5.3.5: Synapses 1

Question 1

What is a synapse?

Answer 1

A junction between two or more neurones, where one neurone can communicate with, or signal to another neurone.

Question 2

What is between two neurones?

Answer 2

The synaptic cleft which is approximately 20 nm wide.

Question 3

What does an action potential in the pre-synaptic neurone cause?

Answer 3

The release of a neurotransmitter that diffuses across the synaptic cleft and generates an action potential in the post-synaptic neurone.

Question 4

What are synapses that use acetylcholine called?

Answer 4

Cholinergic synapses.

Question 5

What does the presynaptic neurone end in?

Answer 5

The pre-synaptic bulb.

Question 6

What specialised features does the pre-synaptic bulb contain?

Answer 6

• Many mitochondria.
• A large amount of smooth endoplasmic reticulum.
• Large amount of vesicles containing acetylcholine.
• A number of voltage-gated calcium ion channels on the cell surface membrane.

Question 7

What does the presence pf many mitochondria in the synaptic bulb indicate?

Answer 7

That an active process needing ATP is involved.

Question 8

Why are there large amount of smooth endoplasmic reticulum in the synaptic bulb?

Answer 8

To package the neurotransmitter into vesicles.

Question 9

What does the post-synaptic membrane contain?

Answer 9

Specialised sodium ion channels that can respond to the neurotransmitter.

Question 10

What are the sodium ion channels on the post-synaptic membrane made up of?

Answer 10

Five polypeptide molecules.

Question 11

How does the presence of acetylcholine cause the sodium ion channels to open?

Answer 11

• Two of these polypeptides have a special receptor site that is specific to acetylcholine.
• The receptor sites have a shape that is complimentary to acetylcholine molecules.
• When acetylcholine binds to the two receptor sites, it causes the sodium ion channel to open.

Question 12

There are steps 1 to 12 that describe the transmission of an impulse across a synaptic cleft.
Step 1…

Answer 12

An action potential arrives at the synaptic bulb.

Question 13

There are steps 1 to 12 that describe the transmission of an impulse across a synaptic cleft.
Step 2:
An action potential arrives at the synaptic bulb…

Answer 13

The voltage-gated calcium ion channels open.

Question 14

There are steps 1 to 12 that describe the transmission of an impulse across a synaptic cleft.
Step 3:
The voltage-gated calcium ion channels open…

Answer 14

Calcium ions diffuse into the synaptic bulb.

Question 15

There are steps 1 to 12 that describe the transmission of an impulse across a synaptic cleft.
Step 4:
Calcium ions diffuse into the synaptic bulb…

Answer 15

The calcium ions cause the synaptic vesicles to move to, and fuse with, the pre-synaptic membrane.

Question 16

There are steps 1 to 12 that describe the transmission of an impulse across a synaptic cleft.
Step 5:
The calcium ions cause the synaptic vesicles to move to, and fuse with, the pre-synaptic membrane…

Answer 16

Acetylcholine is released by exocytosis.

Question 17

There are steps 1 to 12 that describe the transmission of an impulse across a synaptic cleft.
Step 6:
Acetylcholine is released by exocytosis…

Answer 17

Acetylcholine diffuses into the cleft.

Question 18

There are steps 1 to 12 that describe the transmission of an impulse across a synaptic cleft.
Step 7:
Acetylcholine diffuses into the cleft…

Answer 18

Acetylcholine binds to the receptor site on the post-synaptic membrane.

Question 19

There are steps 1 to 12 that describe the transmission of an impulse across a synaptic cleft.
Step 8:
Acetylcholine binds to the receptor site on the post-synaptic membrane…

Answer 19

The sodium ion channels open.

Question 20

There are steps 1 to 12 that describe the transmission of an impulse across a synaptic cleft.
Step 9:
The sodium ion channels open…

Answer 20

Sodium ions diffuse cross the post-synaptic membrane into the post-synaptic neurone.

Question 21

There are steps 1 to 12 that describe the transmission of an impulse across a synaptic cleft.
Step 10:
Sodium ions diffuse cross the post-synaptic membrane into the post-synaptic neurone…

Answer 21

A generator potential or excitatory post-synaptic potential (EPSP) is created.

Question 22

There are steps 1 to 12 that describe the transmission of an impulse across a synaptic cleft.
Step 11:
A generator potential or excitatory post-synaptic potential (EPSP) is created…

Answer 22

If sufficient generator potentials combine then the potential across the post-synaptic membrane reaches the threshold potential.

Question 23

There are steps 1 to 12 that describe the transmission of an impulse across a synaptic cleft.
Step 12:
If sufficient generator potentials combine then the potential across the post-synaptic membrane reaches the threshold potential…

Answer 23

A new action potential is created in the post-synaptic neurone. Once an action potential is created, it will pass down the post-synaptic neurone.

Question 24

What happens if acetylcholine is left in the synaptic cleft?

Answer 24

It will continue to open the sodium channels in the post-synaptic membrane and will continue to cause action potentials.

Question 25

What is acetylcholinesterase?

Answer 25

-An enzyme found in the synaptic cleft.

Question 26

What does acetylcholinesterase do?

Answer 26

• It hydrolyses acetylcholine to ethanoic acid and choline.
• This stops the transmission of signals so that the synapse does not continue to produce action potentials in the post-synaptic neurone.