Synapses

Question 1

What is a synapse?

Answer 1

The junction between a neurone and another neurone / effector cell.

Question 2

What effect does an action potential have on a synapse?

Answer 2

When action potential reaches the end of a neurone, it causes neurotransmitters to be released into the synaptic cleft to diffuse actress to post-synaptic membrane and bind to specific receptors.

Question 3

What happens when neurotransmitters bind with receptors?

Answer 3

• In a neurone - trigger an action potential
• In a muscle cell - cause muscle contraction
• In a gland cell - cause hormone secretion

Question 4

Why are receptors only on the post-synaptic membrane?

Answer 4

To ensure impulses are unidirectional.

Question 5

What happens to the neurotransmitters after they have binded with receptors?

Answer 5

Removed from cleft so response doesn’t keep happening. Taken back into presynaptic neurone or broken down by enzymes and products taken into membrane.

Question 6

How is a nerve transmitted across a cholinergic synapse?

Answer 6

1. Action potential arrives at synaptic knob and stimulates Ca²⁺ voltage-gated channels. Ca²⁺ diffuse into synaptic knob. (Removed after by active transport)
2. Influx of Ca²⁺ cause vesicles to fuse with presynaptic membrane. Acetylcholine released from vesicles by exocytosis.
3. ACh diffuses across cleft, binding with specific receptors. This causes Na⁺ channels to open and move in. Influx of Na⁺ causes an action potential (if threshold reached). ACh removed from cleft.

Question 7

What is a neuromuscular junction?

Answer 7

A specialised cholinergic synapse between a motor neurone and a muscle cell.

Question 8

How is a neuromuscular junction different to a cholinergic synapse?

Answer 8

1. Postsynaptic membrane has lots of folds that form clefts to store the enzyme AChE that breaks down ACh.
2. Postsynaptic membrane has more receptors.
3. When a motor neurone fires an action potential, it always causes a response.

Question 9

What is the difference between excitatory and inhibitory neurotransmitters?

Answer 9

Excitatory - depolarise the postsynaptic membrane, making it fire an action potential if threshold is reached. e.g. ACh
Inhibitory - hyper-polarise the postsynaptic membrane, preventing an action potential being fired. e.g. GABA