The plasma membrane of the synaptic knob is called the PRESYNAPTIC MEMBRANE. It lies close to the POSTSYNAPTIC MEMBRANE of the adjacent cell. Between the two membranes is a 20nm wide SYNAPTIC CLEFT.
Function of Synapses
Mechanism:
As the action potential reaches the end of an axon, it triggers an influx of CA2+ into the axon terminal, this in turn causes some of the vesicles to discharge their contents into the synaptic cleft. The transmitter molecules cross the cleft and bind to RECEPTOR molecules in the Postsynaptic membranes.
PERMEABILITY CHANNEL RECEPTORS: Some receptors are associated with chemically gated permeability channels in the postsynaptic membrane. Binding of the neurotransmitter to these receptors OPENS THE GATE and allows ions to flow through the channel ... changing the membrane's electrical potential.
If the binding of the neurotransmitter courses depolarisation of the POSTSYNAPTIC MEMBRANE the results is a local EXCITATORY POSTSYNAPTIC POTENTIAL.
NB Only if the postsynaptic cell is depolarised enough to EXCEED ITS OWN AXON's THRESHOLD will it transmit an action potential down its axon to another synapse. (Often a postsynaptic cell will not fire an ACTION POTENTIAL if it received information from only ONE presynaptic cell).
INHIBITORY SYNAPSES: Not all synapses are excitatory. Sometimes the binding of transmitter molecules may cause the postsynaptic membrane to become HYPERPOLARISED or alternatively become more permeable to Ca2+. i.e. the transmitter molecules decrease the activity of the next cell
After they have acted, transmitter molecules must be removed or destroyed. Otherwise, their action would continue indefinitely and all information would be lost. e.g. ACETYLCHOLINE is broken down by the enzyme ACETYLCHOLINESTERASE
Many insecticides & nerve gases (e.g. organophosphates) INHIBIT acetylcholinesterase. In the presence of such inhibitors acetylcholine keeps stimulating the postsynaptic membranes and the nervous system soon goes wild, causing contraction of the muscles in uncontrollable spasms and eventually death.
Nor-adrenaline
This is another transmitter substance which may be in some synapses instead of Acetylcholine, e.g. some human brain synapses & sympathetic nervous system synapses.
Synapses result in an appreciable delay, up to one millisec. Therefore slows down transmission in nervous system.
Synapses are highly susceptible to drugs and fatigue e.g.:-
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Synapses where acetylecholine is the neurotransmitter = cholinergic synapses
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Synapses where noradreneline is the neurotransmitter = adrenergic synapses