Synaptic Transmission

Question 1

What are cholinergic synapses

Answer 1

Synapses that use the neurotransmitter acetylcholine (ACh)

Question 2

Transmission across a cholinergic synapse at presynaptic neurone

Answer 2

Depolarisation of pre-synaptic membrane causes opening of voltage-gated Ca2+ channels

Ca2+ diffuse into pre-synaptic neurone

Causing vesicles containing ACh to move and fuse with pre-synaptic membrane

Releasing ACh into the synaptic cleft by exocytosis

Question 3

Transmission across a cholinergic synapse at postynaptic neurone

Answer 3

ACh diffuses across synaptic cleft to bind to specific receptors on post-synaptic membrane

Causing Na+ channels to open

Na+ diffuse into post-synaptic knob causing depolarisation

If threshold is met, an action potential is initiated

Question 4

What happens to acetylcholine after synaptic transmission

Answer 4

It is hydrolysed by acetylcholinesterase

Products are reabsorbed by the presynaptic neurone

To stop overstimulation - if not removed it would keep binding to receptors, causing depolarisation

Question 5

Explain how synapses result in unidirectional nerve impulses

Answer 5

Neurotransmitter only released from pre-synaptic neurone

Receptors only on post-synaptic membrane

Question 6

Explain summation by synapses

Answer 6

Addition of a number of impulses converging on a single post-synaptic neurone

Causing rapid buildup of neurotransmitter

So threshold more likely to be reached to generate an action potential

Question 7

Importance of summation

Answer 7

Low frequency action potentials release insufficient neurotransmitter to exceed threshold

Question 8

Describe spatial summation

Answer 8

Many pre-synaptic neurones share one post-synaptic neurone

Collectively release sufficient neurotransmitter to reach threshold to trigger an action potential

Question 9

Describe temporal summation

Answer 9

One pre-synaptic neurone releases neurotransmitter many times over a short time

Sufficient neurotransmitters to reach threshold to trigger an action potential

Question 10

Describe inhibition by inhibitory synapses

Answer 10

Inhibitory neurotransmitters hyperpolarise postsynaptic membrane

Cl- channels open → Cl- diffuse in

K+ channels open → K+ diffuse out

More Na+ required for depolarisation

Reduces likelihood of action potential formation at post-synaptic membranes

Question 11

Importance of inhibition by inhibitory synapses

Answer 11

Both excitatory and inhibitory neurones forming synapses with the same post-synaptic membrane gives control of whether it ‘fires’ an action potential.

Question 12

Describe the structure of a neuromuscular junction

Answer 12

Receptors are on muscle fibre instead of postsynaptic membrane and there are more

Muscle fibre forms clefts to store enzyme to break down neurotransmitter

Question 13

Compare transmission across cholinergic synapses and neuromuscular junctions

Answer 13

Transmission unidirectional in both

Cholinergic - Neurone to neurone
Neuromuscular - Neurone to muscle

Cholinergic - NT can be excitatory or inhibitory
Neuromuscular - NT always excitatory

Cholinergic - AP may be initiated in postsynaptic neurone
Neuromuscular - AP propagates along sarcolemma down T tubules

Question 14

Explain the effect of drugs on a synapse

(stimulating)

Answer 14

Some drugs stimulate the nervous system leading to more APs

Similar shape to neurotransmitter

Stimulate release of more neurotransmitter

Inhibit enzyme that breaks down neurotransmitter → Na+ continues to enter

Question 15

Explain the effect of drugs on a synapse

(inhibiting)

Answer 15

Some drugs inhibit the nervous system leading to fewer action potentials

Inhibit release of neurotransmitter

Block receptors by mimicking shape of neurotransmitter