Why do neurones use chemical signalling at synaptic junctions?
When a neurone fires, it sends an electrical signal that jumps down the axon via the nodes of Ranvier very rapidly. At a synaptic junction, chemical Brownian diffusion signalling with receptor surface proteins is relatively slow and is often exploited by venoms and susceptible to toxins (on the plus side it's the reason a lot of medical drugs work). It seems flawed for evolution to have selected for this rather than some alternative quicker and more direct electrical interface.**Question.** Why are chemical synaptic interfaces used in higher organisms at the synaptic junction?
Chemical synapses have some advantages that gap junctions do not provide.
A synapse allows one set of ionic currents to be transformed into another set, showing ionic selectivity. For example, sodium and calcium influx at the presynaptic terminal might translate to chloride influx to make the synapse inhibitory.
Gap junctions increase the conductance of the cell membrane, which can reduce the fidelity of signals as they propagate along the dendrite. In contrast, an inactive synapse does not pass currents, virtually having no effect on the postsynaptic cell.
Because chemical synapses generate their own ionic currents, they are also free to modify the speed of those currents. This is difficult to achieve with gap junctions.