The cellular response to action potentials

Question 1

Which ion channels are present at the axon terminal?

Answer 1

• Voltage-gated Na+ channels
• Voltage-gated K+ channels
• Voltage-gated Ca2+ channels

Question 2

Which channels at the nerve terminal open in response to depolarisation and mediate neurotransmitter release?

Answer 2

Voltage gated Ca2+ channels

Question 3

What happens at the nerve terminal in response to an action potential, and how does this contribute to neurotransmitter release?

Answer 3

Depolarisation
Voltage gated Ca2+ channels open
Ca2+ entry
Increase in extracellular Ca2+
Ca2+ binds to synaptotagmin
Vesicle brought close to membrane
Snare complex make a fusion pore
Transmitter released through this pore

Question 4

What is the equilibrium potential of Ca2+?

Answer 4

From the Nernst eqn:
ECa = (61 / 2) x log10(1x10-3/ 1x10-7)
ECa = +122 mV

Question 5

Why is it important to neuronal signalling to keep the resting intracellular [Ca2+] low?

Answer 5

Because the concentration of Ca2+ inside is so low, the small amount of Ca2+ influx through Ca2+ channels can raise the internal concentration of Ca2+ significantly, resulting in a response to the appropriate stimulus.

Question 6

What are dihydropyridines?

Answer 6

Dihydropyridines are specific blockers of L-type Ca2+ channels, e.g. nifedipine.

Question 7

What are the isoforms and blockers of the different types of calcium channel?

Answer 7

Isoform Type Blockers
alpha1C,D,S L DHP
alpha1B N w-CTx-GVIA
alpha1A P/Q w-Aga-IVA
alpha1E R Ni2+
alpha1G,H,I T Ni2+

Question 8

What are the primary locations of the different types of calcium channel?

Answer 8

L- all muscles, neurones, lung
N- neurones
P/Q- neurones
R- neurones, heart (?)
T- neurones, heart

Question 9

Which voltage gated ion channel does the alpha subunit of the calcium channel resemble?

Answer 9

Voltage-gated Na+ channel.

Question 10

How is ACh broken down in the synapse?

Answer 10

Acetylcholinesterase.

Question 11

Where are the nAChRs located in the NMJ?

Answer 11

Around the tops of the junctional folds on the postsynaptic membrane.

Question 12

What is reversal potential?

Answer 12

In a biological membrane, the reversal potential (also known as the Nernst potential) of an ion is the membrane potential at which there is no net (overall) flow of that particular ion from one side of the membrane to the other.

Question 13

Why is a high concentration of Ca2+ channels at the axon terminal important?

Answer 13

The high density of Ca2+ channels at the nerve terminal provides enough Ca2+ influx during an action potential to trigger neurotransmitter release. This occurs by exocytosis.

Question 14

How is a postsynaptic response elicited in the NMJ?

Answer 14

The ACh will bind to the nicotinic ACh receptor on the post-junctional membrane to produce an end-plate potential; this depolarisation in turn raises the muscle above threshold so that an action potential is produced in the muscle membrane.

Question 15

Give 2 examples of blockers of nicotinic ACh receptors, and state how they work.

Answer 15

Tubocurarine- competitive antagonist (blocker), competes with ACh for the active site of the channel and prevents the channel from opening. Increasing ACh concentration can overcome this.
Succinylcholine- depolarising blocker- binds to nAChRs, causing them to open, followed by desensitisation/inactivation of the channels, preventing them from opening. The maintained depolarization will fail to activate adjacent Na+
channels due to inactivation.

Question 16

How many subunits does a nicotinic ACh receptor have?

Answer 16

5:

2 alpha, 1 beta, 1 delta, 1 gamma

Question 17

Which subunit of the nAChR does ACh bind to, how many molecules are needed to open the channel?

Answer 17

ACh binds to the alpha subunit, therefore 2 molecules are needed for the channel to undergo the conformational change that causes it to open.

Question 18

What is the end plate potential?

Answer 18

The depolarisation to threshold due to ACh acting on nACHRs at the neuromuscular junction.

Question 19

What is excitation-contraction coupling?

Answer 19

Contraction caused by action potentials, that move along the muscle fibre.

Question 20

Where is the end plate potential recorded?

Answer 20

In the muscle fibre.

Question 21

How does calcium concentration affect end plate potential?

Answer 21

Transmitter release is dependent on and proportional to Ca2+ entry – end plate potentials decrease in amplitude as external Ca2+ is lowered.

Question 22

How is an action potential in the NMJ elicited?

Answer 22

Brief depolarization (the end-plate potential) caused by activation of nAChR by Ach binding will activate adjacent Na+ channels due to local spread of charge causing a muscle AP.

Question 23

What is a miniature end-plate potential?

Answer 23

A small depolarisation caused by spontaneous release of neurotransmitter (about 1 per second). These are not mediated by calcium influx.

Question 24

What is myasthenia gravis?

Answer 24

An autoimmune disease that targets nicotinic ACh receptors:
• Patients suffer profound weakness
• Weakness increases with exercise
• Caused by antibodies directed against nAChR on postsynaptic membrane of skeletal muscle
• Antibodies lead to loss of functional nAChR by complement mediated lysis and receptor degredation
• Endplate potentials are reduced in amplitude leading to muscle weakness and fatigue

Question 25

Why is a postsynaptic response in the NMJ depleted in myasthenia gravis?

Answer 25

Each quantum of ACh released produces a
smaller response than in the normal muscle
because there are fewer nACh receptors available.
Miniature end-plate potentials in patients with myasthenia gravis are much smaller than normal.

Question 26

What is the difference between neuromuscular blockers and anaesthetics?

Answer 26

Anaesthetics cause numbing of pain, and general anaesthetics also cause a loss of consciousness.
Neuromuscular blockers do not cause loss of consciousness or pain relief, but will cause paralysis. NMBs are used in conjunction with anaesthetics in surgery to prevent the patient from moving.

Question 27

What is a muscarinic ACh receptor?

Answer 27

A G-protein coupled receptor that triggers a signalling cascade in the cell. Activity of the target effector is modified by the Galpha subunit. mAChRs have a slower response than nAChRs.

Question 28

What are the subtypes of muscarinic acetylcholine receptors and which G-proteins do they use?

Answer 28

M1-M5
M1, 3 and 5- Gq
M2 and 4- Gi/o