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Journal of Neuroscience, Vol 9, 943-958, Copyright © 1989 by Society for Neuroscience
Motor activity in the isolated spinal cord of the chick embryo: synaptic drive and firing pattern of single motoneurons
MJ O'Donovan
Department of Physiology, University of Iowa, Iowa City 52242.
The cellular mechanisms underlying embryonic motility were investigated
using intracellular recording from motoneurons and electrotonic recording
from muscle nerves during motor activity generated by an isolated spinal
cord preparation of 12- to 15-d-old chick embryos. DC- coupled recordings
from sartorius (a flexor) and femorotibialis (an extensor) muscle nerves
revealed that both sets of motoneurons were depolarized at the same time in
each cycle even when the motoneurons fired out of phase. Sartorius
motoneurons fired briefly on the rising phase of the depolarization and
then stopped firing before discharging a second burst of spikes as the
depolarization decayed. By contrast, femorotibialis motoneurons fired at
the peak of their depolarization, which was coincident with the
interruption in sartorius activity. Intracellular recordings from
antidromically identified motoneurons confirmed that flexor and extensor
motoneurons were depolarized at the same time during each cycle of
activity. The discharge of femorotibialis motoneurons, and others presumed
to be extensors, followed changes in membrane potential so that maximal
firing occurred during peak depolarization. The relationship between
discharge and membrane potential was different in sartorius motoneurons
(and in others presumed to be flexors) because they fired briefly on the
rising phase of the depolarization and then stopped firing during peak
depolarization. In some of these cells firing resumed as the membrane
potential decayed back to rest. Intracellular injection of depolarizing
current into sartorius motoneurons during motor activity reversed the
direction of the membrane potential change from depolarizing to
hyperpolarizing during the pause in sartorius discharge. In addition, the
discharge evoked by the depolarizing current was blocked during the
reversed part of the synaptic potential revealing its inhibitory nature.
The occurrence of the IPSP was accompanied by a large reduction in
motoneuronal input impedance. Injection of depolarizing current steps into
motoneurons produced steady firing with no evidence of a pause in
discharge, indicating that the depolarization accompanying synaptic
activity was not responsible for the pause in firing of flexor motoneurons.
These results suggest that flexor and extensor motoneurons receive a
similar depolarizing drive from a common set of excitatory premotor
interneurons. The alternating pattern of flexor and extensor discharge is
produced, in part, by the timing of a depolarizing IPSP coincident with
extensor activity that silences flexor discharge.(ABSTRACT TRUNCATED AT 400
WORDS)
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