13.5 Synapses

Question 1

What is a synapse?

Answer 1

junction between

2 neurones

or a neurone & effector

Question 2

How are electrical impulses transmitted across synapses? (1 mark)

Answer 2

using neurotransmitter

Question 3

What are the roles of synapses?

Answer 3

Ensure impulses are UNIDIRECTIONAL (receptors only present on postsynaptic membrane, so impulse can only travel in 1 direction)

SUMMATION - allow impulses & neurones to be MANY TO 1 OR 1 TO MANY

Question 4

What are the types of neurotransmitter?

Answer 4

• EXCITATORY
  
  • causes depolarisation of postsynaptic neurone
• INHIBITORY
  
  • causes hyperpolarisation of postsynaptic neurone, inhibiting triggering of action potential

Question 5

Give an example of an excitatory neurotransmitter:

Answer 5

acetylcholine

Question 6

Give an example of an inhibitory neurotransmitter:

Answer 6

GABA (Gamma-aminobutyric acid)

Question 7

Describe & explain how impulses are transmitted across cholinergic synapses:

Answer 7

(action potential reaches end of presynaptic neurone)

1. Depolarisation of presynaptic membrane >> Ca²⁺ channels open >> Ca²⁺ diffuse into synaptic knob
2. Synaptic vesicles containing acetylcholine fuse with presynaptic membrane >> releases by exocytosis >> diffuses across synaptic cleft
3. Acetylcholine molecules bind with specific receptors on postsynaptic membrane >> Na⁺ channels open
4. Na⁺ (in cynaptic cleft) diffuses into postsynaptic neurone >> triggers action potential
5. Acetylcholinesterase hydrolyses acetylcholine ⟶ choline + ethanoic acid >> components diffuse back across synaptic cleft & into synaptic knob
6. Reform acetylcholine using ATP + stored in presynaptic vesicles - recycling

Question 8

Why is it important that neurotransmitter molecules are removed immediately after binding with the receptor on the postsynaptic membrane?

Answer 8

so stimulus is not maintained

& another stimulus can be transmitted

Question 9

What is the name of the enzyme which hydrolyses acetylcholine?

Answer 9

acetylcholinesterase

Question 10

Write a word equation for the hydrolysis of acetylcholine:

Answer 10

acetylcholine ⟶ choline + ethanoic acid

(catalysed by acetylcholinesterase)

Question 11

What is summation?

Answer 11

accumulation of neurotransmitter in synapse

to sufficient level to trigger action potential

Question 12

What are the 2 types of summation?

Answer 12

1. SPATIAL (many to one)
2. TEMPORAL (one to one)

Question 13

Describe & explain the mechanism of spatial summation:

Answer 13

many presynaptic neurones connect to 1 postsynatic neurone (many to one)

action potential triggered when each releases neurotransmitter, reaching threshold level

Question 14

Describe & explain the mechanism of temporal summation:

Answer 14

single presynaptic neurone is connected to single postsynaptic neurone (one to one)

action potential triggered when presynaptic neurone releases neurotransmitter multiple times in quick succession, reaching threshold level