ICPP - Electrical Excitability

Question 1

What is a neuromuscular junction?

Answer 1

The synapse between a nerve and a skeletal muscle fibre.

Question 2

What role does Ca2+ play at the nerve terminal?

Answer 2

Depolarisation opens the voltage-gates Ca2+ channels, leading to Ca2+ entry. This increases the intracellular Ca2+ concentration, leading to neurotransmitter release from the vesicles.

Question 3

How is increase in frequency of action potentials linked with amount of neurotransmitter released?

Answer 3

More action potentials = increase in intracellular [Ca2+] = more transmitter released.

Question 4

What is the purpose of the other associated subunits attached to the alpha subunit in the Ca2+ channel?

Answer 4

Fine-tunes properties and enables correct regulation of channel activity

Question 5

Which activates faster - Ca2+ or Na2+ channels?

Answer 5

Na2+ channels are much faster than Ca2+ channels.

Question 6

How can snail neurones be used to show that Ca2+ is responsible for Ca2+ channel inactivation?

Answer 6

Snails have Ba2+ as well, which will pass through Ca2+ channels instead of Ca2+. When it does this, much less inactivation of channels is seen, so it must be increased intracellular [Ca2+] that leads to inactivation of Ca2+ channels

Question 7

What are motor end plates?

Answer 7

The point at which the axon connects to the myofibres

Question 8

What are snare proteins?

Answer 8

These form snare complexes are direct neurontransmitter to be released. They do this by bringing the vesicles close to the membrane and forming a “fusion pore”, through which neurotransmitter is released.

Question 9

How many molecules of ACh must bind to nicotinic ACh receptors to make them open?

Answer 9

2

Question 10

Which ions are allowed through nACh receptors?

Answer 10

Na+ is allowed in, and K+ is allowed out.

Question 11

What happens to a muscle action potential once it reaches an end plate?

Answer 11

It is initiated adjacent to the end-plate and propagates along the muscle fibre, initiating contraction of the skeletal muscle fibre.

Question 12

What are the two types of blockers of nicotinic ACh receptors?

Answer 12

Competitive blockers eg tubocurarine, and depolarising blockers eg succinylcholine

Question 13

How can competitive blocking be overcome?

Answer 13

Increase the concentration of ACh.

Question 14

How does a depolarising block work?

Answer 14

Channels become depolarised by blocker, meaning that they cannot be depolarised by ACh.

Question 15

What is myasthenia gravis?

Answer 15

Autoimmune disease where antibodies lead to loss of functional nAChR by complement mediated lysis and receptor degradation.

Question 16

What are the symptoms of myasthenia gravis?

Answer 16

Endplate potentials are reduced in amplitude leading to muscle weakness and fatigue which worsens with exercise.

Question 17

What is the difference in response times of nAChR and mAChR?

Answer 17

Nicotinic AChR produce a fast depolarisation because they are ligand gated. mAChR produce a slower cascade response because they are coupled to G-proteins which are slower to respond.