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Journal of Neuroscience, Vol 11, 2564-2573, Copyright © 1991 by Society for Neuroscience
Organization of hindlimb muscle afferent projections to lumbosacral motoneurons in the chick embryo
MT Lee and MJ O'Donovan
Department of Physiology and Biophysics, University of Iowa, Iowa City 52242.
We have examined the organization of muscle afferent projections to
motoneurons in the lumbosacral spinal cord of chick embryos between stage
37, when muscle afferents first reach the motor nucleus, and stage 44,
which is just before hatching. Connectivity between afferents and
motoneurons was assessed by stimulating individual muscle nerves and
recording the resulting motoneuron synaptic potentials intracellularly or
electrotonically from other muscle nerves. Most of the recordings were made
in the presence of DL-2-amino-5- phosphonovaleric acid (APV), picrotoxin,
and strychnine to block long- latency excitatory and inhibitory pathways.
Activation of muscle afferents evoked slow, positive potentials in muscle
nerves but not in cutaneous nerves. These potentials were abolished in 0 mM
Ca2+, 2mM Mn2+ solutions, indicating that they were generated by the action
of chemical synapses. The muscle nerve recordings revealed a wide-spread
pattern of excitatory connections between afferents and motoneurons
innervating six different thigh muscles, which were not organized according
to synergist-antagonist relationships. This pattern of connectivity was
confirmed using intracellular recording from identified motoneurons, which
allowed the latency of the responses to be determined. Short-latency
potentials in motoneurons were produced by activation of homonymous
afferents and the heteronymous afferents innervating the hip flexors
sartorius and anterior iliotibialis. Stimulation of anterior iliotibialis
afferents also resulted in some short-latency excitatory postsynaptic
potentials (EPSPs) in motoneurons innervating the knee extensor
femorotibialis, though other connections were of longer latency. Afferents
from the adductor, a hip extensor, did not evoke short-latency EPSPs in any
of these three types of motoneurons. Short-latency, but not long-latency
EPSPs, persisted during repetitive stimulation at 5 Hz, suggesting that
they were mediated monosynaptically. Long-latency, fatigue-sensitive
potentials were maintained in the presence of APV, picrotoxin, and
strychnine, suggesting that polysynaptic pathways utilize non-NMDA
receptors as well as NMDA receptors. We found no difference in the pattern
of inputs to femorotibialis motoneurons between stage 37-39 and near
hatching at stage 44, suggesting muscle afferent projections to these
motoneurons are correct at stage 37, when the afferents first reach the
lateral motor column in substantial numbers.
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