Journal of Neurophysiology, Vol 50, Issue 5 1122-1142, Copyright © 1983 by APS
Integrative properties of crayfish medial giant neuron: steady-state model
R. M. Glantz and T. Viancour
A quantitative morphological description of the crayfish medial giant (MG)
neuron was obtained on the basis of 14 individual cells filled with lucifer
yellow. The cable parameters of seven MGs were measured in the axon with
two or three microelectrodes. The length constant is 4.3 +/- 0.8 mm, the
membrane time constant is 3.1 +/- 0.8 ms, the input resistance is 3.6 +/-
0.2 (X10(4) alpha, the mean axon diameter is 208 +/- 34 microns. The
specific membrane resistance (Rm) is 2,000 +/- 204 alpha x cm2. The
specific axoplasmic resistance (Ri) is 60 +/- 29 alpha x cm and the
membrane capacitance is 1.6 +/- microF/cm2. The MG is electrotonically
coupled to its symmetrical homolog. The coupling coefficient is 0.46 for
both steady-state signals and spike transmission. A steady-state cable
model of the MG was calculated on the basis of the axonal Rm and Ri values,
the electrotonic coupling coefficient to the MG symmetrical homolog, and
the mean dimensions of the 14 MG neurites. The model successfully predicts
geometrically determined variations in input resistance and steady-state
signal attenuation. A salient feature of the MG steady-state model is the
extent to which dendritic input conductance and the attenuation of
steady-state voltages is determined by the large input conductance of the
axon. Because of the large diameter of the MG neurites, electrotonic
distances between dendrite terminals and the integrating segment are short
(0.3-0.74 lambda) and the principal basis of excitatory postsynaptic
potential (EPSP) attenuation is variation is local input resistance. The
time integral of the electrotonic coupling potential between the MGs is
diminished by 9.5-21.4% during a monosynaptic sensory EPSP. The magnitude
of the inferred synaptic conductance seen at the decussation is inversely
related to the electrotonic distance between the decussation and the active
dendritic branch. The modest change in input resistance near the
electrotonic junction is consistent with the morphological and electrotonic
separation of the electrotonic junction and the sites of synaptic action on
the dendrites. When the reversal potentials of monosynaptic EPSPs are
measured at the integrating segment, the measurements vary systematically
with the input pathway selected and overestimate the dendritic reversal
potentials by up to 60%.(ABSTRACT TRUNCATED AT 400 WORDS)
