Synapse

The junction between two neurons or neuron and muscle or gland is termed as synapse. Physically it is a cleft, a microscopic gap. The terminal ends of the axon and soma or dentrites of another neuron forms contact-points with each other. The initial end of synapse is called presynaptic terminal whereas postsynaptic terminals are the other end of synapse. A cleft of an average width of 200Å separates the two terminals. A wide variety of possibilities exist in terms of nature of the synapse. A single neuron may form synapse with the other neuron or many presynaptic end of neurons may converge on a single neuron’s postsynaptic terminal or the presynaptic end of a neuron may branchoff and form synapse at multiple postsynaptic terminals.

	Figure 3.6: Scanning electron microscopic photograph of synaptic knobs at the end of an axon.

Based on the nature of connection, synapses are classified as axosomatic, axodendritic, and axo-axonic. As reflected in the nomenclature, axosomatic synapse is the junction of presynaptic axon of the first neuron and soma of the other neuron. The connection between basket and Purkinje cells in the cerebellum is an example of axosomatic synapse. Axodendritic synapse is the junction of presynaptic axon and dendrites of the postsynaptic cell. The climbing fibres and the dendrites of the Purkinje cells form axodendritic connection in the cerebellum. When the axon of the presynaptic cell ends in the axon of postsynaptic neuron it is called axo-axonic synapse. On the other hand, they are categorized as chemical and electrical synapse on the basis of functional characteristics. There are small protein tubular structures that allow ions to move from interior of one neuron to the other one. They are known as gap junctions. Action potentials are transmitted from a smooth muscle fibre to another visceral smooth muscle by way of such gap junctions. Such junctions are electrical synapses. In case of chemical synapses the presynaptic end of the neuron secretes a neurotransmitter which acts on receptor proteins in the membrane of the postsynaptic neuron.