The gray matter of the cerebral cortex contains about 10 billion neurons. It varies in thickness from about 4.5 mm at the crest of a gyrus, to about 1.5 mmin the recess of a sulcus. The cortex contains five different types of cells: pyramidal, fusiform, horizontal cells of Cajal, stellate, and cells of Martinotti. The cortex has been divided into six layers according to the density and arrangement of the different types of cells.
The most superficial layer is the molecular (plexiform) layer. It has a dense network of tangentially oriented fibers and cells, made of axons of cells of Martinotti, stellate cells, and apical dendrites of pyramidal cells and fusiform cells. Afferent fibers from the thalamus terminate here, as do many commissural fibers. This is a layer of intense synapsing. The external granular layer has several small stellate and pyramidal cells, and the external pyramidal layer has larger pyramidal cell bodies than in more superficial layers. Their apical dendrites reach into the molecular layer, and their axons descend into the white matter as projection, commissural or association fibers.
The internal granular layer is densely packed with stellate cells, and there is a horizontal band of fibers called the external band of Baillarger. The internal pyramidal (ganglionic) layer contains medium- sized and large pyramidal cells. In between these cells are cells of Martinotti and stellate cells. There is also another band of fibers called the inner band of Baillarger. The internal pyramidal layer in the precentral gyrus of the motor cortex contains very large pyramidal cells, called Betz cells. The axons of these cells contribute about 3-4% of the pyramidal or corticospinal tract. The innermost layer of the cerebral cortex is the multiform layer of polymorphic cells. Most of the cells in this layer are fusiform cells but pyramidal cells and cells of Martinotti are also present.
Pyramidal cells are so called because of the shape of the cell body. The apex is oriented towards the outer layers and from it a thick apical dendrite projects upwards, giving out several collaterals. Dendrites possess many dendritic spines for synapsing with other cells. An axon projects down from the base of the cell body and may terminate in deeper cortical layers, but more usually descends into the white matter as a projection, commissural, or association fiber.
Stellate cells have small polygonal cell bodies and radiate several dendrites and a short axon which may terminate in the same or a neighboring layer. Horizontal cells of Cajal are small cells horizontally oriented in the superficial layers. Fusiform cells (fusiform means spindle shaped or tapering at both ends) are oriented perpendicular to the layers, have dendritic projections from each pole, and occur principally in deeper cortical layers. Cells of Martinotti are small multipolar cells, with an axon projecting upwards to the surface, and short dendrites.
The bands of Baillarger are made up principally of collateral nerve fibers given out by incoming afferents, and of stellate cells and horizontal cells of Cajal. They include some pyramidal and fusiform collaterals as well. They are prominent in sensory cortical areas because of high densities of thalamocortical fiber terminations. The outer band of Baillarger is especially prominent in the visual cortex, where it is sometimes called the stria of Gennari.