Chemical synapses are specialized junctions that are used for communication between neurons, neurons and muscle or gland cells. The synapse involves a pre-synaptic neuron and a post-synaptic neuron, muscle cell or glad cell. The pre and the post-synaptic cell are separated by a gap of 20nm called the synaptic cleft. The signals pass in a unidirection from pre-synaptic to post-synaptic. The pre-synaptic neuron communicates via the release of neurotransmitter which bind the receptors on the post-synaptic cell. The process is initiated when an action potential invades the terminal of the presynaptic neuron.
Action potentials conduct signals along the axon. They occur in electrically excitable cells such as neurons and cardiac muscle cells. They travel in a wave along the membrane causing voltage sensitive channels to open allowing the influx of Na+ thereby causing the conduction of the signal along the axon. The resting membrane potential of cells including neurons is -70mv. An action potential is generated by a change in the membrane potential from -70mv to +40mv.
The change in membrane potential leads to the opening of voltage-gated calcium channels in the presynaptic membrane. The external Ca2+ concentration is approximately 10–3 M while the internal Ca2+ concentration is approximately 10–7 M. Opening of calcium channels causes a rapid influx of Ca2+ into the presynaptic terminal. The elevated presynaptic Ca2+ concentration allows synaptic vesicles to fuse with the plasma membrane of the presynaptic neuron and release their contents, most importantly neurotransmitters, into the synaptic cleft. These diffuse across the synaptic cleft and bind to specific receptors on the membrane of the postsynaptic neuron, causes channels in the postsynaptic membrane to open (or sometimes close), changing the ability of ions to flow in (or out) of the postsynaptic cells. This neurotransmitter-induced current of ions alters the conductance and membrane potential of the postsynaptic neuron, altering the probability that the neuron will fire an action potential.