Motor Neurone Physiology Flashcards
where are UMN and LMN found
UMN - within the brain
LMN - brain stem and ventral horn of the spinal cord
what are LMNs comprised of
alpha motor neurones
gamma motor neurones
what do the alpha motor neurones in LMN do
innervate the bulk of fibres within a muscle that generate force
what do the gamma motor neurones in LMN do
innervate a sensory organ within the muscle known as a muscle spindle
what is meant by synergistic muscles and what are examples of these
groups of muscles that contract together to accomplish the same body movement
biceps brachii and brachialis
how do axons of LMN exit the spinal cord
via the ventral roots (or cranial nerves)
what does each ventral (anterior) root join with and what does this form
a dorsal (posterior) root to form a mixed spinal nerve (sensory and motor fibres)
what is a motor unit
an alpha-motor neurone and all of the skeletal muscle fibres that it innervates
what is the smallest functional component of the motor system
a motor unit
what is the collection of alpha motor neurones that innervate a single muscle called
a motor neurone pool
how is the force of muscle contraction graded by an alpha-MN
- freq of AP discharge of the alpha-MN
- recruitment of additional, synergistic motor units
what are the 3 sources of input to an alpha-MN that regulate its activity
- spinal interneurones
- UMNs
- dorsal root ganglion cells
what does muscle strength depend on
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
what would a single AP in an alpha-MN cause
muscle fibre to twitch
need summation of twitches to cause decent movement
where on a muscle fibre is it innervated by a motor axon
endplate - neuromuscular junction
what are the major types of skeletal muscle fibres
Type I - slow-oxidative fibres
Type II - fast fibres
– Type IIa and IIx
what are the features of Type I fibres
ATP derived from oxidative phosphorylation.
Slow contraction and relaxation.
Fatigue resistant.
Red fibres due to high myoglobin content.
what are the features of Type IIa fibres
ATP derived from oxidative phosphorylation.
Fast contraction and relaxation.
Fatigue resistant.
Red fibres and reasonably well vascularised.
what are the features of Type IIx fibres
ATP derived from glycolysis.
Fast contraction but NOT fatigue resistant
Pale in colour and poorly vascularised. “white meat”
what causes the differences in Type I and II fibres
how fast myosin ATPase splits ATP to provide energy for cross bridge cycling
in what type of fibres is there bigger motor units
Fast Fatiguing
i.e. Type IIx fibres
which size of motor unit has a smaller threshold
smaller ones
what is the myotatic reflex
when a skeletal muscle is pulled, it pulls back
what does the muscle spindle do
registers the change in length and rate of change in a muscle
contributes to non-conscious proprioception
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
what are the spinal levels of the main deep tendon reflexes
supinator - C5-C6 biceps - C5-C6 triceps - C7 quadriceps - L3-L4 gastrocnemius - S1
what fibres of muscles spindles are innervated by gamma motor neurones
intrafusal fibres
how to intrafusal fibres contract in relation to extrafusal
in parallel
what is the sensory function of intrafusal fibres
serve as proprioceptors that detect the amount and rate of change in length of a muscle
where are golgi tendon organs located
at the junction of muscle and tendon
what is the function of golgi tendon organs
monitor and regulate changes in muscle tension
are in series to extrafusal fibres
why are golgi tendon organs needed to regulate muscle tension
- protect muscle from overload
- regulate muscle tension to an optimal range
where do spinal interneurones receive input from
- primary sensory axons
- descending axons from the brain
- collateral branches of LMN
- other interneurones
what do inhibitory interneurones mediate
- the inverse myotatic response
- reciprocal inhibition between extensor and flexors muscles
what is the inverse myotatic response (a.k.a golgi tendon reflex)
skeletal muscle contraction causes the agonist muscle to simultaneously lengthen and relax
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
what do excitatory interneurones mediate
flexor reflex
crossed extensor reflex
- allows for co-ordinated control of flexors and extensors
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
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
what is the motor control hierarchy in the brain
Somatomotor cortex/Basal ganglia»_space; Motor cortex/Cerebellum»_space; Brainstem/spinal cord
where do motor neurones cell bodies arise
in the ventral horn of the spinal corn
what are the synopsis formed between motor neurones and muscles called
the motor end plate
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
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
what is acetyl choline derived from
Acetylcholinesterase
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
what causes botulism
bacteria - clostridium botulinum
found in soil
who commonly gets botulism
IV drug users - black tar heroin
what does the botulinum toxin do
cleave presynaptic proteins involved in vesicle formation and block vesicle docking with the presynaptic membrane.
what are the Sx of botulism
Rapid onset weakness without sensory loss.
what happens in Lambert Eaton Myasthenic Syndrome
ntibodies to presynaptic calcium channels leads to less vesicle release
what does Lambert Eaton Myasthenic Syndrome have a strong association with
small cell carcinoma
what is an example of a post synaptic disorder
myasthenia gravis
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
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
what are the clinical features of myasthenia gravis
weakness - fluctuates, worse through the day
extraocular weakness
facial and bulbar weakness
proximal limb weakness
Tx for myasthenia gravis
1st line - acetylcholinesterase inhibitor e.g. pyridostigmine
2nd line - thymectomy
what causes morbidity in myasthenia gravis
resp failure
aspiration pneumonia
what are the 3 types of muscle
skeletal
smooth
cardiac
what are the features of skeletal muscle
long, cylindrical, containing nucleii, mitochondria, sarcomeres
what is each skeletal muscle fibre surrounded by
endomysium
what do skeletal muscles group together to form and what is that encapsulated by
form a fascicle
surrounded by perimysium
what forms an individual muscle
groups of fascicles surrounded by epimysium
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
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
what is a fasciculation
Visible, fast, fine , spontaneous twitch
can occur in health muscle due to stress, caffeine or fatigue
what is myotonia and what is affected in it
failure of muscle relaxation after use
dysfunction in the chloride channel
what is dystrophia myotonica
autosomal dominant Cl- channelopathy
DM1 more common - caused by mutation in DMPK gene
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
what are other Sx of myotonic dystrophy
male frontal baldness cataracts testis/ovary atrophy cardiomyopathy decreased cognition
what are Sx and signs of muscle disease
myalgia
muscle weakness
wasting
hyporeflexia
what are features of polymyositis
symmetrical, progressive proximal weakness
muscle pain
joint pain
what is raised in polymyositis and what confirms the diagnosis
CK
muscle biopsy
what is polymyositis associated with
anti-jo-1 antibody
how is polymyositis Tx
prednisolone
what is dermatomyositis
same as polymyositis with skin features
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)
what myositis is more commonly associated with malignancy
dermatomyositis
what is inclusion body myositis
slowly progressive inflammatory myopathy thought now to be degenerative
Sx of IMB
slowly progressive, asymmetrical weakness thumb sparing dysphagia fatigue diminished deep tendon reflex
how does weakness progress in IMB
starts with quads, finger flexors or pharyngeal muscle
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
Sx of rhabdomyolysis
myalgia (muscle pain)
muscle weakness
myoglobinuria (black, brown urine)
