New views of the primary motor cortex

For 100 years, from the 1870s to the 1970s, somatotopic organization was considered the hallmark of the primary motor cortex (M1). M1 neurons were viewed as upper motor neurons, implying that their organization and function were upstream versions of the spinal motoneurons to which they project. Taken together, the notions of somatotopic organization and upper motor neurons established a view of M1 as a sheet of somatotopically arrayed neurons that controlled either the muscles or the movements of different body parts. Evidence accumulating since the 1970s, however, has generated new views of M1 at an accelerating pace. Here, briefly review evidence leading to three new views of M1. First, whereas the gross somatotopic organization of M1-with the head represented laterally, the lower extremity medially, and the upper extremity in between-is unquestioned, we now view representation within the upper extremity region (from which the most evidence is available) as widely distributed. Second, rather than a fixed array of representation, we now view M1 as capable of considerable, and surprisingly rapid, reorganization. And third, rather than simply controlling the parameters of movement execution, we now view M1 as participating in aspects of sensorimotor transformation that include some representation of the sensory cues leading to voluntary movement. Although these new views account for a good deal of recent experimental evidence, they also open many new questions about the primary motor cortex.