3.6.2.2 Synaptic transmission (Unit 6 Responding to Environments)

Question 1

What is the myelin made up of?

Answer 1

Schwann cells

Question 2

What is saltatory conduction?

Answer 2

In myelinated neurones, depolarisation can only occur at the nodes of Ranvier, so the impulse ‘jumps’ from node to node.

Question 3

What are the nodes of Ranvier?

Answer 3

Bare areas of neurones that are not insulated with myelin.

Question 4

Give three factors that affect the speed of conduction of action potentials.

Answer 4

1. Axon diameter
2. Myelination
3. Temperature

Question 5

Why does increasing temperature increase the speed of transmission?

Answer 5

Ions diffuse faster when temperature increases.

Question 6

Give one function of the refractory period

Answer 6

It produces discrete impulses that are unidirectional.

Question 7

What is a synapse?

Answer 7

The junction between a neurone and the next cell.

Question 8

What ensures that impulses cross the synapse in one direct only?

Answer 8

Vesicles of neurotransmitter are only found in the presynaptic knob.

Receptors for the neurotransmitter is only found on the post synaptic membrane.

Question 9

What are Cholinergic synapses?

Answer 9

Synapses that use acetylcholine.

Question 10

What happens to the neurotransmitter once they have attached to the receptors on the post synaptic membrane?

Answer 10

They are broken down by enzymes and their products are taken back into the neurone.

Question 11

What stimulates the synaptic vesicles to move towards the presynaptic membrane in a cholinergic synapse?

Answer 11

Calcium ions diffuse into the synaptic knob causing the synaptic vesicles to move towards and fuse with the presynaptic membrane.

Question 12

What happens when the acetylcholine bind with the receptors on the post synaptic membrane?

Answer 12

Sodium ion channels open and sodium ions flood in. If the threshold is reached and action potential is generated.

Question 13

How is acetylecholine removed from the synaptic cleft?

Answer 13

Acetylcholinesterase breaks acetylcholine and the products are reabsorbed to make more acetylcholine.

Question 14

What is temporal summation?

Answer 14

It is where there are two or more nerve impulses that arrive in quick succession from the same presynaptic membrane and action potential is more likely to occur..

Question 15

What is spatial summation?

Answer 15

When many neurones connect to one neurone. The small amount of neurotransmitter released from each of these neurones can be enough to reace the threshold value.

Question 16

Describe how calcium ions are involved in synaptic transmission.

Answer 16

(Nerve impulse causes) Ca2+ to enter presynaptic neurone/membrane;

(Ca2+ entry) causes fusion of vesicles with presynaptic membrane /
causes exocytosis / release of transmitter;

Question 17

Describe how a nerve impulse causes the release of neurotransmitter from the vesicles in the presynaptic knob

Answer 17

1. (Nerve impulse/depolarisation of membrane) causes Ca2+channel (proteins) to open;
2. Ca2+enter by (facilitated) diffusion;
3. Causes (synaptic) vesicles to fuse with (presynaptic) membrane;

Question 18

GABA causes negatively charged chloride ions to enter post synaptic neurones - How does this inhibit post synaptic neurones?

Answer 18

1. neurone becomes more negative/hyperpolarised
2. not enough sodium ions enter (to reach threshold)
3. for depolarisation to take place

Question 19

Describe how impulses are transmitted across a synapse

Answer 19

Action potential reaches pre-synaptic membrane allowing Ca2+ channels to open and Ca2+ enters by facilitated diffusion.
Vesicles containing Acetylcholine (Ach) fuse with the pre-synaptic membrane.
Ach diffuses across the synaptic cleft
Ach binds to the receptors on the post-synaptic membrane.
Na+ channels open and Na+ moves in by facilitated diffusion
Leading to depolarisation
Acetylcholine esterase hydrolyses Ach to acetic acid and choline to be returned to the presynaptic neurone

Question 20

How can more chloride ions entering the post synaptic neuron affect action potential propagation?

Answer 20

Cl-are negatively charged
They make the post synaptic neurone hyperpolarised
More Na+ needs to enter the post synaptic neuron for depolarisation of the neurone to pass threshold
Action potential is inhibited

Question 21

How can potassium ions leaving the post synaptic neuron affect action potential propagation?

Answer 21

K+ are positively charged
When they move out they make the post synaptic neurone hyperpolarised
More Na+ needs to enter the post synaptic neuron for depolarisation of the neurone to pass threshold
Action potential is inhibited

Question 22

How can drugs that bind to synaptic receptors affect neurotransmission?

Answer 22

Drugs bind receptors instead of Ach because they are complementary – they prevent Na+ from entering, threshold isn’t reached reducing action potentials.

Question 23

How can drugs that inhibit acetylcholinesterase affective neurotransmission?

Answer 23

Drugs prevent acetylcholinesterase from breaking down Ach by aching as an inhibitor (inhibitor model answer applies here e.g. blocks active site or changes shape so the active site is no longer complementary) this prevents Ach from being removed from receptors, more Na+ diffuses in increases action potentials.

Question 24

If a drug is complementary to the neurotransmitter, how might this affect neurotransmission?

Answer 24

Drugs may be complementary to the NT (substrate) and bind it, this changes shape of the substrate and means it can’t bind the receptor. Less Na+ moves in, doesn’t pass threshold reducing action potentials.

Question 25

Describe the events at a NMJ (neuromuscular junction)

Answer 25

Action potential reaches pre-synaptic membrane allowing Ca2+ channels to open and Ca2+ enters by facilitated diffusion.
Vesicles of neurotransmitter fuse with the pre-synaptic membrane
Neurotransmitter e.g. Acetylcholine diffuse across the neuromuscular junction
Neurotransmitters bind to the Na+ receptors causing Na+ to enter the sarcolemma (muscle tissue) depolarising it
Depolarisation is transmitted through the T-tubules causing Ca2+ ions to be released from the sarcoplasmic reticulum
Ca2+ binds to tropomyosin in the leading to muscle contraction