POSTSYNAPTIC ELECTROGENESIS IN SEPTATE GIANT AXONS. II. COMPARISON OF MEDIAL AND LATERAL GIANT AXONS OF CRAYFISH

Abstract

Electrical activities of the non-septate medial and septate lateral giant axons of crayfish were studied by correlating activities recorded on the surface of isolated ventral nerve cords with those recorded from individual axons with capillary microelectrodes. The conductile properties of the medial and lateral giant axons are similar, the mean resting potential being 80.75 mv, and the mean spike height being 118.25 mv. A long-lasting low amplitude depolarization following the spike is apparent in the lateral but not the medial axon, and this has been shown to be due largely to postsynaptic potentials that occur along the length of lateral giants. They are recordable in isolated preparations because the lateral giants originated peripherally in contrast to the medial giants whose cell bodies are centrally located. Various consequences of synaptic interplay, such as facilitation, inhibition, and repetitive discharges were observed. In two respects, the activities of crayfish lateral giants differ from septate median giant axon in earthworm. There are more repetitive discharges and there are no re-entry-type of after-discharges in the crayfish; both differences were ascribed to evolutionary advance and an increased number of neural elements. The septum was found to be ineffective attenuators of local circuit current, and excitation spreads from one segment to another solely by electrical current. Since each segment is a distinct cell, propagation in a septate axon constitutes a unique example of electrical transmission from prejunctional to postjunctional cells.