vesicle docking with the presynaptic membrane.

50
Q

what are the Sx of botulism

A

Rapid onset weakness without sensory loss.

51
Q

what happens in Lambert Eaton Myasthenic Syndrome

A

ntibodies to presynaptic calcium channels leads to less vesicle release

52
Q

what does Lambert Eaton Myasthenic Syndrome have a strong association with

A

small cell carcinoma

53
Q

what is an example of a post synaptic disorder

A

myasthenia gravis

54
Q

what happens in myasthenia gravis

A

body produces antibodies to acetyl choline receptors (AChR)

causes Reduced number of functioning receptors leads to muscle weakness and fatiguability

55
Q

what is the pathophysiology of myasthenia gravis

A

reduced number of ACh receptors and flattening of endplate folds

even with normal amounts of ACh transmission becomes inefficient

symptoms start when ACh receptors reduced to 30% of normal

56
Q

what are the clinical features of myasthenia gravis

A

weakness - fluctuates, worse through the day

extraocular weakness
facial and bulbar weakness
proximal limb weakness

57
Q

Tx for myasthenia gravis

A

1st line - acetylcholinesterase inhibitor e.g. pyridostigmine

2nd line - thymectomy

58
Q

what causes morbidity in myasthenia gravis

A

resp failure

aspiration pneumonia

59
Q

what are the 3 types of muscle

A

skeletal
smooth
cardiac

60
Q

what are the features of skeletal muscle

A

long, cylindrical, containing nucleii, mitochondria, sarcomeres

61
Q

what is each skeletal muscle fibre surrounded by

A

endomysium

62
Q

what do skeletal muscles group together to form and what is that encapsulated by

A

form a fascicle

surrounded by perimysium

63
Q

what forms an individual muscle

A

groups of fascicles surrounded by epimysium

64
Q

what are features of smooth muscle

A 
Cells not striated
Single central nucleus
Gap junctions between cells
Significant connective tissue around them
No sarcomeres
Actin myosin ratio 10:1
65
Q

the 3 muscle fibre types are Type I, Type IIa and Type IIb - what are there features

A

Type I – slow oxidative – dense capilarry network, myoglobin, resist fatigue

Type IIa – fast oxidative – aerobic metabolism

Type IIb – fast glycolytic, easily fatigued

66
Q

what is a fasciculation

A

Visible, fast, fine , spontaneous twitch

can occur in health muscle due to stress, caffeine or fatigue

67
Q

what is myotonia and what is affected in it

A

failure of muscle relaxation after use

dysfunction in the chloride channel

68
Q

what is dystrophia myotonica

A

autosomal dominant Cl- channelopathy

DM1 more common - caused by mutation in DMPK gene

69
Q

what are Sx of myotonic dystrophy relating to muscles

A 
onset 25y/o
distal-onset weakness (hand/foot drop)
weak sternomastoids
myotonia 
facial weakness
muscle wasting
70
Q

what are other Sx of myotonic dystrophy

A 
male frontal baldness
cataracts 
testis/ovary atrophy
cardiomyopathy 
decreased cognition
71
Q

what are Sx and signs of muscle disease

A

myalgia
muscle weakness
wasting
hyporeflexia

72
Q

what are features of polymyositis

A

symmetrical, progressive proximal weakness
muscle pain
joint pain

73
Q

what is raised in polymyositis and what confirms the diagnosis

A

CK

muscle biopsy

74
Q

what is polymyositis associated with

A

anti-jo-1 antibody

75
Q

how is polymyositis Tx

A

prednisolone

76
Q

what is dermatomyositis

A

same as polymyositis with skin features

77
Q

what skin changes are seen in dermotomyositis

A

Heliotrope rash on the eyelids
Gottron’s papules over knuckles
Macular rash e.g. Shawl sign (across back and shoulders)

78
Q

what myositis is more commonly associated with malignancy

A

dermatomyositis

79
Q

what is inclusion body myositis

A

slowly progressive inflammatory myopathy thought now to be degenerative

80
Q

Sx of IMB

A 
slowly progressive, asymmetrical weakness
thumb sparing 
dysphagia 
fatigue 
diminished deep tendon reflex
81
Q

how does weakness progress in IMB

A

starts with quads, finger flexors or pharyngeal muscle

82
Q

what is rhabdomyolysis and causes of it

A

damage to skeletal muscle
causes leakage of large quantities of toxic intracellular contents into plasma.

crush injuries
toxins
past convulsions
acute renal failure

83
Q

Sx of rhabdomyolysis

A

myalgia (muscle pain)
muscle weakness
myoglobinuria (black, brown urine)

Neurology SB (23 decks)

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