Muscle physiology

Question 1

What is a muscle unit made up of?

Answer 1

Single motor neurone + muscles fibres innervated by them

Question 2

What is sarcoplasm?

Answer 2

Muscular cytoplasm

Question 3

What is a sarcolemma?

Answer 3

A muscle fibre membrane that is made up of 2 parts

1) outer coat (thin polysaccharide/collagen layer that attaches to tendons)
2) T-tubules

Question 4

What are T-tubules

Answer 4

Deep invaginations of sarcolemma into the centre of the fibre that allows speedy depolarisation of the interior

Question 5

Sarcoplasmic reticulum

Answer 5

Is enlarged endoplasmic reticulum associated with T-tubules, releases Ca2+ on stimulation

Question 6

Motor end plate of muscle

Answer 6

Sarcolemma directly under an axon, with nicotinic receptors and in-foldings to increase the surface area

Question 7

What are the 5 stages of action potential transmission across a neuromuscular junction?

Answer 7

1) Action potential in an axon reaches the terminal bouton, Na+ influxes and causes depolarisation.
2) Ca2+ floods into the bouton, binds with a vesicle causing them to exocytose.
3) Ach is released into the cleft and binds to nicotinic receptors.
4) Nicotinic receptors allow Na+ to flood into the muscle sarcoplasm.
5) Threshold is reached and an end plate potential is generated.

Question 8

AP through a muscle fibre - 6 steps?

Answer 8

1 - EPP spreads over sarcolemma (Na+ influx)
2 - AP penetrated transverse tubules (Na+ continues to flood into the sarcoplasm)
3 - Na+ in sarcoplasm stimulates sarcoplasmic reticulum to release Ca2+ which diffuses into myofibrils
4 - Ca2+ binds to troponin
5 - Troponin pulls Tropomyosin out of the way the was so that myosin binding sites are clear
6 - Myosin heads then bind to the Actin forming a cross bridge

Question 9

What is the structure of muscle?

Answer 9

1 - muscle
2 - Fasicle/bundle (surrounded by connective tissue)
3 - Fibre/cell (surrounded by its sarcolemma)
4 - Myofibril (intracellular fibril made up of multiple protein layers)
5 - Myofilaments (single protein chains)

Question 10

Myofibril proteins are?

Answer 10

Myosin
Actin
Tropomyosin
Troponin

Question 11

What is myosin?

Answer 11

Thick protein with multiple heads that bind to actin utilising ATP to re-cock their heads

Question 12

What is Actin?

Answer 12

Thin filaments in myofibrils

Question 13

What does tropomyosin do?

Answer 13

Lies over the actin filaments and prevents myosin heads from binding (muscle at rest)

Question 14

What does troponin do?

Answer 14

Binds to Ca2+, changing its shape so that it pulls tropomyosin out of the way

Question 15

What is a sarcomere band?

Answer 15

The contractile unit of muscle, extends from one z line to another, reduces in length upon contraction.

Question 16

A bands in a sarcomere?

Answer 16

Dark and stay a constant length on contraction (= length of myosin), otherwise known as dark bands

Question 17

What are the I bands?

Answer 17

Light and reduce upon contraction = length of the sarcomere - length of myosin, also known as light bands

Question 18

What is the cross bridge cycle?

Answer 18

1 - ATP binds to myosin head to allow it to dissociate from actin
2- ATP splits to ADP + Pi to re-cock the myosin head
3 - Head binds to actin and Pi dissociates
4 - Power stroke occurs as ADP dissociates

Question 19

Phosphocreatine

Answer 19

= quickest source of ATP in skeletal muscle
1 x PC phosphorylates 1 ADP to ATP
Limited stores, made from ATP at rest during respiration and used to phosphorylate ADP on contraction

Question 20

Anaerobic respiration in skeletal muscle

Answer 20

Glycogen splits into lactic acid releasing 3 x ATP (net)

Occurs especially in white fast twitch (2b) muscle

Question 21

Aerobic respiration in skeletal muscle

Answer 21

Glucose is split into CO2 and H2O producing 38 ATPs (net)

Question 22

FFA oxidation in skeletal muscle

Answer 22

= slowest source of ATP

split into CO2 and H2O producing v high ATP yield, depends on size of FFA (100-150 ATPs)

Question 23

Type I muscle fibre

Answer 23

Slow twitch, oxidative fibre
Red with high myoglobin levels(O2 store), v vascular
V resistant to fatigue

Question 24

Type IIa muscle fibre

Answer 24

Fast twitch, oxidative fibre
Red, high myoglobin and vascular, also contains glycogen stores and machinery for anaerobic respiration
Resistant to fatigue

Question 25

Type IIb muscle fibre

Answer 25

Fast twitch, glycolytic fibre
White, low myoglobin and little vascular input, few mitochondria and oxidative enzymes
large glycogen stores
Fatigues rapidly

Question 26

What is the recruitment order of muscle fibres?

Answer 26

1st = Type 1, smallest motor neurones so depolarise quickest
2nd = Type IIa
3rd = Type IIb as largest motor neurones so depolarise slowest

Question 27

What does contraction strength depend on?

Answer 27

Number of motor units recruited, more = more tension

Frequency of impulses, more = more tension

Question 28

What is peripheral fatigue due to?

Answer 28

Contraction failure - APs don’t conduct along muscle fibre
Lactic acid build up - low pH affects muscle proteins
ADP & Pi build up inhibits cross bridge cycling

Question 29

What is central fatigue caused by?

Answer 29

Cerebral cortex not firing appropriate motor APs due to tiredness, low motivation, etc.