J. Lomeli et al., Effects of PAD on conduction of action potentials within segmental and ascending branches of single muscle afferents in the cat spinal cord, EXP BRAIN R, 135(2), 2000, pp. 204-214
In anesthetized and paralyzed cats under artificial respiration, we examine
d the extent to which primary afferent depolarization (PAD) might affect in
vasion of action potentials in intraspinal axonal and/or terminal branches
of single muscle afferents. To this end, one stimulating micropipette was p
laced at the L6 spinal level within the intermediate or motor nucleus, and
another one at the L3 level, in or close to Clarke's column. Antidromically
conducted responses produced in single muscle afferents by stimulation at
these two spinal levels were recorded from fine lateral gastrocnemius nerve
filaments. In all fibers examined, stimulation of one branch, with strengt
hs producing action potentials, increased the intraspinal threshold of the
other branch when applied at short conditioning testing stimulus intervals
(<1.5-2.0 ms), because of the refractoriness produced by the action potenti
als invading the tested branch. Similar increases in the intraspinal thresh
old were found in branches showing tonic PAD and also during the PAD evoked
by stimulation of group I afferent fibers in muscle nerves. It is conclude
d that during tonic or evoked PAD, axonal branches in the dorsal columns an
d myelinated terminals of muscle afferents ending deep in the L6 and L3 seg
mental levels continue to be invaded by action potentials. These findings s
trengthen the view that presynaptic inhibition of muscle afferents produced
by activation of GABAergic mechanisms is more likely to result from change
s in the synaptic effectiveness of the afferent terminals than from conduct
ion failure because of PAD.