Journal of Neuroscience, Vol 3, 2381-2394, Copyright © 1983 by Society for Neuroscience
Electrical characteristics of dendrites and dendritic spines in intracellularly stained CA3 and dentate hippocampal neurons
DA Turner and PA Schwartzkroin
Theoretical parameters of synaptic efficacy were studied in a detailed
cable model of in vitro hippocampal neurons. CA3 pyramidal cells (n = 9)
and dentate granule neurons (n = 6) were injected with horseradish
peroxidase (HRP) after brief physiological analysis. The dendrites of these
HRP-stained neurons were measured and approximated by a series of
cylindrical segments. Specific electrical values of the neurons were
calculated on a steady-state basis, using a cable analysis of the dendritic
segments. The evaluation was expanded to include idealized dendritic
spines. Average spine dimensions were determined by electron microscopic
measurements. The density distribution of spines was patterned after
reported Golgi measurements of similar neurons. The average electrotonic
length of the CA3 apical dendrites was determined to be 0.69 length
constant. These CA3 neurons were not well approximated by a single
equivalent cylinder coupled to a soma (E-C model). The dentate granule
cells exhibited an average electrotonic length of 1.12 length constants and
could be adequately represented by the E-C model. Synaptic efficacy was
estimated by the transfer of either charge or steady-state voltage from a
spine input to the soma. Charge transfer varied as a function of the
electrotonic distance from the soma to the input site. Voltage transfer,
however, did not vary as a simple function of electrotonic distance.
Voltage and charge transfer averaged less than 10% loss across the
dendritic spine neck. These calculations, based on specific neuronal
anatomy, predict that dendritic spines do not significantly attenuate
steady-state electrical signals.
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