Synapses consist of swelling at the end of nerve fibre called synaptic knob lying in close proximity to the membrane of dendrite.
The cytoplasm of this knob contains numerous mitochondria (for energy) and small synaptic vesicles containing neurotransmitter, acetylcholine for the transmission of the nerve impulse across the synapse. The gap between pre- and post- synaptic membrane is called synaptic cleft.
The presynaptic membrane is modified for the attachment of synaptic vesicles and the release of the transmitter substance into the synaptic cleft.
The postsynaptic membrane contains large protein molecules which act as receptor site for the transmitter substances and numerous channel and pores, for the movement of ions into the post-synaptic neuron.
1. The arrival of nerve impulse depolarizes the presynaptic membrane causing calcium channels to open, increasing the permeability of the membrane to calcium ions.
2. After acquiring the position in synaptic knob, calcium ions cause the synaptic vesicles to fuse with the presynaptic membrane, releasing their contents into the synaptic cleft.
3. The vesicles then return to the cytoplasm where they are refilled with transmitter substances
4. Each vesicle contains about 3000 molecules of acehylcholine.
5. Acetylcholine diffuses across the synaptic cleft and attaches to a receptor site on the postsynaptic membrane that recognized the molecular structure of acetylcholine.
6. Then acetylcholine causes the change in the shape of the receptor site which results in ion channels opening up in the post synaptic membrane in response to depolarisation.
7. Depolarisation of the membrane excites the cell, making it more likely to set up a nerve impulse (action potential).
8. Having produced a change in the permeability of the post synaptic membrane the acetylcholine is immediately removed from the synaptic cleft by enzyme acetyl cholinesterase, (also called cholinesterase).
9. This enzyme hydrolyses the acetylcholine to chorine which is then reabsorbed into the synaptic knob to be recycled into acetylcholine by synthetic pathways in the vesicle using energy from ATP.
These are low molecular mass substances released in minute amounts at interneural neuromuscular and neuroglandular synapses. It may be excitatory (depolarizing postsynaptic membrane) or inhibitory (hyperpolarizing postsynaptic membrane). Examples of neurotransmitter are –
(i) Acetylcholine (excitatory) is released at all neuromuscular junction between motor neurons and skeletal muscle cells, at all synapses between preganglionic and postganglionic in the ANS and at certain synapses between neurons in the CNS.
Enzyme acetyl cholinesterase breaks down acetylcholine into acetate and chorine and terminates the action of the transmitter.
(ii) Norepinephrine (excitatory) is secreted by some neurons of the sympathetic neurons of the CNS. It is usually inactivated by the action of an enzyme monoamine oxidase.
(iii) GABA (Gamma amino butyric acid) – It is released by synaptic knobs of the fibres of some interneurons in the CNS. It inhibits the postsynaptic regeneration of action potential hence it is called inhibitory neurotransmitter. Other example of inhibitory neurotransmitter is glycine.
Other excitatory neurotransmitters are – serotonin, dopamine, histamine, glutamate, 5 hydroxytryptamine etc.