Wednesday, October 26, 2011

Cortical Layers

The neocortex of the brain (cerebral cortex) is stratified into six cortical layers which run parallel to the surface of the hemisphere. The stratification can be demonstrated by silver impregnation , cellular staining according to Nissl, myelin staining, and pigment staining. The layers are distinguished according to the different shapes, sizes and numbers of their neurons and by the different densities of myelinated nerve fibers.

Cellular staining reveals the following features: 1) the outermost layer, the molecular layer (layer I), contains few cells; 2) the external granular layer (layer II) is densely packed with small granule cells; 3) the external pyramidal layer (layer III) contains predominantly mediumsized pyramidal cells; 4) the internal granular layer (layer IV) consists of densely packed small granule cells; 5) the internal pyramidal layer (ganglionic layer) (layer V) contains large pyramidal cells; 6) the multiform layer (layer VI) completes the stratification with a loose mixture of different cell types.

The pyramidal cell is the typical neuron of the neocortex. Its axon takes off from the base of the cell, where the basal dendrites branch off at the margins. One long, thick dendrite, the apical dendrite, ascends to the surface of the cortex. The dendrites have thousands of spines at which other neurons synapse.