What is Chemical and electrical synapses?

At a synapse a neuron’s terminal meets a target — another neuron, a muscle fibre, a gland cell. In an electrical synapse gap junctions join the two cells and current passes directly — fast, bidirectional, without amplification, used where speed and synchrony matter (escape reflexes, the heart). In a chemical synapse the cells are separated by a cleft of $20\,\mathrm{nm}$ and the signal is a molecule. An action potential arriving at the terminal opens voltage-gated calcium channels; calcium enters and, within a fraction of a millisecond, makes vesicles of neurotransmitter fuse with the membrane and empty into the cleft; the transmitter diffuses across in microseconds and binds receptors on the postsynaptic membrane — either ion channels that it opens (ionotropic, fast) or G-protein-coupled receptors (metabotropic, slower, Chapter 19); the resulting current shifts the postsynaptic potential by a few millivolts, up (an excitatory postsynaptic potential, from channels passing sodium) or down (an inhibitory one, from channels passing chloride or potassium); and the transmitter is removed within milliseconds by an enzyme or a transporter. The delay is half a millisecond; the price buys one-way transmission, amplification, inhibition, and modifiability — everything a nervous system needs beyond mere conduction.