P: Skeletal muscle

Question 1

Where do alpha motor neurones originate from?

Answer 1

Ventral horn of spinal cord

Question 2

What are motor units + why are they important?

Answer 2

An alpha motor neurone and all the muscle fibres innervated by that neurone - varies strength of contraction of skeletal muscle

Question 3

Muscle contraction

Answer 3

1. Muscle action triggers release of calcium ions from Ryanodine receptor.
2. Calcium binds to troponin C of actin
3. Calcium binding allows actin-myosin to form a crossbridge-bridge formation and sarcomere shortens
4. Muscle relaxation is associated with detachment of actin/myosin with calcium pumped back into SR.

Question 4

What is the difference between DHPR receptors and RYR receptors

Answer 4

Dihydropyridine receptors are embedded in T-tubules, whereas Ryanodine receptors are embedded in SR

Question 5

What is the function of the DHPR receptor

Answer 5

It is a voltage sensor effectively sensing the arrival of a muscle AP as it tracks along the T-tubule
In doing this it signals the RYR receptors to open and Ca2+ is released into the SR

Question 6

What is the fate of the Ca2+ after release from the SR

Answer 6

Binds to troponin C on the actin filament This uncovers myosin binding sites on actin filaments so that a cross-bridge can be formed

Question 7

Isotonic muscle contraction

Answer 7

Constant force, muscle shortens and can move the load, muscle length can be measured

Question 8

Isometric muscle contraction

Answer 8

Constant length, muscle unable to shorten, force generated is measured

Question 9

What are energy sources for contraction

Answer 9

ATP
Carbohydrates/glycogen
FAs/triglycerides (prolonged exercise)

Question 10

Tetanus?

Answer 10

At high levels of stimulation (temporal summation) (more AP), there is a sustained rise in intracellular calcium which allows for a greater force than during a single twitch

Question 11

Motor unit recruitment - which fibres are recruited first?

Answer 11

Slow-twitch (type 1) MUs are recruited first. As more force is needed, fast twitch MU come into play

Question 12

Advantage of motor unit recruitment

Answer 12

The first muscle fibres to be recruited are those that have high resistance to fatigue
Also the small size of slow twitch MUs allows fine motor control at low levels of force

Question 13

Which is temporal and which is spatial between MU recruitment and tetanus

Answer 13

Tetanus is temporal
MU recruitment is spatial

Question 14

Definition of muscle fatigue

Answer 14

Reduced ability of a muscle to generate power (different from weakness)

Question 15

Glucose enters skeletal muscle via

Answer 15

facilitated diffusion

Question 16

outline briefly the sequence of events during a cross-bridge cycle in a healthy skeletal muscle

Answer 16

The head of myosin binds to ATP and hydrolyzes it to ADP and phosphate while generating energy.
The myosin head is then ready to bind to the active site of actin, Tropomyosin is rolled over by troponin C following calcium binding and exposes the actin active site, the myosin head then binds to actin forming a cross bridge.
ADP is then released along with phosphate. The myosin head then binds to another ATP molecule that detaches the myosin head from the actin filament, hydrolysis then occurs