The Physiology of Excitable Cells

Synapses are junctional regions between neurons, or between a neuron and another cell, where information is passed rapidly from one cell to the other. Electrical activity in the first (presynaptic) cell produces a change in the electrical activity of the second (postsynaptic) cell. There are two general types of synaptic transmission, chemical and electrical. In chemical transmission, by far the commoner mechanism, the presynaptic cell releases a chemical transmitter substance which diffuses across the intercellular space between the two cells and then binds to receptor molecules on the surface of the postsynaptic cell. If the postsynaptic receptors are also ion channels, this produces rapid changes in the flow of ions across the cell membrane; such events imply fast synaptic transmission (fig. 7.1a), the subject of this chapter and the next. Sometimes the postsynaptic receptors are not themselves ion channels, but their activation produces further chemical changes before there are any changes in the electrical activity of the cell; events of this type occur in slow synaptic transmission (fig. 7.1b), examined in chapter 9. In electrical transmission (fig. 7.1c and chapter 12), current flows directly from the presynaptic cell into the postsynaptic cell so as to alter its membrane potential significantly.

The nature of synaptic transmission

With the realization, towards the end of the nineteenth century, that the nervous system was composed of individual cells, the neurons, the problem arose as to how excitation was transmitted across the gap between two of them.