Physiology of the Neuromuscular Junction

Question 1

Which neurotransmitter is used at the neuromuscular junction?

Answer 1

Acetylcholine

Question 2

Acetylcholine acts on which type of receptor at the post synaptic terminal?

Answer 2

Nicotinic receptors

Question 3

When nicotinic acetylcholine receptors open, what enters the post synaptic neurone?

Answer 3

Na⁺ ions

Question 4

When nicotinic acetylcholine receptors open, what exits the postsynaptic neurone?

Answer 4

K⁺ ions

Question 5

Why is neuromuscular transmission terminated so quickly?

Answer 5

The enzyme acetylcholinesterase works very quickly to break down acetylcholine - even as soon as it is released from the pre synpatic vesicles

Question 6

When acetylcholine is broken down by acetylcholinesterase, what is it broken down into?

Answer 6

Acetate and choline

Question 7

When acetylcholine is broken down by acetylcholinesterase, what is the fate of acetate?

Answer 7

It diffuses into the synaptic cleft

Question 8

When acetylcholine is broken down by acetylcholinesterase, what is the fate of choline?

Answer 8

It is reabsorbed

(It re-enters the pre-synaptic terminal due to symport with Na⁺ using a choline transporter)

Question 9

Within the pre-synaptic terminal, how is the reabsorbed choline converted back to acetylcholine, and which enzyme aids the process?

Answer 9

It combines with acetyl CoA to produce acetylcholine.

The enzyme aiding this process is choline acetyltransferase (ChAT)

Question 10

When an action potential enters the pre-synaptic terminal, which ions can now enter the pre-synaptic terminal?

Answer 10

Ca²⁺ ions

(due to activation of voltage actived calcium channels)

Question 11

When an action potential arrives at the pre-synaptic terminal, what does the subsequent influx of Ca²⁺ ions induce?

Answer 11

Active zones (filled with acetylcholine) fuse with the pre-synaptic membrane and by exocytosis, acetylcholine is released into the synaptic cleft

Question 12

How many glycoprotein subunits make up an acetylcholine nicotinic receptor?

Answer 12

5

Question 13

How many acetylcholine molecules are required to bind at each acetylchline nicotinic receptor for it to open?

Answer 13

2

Question 14

After a nicotinic acetylcholine receptor is opened, why is an end plate potential created?

Answer 14

The flow of Na⁺ into the post-synaptic neurone is greater than the K⁺ flow out.

This creates a depolarisation

Question 15

Within the nicotinic receptor channel, what is it that attracts sodium ions into the post-synaptic neurone?

Answer 15

Amino acid residues

(negatively charged)

Question 16

What is the quantity of neurotransmitter held within each vesicle referred to as?

Answer 16

A quantum of neurotransmitter

This amount of neurotransmitter is fairly constant between vesicles

Question 17

Why would the end plate potential quickly die out along the muscle fibre without any further intervention?

Answer 17

Leaky chloride channels along the length of the muscle would balance out the depolarisation back to a resting potential

(this doesn’t actually happen, it is hypothetical)

Question 18

Why does the action potential initiated by the end plate potential not actually due out along the length of the muscle fibre?

Answer 18

Volatge activated sodium channels become activated (due to the end plate potential) and allows for the whole muscle fibre to become excited

Question 19

How does the action potential actually cause muscle contraction when travelling along the muscle fibre?

Answer 19

The action potential travels along the muscle fibre until it reaches invaginations on the surface. These are T tubules, leading to the sarcoplasmic reticulum. When the action potential travels down the T tubules it causes calcium release within the cells which leads to the conformational change of the troponin-tropomyosin complex and the free access for the myosin head to the myosin binding site on actin

Question 20

Why does the action potential last for a much shorter time than the muscle twitch?

Answer 20

• Time is required for calcium release
• Troponin binding to calcium takes time
• Reveal of myosin head binding site takes time
• Sliding of actin and myosin takes time

etc.