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Organisms respond to changes in their internal and external
environments (AQA A2 Biology) PART 5 of 9 TOPICS
The structure of a cholinergic synapse and neuromuscular junction should be known. The
acetylcholine receptor in the first image on the left is more better known as nicotinic cholinergic
In a cholinergic synapse (this is the only synapse you need to know) an action potential increases
permeability of the presynaptic membrane by stimulating the Ca2+ ion gated channels to open. This
causes an influx of Ca2+ ions into the presynaptic knob down its concentration gradient by facilitated
diffusion. The high concentration of Ca2+ ions causes the vesicles of acetylcholine (neurotransmitters)
to fuse with the presynaptic membrane. NB: It is best to say acetylcholine than Ach because it gives
you more of an understanding and helps with questions if it says `acetylcholine' instead of Ach. If
you are going to use Ach it is important that you know what it is. Acetylcholine leaves the
presynaptic knob by exocytosis into the synaptic cleft. Acetylcholine diffuses across the synaptic cleft
and binds to the cholinergic receptors causing the Na ligand gated channels to open. This causes an
influx of Na+ ions into the postsynaptic neurone making the postsynaptic neurone depolarised and if
the threshold is met, an action potential is generated. The acetylcholine is removed from the synaptic
cleft by the enzyme acetylcholine esterase into products by complementary shapes to prevent a
continuous impulse. NB: Acetylcholine esterase can be abbreviated into Ache however it is best also
to refer to this enzyme as acetylcholine esterase as it will help you in questions that have this name.
The products are actively transported into the presynaptic knob by the use of Pi from ATP into vesicles
to make acetylcholine. The Ca2+ ions are actively transported out of the presynaptic knob by the use
of Pi from ATP.
Above is an example of excitatory neurotransmitters. This is where the postsynaptic neurone is
depolarised leading to an action potential being fired when the threshold is met. Neurotransmitters
can also be inhibitory where they hyperpolarise the postsynaptic neurone by opening the K= ion gated
Neuromuscular junctions work in exactly the same way however:
Postsynaptic membrane: The postsynaptic membrane of the muscle is deeply folded to form
clefts. This is where acetylcholine esterase is stored. NB: It is important that you say
postsynaptic membrane of the muscle and not postsynaptic membrane of a neurone as a
postsynaptic neurone is not involved in a neuromuscular junction.
Receptors: There are many more receptors on the postsynaptic membrane of the muscle than
on the postsynaptic membrane of a neurone.
Neurotransmitters: The acetylcholine are excitatory in every neuromuscular junction whereas
in the synapse it can be excitatory or inhibitory.
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Spatial summation is where many presynaptic neurones connect to one postsynaptic neurone. A small
amount of excitatory neurotransmitters can be enough for the threshold to be met in the
postsynaptic neurone and causing an action potential to be created. If some neurotransmitters are
inhibitory then the overall effect may not be an action potential as it will be difficult to meet the
threshold in the postsynaptic neurone.…read more