Preservation of motoneuron electrotonic characteristics during postembryonic growth

Abstract

The postembryonic developmental period of the nervous system involves, among other processes, an increase in the dimensions of individual neurons. In the present study we examine whether the characteristic electronic parameters that underlie the integrative properties of the neuron (L, the electrotonic length; rho, dendritic to somatic conductance ratio; and tau m, the membrane time constant) are established before, during, or after the neurons reach their final adult size. A second aspect that we analyze are the mechanisms by which the neurons adjust their electrotonic parameters during the growth period. The dimensions of the soma and dendrites of the phallic motoneurons (Ph.m.n.) of the cockroach, Periplaneta americana, increase during the last postembryonic developmental stages and metamorphosis to adult by a factor of 1.75 and 1.5, respectively. This increase is not associated with major changes in the morphological outline of the neuron but is associated with a significant decrease in the input resistance (Rin) by a factor of 6 (from 120 to 20 M omega). The dendritic to somatic conductance ratio (rho) is maintained constant. Morphological analysis of cobalt-filled neurons reveals that the neurons can be represented by an equivalent cylinder (Rall, 1969). Calculations of the electrotonic length (L) of the equivalent cylinder from measurements of the time constants tau m and tau 1 (Rall, 1969) revealed that L is constant throughout the period of neuronal growth (1.29 +/- 0.24 lambda in adult, and 1.13 +/- 0.28 lambda in nymph). The mechanism by which the electrotonic parameters are maintained in spite of growth is continuous adjustment of the diameter and length of the various neuronal segments, rather than by changes in the biophysical properties of the neuron. Several implications in relation to mechanisms of integration and input-output relations during growth and development of neurons are discussed in view of our findings.