6.2.2 Synaptic transmission

Question 1

What is a synapse?

Answer 1

The point at which one neurone forms a junction with another neurone, or with an effector (usually a muscle)

Question 2

What happens when an impulse arrives at a synapse?

Answer 2

1. Action potential arrive at axon terminal
2. Voltage-gated Ca2+ channels open
3. Ca2+ enters the cell
4. Ca2+ signals to vesicle
   5.Vesicles move to the membrane
5. Vesicles release neurotransmitter by exocytosis
6. Neurotransmitter diffuses across the synaptic cleft and binds to receptors - movement of Na+ into post synaptic neurone = depolarisation –> action potential
7. Acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid, which diffuses back across the synaptic cleft into the presynaptic neurone (=recycling)
8. ATP released by mitochondria is used to recombine choline and ethanoic acid into acetylcholine. This is stored in synaptic vesicles for future use. Sodium ion channels close in the absence of acetylcholine in the receptor sites

Question 3

What are the functions of synapses?

Answer 3

• Allow an action potential to be transmitted from one neurone to another
• Allows a single action potential travelling along one neurone to be transmitted to a number of different neurones
• A number of different action potentials can be combined at a synapse

Question 4

What are the main features of synapses?

Answer 4

1. Unidirectional
2. Allow summation
3. Can be excitatory or inhibitory

Question 5

Why are synapses unidirectional?

Answer 5

• Presynaptic neurone has the synaptic vesicles
• Post synaptic neurone has Na+ channels with binding sites for neurotransmitter

Question 6

What is spatial summation?

Answer 6

Neurone A releases transmitter but concentration is below threshold to trigger action potential
Neurone B releases transmitter but concentration is below threshold to trigger action potential

Neurone A and B release neurotransmitter concentration is above threshold and so an action potential is triggered in the postsynaptic neurone

Question 7

What is temporal summation?

Answer 7

Low-frequency action potentials lead to neurotransmitter being broken down rapidly. Concentration is below threshold to trigger an action potential in the postsynaptic neurone

High-frequency action potentials lead to the release of neurotransmitter in a short time. Concentration is above threshold to trigger an action potential in the postsynaptic neurone

Question 8

How do inhibitory synapses operate?

Answer 8

• When an impulse arrives at the pre-synaptic neurone a different type of neurotransmitter is released.
• The transmitter substance binds to receptors on the post synaptic membrane, causing Chloride ion channels to open and chloride ions to move into the post synaptic neurone.
• The transmitter substance also opens Potassium channels, causing Potassium ions to move out of the post synaptic neurone.
• Overall this causes the inside of the post synaptic neurone to become more negative, relative to the outside.
• This is called hyperpolarisation, and it makes it less likely that an action potential will be triggered in the post synaptic neurone.

Question 9

Some drugs stimulate the nervous system by creating more action potentials in the post-synaptic neurone. This may be by:

Answer 9

• Mimicking the neurotransmitter
• Stimulate the release for more neurotransmitter
• Inhibiting the breakdown of neurotransmitter

Question 10

Some drugs inhibit the nervous system by creating fewer action potentials in the post-synaptic neurone. This may be by:

Answer 10

• Inhibiting the release of neurotransmitter
• Blocking receptors on the post synaptic membrane