Journal of Neurophysiology, Vol 53, Issue 6 1590-1603, Copyright © 1985 by APS
Axonal projections of single bulbospinal inspiratory neurons revealed by spike-triggered averaging and antidromic activation
T. E. Dick and A. J. Berger
Activity was recorded extracellularly from 26 inspiratory bulbospinal
neurons in anesthetized, paralyzed, artificially ventilated cats. All but
one were located in the ventral respiratory group. A neuron was classified
as either I alpha or I beta by comparing its firing pattern during
inspiratory cycles with lung inflation to its pattern when lung inflation
was withheld during the central inspiratory phase (2, 14, 15). In this
study, the projection and conduction velocity of these axons were
determined using two methods: antidromic activation (AA) of the bulbospinal
neurons and spike-triggered averaging (STA) of the extracellular field
potentials. These methods have been compared directly because the same
electrode was used both for stimulating the axon of the bulbospinal neuron
and recording its axonal potential in the same location. Axonal projections
from these neurons were mapped in the contralateral spinal cord with a
mobile electrode by determining where the lowest stimulus threshold occurs
for AA and greatest axonal potential can be recorded with STA. The
locations of these axons were in the ventral and lateral funiculi. Each
method determined a similar location for an axon. Positions of 10 axons
were determined at both the third (C3) and fourth (C4) cervical segments.
Single axons maintained their positions in either descending tract from
rostral C3 to mid-C4. In five of six cases where two "neighboring"
medullary units were characterized, the axons of each pair projected
together within 350 micron of each other in the cervical spinal cord.
Estimates of mean axonal conduction velocity (CV) from antidromic
activation from a single stimulus site, "single-point AA," were as much as
42% less than corresponding estimates from STA extracellular field
potentials at that point (P less than 0.001). Such single-point estimates
were less than determinations that were calculated from the difference in
conduction time and the difference in conduction distance from two points
in the spinal cord. These two-point determinations averaged 55.4 +/- 13.1
m/s (using AA) and 53.3 +/- 13.1 (using STA) for 10 neurons. These values
were not significantly (P greater than 0.2) different from each other and
are greater than most earlier reports, which used the single-point AA
method. Either method, AA or STA, can be used to determine axonal position
and CV. The advantages and disadvantages of each method are
discussed.(ABSTRACT TRUNCATED AT 400 WORDS)
