Topic 6—B: Nervous Coordination- 2. Synaptic transmission

Question 1

Synapse

Answer 1

A junction between a neurone and another neurone, or between a neurone and an effector cell

Question 2

What are the tiny gaps between the cells at a synapse called?

Answer 2

Synaptic cleft

Question 3

Presynaptic neurone

Answer 3

• The one before the synapse
• It has a swelling called a synaptic knob
• This contains synaptic vesicles filled with chemicals called neurotransmitters

Question 4

Effect of an action potential

Answer 4

• When an action potential reaches the end of a neurone it causes neurotransmitters to be released into the synaptic cleft
• They diffuse across the postsynaptic membrane (one after synapse) and bind to specific receptors.
• When neurotransmitters bind to receptors they might trigger an action potential (in a neurone), cause muscle contraction (in a muscle cell) or cause a hormone to be secreted (from a gland cell)
• Because the receptors are only on the postsynaptic membranes, synapses make sure impulses are unidirectional- (impulse can only travel in one direction)
• Neurotransmitters are removed from the cleft so the response doesn’t keep on happening (taken back into the presynaptic neurone or they’re broken down by enzymes)

Question 5

Acetylcholine

Answer 5

• Type of neurotransmitter
• Binds to cholinergic synapses

Question 6

Cholinergic synapses

Answer 6

1. Action potential arrives at the synaptic knob of the presynaptic neurone
2. The action potential stimulates voltage-gated calcium ion channels in the presynaptic neurone to open and calcium ions diffuse into the synaptic knob
3. Calcium entry into the synaptic knob causes the synaptic vesicles to fuse with the presynaptic membrane
4. The vesicles release the neurotransmitter acetylcholine into the synaptic cleft by exocytosis
5. Acetylcholine diffuses across the synaptic cleft and binds to specific cholinergic receptors on the postsynaptic membrane.
6. This causes sodium ions in the postsynaptic membrane to open
7. Influx of sodium ions into the postsynaptic membrane causes depolarisation (action potential on the postsynaptic membrane is generated if the threshold is reached)
   - Acetylcholine is removed from the synaptic cleft so the response doesn’t keep on happening and its broken down by an enzyme called acetylcholinesterase and the products are re-absorbed by the presynaptic neurone and used to make more acetylcholine

Question 7

What can neurotransmitters either be ?

Answer 7

• excitatory
• inhibitory
• both

Question 8

Excitatory neurotransmitters

Answer 8

• They depolarise the post-synaptic membrane
• This makes it fire an action potential if threshold is reached e.g. - acetylcholine is an excitatory neurotransmitter (it binds to cholinergic receptors to cause an action potential on the postsynaptic membrane

Question 9

Inhibitory neurotransmitters

Answer 9

• They hyperpolarise the post synaptic membrane (make the potential difference more negative) preventing it from firing an action potential
  E.g. GABA is an inhibitory neurotransmitter- when it binds to receptors it causes potassium ion channels to open on the post synaptic membrane by hyperpolarising the neurone
• Acetylcholine is an inhibitory. Neurotransmitter at cholinergic synapses in the heart. When it binds to. Here, it can cause potassium ion channels to open on the postsynaptic membrane hyperpolarising it

Question 10

Inhibitory synapse

Answer 10

• a synapse where inhibitory neurotransmitters are released from the presynaptic membrane following an action potential

Question 11

What happens if a stimulus is weak?

Answer 11

• Only small amount of neurotransmitter will be released from a neurone into the synaptic cleft
• This might not be enough to excite the post synaptic membrane to the threshold level and stimulate an action potential

Question 12

Summation

Answer 12

• Where the effect of neurotransmitters released from many neurones (or one neurone that’s stimulated a lot in a short period of time) is added together
• It means synapses accurately process information, finely turning the response

Question 13

What are the 2 types of summation?

Answer 13

1. Spatial
2. Temporal

Question 14

Spatial summation

Answer 14

• Where 2 or more presynaptic neurones release their neurotransmitters at the same time onto the same postsynaptic neurone.
• The small amount of neurotransmitter released from each of these neurones can be enough altogether to reach the threshold in the postsynaptic neurone and trigger an action potential
• Individually they would not release enough neurotransmitter to reach threshold potential

Question 15

Temporal summation

Answer 15

• When two or more nerve impulses arrive in quick succession from the same presynaptic neurone
• this makes an action potential more likely because more neurotransmitter is released into the synaptic cleft

Question 16

Neuromuscular junction

Answer 16

• specialised cholinergic synapse between a motor neurone and a muscle cell
• They use the neurotransmitter acetylcholine which binds to cholinergic receptors called nicotinic. Cholinergic receptors
• They work in basically the same way as cholinergic synapses i.e. they both release ACh from vesicles in the presynaptic membrane, ACh then diffuses across the synaptic cleft and binds to cholinergic receptors on the post synaptic membrane and this triggers an action potential if the threshold is reached
• In both types of synapse, ACh is broken down in the synaptic cleft by the enzyme acetylcholinesterase

Question 17

Neuromuscular junction (differences with cholinergic synapses)

Answer 17

• The postsynaptic membrane has lots of folds that form clefts. These clefts AChE
• The postsynaptic membrane has lots of folds that form clefts. These clefts store AChE
• ACh is always excitatory so when a motor neurone fires an action potential, it normally triggers a response in a muscle cell. This isn’t always the case for a synapse between two neurones

Question 18

Neuromuscular junction

Answer 18

1. Membrane of axon terminal depolarises
2. Calcium ion channels open and calcium ions diffuse into the axon terminal
3. Calcium ions stimulate exocytosis of ACh
4. ACh diffuses across the cleft
5. ACh attaches to receptors on motor end plate. Sodium ions diffuse in depolarising the membrane
6. Depolarisation spreads across muscle membrane
7. Depolarisation goes down T Tubule to sarcoplasmic reticulum

Question 19

Drugs at synapses

Answer 19

• They can affect synaptic transmission
• e.g. some drugs are the same shape as neurotransmitters so their action at receptors (drugs are called agonists) meaning more receptors are activated
  (Nicotine mimics acetylcholine so binds to nicotinic cholinergic receptors in the brain
• e.g. some drugs block receptors so they can’t be activated by neurotransmitters . This means fewer receptors (if any) can be activated
• Some drugs inhibit the enzyme that breaks down neurotransmitters (they stop it from working). This means there are more neurotransmitters in the synaptic cleft to bind to receptors and they’re there for longer
• Some drugs stimulate the release of neurotransmitter from the presynaptic neurone so more receptors are activated
• Some drugs inhibit the release of neurotransmitters from the presynaptic neurone so fewer receptors are activated