Lecture: Physiology 4: From neuromuscular junction to myocyte

Question 1

What are neuromuscular junctions?

Answer 1

Neuromuscular junctions are specialised intercellular connection between a neuron and a muscle cell (myocyte).

Question 2

How does acetylcholine trigger an action potential across a synapse?

Answer 2

1. Acetylcholine is synthesised and packaged in vesicles
2. Depolarisation of AP causes pre-synaptic calcium channels to open.
3. Increased free calcium across synaptic cleft causes vesicles to fuse
4. Acetylcholine is released
5. Acetylcholine binds to and activates specific post-synaptic ACh receptors on myocyte membrane that may initiate myocyte action potential (i.e. excitatory).

Question 3

How is acetylcholine removed following an action potential?

Answer 3

Acetylcholine is eliminated from synapse via acetylcholinesterase (AChE).

Question 4

What is a myofibre?

Answer 4

One long multi-nucleate muscle cell.

Question 5

What is a myofibril?

Answer 5

An organelle- string of sarcomeres.

Question 6

What is a myofilament?

Answer 6

Thick or thin filament (molecules) .

Question 7

What happens during contract of a muscle fibre?

Answer 7

Ends of sarcomere (z lines) are pulled toward each other by myosin filament pulling actin filaments.

Question 8

What is the A band?

Answer 8

Dark bands; overlap of thick filaments (contains myosin) and thin filament (contains actin)

Question 9

What is the I band?

Answer 9

Light bands; thin filaments (contains actin)

Question 10

What is the H band?

Answer 10

Space where there is no A or I band overlap

Question 11

Describe the action of thin and thick filaments during contraction

Answer 11

Thin filaments are pulled to centre of sarcomere towards the M line and so the overlap between thick and thin filaments increases and Z lines of each sarcomere come closer together. I and H bands progressively get shorter with contraction.

Question 12

What is the role of mysoin?

Answer 12

Myosin is “the motor protein” that pulls actin by bending “neck”.

Question 13

Describe the composition of thin filaments

Answer 13

Thin filaments are composed of actin polymer and are anchored to Z lines. Thin filaments also have tropomyosin polymer (block actin-myosin activity) and troponin complexes (control tropomyosin).

Question 14

What is F-actin?

Answer 14

F-actin are the polymer chains of actin which have a groove of each side that houses tropomyosin.

Question 15

What are the proteins found on thin filaments?

Answer 15

1. Troponin complexes

2. Tropomysoin polymer

Question 16

What is Excitation-Contraction coupling?

Answer 16

Excitation-Contraction coupling is the link between the depolarisation of the membrane (with a tiny influx of calcium) and the consequent huge cytosolic calcium influx that then leads to contraction. POSITIVE FEEDBACK.

Question 17

What are the 4 classes of 2nd messengers?

Answer 17

• Ions
• G proteins
• Phosphorylation (or dephosphorylation)
• Intracellular receptors

Question 18

What is the 2nd messenger in muscle contraction and where is it formed?

Answer 18

In muscle contraction, calcium is a second messenger. Most of the calcium comes from the sarcoplasmic reticulum (SR).

Question 19

What is twitch?

Answer 19

Twitch is a single contraction of a muscle fibre.

Question 20

What is tetany?

Answer 20

A process whereby a myocyte continuously generates its maximal force for a duration lasting at least twice as long as a single twitch, because relaxation is not long enough to pump calcium back into SR.

Question 21

What are the 3 phases of twitch?

Answer 21

3 phases: latent, contraction, and relaxation. Calcium is pumped back into SR during relaxation, thus only one contraction is triggered.

Question 22

Describe the cross-bridge cycling action of actin and mysoin

Answer 22

In the resting state of sarcomere, the myosin heads are blocked from binding to actin by tropomyosin, which occupies the specific binding sites.

Calcium triggers troponin to move tropomyosin from binding site and to allow myosin head to bind to and pull actin.

Myosin head must cleaves ATP to bind to actin and trigger a power stroke.

Question 23

What are the 3 subunits of troponin?

Answer 23

T, C, and I

Question 24

What is the role of Troponin T?

Answer 24

Tropomyosin-binding

Question 25

What is the role of Troponin C?

Answer 25

Calcium-binding; calcium binding causes troponin to change shape and move tropomyosin.

Question 26

What is the role of Troponin I?

Answer 26

Inhibitory- binds to actin

Question 27

What are the 2 types of acetylcholine receptors?

Answer 27

1. Nicotinic receptors

2. Muscarinic receptors

Question 28

How do nicotinic acetylcholine receptors work?

Answer 28

When receptor is activated, there is an increase in voltage inside cell which allows both sodium ions to move into the cell and potassium ions to move out of the cell; “non-selective cation channel”.

Question 29

How do muscarinic acetylcholine receptors work?

Answer 29

When receptor is activated, it triggers a G protein which causes adenylate cyclase to convert ATP to cAMP which triggers a further cascade to trigger cellular response.