What is a synapse?
The junction between a neurone and another neurone / effector cell.
What effect does an action potential have on a synapse?
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.
What happens when neurotransmitters bind with receptors?
- In a neurone - trigger an action potential
- In a muscle cell - cause muscle contraction
- In a gland cell - cause hormone secretion
Why are receptors only on the post-synaptic membrane?
To ensure impulses are unidirectional.
What happens to the neurotransmitters after they have binded with receptors?
Removed from cleft so response doesn’t keep happening. Taken back into presynaptic neurone or broken down by enzymes and products taken into membrane.
How is a nerve transmitted across a cholinergic synapse?
- Action potential arrives at synaptic knob and stimulates Ca²⁺ voltage-gated channels. Ca²⁺ diffuse into synaptic knob. (Removed after by active transport)
- Influx of Ca²⁺ cause vesicles to fuse with presynaptic membrane. Acetylcholine released from vesicles by exocytosis.
- 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.
What is a neuromuscular junction?
A specialised cholinergic synapse between a motor neurone and a muscle cell.
How is a neuromuscular junction different to a cholinergic synapse?
- Postsynaptic membrane has lots of folds that form clefts to store the enzyme AChE that breaks down ACh.
- Postsynaptic membrane has more receptors.
- When a motor neurone fires an action potential, it always causes a response.
What is the difference between excitatory and inhibitory neurotransmitters?
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
