Journal of Neurophysiology, Vol 51, Issue 1 50-63, Copyright © 1984 by APS
Topographic factors in distribution of homonymous group Ia-afferent input to cat medial gastrocnemius motoneurons
S. M. Lucas and M. D. Binder
Experiments were performed to determine whether the topographic
relationships between muscle spindles and their surrounding extrafusal
fibers are preserved in the pattern of homonymous, monosynaptic connections
from Ia-afferents to motoneurons. The medial gastrocnemius (MG) muscle of
adult cats was chosen as a model system because previous studies have shown
that its muscle nerve divides into several branches, each of which
innervates a distinct muscle compartment (20, 27, 28, 38), and that the
Ia-afferent fibers innervating muscle spindles within a compartment are
found in the same nerve branch (27, 28). Thus, we could make intracellular
recordings from MG motoneurons, determine which intramuscular compartment
they innervated, and then compare the synaptic input they received from
Ia-afferents innervating the same compartment with that which they received
from Ia-afferents innervating different compartments. Our results indicate
that homonymous Ia-afferent input is "topographically weighted" within the
MG motor nucleus such that afferents innervating a given intramuscular
compartment exert relatively greater synaptic effects in motoneurons that
project to the same compartment than in other homonymous motoneurons. The
degree of topographic weighting was quite variable in the different
experiments but appeared more prominently in experiments in which a high
proportion of the motoneurons studied were characterized by high-rheobase
values and low input resistances. This suggests that topographic factors
may exert more influence on the distribution of Ia-afferent input to large
motoneurons than to small motoneurons. In the DISCUSSION, the extent of
topographic weighting within the homonymous motoneuron pool is compared
with weighting across synergist motoneuron pools, and alternative models of
topographic weighting are proposed and evaluated.
