Extracellular potential field of antidromically activated CA1 pyramidal neurons

Abstract

The laminar profile of the extracellular potential field set up in a population of hippocampal CA1 pyramids during antidromic invasion was analysed in unanaesthetized guinea-pigs. Antidromic invasion was provided by single shock stimulation of the alveus. (1) Laminar configuration of the extracellular potential field is consistent with a sequential invasion of antidromically activated CA1 pyramidal neurons, initiated in the region of the axon hillock. The spike potential generated in the region of the axon hillock triggers the discharge of the neuron soma and the confining dendritic membrane. (2) While the spike generated at the axon hillock extends electrotonically over the membrane of the soma and initial part of the apical dendritic shaft (about 150 μ) decaying exponentially in amplitude, the spike fraction added by the soma and confining dendritic membrane spreads along the apical and basal dendrites to a variable distance with decreasing amplitude. (3) The dimensions of peak latency shifts along the apical and basal dendrites suggest the active propagation of spike potentials. (4) The asymmetric configuration of the potential field around the basal and apical dendrites is referred to the geometry of dendritic and axonal processes. (5) Conduction velocity of the spike potential falls in a proximo-distal direction from 1.2 m/sec to 0.1 m/sec in the apical, and from 0.5 m/sec to 0.03 m/sec in the basal dendrites. Parallel changes in recovery time of spike parameters were found in the different portions of the pyramidal neuron in two-shock tests.