Premotor Cortex - Anatomy and Structure

Anatomy and Structure

The premotor cortex occupies the part of Brodmann area 6 that lies on the lateral surface of the cerebral hemisphere. The medial extension of area 6, onto the midline surface of the hemisphere, is the site of the supplementary motor area, or SMA.

The premotor cortex can be distinguished from the primary motor cortex, Brodmann area 4, just posterior to it, based on two main anatomical markers. First, the primary motor cortex contains giant pyramidal cells called Betz cells in layer V, whereas giant pyramidal cells are less common and smaller in the premotor cortex. Second, the primary motor cortex is agranular: it lacks a layer IV marked by the presence of granule cells. The premotor cortex is dysgranular: it contains a faint layer IV.

The premotor cortex can be distinguished from Brodmann area 46 of the prefrontal cortex, just anterior to it, by the presence of a fully formed granular layer IV in area 46. The premotor cortex is therefore anatomically a transition between the agranular motor cortex and the granular, six-layered prefrontal cortex.

The premotor cortex has been divided into finer subregions on the basis of cytoarchitecture (the appearance of the cortex under a microscope), cytohistochemistry (the manner in which the cortex appears when stained by various chemical substances), anatomical connectivity to other brain areas, and physiological properties. These divisions are summarized below in Divisions of the premotor cortex.

The connectivity of the premotor cortex is diverse, partly because the premotor cortex itself is heterogenous and different subregions have different connectivity. Generally the premotor cortex has strong afferent (input) and efferent (output) connectivity to the primary motor cortex, the supplementary motor area, the superior and inferior parietal cortex, and prefrontal cortex. Subcortically it projects to the spinal cord, the striatum, and the motor thalamus among other structures.