Component 3.5 - The Synapse

Question 1

What is an electrical synapse?

Answer 1

A junction which is small enough for an electrical impulse to be directly transmitted from one neurone to the next

Question 2

What happens when an impulse arrives at the synaptic end bulb?

Answer 2

• It alters its membrane permeability opening voltage- gated calcium channels
• Calcium diffuses in down conc gradient
• Influx of calcium causes synaptic vesicles to fuse with pre-synaptic membrane

Question 3

What happens after the synaptic vesicles have fused?

Answer 3

• The neurotransmitter diffuses across the synaptic cleft and binds to a receptor
• Two acetylcholine molecules show co-operative binding when they attach

Question 4

What happens when the receptor receives the acetylcholine molecules?

Answer 4

• The receptor protein changes shape
• This opens a channel and sodium ions diffuse in down a conc gradient
• Post-synaptic membrane is depolarised

Question 5

What happens if insufficient acetylcholine is bound?

Answer 5

• The post-synaptic membrane will not be depolarised enough to exceed the threshold potential difference
• An action potential will not be produced

Question 6

How is acetylcholine removed from the synaptic cleft to prevent it binding to the receptor?

Answer 6

1) Reabsorption of acetylcholine into the pre-synaptic neurone
2) Active transport of calcium ions out of the synaptic end bulb so no more exocytosis of acetylcholine occurs

Question 7

Why is it important that acetylcholine is removed from the synaptic cleft?

Answer 7

So that it does not constantly initiate new impulses in the post-synaptic membrane which would not be distinct

Question 8

What organelle is present in high quantities in the synaptic end bulb and why?

Answer 8

• Mitochondria

- Lots of energy is required to re-form these neurotransmitter molecules and for their exocytosis

Question 9

Why do neurones only transmit electrical impulses in one direction?

Answer 9

• Repolarisation happens after an action potential and therefore depolarisation could not also happen at this point
• Synaptic vesicles only occur at the end bulb of the pre-synaptic neurone
• Neurotransmitter receptors only occur on the post-synaptic membrane

Question 10

What are the properties of synapses? (4)

Answer 10

(1) Trasmit info between neurones
(2) Pass impulses in one direction
(3) Protect the response system from overstimulation (impulse is always same size)
(4) Filter out low level stimuli (action potential only initiated when depolarisation is large enough)

Question 11

How is acetylcholinesterase involved with the removal of acetyl choline from the synaptic cleft?

Answer 11

Acetylcholinesterase hydrolyse acetyl choline into choline and ethanoic acid which diffuse back across the synaptic cleft into pre-synaptic neurone and re-form acetylcholine

Question 12

How does organophosphorus effect the transmission of impulses?

Answer 12

Prevent the breakdown of neurotransmitters by inhibiting acetylcholinesterase. This causes repeated firing of post-synaptic neurone.

Question 13

How do psychoactive drugs effect the transmission of impulses?

Answer 13

Affect different neurotransmitters or their receptors, which affects the firing of neurones.