Synaptic transmission

Question 1

Explain transmission across a cholinergic synapse.

Answer 1

1. Action potential arrives at the synaptic knob of the presynaptic neurone where it stimulates voltage-gated calcium ions to open allowing calcium to diffuse into the synapse
2. Increase in concentration of Ca2+ causes synaptic vesicles to move to presynaptic membrane and fuse with it which releases acetycholine into the synaptic clef. This is know as exocytosis.
3. ACh diffuses across the synaptic cleft and binds to the receptors on the post synaptic membrane.
4. This causes Na+ channels to open which allows Na+ to diffuse into the post synaptic neurone resulting in depolarisation. If threshold is reached new action potential is generated.

Question 2

Which enzyme is ACh broken down by?

Answer 2

Acetylcholinesterase

Question 3

Explain how synapses are undirectional

Answer 3

The impulse can only travel in one direction because receptors are only found on the postsynaptic membrane. Neurotransmitters can also only be made and released from the presynaptic neurones side.

Question 4

Explain the 2 types of summation

Answer 4

Spatial summation
- When many presynaptic neurones connect to a single postsynaptic neurone. Together they can release enough neurotransmitters to exceed the threshold value and trigger an action potential

Temporal summation
- When a single presynaptic neurone releases neurotransmitter many times over a short period of time. An action potential is generated if total amount exceeds threshold value.

Question 5

Explain inhibitory synapses

Answer 5

• Some synapses make it less likely that an action potential will be triggered because the neurotransmitter may bind to Cl- channels causing them to open and Cl- diffusing into postsynaptic neurone. This could cause K+ channels to open meaning more K+ leaves and the overall membrane potential is more negative than resting potential (hyperpolarisation)