The origin of corticospinal projections from the premotor areas in the frontal lobe

Abstract

We determined the origin of corticospinal neurons in the frontal lobe. These neurons were labeled by retrograde transport of tracers after injections into either the dorsolateral funiculus at the second cervical segment or the gray matter of the spinal cord throughout the cervical enlargement. Using retrograde transport of tracer from the arm area of the primary motor cortex, we defined the arm representation in each premotor area in another set of animals. We found that corticospinal projections to cervical segments of the spinal cord originate from the primary motor cortex and from the 6 premotor areas in the frontal lobe. These are the same premotor areas that project directly to the arm area of the primary motor cortex. The premotor areas are located in parts of cytoarchitectonic area 6 on the lateral surface and medial wall of the hemisphere, as well as in subfields of areas 23 and 24 in the cingulate sulcus. The total number of corticospinal neurons in the arm representations of the premotor areas equals or exceeds the total number in the arm representation of the primary motor cortex. The premotor areas collectively comprise more than 60% of the cortical area in the frontal lobe that projects to the spinal cord. Like the primary motor cortex, each of the premotor areas contains local regions that have a high density of corticospinal neurons. These observations indicate that a substantial component of the corticospinal system originates from the premotor areas in the frontal lobe. Each of the premotor areas has direct access to the spinal cord, and as a consequence, each has the potential to influence the generation and control of movement independently of the primary motor cortex. These findings raise serious questions about the utility of viewing the primary motor cortex as the “upper motoneuron” or “final common pathway” for the central control of movement.