Lecture 7: Muscle Function Flashcards
what is an alpha motor neurone
Synapses with multiple
muscle fibres
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 is the all or none principle of the motor unit
When a motor unit is stimulated, all muscle fibres in that motor unit will fire
what is a motor neuron pool
all the motor neurons that innervate a single muscle
what is a neuromuscular junction
synapse between neuron and a muscle fibre
what are the 3 parts of the neuromuscular junction
presynaptic terminal
synaptic cleft
postsynaptic muscle fiber (motor endplate)
what are the 5 steps of Neuromuscular Junction Synaptic Transmission
- depolarization of presynaptic membrane (influx of calcium)
- acetylcholine released into synaptic cleft
- acetylcholine binds to receptors on muscle membrane
- depolarization of motor endplate
- endplate potential can trigger action potential
why do Motor neurons release excess acetylcholine to depolarize the postsynaptic membrane
safety factor for transmission:
Increases chance that muscle will contract, even when fatigued
what is the clinical implication of the safety factor
Myasthenia Gravis results in a loss of acetylcholine receptors in the muscle fibre
Reduced membrane depolarization and smaller safety factor → weakness
what is the muscle structure (4 parts)
muscle > fascicle > muscle fibre > myofibril
what is a sarcolemma
Each muscle fibre is surrounded
allows depolarizing current to reach the sarcomeres
what is the Sarcoplasmic reticulum
stores, releases, and retrieves calcium
what are Sarcomeres
the smallest functional unit of the muscle fibre
Consist of Actin and Myosin
what is actin
the thin filament
what is myosin
the thick filament
what are the 4 steps to excitation-contraction coupling
- endplate potential triggers action potential
- action potential travels to muscle via t-tubule
- release of calcium from sarcoplasmic reticulum
- interaction of actin myosin
why is calcium needed for muscle contraction
because it converts an action potential into a muscle contraction
what are the 5 steps of the sliding filament theory
- myosin head is in a cocked position with ATP present
- calcium reveals binding site on actin. myosin head binds with actin to make a cross-bridge
- ‘Powerstroke’ – myosin pulls actin towards the centre of the sarcomere
- When ATP is still present, Myosin dissociates from actin, its head is re-cocked, and it binds to the next active site
- cross-bridge cycling
when does muscle relaxation happen
when stimulation of the motor neuron stops or ATP is no longer available
Calcium is pumped back into the sarcoplasmic reticulum, breaking the link between actin and myosin, causing the muscle to relax
what are the 6 factors that influence the amount of force (tension) a muscle can generate
muscle length
muscle velocity and direction
innervation ratio
motor unit type
size principle
rate coding
how does muscle length affect the force produced by muscles?
Active contraction and passive structures contribute to total tension in the muscle
what is passive contribution of muscle length
- Weak actin-myosin bonds act like an elastic
- Increased tension with increased length
what is active contraction of muscle length
- Smaller force generated when moving away from optimal length during active contraction
(think of trying to contract wrist in extension vs neutral)
- Less actin-myosin overlap
what is total tension in muscle length
it gets higher as muscle is lengthened
when lengthening a muscle is most of the tension coming from passive or active components
passive
how does muscle velocity and direction affect the force produced by muscles?
Eccentric > isometric > concentric
Lengthening (eccentric)
* More tension with increasing velocity and greater passive contribution
Shortening (concentric)
* Less tension with increasing velocity
what is innervation ratio
The number of muscle fibres innervated by a single motor neuron
how does innervation ratio affect the force produced by muscles?
Innervation ratio differs by muscle type
- Power/Force muscles = higher innervation ratio
- Precision muscles = smaller innervation ratio
Collateral sprouting increases the innervation ratio
how does motor unit/ muscle fibre type affect tension?
slow motor unit (type 1)
-Low threshold for activation (activated first) produce the least force
Fast fatigue-resistant motor units (type lla)
-contract less quickly then type llx
fast fatigable (type llx)
high activation threshold (activated last)
produce the most force
how does the size principle of motor unit recruitment affect tension?
With increasing levels of motor activation, motor units are recruited from smallest to largest
(S→FR→FF)
how does rate coding affect tension?
Motor unit firing rate (rate of recruitment)
- Smaller muscles typically rely on rate coding while larger muscles rely more on the size principle for recruitment
how does age impact neuromuscular function
action potential propagation is slowed
Reduced density of active areas in the presynaptic terminal
Loss of motor units (but increased size of remaining ones)
how does ALS affect neuromuscular function
Rapid loss of fast fatigable neurons, followed by loss of fast fatigue-resistant neurons but not type 1 neurons
Loss of strength and power
how does stroke affect neuromuscular function
Loss of motor units on paretic side
Smaller muscle cross-sectional area
Reduced ability to use size principle
how does a spinal cord injury affect neuromuscular function
Extensive atrophy, mostly affects type II fibres
Loss of motor units
Transformation toward type IIx
how does Parkinson disease affect neuromuscular function
loss of type II
motor units
type I muscle fibre hypertrophy
- Loss of strength and power
what can neuromuscular junction changes could be causally linked to as people age
decline in muscle mass and function
How many alpha motor neurons can innervate a single muscle fibre?
1
true or false:
The longer the length of the muscle, the greater the active contribution to total tension
false
true or false
Concentric muscle contractions produce less force when moving faster
true
Which of the following 3 muscle groups would you expect to have the larger innervation ratio?
Muscles controlling shoulder and back movements (e.g. latissimus dorsi)
Muscles controlling eye movement (extra-ocular muscles)
Muscles controlling the thumb (e.g. abductor pollicis brevis)
Both the muscles controlling the thumb and muscles controlling eye movement each have the greatest number per motor unit
Muscles controlling shoulder and back movements (e.g. latissimus dorsi)
Place the following steps in the appropriate order
Graded potential is triggered
Actin and myosin form cross bridges
Current travels down the t-tubules
Sarcoplasmic reticulum releases calcium
Action potential is triggered
Graded potential is triggered
Action potential is triggered
Current travels down the t-tubules
Sarcoplasmic reticulum releases calcium
Actin and myosin form cross bridges
