Integrative properties of crayfish medial giant neuron: steady-state model.

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)