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The Journal of Neurophysiology Vol. 83 No. 5 May 2000, pp. 2931-2945
Copyright ©2000 by the American Physiological Society
1Department of Neuroscience and 2Graduate Program in Biomedical Engineering, University of Minnesota, Minneapolis, Minnesota 55455
Bosco, G.,
R. E. Poppele, and
J. Eian.
Reference Frames for Spinal Proprioception: Limb Endpoint Based
or Joint-Level Based?. J. Neurophysiol. 83: 2931-2945, 2000. Many sensorimotor neurons in the CNS encode
global parameters of limb movement and posture rather than specific
muscle or joint parameters. Our investigations of spinocerebellar
activity have demonstrated that these second-order spinal neurons also may encode proprioceptive information in a limb-based rather than joint-based reference frame. However, our finding that each foot position was determined by a unique combination of joint angles in the
passive limb made it difficult to distinguish unequivocally between a
limb-based and a joint-based representation. In this study, we
decoupled foot position from limb geometry by applying mechanical
constraints to individual hindlimb joints in anesthetized cats. We
quantified the effect of the joint constraints on limb geometry by
analyzing joint-angle covariance in the free and constrained conditions. One type of constraint, a rigid constraint of the knee
angle, both changed the covariance pattern and significantly reduced
the strength of joint-angle covariance. The other type, an elastic
constraint of the ankle angle, changed only the covariance pattern and
not its overall strength. We studied the effect of these constraints on
the activity in 70 dorsal spinocerebellar tract (DSCT) neurons using a
multivariate regression model, with limb axis length and orientation as
predictors of neuronal activity. This model also included an
experimental condition indicator variable that allowed significant
intercept or slope changes in the relationships between foot position
parameters and neuronal activity to be determined across conditions.
The result of this analysis was that the spatial tuning of 37/70
neurons (53%) was unaffected by the constraints, suggesting that they
were somehow able to signal foot position independently from the
specific joint angles. We also investigated the extent to which cell
activity represented individual joint angles by means of a regression
model based on a linear combination of joint angles. A backward
elimination of the insignificant predictors determined the set of
independent joint angles that best described the neuronal activity for
each experimental condition. Finally, by comparing the results of these
two approaches, we could determine whether a DSCT neuron represented
foot position, specific joint angles, or none of these variables
consistently. We found that 10/70 neurons (14%) represented one or
more specific joint-angles. The activity of another 27 neurons (39%)
was significantly affected by limb geometry changes, but 33 neurons
(47%) consistently elaborated a foot position representation in the
coordinates of the limb axis.
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