Synaptic transmission (A-level only)

Question 1

Synapse

Answer 1

Junction between two neurones (e.g. a relay neurone and a motor neurone) or between a neurone and an effector (e.g a motor neurone and a muscle cell)

Question 2

The features of synapses are:

Answer 2

Synaptic cleft
Presynaptic neurone
Synaptic knob
Synaptic vesicles
Neurotransmitters
Postsynaptic membrane

Question 3

Synaptic cleft

Answer 3

At a synapse, there is a gap between the cells.

This gap is called the synaptic cleft.

When an action potential reaches a synapse, it must be transmitted across the synaptic cleft.

Question 4

Presynaptic neurone

Answer 4

The presynaptic neurone is the neurone before the synapse.

When an action potential reaches the end of the neurone, it is transmitted across the presynaptic membrane to the postsynaptic membrane or to an effector cell.

Question 5

Synaptic knob

Answer 5

The end of the axon of the presynaptic neurone is called the synaptic knob.

The synaptic knob is a swelling which contains synaptic vesicles.

The synaptic knob is the location where the nerve impulse is transmitted across the synpatic cleft.

There are also lots of mitochondria in the synaptic knob.

This is because lots of energy is needed to synthesise neurotransmitters.

Question 6

Synaptic vesicles

Answer 6

Synaptic vesicles are vesicles located in the synaptic knob.

The vesicles contain neurotransmitters.

The vesicles fuse with the presynaptic membrane to release neurotransmitters into the synaptic cleft.

Question 7

Neurotransmitters

Answer 7

Neurotransmitters are the chemicals that allow an action potential to be transferred across a synapse.

When neurotransmitters are released from the synaptic vesicles into the synaptic cleft, they bind to specific receptors on the postsynaptic membrane.

Question 8

Postsynaptic membrane

Answer 8

The postsynaptic membrane is the membrane of the postsynaptic neurone or effector cells.

Receptors on the postsynaptic membrane have a complementary shape to the neurotransmitters released from the synaptic knob.

When neurotransmitters bind to their receptors, the action potential continues.

There are only receptors on the postsynaptic membrane.

This ensures the nerve impulse only moves in one direction.

Question 9

2 types of neurotransmitters

Answer 9

Excitatory
Inhibitory

Question 10

Excitatory neurotrasmitters

Answer 10

Excitatory neurotransmitters generate an action potential in the postsynaptic cell.

When the neurotransmitters bind to the receptors on the postsynaptic membrane, the membrane is depolarised.

E.g. when acetylcholine binds to receptors on the postsynaptic membrane in the central nervous system (CNS) an action potential is established.

Question 11

Inhibitory neurotransmitters

Answer 11

Inhibitory neurotransmitters prevent an action potential from being generated in the postsynaptic cell.

When the neurotransmitters bind to the receptors on the postsynaptic membrane, the membrane is hyperpolarised.

E.g. when acetylcholine binds to receptors on the postsynaptic membrane in the heart, potassium ion channels are opened in the membrane.

This prevents an action potential from being established.

Question 12

Summation

Answer 12

Process where neurotransmitters from multiple neurones are summed together to produce a response.

Question 13

Spatial summation

Answer 13

Spatial summation takes place when multiple presynaptic neurones form a junction with a single neurone.

Each presynaptic neurone releases neurotransmitters.

Overall there are many neurotransmitters that bind to the receptors on one postsynaptic membrane.

Together the neurotransmitters can establish a generator potential that reaches the threshold value and an action potential is generated.

Question 14

Temporal summation

Answer 14

Temporal summation takes place when multiple nerve impulses arrive at the same synaptic knob within a short period of time.

More neurotransmitter is released into the synaptic cleft, so more neurotransmitter is available to bind to receptors on the postsynaptic membrane.

Together the neurotransmitters can establish a generator potential that reaches the threshold value and an action potential is generated.

Question 15

Neuromuscular junction

Answer 15

Synapse between a motor neurone and a muscle cell.

Question 16

Stages of transmission of an action potential across a neuromusular junction:

Answer 16

Arrive at the synaptic knob
Release acetylcholine
Binding to receptors
Removal of acetylcholine

Question 17

Arrive at synaptic knob

Answer 17

An action potential arrives at the synaptic knob at the end of the motor neurone.

The action potential depolarises the membrane of the synaptic knob.

This causes voltage-gated calcium (Ca2+) ion channels to open.

Ca2+ ions diffuse into the synaptic knob.

Question 18

Release acetylcholine

Answer 18

The Ca2+ ion concentration inside the synaptic knob begins to increase.

This causes the synaptic vesicles to move and fuse with the presynaptic membrane.

Acetylcholine, the neurotransmitter inside the vesicles, is released into the synaptic cleft.

This process is called exocytosis.

Question 19

Binding to receptors

Answer 19

Acetylcholine binds to specific receptors on the postsynaptic membrane called nicotinic cholinergic receptors.

Binding of the neurotransmitter opens sodium ion channels in the postsynaptic muscle cell.

As Na+ ions diffuse into the cell, the membrane becomes depolarised.

If the potential difference reaches the threshold value, an action potential is generated and flows along the motor cell.

Question 20

Removal of acetylcholine

Answer 20

An enzyme called acetylcholinesterase (AChE) breaks down acetylcholine in the synaptic cleft.

The products of the break down are reabsorbed by the presynaptic neurone and reused to synthesise more acetylcholine.

It is important that the acetylcholine is removed from the receptors.

This stops action potentials from being continuously generated in the postsynaptic cell.

Question 21

Cholinergic synapse

Answer 21

Synapses that use acetylcholine as a neurotransmitter

Question 22

Key differences between a neuromuscular junction and a cholinergic synapse:

Answer 22

Type of postsynaptic cell
Number of receptors
Type of response
Result of depolarisation
Acetylcholinesterase

Question 23

Type of postsynaptic cell

Answer 23

Cholinergic synapses are between two neurones.

Neuromuscular junctions are between a motor neurone and a muscle cell.

Question 24

Number of receptors

Answer 24

There are less receptors in the postsynaptic membrane at a cholinergic synapse than at a neuromuscular junction.

Question 25

Type of response

Answer 25

A cholinergic synapse can trigger an inhibitory or excitatory response in the postsynaptic membrane.

An action potential at a neuromuscular junction always triggers an excitatory response in the muscle cell.

Question 26

Result of depolarisation

Answer 26

In a cholinergic synapse, depolarisation of the postsynaptic membrane results in an action potential.

At a neuromuscular junction, depolarisation of the postsynaptic membrane results in muscle contraction.

Question 27

Acetylcholinesterase

Answer 27

Acetylcholinesterase is the enzyme that breaks down acetylcholine after it has bound to the receptors on the postsynaptic membrane.

In cholinergic synapses, the enzyme is located in the synaptic cleft.

At a neuromuscular junction, the enzyme is stored in clefts in the postsynaptic membrane.

Question 28

Excitatory drugs

Answer 28

Excitatory drugs stimulate the nervous system producing more action potentials on the post-synaptic membrane.

Question 29

Excitatory drugs can:

Answer 29

Mimic neurotransmitters
Inhibit enzymes
Release neurotransmitters

Question 30

Mimic neurotransmitters

Answer 30

Drugs with a similar shape to neurotransmitters can mimic their function.

The drug can bind to receptors on the postsynaptic membrane to produce an action potential.

These drugs are called agonists.

E.g. Nicotine can bind to nicotinic cholinergic receptors in the brain to mimic acetylcholine.

Question 31

Inhibit enzymes

Answer 31

Drugs can bind to enzymes to prevent the breakdown of a neurotransmitter.

The neurotransmitter would continue to generate an action potential in the postsynaptic membrane.

E.g. Nerve gas inhibits acetylcholinesterase and stops the breakdown of acetylcholine. This causes loss of muscle control.

Question 32

Release neurotransmitters

Answer 32

Drugs can cause presynaptic neurones to release neurotransmitters.

More neurotransmitters will activate more receptors and an action potential is more likely to be created.

Question 33

Inhibitory drugs

Answer 33

Inhibitory drugs inhibit the nervous system so that fewer action potentials are produced.

Question 34

Block calcium ion channels

Answer 34

Drugs can block calcium ion channels in the presynaptic membrane.

Blocking calcium ion channels would prevent the release of neurotransmitters from the presynaptic neurone.

E.g. Alcohol.

Question 35

Inhibitory drugs can:

Answer 35

Block calcium ion channels
Block receptors

Question 36

Block receptors

Answer 36

Drugs can block receptors on the postsynaptic membrane.

If the receptors are blocked, neurotransmitters cannot bind and an action potential is not generated in the postsynaptic neurone.

These drugs are called antagonists.

E.g. Curare blocks nicotinic cholinergic receptors causing muscle paralysis.