Synaptic Transmission

Question 1

Name the process that occurs at the presynaptic membrane that causes Ca2+ voltage gated channels to open.

Answer 1

Depolarisation.

Question 2

Name the neurotransmitter present in a cholinergic synapse.

Answer 2

Acetylcholine

Question 3

Name the ion that enters into the presynaptic neurone.
Name the transport process it does this by.

Answer 3

Ca2+, enters by facilitated diffusion.

Question 4

What fuses with the presynaptic membrane in order to release Ach?

Answer 4

Vesicles

Question 5

Name the process that releases Ach into the synaptic cleft.

Answer 5

Exocytosis

Question 6

After Ach has diffused across the synaptic cleft, what is it detected by on the post synaptic cell?

Answer 6

Receptors

Question 7

Detection of Ach on receptors causes which ion channels to open?

Answer 7

Na+

Question 8

How does opening of Na+ channels in the post synaptic cell lead to depolarisation?
Link this to the threshold.

Answer 8

Na+ diffuses in, making the inside of the axon less negative. If enough Na+ diffuses in and the threshold is reached, an action potential is generated.

Question 9

What enzyme hydrolyses Ach?
Name the products.

Answer 9

Acetylcholinesterase.
Acetic acid (ethanoic) and choline

Question 10

How is Ach ‘recycled’?

Answer 10

Products from hydrolysis are reabsorbed by the cell so Ach can be reformed.

Question 11

Why is Ach recycled?

Answer 11

To prevent the neurone from becoming overstimulated.

Question 12

State 2 ways that synaptic transmission is kept unidirectional.

Answer 12

Neurotransmitter only released from the pre-synaptic neurone
Receptors only present on post synaptic membrane.

Question 13

What is summation?

Answer 13

The sum of excitatory and inhibitory effects of neurotransmitters.

Question 14

How does summation increase the likelihood of a neurone firing?

Answer 14

It adds the impulses that converge on a single post synaptic neurone, leading to a rapid build up of neurotransmitter.
More neurotransmitter means threshold is more likely to be reached and an action potential is more likely generated.

Question 15

Why might action potentials in low frequency prevent the next neurone from firing?

Answer 15

They release insufficient neurotransmitter to exceed the threshold.

Question 16

Roughly how many presynaptic neurones does spatial summation involve?
What do these neurones all share?

Answer 16

Several - they all share the same synaptic cleft.

Question 17

How does spatial summation allow an action potential to be triggered?
Link this to threshold.

Answer 17

The presynaptic cells collectively release sufficient neurotransmitter to reach threshold and trigger an action potential.

Question 18

How many pre synaptic cells does temporal summation involve?

Answer 18

One

Question 19

How does temporal summation work?

Answer 19

One pre synaptic neurone releases neurotransmitter many times over a short time period. Sufficient neurotransmitter causes threshold to be reached and an action potential triggered.

Question 20

Inhibitory synapses cause which type of polarisation in post synaptic membranes?
Which channel proteins open?
Which ions move in/out by which transport process?

Answer 20

Hyperpolarisation.
Cl- channels open so it diffuses into the post synaptic membrane.
K+ channels open and it diffuses out.

Question 21

At inhibitory synapses, why is more Na+ required for depolarisation?

Answer 21

Hyperpolarisation means more Na+ is required to raise the potential difference of the inside of the axon. This reduces the likelihood of threshold being met and an action potential being formed at the post synaptic membrane.

Question 22

Where are receptors found at neuromuscular junctions compared to synapses?

Answer 22

Neuromuscular - Receptors found on muscle fibre.
Synapse - Receptors found on post synaptic membrane.

Question 23

Muscle fibres form clefts. What is stored in these clefts?

Answer 23

Muscle fibres form clefts to store enzymes such as acetylcholinesterase, that break down neurotransmitter.

Question 24

Give one similarity between cholinergic synapses and neuromuscular junctions.

Answer 24

Unidirectional.

Question 25

Cholinergic synapses are between 2 …

Answer 25

Neurones

Question 26

Neuromuscular junctions are between…

Answer 26

Motor neurones and muscles

Question 27

Are neurotransmitters at cholinergic synapses excitatory or inhibitory?

Answer 27

Both excitatory and inhibitory.

Question 28

Are neuromuscular junctions excitatory or inhibitory?

Answer 28

Always excitatory.

Question 29

Where does the action potential move from in pre synaptic cells at cholinergic synapses?

Answer 29

Action potential moves from pre to post synaptic cell.

Question 30

Where does the action potential move from in neuromuscular junctions?

Answer 30

It propagates along the sarcolemma down T tubules.

Question 31

Briefly explain how a drug may stimulate the nervous system to fire more action potentials.
How do they lead to more Na+ entering?

Answer 31

Drugs are a similar shape to the neurotransmitter which stimulate the release of more neurotransmitter.
They inhibit the enzyme that breaks down neurotransmitter so Na+ continues to enter.

Question 32

Briefly explain how a drug may inhibit the nervous system to reduce action potentials.
How do they inhibit release of neurotransmitter?

Answer 32

They inhibit release of neurotransmitter by, for example, preventing the opening of Ca2+ channels. They also block receptors by mimicking the shape of neurotransmitter.