POSTSYNAPTIC ELECTROGENESIS IN SEPTATE GIANT AXONS. I. EARTHWORM MEDIAN GIANT AXON

Abstract

One or several intracellular microelectrodes as well as external recording from the nerve cord were used to study the activity of the segmented, septate median giant axon of earthworm. Its spike can be evoked independently of responses in the 2 lateral fibers, and activity of the latter is not picked up electrotonically in the median giant axon. Under appropriate experimental conditions, the directly-evoked spike may be a sequence of small but graded potentials lasting up to about 10 msec. These differ in amplitude and form in different parts of the axon and may give rise at various regions to repetitive spikes. They are postsynaptic potentials probably evoked by activity of other fibers of the nerve cord which activate the median giant synaptically. The median fiber itself also excites activity in small fibers of the cord which can be detected for about 40 msec. These intricate efferent and afferent synaptic connections of the median giant axon, which are associated with its segmental origin, probably play a role in the inte-grative activity of the animal. The postsynaptic potentials recorded in the axons are not generated at the septal membranes. Evidence from the present studies, and other more detailed findings from similar experiments on the septate, lateral giant axons of crayfish lead to the conclusion that the septa are merely the ontogenetic vestiges of the segmental origin of the different parts of these axons, perhaps modified by diminished membrane resistance and behaving as ephatic junctions, without active junctional transfer such as occurs with the development of electrically inexcitable postsynaptic potentials at synapses.