Nerve junction

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Nerve Junctions
A nerve junction occurs between two neurones. This is called a synapse and using
specialised processes will allow two or more neurones to communicate and send signals to
one another.
A cholinergic synapse are those that use acetylcholine as their transmitter substance.
A neurotransmitter is a chemical that diffuses across the cleft of the synapse to transmit a
signal to the postsynaptic neurone.
Structure of a cholinergic synapse
A synapse is a junction between two or more neurone.
At a synaptic junction you will find the synaptic knob on the neurone sending the signal
(the pre-synaptic neurone), which communicates with the post-synaptic neurone.
Between the two neurones is a small space called the synaptic cleft or synaptic gap
(approx. 20nm wide).
The synaptic knob
The presynaptic neurone ends in a swelling called the synaptic knob. Features of the knob
include:
many mitochondria
a large amount of smooth endoplasmic reticulum
vesicles of ACh
voltage-gated calcium ion channels
INSERT PICTURE OF SYNAPTIC KNOB
The postsynaptic membrane
The postsynaptic membrane contains specialised sodium ion channels that can respond to
the transmitter substance.
The channels consists of the following:
five polypeptide molecules
two of the polypeptides have a special receptor that is specific to ACh
receptors have a complementary shape to ACh
ACh will fit and bind to receptors
when ACh binds to the receptors, the sodium ion channels open
Damilola Fasoyiro A2 Biology
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Transmission across the synapse
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The role of acetylcholinesterase
an enzyme found in the synaptic cleft
hydrolyses ACh to ethanoic acid and choline via a hydrolysis reaction
stops transmission of signals so that the synapse does not continue to produce action
potentials in the postsynaptic neurone
the ethanoic acid and choline are recycled: by re-entering the synaptic knob by diffusion
then recombined to ACh using ATP
the recycled ACh is stored in the synaptic vessels for future use
Keeping the gradients constant
the processes outlined above provide perfect concentration gradients…read more

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