Morphology of chemical synapses

The electron microscopy has revealed many morphologically distinct kinds of synapse, but all share common features as shown in figure. Clear, spherical, small synaptic vesicles (SSVs), 50 nm across, store neurotransmitter, and are scattered all through the terminal in relationship with microtubules which transport them to the presynaptic membrane. Presynaptic membrane is thicken and may have dense projections that are included in the docking of synaptic vesicles at the active zone, the area from which transmitter discharge occurs. The dendrite cell membrane at the synapse is thickened as the postsynaptic density, created by the proteins which constitute the postsynaptic machinery.

                                                      Figure: The structure of a chemical synapse.

The Cortical axodendritic synapses are asymmetrical in that they have an extremely well developed postsynaptic density, they are generally excitatory. The Axosomatic synapses are symmetrical in having pre and postsynaptic densities of comparable thickness, they are generally inhibitory. The cerebral cortex may have as many as 1013 synapses.

Several synapses lack obvious particular contact zones on both pre- and postsynaptic sides and have extremely wide (100–500 nm) synaptic clefts. These frequently secrete a catecholamine and have large dense-core vesicles (LDCVs) 40–120 nm across. The LDCVs are also found in peptide-secreting neurons.Most synapses hold both SSVs and LDCVs as shown in figure. This structural evidence supports physiological studies displaying that many neurons secrete more than one transmitter.

                               Figure: A type I synapse holding both small clear vesicles and large dense-core vesicles.