Sophisticated Movements

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Science  23 Jan 2009:
Vol. 323, Issue 5913, pp. 439
DOI: 10.1126/science.323.5913.439a

The ability of human beings to exert fine motor control is astonishing. Just think of the precision movements executed by a brain surgeon or a concert pianist or even a top-flight tennis player. What enables us to control the muscles of our upper extremities to such an unmatched degree?

To establish the anatomical underpinnings of these abilities, Rathelot and Strick injected a strain of rabies virus into single shoulder and arm muscles of monkeys and also reanalyzed previously published results from similar injections into finger muscles. Retrograde transneuronal transport of the virus demonstrated that the primary motor cortex (area M1 in the brain) contains two subdivisions. A rostral region of M1 contains corticospinal cells that project to spinal interneurons; these cells do not make direct connections onto motoneurons and represent a phylogenetically old part of M1 that is standard for most mammals. In contrast, a caudal region of M1 contains neurons that do synapse directly onto motoneurons; this region represents a phylogenetically younger part of M1 that is present only in some higher primates and in humans. The direct access to motoneurons afforded by corticomotoneuronal cells enables this younger part of M1 to bypass spinal cord mechanisms and to sculpt idiosyncratic patterns of motor output that are revealed as highly skilled movements. — PRS

Proc. Natl. Acad. Sci. U.S.A. 106, 918 (2009).

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