Lecture 7: Muscle Function

Question 1

what is an alpha motor neurone

Answer 1

Synapses with multiple
muscle fibres

Provides excitation for muscle fibres to generate tension

Question 2

what is a motor unit

Answer 2

The alpha motor neuron and the skeletal muscle fibres it innervates

Question 3

what is the all or none principle of the motor unit

Answer 3

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

Question 4

what is a motor neuron pool

Answer 4

all the motor neurons that innervate a single muscle

Question 5

what is a neuromuscular junction

Answer 5

synapse between neuron and a muscle fibre

Question 6

what are the 3 parts of the neuromuscular junction

Answer 6

presynaptic terminal

synaptic cleft

postsynaptic muscle fiber (motor endplate)

Question 7

what are the 5 steps of Neuromuscular Junction Synaptic Transmission

Answer 7

1. depolarization of presynaptic membrane (influx of calcium)
2. acetylcholine released into synaptic cleft
3. acetylcholine binds to receptors on muscle membrane
4. depolarization of motor endplate
5. endplate potential can trigger action potential

Question 8

why do Motor neurons release excess acetylcholine to depolarize the postsynaptic membrane

Answer 8

safety factor for transmission:

Increases chance that muscle will contract, even when fatigued

Question 9

what is the clinical implication of the safety factor

Answer 9

Myasthenia Gravis results in a loss of acetylcholine receptors in the muscle fibre

Reduced membrane depolarization and smaller safety factor → weakness

Question 10

what is the muscle structure (4 parts)

Answer 10

muscle > fascicle > muscle fibre > myofibril

Question 11

what is a sarcolemma

Answer 11

Each muscle fibre is surrounded

allows depolarizing current to reach the sarcomeres

Question 12

what is the Sarcoplasmic reticulum

Answer 12

stores, releases, and retrieves calcium

Question 13

what are Sarcomeres

Answer 13

the smallest functional unit of the muscle fibre

Consist of Actin and Myosin

Question 14

what is actin

Answer 14

the thin filament

Question 15

what is myosin

Answer 15

the thick filament

Question 16

what are the 4 steps to excitation-contraction coupling

Answer 16

1. endplate potential triggers action potential
2. action potential travels to muscle via t-tubule
3. release of calcium from sarcoplasmic reticulum
4. interaction of actin myosin

Question 17

why is calcium needed for muscle contraction

Answer 17

because it converts an action potential into a muscle contraction

Question 18

what are the 5 steps of the sliding filament theory

Answer 18

1. myosin head is in a cocked position with ATP present
2. calcium reveals binding site on actin. myosin head binds with actin to make a cross-bridge
3. ‘Powerstroke’ – myosin pulls actin towards the centre of the sarcomere
4. When ATP is still present, Myosin dissociates from actin, its head is re-cocked, and it binds to the next active site
5. cross-bridge cycling

Question 19

when does muscle relaxation happen

Answer 19

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

Question 20

what are the 6 factors that influence the amount of force (tension) a muscle can generate

Answer 20

muscle length

muscle velocity and direction

innervation ratio

motor unit type

size principle

rate coding

Question 21

how does muscle length affect the force produced by muscles?

Answer 21

Active contraction and passive structures contribute to total tension in the muscle

Question 22

what is passive contribution of muscle length

Answer 22

• Weak actin-myosin bonds act like an elastic
• Increased tension with increased length

Question 23

what is active contraction of muscle length

Answer 23

• 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

Question 24

what is total tension in muscle length

Answer 24

it gets higher as muscle is lengthened

Question 25

when lengthening a muscle is most of the tension coming from passive or active components

Answer 25

passive

Question 26

how does muscle velocity and direction affect the force produced by muscles?

Answer 26

Eccentric > isometric > concentric Lengthening (eccentric) * More tension with increasing velocity and greater passive contribution Shortening (concentric) * Less tension with increasing velocity

Question 27

what is innervation ratio

Answer 27

The number of muscle fibres innervated by a single motor neuron

Question 28

how does innervation ratio affect the force produced by muscles?

Answer 28

Innervation ratio differs by muscle type * Power/Force muscles = higher innervation ratio * Precision muscles = smaller innervation ratio Collateral sprouting increases the innervation ratio

Question 29

how does motor unit/ muscle fibre type affect tension?

Answer 29

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

Question 30

how does the size principle of motor unit recruitment affect tension?

Answer 30

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

Question 31

how does rate coding affect tension?

Answer 31

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

Question 32

how does age impact neuromuscular function

Answer 32

action potential propagation is slowed Reduced density of active areas in the presynaptic terminal Loss of motor units (but increased size of remaining ones)

Question 33

how does ALS affect neuromuscular function

Answer 33

Rapid loss of fast fatigable neurons, followed by loss of fast fatigue-resistant neurons but not type 1 neurons Loss of strength and power

Question 34

how does stroke affect neuromuscular function

Answer 34

Loss of motor units on paretic side Smaller muscle cross-sectional area Reduced ability to use size principle

Question 35

how does a spinal cord injury affect neuromuscular function

Answer 35

Extensive atrophy, mostly affects type II fibres Loss of motor units Transformation toward type IIx

Question 36

how does Parkinson disease affect neuromuscular function

Answer 36

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

Question 37

what can neuromuscular junction changes could be causally linked to as people age

Answer 37

decline in muscle mass and function

Question 38

How many alpha motor neurons can innervate a single muscle fibre?

Answer 38

1

Question 39

true or false: The longer the length of the muscle, the greater the active contribution to total tension

Answer 39

false

Question 40

true or false Concentric muscle contractions produce less force when moving faster

Answer 40

true

Question 41

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

Answer 41

Muscles controlling shoulder and back movements (e.g. latissimus dorsi)

Question 42

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

Answer 42

Graded potential is triggered Action potential is triggered Current travels down the t-tubules Sarcoplasmic reticulum releases calcium Actin and myosin form cross bridges