EXCITATION & CONDUCTION

Nerve cells have a low threshold for excitation. The stimulus may be electrical, chemical, or mechanical. Two types of physicochemical disturbances are produced: local, nonpropagated potentials called, depending on their location, synaptic, generator, or electrotonic potentials; and propagated disturbances, the action potentials (or nerve impulses). These are the only electrical responses of neurons and other excitable tissues, and they are the main language of the nervous system. They are due to changes in the conduction of ions across the cell membrane that are produced by alterations in ion channels.

The impulse is normally transmitted (conducted) along the axon to its termination. Nerves are not "telephone wires" that transmit impulses passively; conduction of nerve impulses, although rapid, is much slower than that of electricity. Nerve tissue is in fact a relatively poor passive conductor, and it would take a potential of many volts to produce a signal of a fraction of 1 V at the other end of a 1-m axon in the absence of active processes in the nerve. Conduction is an active, self-propagating process, and the impulse moves along the nerve at a constant amplitude and velocity. The process is often compared to what happens when a match is applied to one end of a train of gunpowder; by igniting the powder particles immediately in front of it, the flame moves steadily down the train to its end.

The electrical events in neurons are rapid, being measured in milliseconds (ms); and the potential changes are small, being measured in millivolts (mV). In addition to development of microelectrodes with a tip diameter of less than 1 um, the principal advances that made detailed study of the electrical activity in nerves possible were the development of electronic amplifiers and the cathode ray oscilloscope. Modern amplifiers magnify potential changes 1000 times or more, and the cathode ray oscilloscope provides an almost inertia-less and almost instantaneously responding "lever" for recording electrical events.