The synapse is the small gap separating two neurons, the presynaptic neuron (neuron that carries the impulse to the synapse,) and postsynaptic neuron (neuron that carries the impulse away from the synapse.) It separates the axon terminals of the presynaptic neuron from the postsynaptic neuron. The synapse is made of three major parts: a presynaptic neuron, a postsynaptic neuron, and a synaptic cleft. The presynaptic neuron contains the neurotransmitters, mitochondria, endoplasmic reticulum, and other cell organelles. The postsynaptic neuron contains receptor sites for the neurotransmitters in the presynaptic neuron. The synaptic cleft is the space between the presynaptic and postsynaptic neuron.
The arrival of an action potential normally causes the release of neurotransmitters from the presynaptic neuron. The action potential travels down to the axon terminal of the presynaptic neuron. Each axon terminal becomes swollen forming a presynaptic knob. There is a depolarisation of the presynaptic membrane resulting from the action potential. This depolarisation causes an increase in the permeability to sodium and calcium ions. The presynaptic knob is then filled with membrane-bound vesicles; each filled with a neurotransmitter. Calcium ions then flood into the presynaptic knob by diffusion. The influx of calcium ions triggers the exocytosis of the synaptic vesicles. The neurotransmitters are then released into the synaptic cleft. The neurotransmitters travel across the synaptic cleft towards the receptors by diffusion.
There are two main categories of transmissions, excitatory transmissions and inhibitory transmissions. Excitatory transmissions occur when the neurotransmitter at a synapse depolarises the postsynaptic membrane. Chemically regulated channels are the receptors where the neurotransmitters bind to at the postsynaptic membrane. Inhibitory transmissions occur when the