Relationships between the spike components and the delayed depolarization in cat spinal neurones

1. Changes in the delayed depolarization (DD) following composite (IS-SD) intracellular spikes in motoneurones and neurones of the ventral spinocerebellar tract were recorded in a variety of experimental conditions. Cell activation was either antidromic or by direct intracellular stimulation. 2. It was observed that under all conditions in which IS-SD coupling changes took place (as a consequence of spontaneous fluctuations, membrane conductance variations, variations of direct-stimulation parameters, changes in steady membrane polarization), SD spike delays were always accompanied by a progressive concomitant reduction of the DD depolarizing hump amplitude. 3. Under the same conditions the latency of the DD peak from the stimulus artifact remained constant. Accordingly, any increase of the SD delay was accompanied by a reciprocal reduction of the time interval between the SD spike and the DD peak. This variability of temporal relationships between SD spike and DD would appear to contradict the hypothesis that the DD might represent the image of the excitation spreading from the soma to the dendrites (Kernell, 1964; Nelson & Burke, 1967). 4. as the gradual reduction of the DD hump progressed, the time course of the decay phase of the afterhyperpotential more and more closely approximated the decay phase of the IS spike. As an alternative hypothesis it is suggested that the DD might originate from the current which generates the IS spike.