Journal of Neurophysiology, Vol 74, Issue 6 2415-2426, Copyright © 1995 by APS
Computation of long-distance propagation of impulses elicited by Poisson-process stimulation
K. Moradmand and M. D. Goldfinger
Department of Physiology & Biophysics, Wright State University, Dayton, Ohio 45401-0927, USA.
1. The purpose of this work was to determine whether computed temporally
coded axonal information generated by Poisson process stimulation were
modified during long-distance propagation, as originally suggested by S. A.
George. Propagated impulses were computed with the use of the
Hodgkin-Huxley equations and cable theory to simulate excitation and
current spread in 100-microns-diam unmyelinated axons, whose total length
was 8.1 cm (25 lambda) or 101.4 cm (312.5 lambda). Differential equations
were solved numerically, with the use of trapezoidal integration over
small, constant electrotonic and temporal steps (0.125 lambda and 1.0
microsecond, respectively). 2. Using dual-pulse stimulation, we confirmed
that for interstimulus intervals between 5 and 11 ms, the conduction
velocity of the second of a short-interval pair of impulses was slower than
that of the first impulse. Further, with sufficiently long propagation
distance, the second impulse's conduction velocity increased steadily and
eventually approached that of the first impulse. This effect caused a
spatially varying interspike interval: as propagation proceeded, the
interspike interval increased and eventually approached stabilization. 3.
With Poisson stimulation, the peak amplitude of propagating action
potentials varied with interspike interval durations between 5 and 11 ms.
Such amplitude attenuation was caused by the incomplete relaxation of
parameters n (macroscopic K-conductance activation) and h (macroscopic
Na-conductance inactivation) during the interspike period. 4. The
stochastic properties of the impulse train became less Poisson-like with
propagation distance. In cases of propagation over 99.4 cm, the impulse
trains developed marked periodicities in Interevent Interval Distribution
and Expectation Density function because of the axially modulated
transformation of interspike intervals. 5. Despite these changes in impulse
train parameters, the arithmetic value of the mean interspike interval did
not change as a function of propagation distance. This work showed that in
theory, whereas the pattern of Poisson-like impulse codes was modified
during long-distance propagation, their mean rate was conserved.
