22 Cerebral Cortex
Most Cerebral Cortex Is Neocortex
Most areas of cerebral cortex are neocortex, meaning that they have six more or less distinct layers (numbered I through VI from the surface down). About 80% of all cortical neurons are pyramidal cells, shaped as their name implies. They have a long apical dendrite ascending toward the cortical surface, a series of basal dendrites, and an axon emerging from the base of the cell body. Nearly all of the axons that leave the cerebral cortex are axons of pyramidal cells. The remaining 20% of cortical neurons is an assortment of nonpyramidal cells, most of them small and most of them inhibitory interneurons with axons that do not leave the cortex.
Layer I contains few cells and many synapses (just as the superficial layer of cerebellar cortex (see Chapter 20) is a place where mossy and climbing fibers synapse on the dendrites of Purkinje cells). Layer VI contains spindle-shaped modified pyramidal cells. The four middle layers of neocortex are alternating layers of mostly small cells and mostly large pyramidal cells (THB6 Figure 22-5, p. 545). Cortical areas that do not emit many long axons, such as primary sensory areas, are full of small pyramidal and nonpyramidal cells and are called granular areas. Cortical areas that emit many long axons, such as motor cortex, have many large pyramidal cells and are called agranular areas.
Different Neocortical Layers Have Distinctive Connections
Neocortical Areas Are Specialized for Different Functions
Although neocortex has the same basic structure everywhere in terms of percentages of pyramidal and nonpyramidal cells, layering, and columns, areas still differ from one another in terms of things like the sizes of cells and the thickness of layers. These differences turn out to be correlated with differences in function and connections, and so they have led to systematic maps of cortical areas. Several mapping systems have been devised, and some of the numbers in the map devised by Brodmann are in common use (THB6 Figure 22-10, p. 550). Important Brodmann’s numbers are indicated in parentheses in the figures in this chapter.