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Abstract
The specific membrane capacitance (C(m)) of a neuron influences synaptic efficacy
and determines the speed with which electrical signals propagate along dendrites and
unmyelinated axons. The value of this important parameter remains controversial. In
this study, C(m) was estimated for the somatic membrane of cortical pyramidal neurons,
spinal cord neurons, and hippocampal neurons. A nucleated patch was pulled and a voltage-clamp
step was applied. The exponential decay of the capacitative charging current was analyzed
to give the total membrane capacitance, which was then divided by the observed surface
area of the patch. C(m) was 0.9 microF/cm(2) for each class of neuron. To test the
possibility that membrane proteins may alter C(m), embryonic kidney cells (HEK-293)
were studied before and after transfection with a plasmid coding for glycine receptor/channels.
The value of C(m) was indistinguishable in untransfected cells and in transfected
cells expressing a high level of glycine channels, indicating that differences in
transmembrane protein content do not significantly affect C(m). Thus, to a first approximation,
C(m) may be treated as a "biological constant" across many classes of neuron.