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The Journal of Neurophysiology Vol. 86 No. 5 November 2001, pp. 2125-2143
Copyright ©2001 by the American Physiological Society
INVITED REVIEW
Departments of Neurology, Neurobiology and Anatomy, Brain and Cognitive Science, and Physical Medicine and Rehabilitation, the Center for Visual Science, and the Brain Injury Rehabilitation Program at St. Mary's Hospital, University of Rochester School of Medicine and Dentistry, Rochester, New York 14642
Schieber, Marc H.
Constraints on Somatotopic Organization in the Primary Motor
Cortex. J. Neurophysiol. 86: 2125-2143, 2001. Since the 1870s, the primary motor cortex (M1) has been known
to have a somatotopic organization, with different regions of cortex
participating in control of face, arm, and leg movements. Through the
middle of the 20th century, it seemed possible that the principle of
somatotopic organization extended to the detailed representation of
different body parts within each of the three major representations.
The arm region of M1, for example, was thought to contain a
well-ordered, point-to-point representation of the movements or muscles
of the thumb, index, middle, ring, and little fingers, the wrist,
elbow, and shoulder, as conveyed by the iconic homunculus and
simiusculus. In the last quarter of the 20th century, however,
experimental evidence has accumulated indicating that within-limb
somatotopy in M1 is not spatially discrete nor sequentially ordered.
Rather, beneath gradual somatotopic gradients of representation, the
representations of different smaller body parts or muscles each are
distributed widely within the face, arm, or leg representation, such
that the representations of any two smaller parts overlap extensively.
Appreciation of this underlying organization will be essential to
further understanding of the contribution to control of movement made
by M1. Because no single experiment disproves a well-ordered
within-limb somatotopic organization in M1, here I review the
accumulated evidence, using a framework of six major features that
constrain the somatotopic organization of M1: convergence of output,
divergence of output, horizontal interconnections, distributed
activation, effects of lesions, and ability to reorganize. Review of
the classic experiments that led to development of the homunculus and
simiusculus shows that these data too were consistent with distributed
within-limb somatotopy. I conclude with speculations on what the
constrained somatotopy of M1 might tell us about its contribution to
control of movement.
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