APA Dictionary of Psychology

the layers of neurons that constitute the structure of the cerebral cortex and cerebellar cortex. In the cerebral cortex the number of layers varies, reaching a maximum of six in the neocortex. These six layers, identified by Roman numerals and starting from the outer surface, are I, the plexiform molecular layer, a narrow band of myelinated fibers; II, the external granular layer, containing granule cells and pyramidal cells; III, the external pyramidal layer, with medium-sized pyramidal cells in the outer zone and larger pyramidal cells in the inner zone; IV, the internal granular layer, which contains synapses of layer-III cells along with stellate cells categorized as granule cells; V, the ganglionic (or internal pyramidal) layer, which includes large pyramidal cells and the giant Betz cells; and VI, the polymorphic fusiform (or multiform) layer, which contains cells of many shapes but mainly spindle-shaped and pyramidal cells.