synaptic transmission

Question 1

Describe the movement of calcium ions following the arrival of an action potential at a presynaptic neurone.

Answer 1

The action potential stimulates voltage-gated calcium ion channels in the presynaptic neurone to open, so calcium ions diffuse into the synaptic knob.

Question 2

Explain how acetylcholine (ACh) leaves the presynaptic neurone and causes an action potential in the postsynaptic neurone.

Answer 2

Action potential arrives at the presynaptic membrane. This causes calcium ion channels to open on the presynaptic membrane. The influx of calcium ions into the synaptic knob causes the synaptic vesicles to fuse with the presynaptic membrane. The vesicles release ACh into the synaptic cleft by exocytosis. ACh diffuses across the synaptic cleft and binds to specific cholinergic receptors on the postsynaptic membrane. This causes sodium ions channels in the postsynaptic neurone to open. Sodium ions diffuse into/enter the postsynaptic neurone. If the threshold is reached, the voltage-gated ion channels open and causes an action potential/depolarisation.

Question 3

Describe how a neurotransmitter stimulates the production of nerve impulses in postsynaptic neurones.

Answer 3

Neurotransmitter diffuses across the synapse; Attaches to receptors on postsynaptic membrane; and stimulates entry of sodium ions and action potential;

Question 4

Explain why impulses at a synapse are unidirectional

Answer 4

Vesicles of neurotransmitter are only found in the synaptic knob. Neurotransmitter receptors are only found on the postsynaptic membranes. This means the neurotransmitter can’t activate an action potential back along the presynaptic neurone.

Question 5

Why is it important that ACh is removed from the synaptic cleft by being broken down by the enzyme AChE?

Answer 5

So the response doesn’t keep happening- so there is no ACh to bind to the receptors and the sodium ion channels close.

Question 6

What effect does an inhibitory neurotransmitter have on a postsynaptic membrane?

Answer 6

Inhibitory neurotransmitters cause the inside of the post synaptic neurone/membrane/axon to remain more negative. This means the membrane potential is below resting potential. This occurs because chloride ion channels to open and chloride ions to move into the neurone. It also causes more potassium channels to open causing more potassium ions to leave the neurone. The inside of the postsynaptic neurone becomes more negative due to hyperpolarisation; So more sodium ions required to reach the threshold and an action potential in the postsynaptic membrane;

Question 7

Describe spatial summation.

Answer 7

When more than one presynaptic neurone releases neurotransmitter into the same synapse, allowing threshold potential to be reached.

Question 8

Describe temporal summation

Answer 8

When one presynaptic neurone releases many small amounts of neurotransmitter into the same synapse following repeated action potentials, allowing threshold potential to be reached.