❤️ 3.6 - 3.6.2 - 3.6.2.2 Synaptic transmission (A-level only) Flashcards
What is a synapse?
It is the junction between a neurone and another neurone, or between a neurone and an effector cell, e.g. a muscle or gland cell.
What is the name given to the tiny gap between the cells at a synapse?
The synaptic cleft.
What does a synaptic knob contain?
Synaptic vesicles filled with chemicals called neurotransmitters.
What happens when an action potential reaches the end of a neurone?
It causes neurotransmitters to be released into the synaptic cleft. They then diffuse across to the postsynaptic membrane (the one after the synapse) and bind to specific receptors.
What happens 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).
How do synapses make sure that impulses are unidirectional (impulse can only travel in one direction)?
By the receptors only being on the postsynaptic membrane.
What are removed from the cleft so the response doesn’t keep happening?
Neurotransmitters.
E.g. They are taken back into the presynaptic neurone or they are broken down by enzymes (and the products are taken into the neurone).
What are synapses that use acetylcholine called?
Cholinergic synapses.
What are 2 examples of neurotransmitters?
Acetylcholine (ACh) and noradrenaline.
How does ACh transmit a nerve impulse across a cholinergic synapse?
1) An AP arrives at the SK of the presynaptic N.
2) The AP stimulates the voltage-gated Ca ion channels in the presynaptic neurone to open.
3) Ca ions diffuse into the SK (they’re pumped out afterwards by AT).
4) The influx of calcium ions into the SK causes the synaptic vesicles to move to the presynaptic membrane. They then fuse with the presynaptic membrane.
5) The vesicles release the neurotransmitter ACh into the SC - this is called exocytosis.
6) ACh diffuses across the SC and binds to specific cholinergic receptors on the postsynaptic membrane.
7) This causes Na ions channels in the postsynaptic neurone to open.
8) The influx of Na ions into the postsynaptic membrane causes depolarisation. An AP on the postsynaptic membrane is generated if the threshold is reached.
9) ACh is removed from the SC so the response doesn’t keep happening. It is broken down by an enzyme called acetylcholinesterase (AChE) and the products are re-absorbed by the presynaptic neurone and used to make more ACh.
What does it mean by voltage-gated ion channels?
It means that they open at a certain voltage.
Neurotransmitters can be excitatory, inhibitory or both.
What does it mean if a neurotransmitter is excitatory?
- including an example -
The neurotransmitters depolarise the postsynaptic membrane, making it fire an action potential if the threshold is reached.
E.g. Acetylcholine is an excitatory neurotransmitter at cholinergic synapses in the CNS - it binds to cholinergic receptors to cause an action potential in the postsynaptic membrane - and at neuromuscular junctions.
Neurotransmitters can be excitatory, inhibitory or both.
What does it mean if a neurotransmitter is inhibitory?
- including an example -
The neurotransmitters hyperpolarise the postsynaptic membrane (make the potential difference more negative), preventing it from firing an action potential.
E.g. Acetylcholine is an inhibitory neurotransmitter at cholinergic synapses in the heart. When it binds to receptors here, it can cause potassium ion channels to open on the postsynaptic membrane, hyperpolarising it.
Does the strength of the stimulus determine the amount of neurotransmitter that is released?
Yes. If the stimulus is weak, only a small amount of neurotransmitter will be released from a neurone into the synaptic cleft. This might not be enough to excite the postsynaptic membrane to the threshold level and stimulate an action potential.
Give a definition for Summation.
It is where the effect of neurotransmitter released from many neurones (or one neurone that is stimulated a lot in a short period of time) is added together.
Name the 2 types of summation.
1) Spatial summation.
2) Temporal summation.
What is spatial summation?
It is sometimes where many neurones connect to one 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. If some neurone release an inhibitory neurotransmitter then the total effect of all the neurotransmitter might be no action potential.
What is temporal summation?
It is where 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.
What types of summation mean synapses accurately process information, finely tuning the response?
Both, spatial and temporal.
What are neuromuscular junctions?
Synapses between a motor neurones and muscle cells.
What neurotransmitter do neuromuscular junctions use?
What does the neurotransmitter bind to?
Acetylcholine (ACh).
It binds to cholinergic receptors called nicotinic cholinergic receptors.
What are the 3 differences between neuromuscular junctions and cholinergic receptors?
In neuromuscular junctions …
1) The postsynaptic membrane has lots of folds that form clefts. These clefts store the enzyme that breaks down ACh (acetylcholinesterase - AshE).
2) The postsynaptic membrane has more receptors than other synapses.
3) ACh is always excitatory at a neuromuscular junction. 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.
What can drugs affect the action of?
Neurotransmitters at synapses.
What are 5 examples of how drugs can affect synaptic transmission?
1) Some drugs are the same shape as neurotransmitters so they mimic their action at receptors (drugs called = agonists).
2) Some drugs block receptors so they can’t be activated by neurotransmitters (drugs called = agonists). This means that fewer receptors (if any) can be activated. This results in the muscle being paralysed.
3) Some drugs inhibit the enzyme that breaks down neurotransmitters (they stop them from working). This means there are more neurotransmitters in the synaptic cleft to bind to receptors and they’re there for longer. This can lead to loss of muscle control.
4) Some drugs stimulate the release of neurotransmitter from the presynaptic neurone so more receptors are activated.
5) Some drugs inhibit the release of neurotransmitters from the presynaptic neurone so fewer receptors are activated.
