I/E: Synaptic Transmission

Question 1

How do synapses ensure that impulses on a neurone are unidirectional?

Answer 1

Receptors are only on the postsynaptic membranes.

Question 2

How do nerve impulses transmit across a cholinergic synapse?

Answer 2

1. AP arrives at synaptic knob of presynaptic neurone.
2. Stimulates voltage-gated calcium ion channels in presynaptic neurone to open.
3. Calcium ions diffuse into the synaptic knob.
4. Influx of calcium ions causes the synaptic vesicles to move to the presynaptic membrane and fuse with the membrane.
5. Vesicles release acetylcholine into synaptic cleft - exocytosis.
6. ACh diffuses across the synaptic cleft and binds to specific cholinergic receptors on the postsynaptic membrane.
7. Sodium ion channels in postsynaptic neurone open.
8. Influx of sodium ions causes depolarisation and AP occurs if threshold is reached.
9. ACh is removed from cleft by being broken down by acetylcholinesterase and the products are re-absorbed by the presynaptic neurone and used to make more ACh.

Question 3

What are the different types of neurotransmitters?

Answer 3

Excitatory, inhibitory or both.

Question 4

What do excitatory neurotransmitters do to the postsynaptic membrane?

Answer 4

Depolarise the postsynaptic membrane.

Question 5

What do inhibitory neurotransmitters do to the postsynaptic membrane?

Answer 5

Hyperpolarise

Question 6

What are the two types of summation?

Answer 6

Spatial summation

Temporal summation

Question 7

Describe spatial summation:

Answer 7

Many neurones connect to one neurone.

Total amount of neurotransmitter released from all neurones might be enough to reach the threshold - BUT if some neurotransmitters are inhibitory then no action potential might be triggered.

Question 8

Describe temporal summation:

Answer 8

Two or more nerve impulses arrive in quick succession from the same presynaptic neurone.

Question 9

How does temporal summation make an action potential more likely?

Answer 9

More neurotransmitter is released into the synaptic cleft, as impulses arrive in quick succession.

Question 10

How does spatial summation make an action potential more likely?

Answer 10

Additive effect of all neurones means that threshold may be reached even if action potential is small.

Question 11

Neuromuscular junction

Answer 11

A synapse between a motor neurone and a muscle cell, which uses the neurotransmitter ACh.

Question 12

In neuromuscular junctions, what does ACh bind to?

Answer 12

Nicotinic cholinergic receptors.

Question 13

How is a neuromuscular junction different to a cholinergic synapse?

Answer 13

• Postsynaptic membrane has folds that form clefts - these store acetylcholinesterase.
• Postsynaptic membrane has more receptors than other synapses.
• ACh is always excitatory.

Question 14

Give 5 ways that drugs can affect the action of neurotransmitters at synapses:

Answer 14

1. Agonists - same shape as neurotransmitters and so mimic their action so more receptors are activated.
2. Antagonists - block receptors so they can’t be activated and so fewer receptors are activated and muscles become paralysed.
3. Enzyme inhibitors - neurotransmitters aren’t broken down in synaptic cleft, so more bind to receptors and are there for longer - can lead to loss of muscle control.
4. Stimulate neurotransmitters release so more receptors are activated.
5. Inhibit release of neurotransmitters so fewer receptors are activated.