Spontaneous Neural Activity

Abstract

Single units which discharged with regular spontaneous rhythms without intentional stimulation were isolated from the ventral nerve cord of the crayfish by intracellular recording methods in the proximity of the sixth abdominal ganglion. These units were divided into two groups: group A units in which interspike intervals varied less than 10 msec; group B units in which interspike intervals varied within a ringe of 10–30 msec. Group A units maintained “constant” interspike intervals and could not be discharged by sensory inputs, while the group B units could be discharged by appropriate sensory-nerve stimulation. Both group A and B units discharged to direct stimulation of the impaled unit, and the evoked direct single-spike activity reset the spontaneous activity. In Group B units presynaptic volleys reset the spontaneous rhythm of some units and in other units synaptically evoked spikes were interpolated within the spontaneous rhythm without resetting the rhythms. In a few units it was possible to initiate ongoing spontaneous activity by direct stimulation of the impaled unit. Spontaneously active units demonstrated the property of enhancement when driven by repetitive direct stimulation. It is concluded that endogenous pacemaker activity is responsible for much of the ongoing regular spontaneous rhythms of the crayfish central neurons and that interaction of evoked responses with endogenous pacemaker sites is responsible for the observed results.

This study was done in collaboration with Dr. Donald Kennedy, Department of Biological Sciences, Stanford University.