Synapses Flashcards
Synapses are unidirectional. Explain how acetylcholine contributes to a synapse being unidirectional.
- (Acetylcholine) released from/in presynaptic side;
- Diffusion from higher concentration/to lower concentration;
- Receptors in postsynaptic (side) / binds on postsynaptic
(side) ;
What is a synapse?
The junction between a neurone and another neurone, or between a neurone and an effector cell. The tiny gap between the cells at a synapse is called the synaptic cleft.
What is a presynaptic neurone?
The one before the synapse- it has a swelling called a synaptic knob. This contains synaptic vesicles filled with chemicals called neurotransmitters.
How is a nerve impulse transmitted across a cholinergenic synapse?
Action potential arrives at synaptic knob of the presynaptic neurone.
This stimulates voltage gated calcium ion channels in the presynaptic neurone to open.
Calcium ions diffuse into the synaptic knob.
This causes the synaptic vesicles fuse with the presynaptic membrane.
The vesicles releases the neurotransmitter acetylcholine into the synaptic cleft by exocytosis.
The ACh diffuses across the synaptic cleft and binds to specific cholinergic receptors on the postsynaptic membrane.
This causes sodium ion channels in the postsynaptic neurone to open.
The influx sodium ions causes an action potential in the postsynaptic neurone (if threshold).
ACh is eventually broken down by acetylcholinesterase.
What is a neuromuscular junction?
A neuromuscular junction is a specialised synapse between a motor neurone and a muscle cell.
Neuromuscular junctions work in the same way as cholinergenic synapses, but there are a few differences…
- the postsynaptic membrane has lots of folds that form clefts. These clefts store the enzyme that breaks down ACh.
- the postsynaptic membrane has more receptors than other synapses.
- when a motor neurone fires an action potential, it always triggers a response in a muscle cell. This isn’t always the case for a synapse between two neurones.
What is an excitatory neurotransmitter?
They depolarise the postsynaptic membrane, making it fire an action potential if the threshold is reached.
e.g acetylcholine
What is an inhibitory neurotransmitter?
They hyperpolarise the postsynaptic membrane, preventing it from firing an action potential.
e.g GABA
What is summation at synapses?
Summation is where the effect of neurotransmitters released from many neurones is added together.
It means synapses accurately process information, finely tuning the response.
What are the two types of summation?
Spatial summation
Temporal summation
What is spatial summation?
Where two or more presynaptic neurones release their neurotransmitter at the same time onto the same postsynaptic neurone.
The small amount of neurotransmitter released from each of these neurones can be altogether enough to reach the action potential threshold.
What different ways can drugs affect synaptic transmission?
Some drugs block receptors so they can’t be activated by neurotransmitters.
Some inhibit the enzyme that breaks down neurotransmitters.
Some stimulate the release of neurotransmitter from the postsynaptic neurone so more receptors are activated.
What is temporal summation?
Where two or more nerve impulses arrive in quick succession from the presynaptic neurone. This makes an action potential more likely because more neurotransmitter is released into the synaptic cleft.
