Y. Cai*; M. Flynn; D.A. Baxter; T. Crow

1. Dept Neurobiol Anat, Univ Texas Houston, Houston, TX, USA

It is common to infer that features of the action potential recorded from the soma reflect similar features of spikes occurring in axons or synaptic terminals. Moreover, it is often assumed that changes in spikes measured in the soma reflect changes in axons and synapses. To test the validity of these assumptions, we developed two multi-compartmental models, one of type-B photoreceptors of Hermissenda and one of octopus cells in the cochlear nucleus. One feature shared by these cells is their small somatic spikes (15-30 mV). Both models included Na⁺ channels in their axons but no or lower density Na⁺ in somas. The two models used different types of K⁺ currents, which were included in their somatic and axonal compartments. Experimental data showed that blocking I_K,A results in spike broadening in B photoreceptors (Gandhi and Matzel, 2000). When K⁺ conductances were reduced, both models exhibited broadening in somatic spikes but no or lesser broadening in axonal spikes. These results can be explained, in part, by the different membrane potential ranges over which the K⁺ channels operate. During the spike, the membrane potential in the axon is higher than in the soma. At higher membrane potentials, the I-V curve of the K⁺ channel is steeper and the K⁺ channels activate faster. Thus, the recruitment of additional K⁺ channels in the axon due to the increase in membrane potential is able to compensate for the decrease in K⁺ conductances, yielding less spike broadening. Since such an I-V pattern is a general characteristic of K⁺ currents, these results suggest that what happens in the axon may not be reliably inferred based on recordings from the soma in cells with small soma spikes.

Supported by: NIH grant P01 NS38310

Citation for this abstract:

Cai, Y., M. Flynn, D.A. Baxter, and T. Crow (2002). Changes in spike width in soma may not reflect changes in axon. Soc. Neurosci. Abstr. 28, 446.19.