Muscle Function Flashcards
what is an alpha motor neuron?
A neuron that synapses with multiple
muscle fibres
what does an alpha motor neuron do?
Provides excitation for muscle
fibres to generate tension
what is a motor unit?
The alpha motor neuron and the
skeletal muscle fibres it innervates
what principle does a motor unit follow? And what does this mean?
an all or none principle
When a motor unit is
stimulated, all muscle fibres in
that motor unit will fire
where ar motor neurons grouped together?
on the longitudinal axis or the spinal cord
what is a motor neuron pool?
all the motor neurons that innervate a single muscle
what are the 3 parts of the neuromuscular junction?
- presynaptic terminal
- synaptic cleft
- post synaptic muscle fiber (motor endplate)
what are the steps of synaptic transmission at the NMJ?
- Depolarization of the presynaptic
membrane causes influx of calcium - Acetylcholine is released into
synaptic cleft via vesicles - Acetylcholine binds to receptors on
muscle membrane - Causes depolarization of motor
endplate (end plate potential) - End plate potential triggers an action potential if reaches threshold
what happens to acetylcholine after synaptic transmission?
its broken down by the enzyme acetylcholinesterase and the vesicles are recycled
why do motor neurons release extra acetylcholine?
to depolarize the post synaptic membrane
what is the purpose of the safety factor or transmission?
it increases the chance that
the muscle will contract, even
when fatigued
what happens to acetylcholine with Myasthenia Gravis?
results in a loss of acetylcholine receptors in the muscle fibre
Reduced membrane depolarization and smaller safety factor → weakness
list the parts of a muscle from largest to smallest
muscle-fascicle- muscle fiber- myofibril
what is each muscle fibre surrounded by?
sarcolemma
what is the function of the sarcolemma?
allows depolarizing current to
reach the sarcomeres
what are the openings in the sarcolemma?
T-tubules
what is the function of the sarcoplasmic reticulum?
stores, releases, and retrieves calcium
what is the smallest functional unit of the muscle fiber?
sarcomeres
what do sarcomeres consist of?
actin and myosin
how are sarcomeres arranged?
in series
what are the steps of exctitation-contraction coupling?
- Endplate potential
triggers action potential - Action potential travels into
muscle via t-tubules - Release of Ca2+ from sarcoplasmic
reticulum - Interaction of actin and myosin
what is the secondary messenger from action potential to muscle contraction?
calcium
what is a muscle contraction dependent on?
release of calcium from the sarcoplasmic reticulum
describe the steps of the sliding filament theory
- Myosin head is in a ‘cocked’
position with ATP present - Ca2+ reveals binding site on
actin. Myosin head binds
with actin (cross-bridge) - ‘Powerstroke’ – myosin pulls actin
towards the centre of the sarcomere
4.When ATP is still present:
* Myosin dissociates from actin
* Myosin head is ‘re-cocked’
* Myosin binds to next active site
- Cross-bridge cycling
when does muscle relaxation occur?
when stimulation of the motor neuron stops or ATP is no longer available
what breaks the link between actin and myosin causing the muscle to relax?
calcium being pumped back into the sarcoplasmic reticulum
what contributes to total tension in the muscle?
active contraction and passive structures
what acts like an elastic in passive muscle contribution?
weak actin-myosin bonds, connective tissue and Titin
is there a larger or smaller force generated when moving away from optimal length during active contraction?
smaller force
list the muscles contraction types from most least tension to most tension
concentric- isometric- eccentric
increasing velocity does what to muscle tension?
less muscle tension because the cross bridges cannot cycle quickly enough
why do eccentric muscle contractions have more tension with increasing velocity?
because a greater percentage of cross bridges remain attached
what is innervation ratio?
The number of muscle fibres innervated by a single motor neuron
how many motor neurons synapse with a muscle fiber?
1
what does innervation ratio differ by?
muscle type
what muscles have a higher innervation ratio?
power/force muscles higher innervation ratio (ex. glutes)
what muscles have a lower innervation ratio?
precision muscles (ex:hand muscles)
what increases the innervation ratio?
collateral sprouting
what type of motor units have a low threshold and are activated first?
slow motor units (fatigue resistant)
list the order of which motor units are activated from first to last
slow fatigue resistant (Type 1)- fast fatigue resistant (type 2)- fast fatiguable (Type 2x)
what is the size principle of motor unit recruitment?
With increasing levels of
motor activation, motor
units are recruited from
smallest to largest
(S→FR→FF)
what is rate coding?
Motor unit firing rate
what do smaller muscles usually rely on for recruitment?
rate coding
what do larger muscles typically rely on for recruitment?
size principle
what are the impacts of aging on neuromuscular function?
Action potential propagation is slowed along alpha motor neuron axons
Reduced density of active areas in the
presynaptic terminal
Loss of motor units (but increased size of remaining ones)
what could be causally linked to the decline of muscle mass as people age?
NMJ changes
what neurons does ALS affect?
Rapid loss of fast fatigable
neurons, followed by loss of fast
fatigue-resistant neurons
what neurons are spared in ALS?
Type 1 slow fatigue resistant
what is the clinical implication of Type 2 and 2X neurons being impacted in ALS?
loss of strength and power
what is the impact of stroke on neuromuscular function?
loss of motor units on paretic side
smaller muscle cross sectional area
reduce ability to use size principle
what is the impact of a SCI on neuromuscular function?
Extensive atrophy
* Preferentially affects type II
fibres
Transformation toward type IIx
fibres in most individuals
Loss of motor units (50-90%)
* Increased size of remaining
motor units
what is the impact or Parkinson’s disease on neuromuscular function?
Loss of motor units
* Selective loss of type II
motor units
* Compensatory type I
muscle fibre hypertrophy
Loss of strength and power
