ELECTRICAL CONSEQUENCES OF SPINE DIMENSIONS IN A MODEL OF A CORTICAL SPINY STELLATE CELL COMPLETELY RECONSTRUCTED FROM SERIAL THIN-SECTIONS

We built a passive compartmental model of a cortical spiny stellate ce ll from the barrel cortex of the mouse that had been reconstructed in its entirety from electron microscopic analysis of serial thin section s (White and Rock, 1980). Morphological data included dimensions of so ma and all five dendrites, neck lengths and head diameters of all 380 spines (a uniform neck diameter of 0.1 mu m was assumed), locations of all symmetrical and asymmetrical (axo-spinous) synapses, and location s of all 43 thalamocortical (TC) synapses (as identified from the cons equences of a prior thalamic lesion). In the model, unitary excitatory synaptic inputs had a peak conductance change of 0.5 nS at 0.2 msec; conclusions were robust over a wide range of assumed passive-membrane parameters. When recorded at the soma, all unitary EPSPs, which were i nitiated at the spine heads, were relatively iso-efficient; each produ ced about 1 mV somatic depolarization regardless of spine location or geometry. However, in the spine heads there was a twentyfold variation in EPSP amplitudes, largely reflecting the variation in spine neck le ngths. Synchronous activation of the TC synapses produced a somatic de polarization probably sufficient to fire the neuron; doubling or halvi ng the TC spine neck diameters had only minimal effect on the amplitud e of the composite TC-EPSP. As have others, we also conclude that from a somato-centric viewpoint, changes in spine geometry would have rela tively little direct influence on amplitudes of EPSPs recorded at the soma, especially for a distributed, synchronously activated input such as the TC pathway. However, consideration of the detailed morphology of an entire neuron indicates that, from a dendro-centric point of vie w, changes in spine dimension can have a very significant electrical i mpact on local processing near the sites of input.