Journal of Neurophysiology, Vol 65, Issue 3 468-476, Copyright © 1991 by APS
Strength-duration and activity-dependent excitability properties of frog afferent axons and their intraspinal projections
N. C. Tkacs and R. D. Wurster
Department of Physiology, Loyola University Medical Center, Maywood, Illinois 60153.
1. Excitability properties of afferent axons and terminal regions in frog
dorsal roots (DR) and spinal cords in vitro were investigated by antidromic
activation from three sites--the root, the entry zone (dorsal white matter
or DW), and deep within the dorsal horn (DH)--while recordings were made
from the DR. 2. Two approaches were used to assess physiological
differences between telodendria and trunk axons. Rheobases and
strength-duration time constants (tau sd) of single DR fibers were measured
by stimulation in the DH or in the DW. Conduction velocity was estimated on
the basis of onset latencies of evoked spikes (the time from stimulation to
action potential arrival at the recording electrodes). Population
supernormality was evaluated on the basis of responses to conditioned and
unconditioned submaximal stimuli delivered to the DH or to the proximal end
of isolated DRs. 3. Single-fiber action potentials occurred at longer
latencies after DH stimulation than after DW stimulation. Estimated
intraspinal conduction velocity was congruent to 0.6 m/s. Extraspinal
conduction velocity in these fibers averaged 22.2 m/s. Average tau sd was
longer in the DH than in the DW (670 microseconds vs. 204 microseconds). 4.
DH and DR test responses evoked 10-150 ms after a conditioning stimulus had
increased areas relative to unconditioned test responses.
Conditioning-associated changes in evoked responses were greater with the
DH stimulation site than with the DR stimulation site, and these changes
were not altered by treatment designed to block synaptic transmission. 5.
We conclude that membrane properties determining tau sd differ between
large afferent axons and fine terminal regions of those axons.(ABSTRACT
TRUNCATED AT 250 WORDS)
