3.6.2 Synaptic transmission
Cholinergic synapses
Cholinergic synapses use the neurotransmitter acetylcholine
Unidirectionally
Synapses can only transport information in one direction across a synapse from the presynaptic
neurone to the post synaptic neurone.
The receptors are only on the postsynaptic membrane
Summation
The process by which the excitatory and inhibitory signals together are able to generate an action
potential or not
Temporal summation
A single presynaptic neurone releases neurotransmitters many times over a short period which can
cause concentration of neurotransmitter to exceed the threshold value of the postsynaptic neurone a
new action potential is then triggered.
Large amount of acetylcholine is released into synaptic cleft
Large number of gated ion channels open
Sufficient number of sodium ions pass through membrane
Spatial summation
When multiple different presynaptic neurones release enough neurotransmitters at once to exceed
threshold value and a new action potential is triggered in posy synaptic neurone
Benefits of summation:
Allows effect of stimulus to be magnified
Combination of different stimuli can trigger a response
Avoids nervous system from being overwhelmed by impulses
Transmission across a cholinergic synapse:
1. Arrival of AP, depolarisation of presynaptic membrane, voltage gated sodium ion channels
open
2. Calcium ions diffuse down electrochemical gradient from tissue fluid around synapse (high
concentration) to cytoplasm of presynaptic neurone (low concentration)
3. Stimulating acetylcholine vesicles to fuse with presynaptic membrane, releasing
acetylcholine in the synaptic cleft
4. Acetylcholine diffuses over synaptic cleft and temporarily binds to ligand-gated sodium ion
channels (receptor proteins) in postsynaptic membrane
5. Receptor proteins open, allowing sodium ions to diffuse down electrochemical gradient into
cytoplasm of postsynaptic neurone
Cholinergic synapses
Cholinergic synapses use the neurotransmitter acetylcholine
Unidirectionally
Synapses can only transport information in one direction across a synapse from the presynaptic
neurone to the post synaptic neurone.
The receptors are only on the postsynaptic membrane
Summation
The process by which the excitatory and inhibitory signals together are able to generate an action
potential or not
Temporal summation
A single presynaptic neurone releases neurotransmitters many times over a short period which can
cause concentration of neurotransmitter to exceed the threshold value of the postsynaptic neurone a
new action potential is then triggered.
Large amount of acetylcholine is released into synaptic cleft
Large number of gated ion channels open
Sufficient number of sodium ions pass through membrane
Spatial summation
When multiple different presynaptic neurones release enough neurotransmitters at once to exceed
threshold value and a new action potential is triggered in posy synaptic neurone
Benefits of summation:
Allows effect of stimulus to be magnified
Combination of different stimuli can trigger a response
Avoids nervous system from being overwhelmed by impulses
Transmission across a cholinergic synapse:
1. Arrival of AP, depolarisation of presynaptic membrane, voltage gated sodium ion channels
open
2. Calcium ions diffuse down electrochemical gradient from tissue fluid around synapse (high
concentration) to cytoplasm of presynaptic neurone (low concentration)
3. Stimulating acetylcholine vesicles to fuse with presynaptic membrane, releasing
acetylcholine in the synaptic cleft
4. Acetylcholine diffuses over synaptic cleft and temporarily binds to ligand-gated sodium ion
channels (receptor proteins) in postsynaptic membrane
5. Receptor proteins open, allowing sodium ions to diffuse down electrochemical gradient into
cytoplasm of postsynaptic neurone
