Muscle Function

Question 1

what is an alpha motor neuron?

Answer 1

A neuron that synapses with multiple
muscle fibres

Question 2

what does an alpha motor neuron do?

Answer 2

Provides excitation for muscle
fibres to generate tension

Question 3

what is a motor unit?

Answer 3

The alpha motor neuron and the
skeletal muscle fibres it innervates

Question 4

what principle does a motor unit follow? And what does this mean?

Answer 4

an all or none principle

When a motor unit is
stimulated, all muscle fibres in
that motor unit will fire

Question 5

where ar motor neurons grouped together?

Answer 5

on the longitudinal axis or the spinal cord

Question 6

what is a motor neuron pool?

Answer 6

all the motor neurons that innervate a single muscle

Question 7

what are the 3 parts of the neuromuscular junction?

Answer 7

1. presynaptic terminal
2. synaptic cleft
3. post synaptic muscle fiber (motor endplate)

Question 8

what are the steps of synaptic transmission at the NMJ?

Answer 8

1. Depolarization of the presynaptic
   membrane causes influx of calcium
2. Acetylcholine is released into
   synaptic cleft via vesicles
3. Acetylcholine binds to receptors on
   muscle membrane
4. Causes depolarization of motor
   endplate (end plate potential)
5. End plate potential triggers an action potential if reaches threshold

Question 9

what happens to acetylcholine after synaptic transmission?

Answer 9

its broken down by the enzyme acetylcholinesterase and the vesicles are recycled

Question 10

why do motor neurons release extra acetylcholine?

Answer 10

to depolarize the post synaptic membrane

Question 11

what is the purpose of the safety factor or transmission?

Answer 11

it increases the chance that
the muscle will contract, even
when fatigued

Question 12

what happens to acetylcholine with Myasthenia Gravis?

Answer 12

results in a loss of acetylcholine receptors in the muscle fibre

Reduced membrane depolarization and smaller safety factor → weakness

Question 13

list the parts of a muscle from largest to smallest

Answer 13

muscle-fascicle- muscle fiber- myofibril

Question 14

what is each muscle fibre surrounded by?

Answer 14

sarcolemma

Question 15

what is the function of the sarcolemma?

Answer 15

allows depolarizing current to
reach the sarcomeres

Question 16

what are the openings in the sarcolemma?

Answer 16

T-tubules

Question 17

what is the function of the sarcoplasmic reticulum?

Answer 17

stores, releases, and retrieves calcium

Question 18

what is the smallest functional unit of the muscle fiber?

Answer 18

sarcomeres

Question 19

what do sarcomeres consist of?

Answer 19

actin and myosin

Question 20

how are sarcomeres arranged?

Answer 20

in series

Question 21

what are the steps of exctitation-contraction coupling?

Answer 21

1. Endplate potential
   triggers action potential
2. Action potential travels into
   muscle via t-tubules
3. Release of Ca2+ from sarcoplasmic
   reticulum
4. Interaction of actin and myosin

Question 22

what is the secondary messenger from action potential to muscle contraction?

Answer 22

calcium

Question 23

what is a muscle contraction dependent on?

Answer 23

release of calcium from the sarcoplasmic reticulum

Question 24

describe the steps of the sliding filament theory

Answer 24

1. Myosin head is in a ‘cocked’
   position with ATP present
2. Ca2+ reveals binding site on
   actin. Myosin head binds
   with actin (cross-bridge)
3. ‘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

5. Cross-bridge cycling

Question 25

when does muscle relaxation occur?

Answer 25

when stimulation of the motor neuron stops or ATP is no longer available

Question 26

what breaks the link between actin and myosin causing the muscle to relax?

Answer 26

calcium being pumped back into the sarcoplasmic reticulum

Question 27

what contributes to total tension in the muscle?

Answer 27

active contraction and passive structures

Question 28

what acts like an elastic in passive muscle contribution?

Answer 28

weak actin-myosin bonds, connective tissue and Titin

Question 29

is there a larger or smaller force generated when moving away from optimal length during active contraction?

Answer 29

smaller force

Question 30

list the muscles contraction types from most least tension to most tension

Answer 30

concentric- isometric- eccentric

Question 31

increasing velocity does what to muscle tension?

Answer 31

less muscle tension because the cross bridges cannot cycle quickly enough

Question 32

why do eccentric muscle contractions have more tension with increasing velocity?

Answer 32

because a greater percentage of cross bridges remain attached

Question 33

what is innervation ratio?

Answer 33

The number of muscle fibres innervated by a single motor neuron

Question 34

how many motor neurons synapse with a muscle fiber?

Answer 34

1

Question 35

what does innervation ratio differ by?

Answer 35

muscle type

Question 36

what muscles have a higher innervation ratio?

Answer 36

power/force muscles higher innervation ratio (ex. glutes)

Question 37

what muscles have a lower innervation ratio?

Answer 37

precision muscles (ex:hand muscles)

Question 38

what increases the innervation ratio?

Answer 38

collateral sprouting

Question 39

what type of motor units have a low threshold and are activated first?

Answer 39

slow motor units (fatigue resistant)

Question 40

list the order of which motor units are activated from first to last

Answer 40

slow fatigue resistant (Type 1)- fast fatigue resistant (type 2)- fast fatiguable (Type 2x)

Question 41

what is the size principle of motor unit recruitment?

Answer 41

With increasing levels of motor activation, motor units are recruited from smallest to largest (S→FR→FF)

Question 42

what is rate coding?

Answer 42

Motor unit firing rate

Question 43

what do smaller muscles usually rely on for recruitment?

Answer 43

rate coding

Question 44

what do larger muscles typically rely on for recruitment?

Answer 44

size principle

Question 45

what are the impacts of aging on neuromuscular function?

Answer 45

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)

Question 46

what could be causally linked to the decline of muscle mass as people age?

Answer 46

NMJ changes

Question 47

what neurons does ALS affect?

Answer 47

Rapid loss of fast fatigable neurons, followed by loss of fast fatigue-resistant neurons

Question 48

what neurons are spared in ALS?

Answer 48

Type 1 slow fatigue resistant

Question 49

what is the clinical implication of Type 2 and 2X neurons being impacted in ALS?

Answer 49

loss of strength and power

Question 50

what is the impact of stroke on neuromuscular function?

Answer 50

loss of motor units on paretic side smaller muscle cross sectional area reduce ability to use size principle

Question 51

what is the impact of a SCI on neuromuscular function?

Answer 51

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

Question 52

what is the impact or Parkinson's disease on neuromuscular function?

Answer 52

Loss of motor units * Selective loss of type II motor units * Compensatory type I muscle fibre hypertrophy Loss of strength and power