Synaptic Transmission

Question 1

Transmission across a cholinergic synapse

Answer 1

1)Action potential arrives causing calcium ion channels to open—> Ca2+ diffuse into pre-synaptic neurone
2)Causing vesicles containing neurotransmitter to fuse with pre-synaptic membrane
3)Neurotransmitters diffuse across synaptic cleft—–> bind to specific neurotransmitter receptors found only on post synaptic membrane.
4)Na+ channels open—–> NA+ diffuse into post synaptic knob—->depolarisation initiates action potential (excitatory synapse)
5)Neurotransmitter removed from cleft so response doesn’t keep occurring.

Note: enzyme acetylcholinesterase breaks down Acetylcholine and the products are reabsorbed by the presynaptic neurone

Question 2

Compare the cholinergic synapse with neuromuscular synapse

Answer 2

-Cholinergic synapse- neurone to neurone whereas neuromuscular is neurone to muscle
-Neuromuscular junction acetylcholine is always excitatory and never inhibitory, so always triggers action potential
-Neuromuscular junction the post synaptic membrane has more receptors than other synapses
-Neuromuscular junction- lots of folds on post synaptic membrane which forms clefts to store enzyme to break down neurotransmitter

Question 3

Why do synapses result in unidirectional nerve impulses

Answer 3

Neurotransmitter only made in pre synaptic neurone.
Also receptors are only on post synaptic membrane

Question 4

Summation

Answer 4

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

Question 5

Spatial summation

Answer 5

Many pre-synaptic neurones share the same synaptic cleft
Collectively release sufficient neurotransmitter to reach threshold to trigger an action potential.

Question 6

Temporal summation

Answer 6

one pre-synaptic neurone releases neurotransmitters many times over a short period.
Sufficient neurotransmitter to reach threshold to trigger an action potential.

Question 7

Importance of summation

Answer 7

Low frequency action potential often releases insufficient amounts of neurotransmitters to exceed threshold in post synaptic neurone—–> summation allows for action potential to be generated by the build up of neurotransmitter

Question 8

Inhibition by inhibitory synapses

Answer 8

Inhibitory neurotransmitters hyperpolarise the post synaptic membrane
-K+ channels open= K+ diffuse out
-Cl- channels open= Cl- diffuses out.

-this inhibits formation of action potential
-so no depolarisation
-reduces the effect of Na+ entering so much less likely to reach threshold.

Question 9

Why is it important to have both excitatory and inhibitory neurones.

Answer 9

It gives control of whether post-synaptic membrane fires or not, therefore firing is not inevitable and stimulation can be overridden.

Question 10

What are the two ways that drugs can effect synapses

Answer 10

1) Stimulate NS so more action potentials e.g. mimic neurotransmitters or stimulate release of more neurotransmitters.
Also inhibits enzyme that breaks down neurotransmitters

2)Inhibit NS so it becomes hyperpolarised so fewer action potentials
e.g. inhibit release of neurotransmitter/ block receptors