6.2.2 Synaptic transmission

Question 1

What is the function of synapses?

Answer 1

• Electrical impulse cannot travel over junction between neurons
• Neurotransmitters send impulses between neurons / from neurons to effectors
• New impulses can be initiated in several different neurons for multiple simultaneous responses

Question 2

Describe the structure of a synapse

Answer 2

Presynaptic neuron ends in synaptic knob: contains lots of mitochondria, endoplasmic reticulum and vesicles of neurotransmitter
Synaptic cleft: 20-30 nm gap between neurons
Postsynaptic neuron: has complementary receptors to neurotransmitter (ligand-gated Na+ channels)

Question 3

Outline what happens in the presynaptic neuron when an action potential is transmitted from one neuron to another

Answer 3

1. Wave of depolarisation travels down presynaptic neuron, causing voltage-gated Ca 2+ channels to open
2. Vesicles move towards and fuse with presynaptic membrane
3. Exocytosis of neurotransmitter into synaptic cleft

Question 4

How do neurotransmitters cross the synaptic cleft?

Answer 4

Via simple diffusion

Question 5

Outline what happens in the postsynaptic neuron when an action potential is transmitted from one neuron to another

Answer 5

1. Neurotransmitter binds to specific receptor on postsynaptic membrane
2. Ligand-gated NA+ channels open
3. If influx of Na+ ions raises membrane to threshold potential, action potential is generated

Question 6

Explain why synaptic transmission is unidirectional

Answer 6

Only presynaptic neuron contains vesicles of neurotransmitter and only postsynaptic membrane has complementary receptors
So impulse always travels presynaptic –> postsynaptic

Question 7

Define summation and name the 2 types

Answer 7

Neurotransmitter from several sub-threshold impulses accumulates to generate action potential:
- temporal summation
- spatial summation

Question 8

What is the difference between temporal and spatial summation?

Answer 8

Temporal: one presynaptic neuron releases neurotransmitter several times in quick succession
Spatial: multiple presynaptic neurons release neurotransmitter

Question 9

What are cholinergic synapses?

Answer 9

Use acetylcholine as primary neurotransmitter. Excitatory or inhibitory. Located at:
- Motor end plate (muscle contraction)
- Preganglionic neurons (excitation)
- Parasympathetic postganglionic neurons (inhibition e.g. of heart of breathing rate)

Question 10

What happens to acetylcholine from synaptic cleft?

Answer 10

1. Hydrolysis into acetyl and choline by acetylcholinesterase (AChE)
2. Acetyl and choline diffuse back into presynaptic membrane
3. ATP is used to reform acetylcholine for storage in vesicles

Question 11

Explain the importance of AChE

Answer 11

• prevents overstimulation of skeletal muscle cells
• enables acetyl and choline to be recycled

Question 12

What happens in an inhibitory synapse?

Answer 12

1. Neurotransmitter binds to and opens Cl- channels on postsynaptic membrane and triggers K+ channels to open
2. Cl- moves in and K+ moves out via facilitated diffusion
3. Potential difference becomes more negative: hyperpolarisation

Question 13

Describe the structure of a neuromuscular junction

Answer 13

Synaptic cleft between a presynaptic neuron and a skeletal muscle cell

Question 14

How might drugs increase synaptic transmission?

Answer 14

• inhibit AChE
• Mimic shape of neurotransmitter