15 Synapses

Structure of a synapse

Synapses transmit information, not impulses
The synaptic cleft is 20-30nm wide
The presynaptic neurone has a bulged synaptic knob and has lots of mitochondria and a lot of endoplasmic reticulum to make the neurotransmitters
The neurotransmitter is stored in synaptic vesicles
The neurotransmitter diffuses across the cleft to the postsynaptic neurone on to specif protein receptors
The presynaptic neurone has multiple calcium ion channels

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Unidirectionality - information can only be transmitted in one direction due to the receptor proteins only being on the postsynaptic neurone, acts as a valve
Summation - low-frequency action potentials are often unable to generate enough neurotransmitter to cause a new action potential as they cannot overcome the threshold value
Spatial summation - multiple presynaptic neurones release neurotransmitter together to trigger a new action potential by overcoming the threshold value
Temporal summation - a single presynaptic neurone releases neurotransmitter multiple times over a short time period to overcome the threshold value

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The presynaptic neurone releases a neurotransmitter that binds to chlorine ion channels on the postsynaptic neurone
This opens the chlorine ion channels
Chlorine ions move into the postsynaptic neurone
This opens potassium ion channels
Potassium ions move out of the postsynaptic neurone
This makes the inside of the postsynaptic neurone very negative and the outside more positive
This results in the new membrane potential being -80mV compared to the usual -65mV at resting potential
This is called hyperpolarisation and decreases the chance of an action potential being generated and a larger influx of sodium ions will be needed

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A cholinergic synapse is where the neurotransmitter is acetylcholine
Cholinergic synapses are very common in vertebrates where they happen in the CNS and at neuromuscular junctions

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The arrival of the action potential at the presynaptic knob opens calcium ion channels and calcium ions flow into the presynaptic neurone by facilitated diffusion
This influx of calcium ions causes synaptic vesicles containing neurotransmitter to fuse with the presynaptic membrane releasing acetylcholine into the synaptic cleft
As the acetylcholine diffuses across the cleft, it binds to receptor sites on the postsynaptic neurone. These receptor sites are sodium ion channels
The binding causes the sodium ion channels to open allowing sodium ions to rapidly flow into the postsynaptic neurone
This generates an action potential in the postsynaptic neurone
Acetylcholinesterase hydrolyses the acetylcholine back into acetyl and choline molecules which diffuse back across the synaptic cleft and diffuse back into the presynaptic neurone
This recycling prevents the acetylcholine from constantly generating an action potential in the postsynaptic neurone
ATP released by mitochondria recombines the acetyl and choline back into acetylcholine which is then stored in synaptic vesicles for the future
The sodium ion channels close when no acetylcholine is present

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