Synaptic Transmisson

Question 1

What are cholinergic synapses?

Answer 1

Synapses that use the neurotransmitter acetylcholine (ACh)

Question 2

Describe transmission across a cholinergic synapse- pre synaptic membrane

Answer 2

Depolarisation of pre-synaptic membrane causes opening of voltage-gated Calcium ion channels
-○ calcium ions diffuse into pre-synaptic neurone/knob
-Causing vesicles containing ACh to move and fuse with pre-synaptic membrane
Releasing ACh into the synaptic cleft (by exocytosis)

Question 3

Describe transmission across a cholinergic synapse- post synaptic neurone

Answer 3

ACh diffuses across synaptic cleft to bind to specific receptors on post-synaptic membrane
4. Causing sodium ion channels to open
- sodium ions diffuse into post synaptic knob causing depolarisation
- if a threshold is met an action potential is generated

Question 4

Explain how synapses result in unidirectional nerve impulses

Answer 4

● Neurotransmitter only made in / released from pre-synaptic neurone
● Receptors only on post-synaptic membrane

Question 5

Explain what happens to acetylcholine after synaptic transmission

Answer 5

● 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 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 (NT)
● So threshold more likely to be reached to generate an action potential

Question 7

Describe spatial summation

Answer 7

● Many pre-synaptic neurones share
one synaptic cleft / post-synaptic neurone
● Collectively release sufficient neurotransmitter to reach threshold to trigger an action potential

Question 8

Describe temporal summation

Answer 8

● One pre-synaptic neurone releases
neurotransmitter many times over a short time
● Sufficient,neurotransmitter to reach threshold to trigger an action potential

Question 9

Describe inhibition by inhibitory synapses

Answer 9

● Inhibitory neurotransmitters hyperpolarise postsynaptic membrane as:
○ chloride ion channels open → chloride ions diffuse in
○ potassium ion channels open → potassium ions diffuse out
● This means inside of axon has a more negative charge relative to outside / below resting potential
● So more sodium ions required to enter for depolarisation
● Reduces likelihood of threshold being met / action potential formation at post-synaptic membranes

Question 10

Describe the structure of a neuromuscular junction

Answer 10

● Receptors are on muscle fibre sarcolemma instead of postsynaptic membrane and there are more
● Muscle fibre forms clefts to store enzyme eg. acetylcholinesterase to break down neurotransmitter

Question 11

Compare transmission across cholinergic synapses and neuromuscular
junctions

Answer 11

Both- transmissions is unidirectional

Cholinergic synapse :
-Neurone to neurone (or effectors, glands)
-Neurotransmitters can be excitatory or inhibitory
-Action potential may be initiated in postsynaptic
neurone

Neuromuscular junction
-Motor neurone to muscle
-Neurotransmitter Always excitatory
-Action potential propagates along sarcolemma
down T tubules

Question 12

Use examples to explain the effect of drugs on a synapse

Answer 12

● Some drugs stimulate the nervous system, leading to more action potentials, eg.:
○ Similar shape to neurotransmitter
○ Stimulate release of more neurotransmitter
○ Inhibit enzyme that breaks down neurotransmitter → Na+
continues to enter
● Some drugs inhibit the nervous system, leading to fewer action potentials, eg.:
○ Inhibit release of neurotransmitter eg. prevent opening of calcium ion channels
○ Block receptors by mimicking shape of neurotransmitter