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Neurology SB > Motor Neurone Physiology > Flashcards

Motor Neurone Physiology Flashcards

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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
what happens during the myotatic reflex
``` 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
26
what are the spinal levels of the main deep tendon reflexes
``` supinator - C5-C6 biceps - C5-C6 triceps - C7 quadriceps - L3-L4 gastrocnemius - S1 ```
27
what fibres of muscles spindles are innervated by gamma motor neurones
intrafusal fibres
28
how to intrafusal fibres contract in relation to extrafusal
in parallel
29
what is the sensory function of intrafusal fibres
serve as proprioceptors that detect the amount and rate of change in length of a muscle
30
where are golgi tendon organs located
at the junction of muscle and tendon
31
what is the function of golgi tendon organs
monitor and regulate changes in muscle tension | are in series to extrafusal fibres
32
why are golgi tendon organs needed to regulate muscle tension
- protect muscle from overload | - regulate muscle tension to an optimal range
33
where do spinal interneurones receive input from
- primary sensory axons - descending axons from the brain - collateral branches of LMN - other interneurones
34
what do inhibitory interneurones mediate
- the inverse myotatic response | - reciprocal inhibition between extensor and flexors muscles
35
what is the inverse myotatic response (a.k.a golgi tendon reflex)
skeletal muscle contraction causes the agonist muscle to simultaneously lengthen and relax
36
what is reciprocal inhibition
- 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
what do excitatory interneurones mediate
flexor reflex crossed extensor reflex - allows for co-ordinated control of flexors and extensors
38
what is the flexor reflex
stimulus causes limb to flex by: - contraction of flexor muscles via excitatory interneurones - relaxation of extensor muscles via excitatory and inhibitory interneurones
39
what is the crossed extensor reflex
stimulus causes limb to extend by: - contraction of extensor muscles via excitatory interneurones - relaxation of flexor muscles via excitatory and inhibitory interneurones
40
what is the motor control hierarchy in the brain
Somatomotor cortex/Basal ganglia >> Motor cortex/Cerebellum >> Brainstem/spinal cord
41
where do motor neurones cell bodies arise
in the ventral horn of the spinal corn
42
what are the synopsis formed between motor neurones and muscles called
the motor end plate
43
how does the relationship between a single motor neurone work
A single motor neuron may control many muscle cells but: | Each muscle cell responds to only one motor neurone
44
what is the overview of muscle movement
- 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
what is acetyl choline derived from
Acetylcholinesterase
46
what drug can cause neuromuscular junction dysfunction
- curare - occupies same position on ACh receptor but does not open ion channel - no muscle contraction = no respiration
47
what causes botulism
bacteria - clostridium botulinum | found in soil
48
who commonly gets botulism
IV drug users - black tar heroin
49
what does the botulinum toxin do
cleave presynaptic proteins involved in vesicle formation and block vesicle docking with the presynaptic membrane.
50
what are the Sx of botulism
Rapid onset weakness without sensory loss.
51
what happens in Lambert Eaton Myasthenic Syndrome
ntibodies to presynaptic calcium channels leads to less vesicle release
52
what does Lambert Eaton Myasthenic Syndrome have a strong association with
small cell carcinoma
53
what is an example of a post synaptic disorder
myasthenia gravis
54
what happens in myasthenia gravis
body produces antibodies to acetyl choline receptors (AChR) causes Reduced number of functioning receptors leads to muscle weakness and fatiguability
55
what is the pathophysiology of myasthenia gravis
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
what are the clinical features of myasthenia gravis
weakness - fluctuates, worse through the day extraocular weakness facial and bulbar weakness proximal limb weakness
57
Tx for myasthenia gravis
1st line - acetylcholinesterase inhibitor e.g. pyridostigmine 2nd line - thymectomy
58
what causes morbidity in myasthenia gravis
resp failure | aspiration pneumonia
59
what are the 3 types of muscle
skeletal smooth cardiac
60
what are the features of skeletal muscle
long, cylindrical, containing nucleii, mitochondria, sarcomeres
61
what is each skeletal muscle fibre surrounded by
endomysium
62
what do skeletal muscles group together to form and what is that encapsulated by
form a fascicle surrounded by perimysium
63
what forms an individual muscle
groups of fascicles surrounded by epimysium
64
what are features of smooth muscle
``` Cells not striated Single central nucleus Gap junctions between cells Significant connective tissue around them No sarcomeres Actin myosin ratio 10:1 ```
65
the 3 muscle fibre types are Type I, Type IIa and Type IIb - what are there features
Type I – slow oxidative – dense capilarry network, myoglobin, resist fatigue Type IIa – fast oxidative – aerobic metabolism Type IIb – fast glycolytic, easily fatigued
66
what is a fasciculation
Visible, fast, fine , spontaneous twitch can occur in health muscle due to stress, caffeine or fatigue
67
what is myotonia and what is affected in it
failure of muscle relaxation after use dysfunction in the chloride channel
68
what is dystrophia myotonica
autosomal dominant Cl- channelopathy | DM1 more common - caused by mutation in DMPK gene
69
what are Sx of myotonic dystrophy relating to muscles
``` onset 25y/o distal-onset weakness (hand/foot drop) weak sternomastoids myotonia facial weakness muscle wasting ```
70
what are other Sx of myotonic dystrophy
``` male frontal baldness cataracts testis/ovary atrophy cardiomyopathy decreased cognition ```
71
what are Sx and signs of muscle disease
myalgia muscle weakness wasting hyporeflexia
72
what are features of polymyositis
symmetrical, progressive proximal weakness muscle pain joint pain
73
what is raised in polymyositis and what confirms the diagnosis
CK muscle biopsy
74
what is polymyositis associated with
anti-jo-1 antibody
75
how is polymyositis Tx
prednisolone
76
what is dermatomyositis
same as polymyositis with skin features
77
what skin changes are seen in dermotomyositis
Heliotrope rash on the eyelids Gottron’s papules over knuckles Macular rash e.g. Shawl sign (across back and shoulders)
78
what myositis is more commonly associated with malignancy
dermatomyositis
79
what is inclusion body myositis
slowly progressive inflammatory myopathy thought now to be degenerative
80
Sx of IMB
``` slowly progressive, asymmetrical weakness thumb sparing dysphagia fatigue diminished deep tendon reflex ```
81
how does weakness progress in IMB
starts with quads, finger flexors or pharyngeal muscle
82
what is rhabdomyolysis and causes of it
damage to skeletal muscle causes leakage of large quantities of toxic intracellular contents into plasma. crush injuries toxins past convulsions acute renal failure
83
Sx of rhabdomyolysis
myalgia (muscle pain) muscle weakness myoglobinuria (black, brown urine)

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