13.5 Synapses

Question 1

What is a synapse?

Answer 1

The area which connects 2 neurones

Question 2

What chemicals are used to transmit impulses across a synapse?

Answer 2

Neurotransmitters

Question 3

What are the 6 key structures of a synapse?

Answer 3

1. Synaptic cleft
2. Presynaptic neuron
3. Postsynaptic neuron
4. Synaptic knob
5. Synaptic vesicles
6. Neurotransmitter receptors

Question 4

Describe what the synaptic cleft is.

Answer 4

Gap which separates the axon of one neuron from the dendrite of another neuron
20-30nm across.

Question 5

Describe what the presynaptic neuron is.

Answer 5

Neuron along which impulse has arrived

Question 6

Describe what the postsynaptic neuron is.

Answer 6

Neuron that receives the neurotransmitter.

Question 7

Describe what the synaptic knob is.

Answer 7

The swollen end of a presynaptic neuron

Question 8

What does the synaptic knob contain?

Answer 8

Lots of mitochondria and endoplasmic reticulum to enable it to make neurotransmitters.

Question 9

Describe what the synaptic vesicle is.

Answer 9

Vesicles containing neurotransmitters.

Question 10

How do synaptic vesicles release their contents into the synaptic cleft?

Answer 10

By fusing with the presynaptic membrane.

Question 11

What happens when a synaptic vesicle fuses with the presynaptic membrane?

Answer 11

The vesicle releases its contents into the synaptic cleft.

Question 12

Describe what neurotransmitter receptors are.

Answer 12

Receptors which neurotransmitters bind to in the postsynaptic membrane.

Question 13

What are the 2 types of neurotransmitters?

Answer 13

1. Excitatory

2. Inhibitory

Question 14

What is the function of excitatory neurotransmitters?

Answer 14

Cause depolarisation of the postsynaptic neuron.

If the threshold is reached in the postsynaptic membrane, an action potential is triggered.

Question 15

What is one example of an excitatory neurotransmitter?

Answer 15

Acetylcholine.

Question 16

What is the function of inhibitory neurotransmitters?

Answer 16

Cause hyperpolarisation of postsynaptic membrane.

This prevents an action potential being triggered.

Question 17

What is one example of an inhibitory neurotransmitter?

Answer 17

Gamma-aminobutyric acid (GABA), found in parts of brain

Question 18

What are the 8 stages in synaptic transmission?

Answer 18

1. AP reaches end of presynaptic neuron
2. Depolarisation of presynaptic membrane = Ca ion channels open
3. Ca ions diffuse into the presynaptic knob
4. This causes synaptic vesicles to fuse w/ presynaptic membrane. Neurotransmitter is released into the synaptic cleft by exocytosis.
5. Neurotransmitter diffuses across synaptic cleft and binds w/ its specific receptor molecule on postsynaptic membrane
6. This causes Na ion channels to open
7. Na ions diffuse into the postsynaptic neuron
8. This triggers an AP and impulse is propagated along the postsynaptic neurone

Question 19

What happens after a neurotransmitter has triggered an action potential?

Answer 19

• The neurotransmitter must be removed from the postsynaptic neurone so the stimulus isn’t maintained
• The neurotransmitter is broken down by enzymes and the products are absorbed into the presynaptic knob

Question 20

What neurotransmitter do cholinergic synapses use?

Answer 20

Acetylcholine

Question 21

Where are cholinergic synapses found?

Answer 21

Neuromuscular junctions

Question 22

What is a neuromuscular junction?

Answer 22

Where a motor neurone meets with a muscle cell (effector)

Question 23

How does transmission across cholinergic synapses work? 6 stages.

Answer 23

1. Arrival of AP at presynaptic neurone causes Ca ion channels to open. Ca ions enter synaptic knob.
2. Influx of Ca ions causes synaptic vesicles to fuse w/ presynaptic membrane, releasing acetylcholine into synaptic cleft.
3. Acetylcholine fuses w/ receptor sites on Na ion channel in postsynaptic membrane. This causes Na ion channels to open, allowing Na ions to rapidly diffuse along a conc. gradient
4. Influx of Na ions generates new AP in postsynaptic neurone
5. Acetylcholinesterase hydrolyses ACE into choline and ethanoic acid, which diffuses back across the synaptic cleft into the presynaptic neurone.
6. ATP released by mitochondria is used to recombine choline+ethanoic acid into ACE, stored for future use. Na ion channels close.

Question 24

What enzyme hydrolyses acetylcholine?

Answer 24

Acetylcholinesterase

Question 25

In addition to acetylcholinesterase breaking down acetylcholine into choline and ethanoic acid, what else does the break down of acetylcholine do?

Answer 25

Prevents ACE continuously generating a new AP in the postsynaptic neurone

Question 26

What are the 3 roles of neurones?

Answer 26

1. Ensure impulses are unidirectional. NT receptors are only on the postsynaptic membrane - impulses can only travel from presynaptic to postsynaptic
2. Allow an impulse from one neurone to be transmitted to multiple neurones at multiple synapses. This results in a single stimulus creating multiple responses at the same time.
3. Multiple neurones can feed back into the same synapse with a single postsynpatic neurone. This results in stimuli from different receptors combining to produce a single result.

Question 27

Define summation.

Answer 27

When the amount of NT from a single impulse isn’t enough to trigger an AP, so the NT builds up until threshold value is reached and AP is generated.

Question 28

What is spatial summation?

Answer 28

Occurs when multiple presynaptic neurones are connected to one postsynaptic neurone. Each one releases NT, which builds up until an AP is triggered in the postsynaptic neurone.

Question 29

What is temporal summation?

Answer 29

Occurs when a single presynaptic neurone releases NT as a result of an AP occurring several times over a short period. This builds up in the synapse until the threshold is reached and an AP is triggered,