the synapse

Question 1

what is the synapse?

Answer 1

the synapse is a functional connection between neurones (or a neurone and a muscle)

Question 2

what does the synaptic knob have?

Answer 2

numerous synaptic vesicles containing a neurotransmitter chemical e.g. acetylcholine.

numerous mitochondria which provide ATP for the synthesis of the neurotransmitter.

a thickened membrane known as the presynaptic membrane.

Question 3

describe the structure of the synaptic cleft

Answer 3

the synaptic gap (cleft) separates the presynaptic membrane from the postsynaptic membrane of the dendrite on the following neurone. the postsynaptic membrane contains specific protein receptor sites.

Question 4

draw a diagram of the synapse

Answer 4

see diagram

Question 5

neurones possessing the neurotransmitter acetylcholine are known as …

Answer 5

neurones possessing the neurotransmitter acetylcholine are known as cholinergic neurones

Question 6

what is the mechanism of synaptic transmission?

Answer 6

1. arrival of action potentials (impulses) at synaptic knob causes depolarisation of presynaptic membrane
2. this stimulates the opening of calcium ion channels. calcium ions move into the synaptic knob by facilitated diffusion
3. this causes synaptic vesicles to fuse with presynaptic membrane and break open
4. neurotransmitter (e.g. acetylcholine) is released and diffuses across the synaptic cleft
5. acetylcholine attaches to specific protein receptor sites on the postsynaptic membrane
6. this attachment stimulates the entry of sodium ions which produce EPSP’s (excitatory postsynaptic potentials)
7. as more acetylcholine molecules attach and sodium ions enter, EPSP’s build up eventually leading to depolarisation of the postsynaptic neurone and transmission of an impulse
8. the acetylcholine is broken down in the postsynaptic membrane by the enzyme acetyl cholinesterase forming ethanoic acid and choline
9. ethanoic acid and choline are taken up into the synaptic knob by active transport and acetylcholine is resynthesised. active transport and resynthesis require ATP yielded from the mitochondria.

Question 7

how does the synapse ensure that nervous impulses are uni-directional?

Answer 7

only the pre-synaptic neurone releases neurotransmitter into the synapse, and only the post-synaptic neurone has receptors for the neurotransmitter. thus the synapse also ensures that nervous impulses are uni-directional.

Question 8

draw a diagram of synapse structure and function

Answer 8

see diagram

Question 9

what is summation?

Answer 9

this refers to the additive effect of several EPSPs causing depolarisation of the postsynaptic membrane

Question 10

what are the two types of summation?

Answer 10

spatial summation and temporal summation

Question 11

what is spatial summation?

Answer 11

1. two or more impulses from different synaptic knobs arrive simultaneously at different regions on the same neurone.
2. sufficient neurotransmitter binds to the receptors to open sodium channels.
3. sodium influx is sufficient for EPSP’s to summate in the postsynaptic membrane and reach the threshold level leading to depolarisation and initiation of an action potential.

Question 12

what is temporal summation?

Answer 12

1. the impulses arrive at the same place on the axon within a short time period.
2. this results in an action potential in the post synaptic neurone as described above - see points 2 and 3.

Question 13

draw a graph to show the additive effect of EPSPs produced by temporal or spatial summation

Answer 13

see graph

Question 14

what are inhibitory synapses?

Answer 14

some synapses have an inhibitory effect on the postsynaptic neurone.

this is usually achieved by stimulating the influx of negative chloride ions, or by stimulating removal of positive potassium ions, which causes a more negative charge inside the axon.

consequently reaching the threshold level by the influx of positive sodium ions during depolarisation is more difficult to achieve.