Chemical synapses are formed by the close association of an axon terminal of the presynaptic cell with some part of the post synaptic cell. The synaptic cleft is typically 20nm across.
Axodendritic synapses are made on dendrites with spiny neurons, the synapses are on the spines. Axosomatic synapses are made on the cell body. Axoaxonal synapses are the synapses between axon terminals and axons of post synaptic neurons.
Small synaptic vesicles, 50nm across store neurotransmitters and are scattered through the synaptic terminal in association with microtubules that transport them to the presynaptic membrane. This membrane is thickened and has dense projections involved in the docking of synaptic vesicles at the active zone.
Cortical axodendritic synapses are asymmetrical, having well developed post synaptic density. These are normally excitatory.
Axosomatic synapses are symmetrical with the pre/post synaptic densities of comparable thickness and are usually inhibitory.
Large dense core vesicles are found in peptide-secreting neurons and in wide synaptic clefts.
Transmitter release requires a rise in intracellular Ca2+ as the Ca2+ entry into the axon terminal via the voltage-dependent Ca2+ channels. It takes 5us for the transmitter to diffuse across. The binding of the transmitter will then…