It is ironic that of all the parts of the brain, we know least about the functions of the cerebral cortex, even though it is by far the largest portion of the nervous system. But we do know the effects of damage or specific stimulation in various portions of the cortex. In the first part of this chapter, the facts known about cortical function are discussed; then basic theories of neuronal mechanisms involved in thought processes, memory, analysis of sensory information, and so forth are presented briefly.
The functional part of the cerebral cortex is a thin layer of neurons covering the surface of all the convolutions of the cerebrum. This layer is only 2 to 5 millimeters thick, with a total area of about one quarter of a square meter. The total cerebral cortex contains about 100 billion neurons.
Figure 57-1 shows the typical histological structure of the neuronal surface of the cerebral cortex, with its successive layers of different types of neurons. Most of the neurons are of three types: (1) granular (also called stellate), (2) fusiform, and (3) pyramidal, the last named for their characteristic pyramidal shape.
The granular neurons generally have short axons and, therefore, function mainly as interneurons that transmit neural signals only short distances within the cortex itself. Some are excitatory, releasing mainly the excitatory neurotransmitter glutamate; others are inhibitory and release mainly the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). The sensory areas of the cortex as well as the association areas between sensory and motor areas have large concentrations of these granule cells, suggesting a high degree of intracortical processing of incoming sensory signals within the sensory areas and association areas.
The pyramidal and fusiform cells give rise to almost all the output fibers from the cortex. The pyramidal cells are larger and more numerous than the fusiform cells. They are the source of the long, large nerve fibers that go all the way to the spinal cord. They also give rise to most of the large subcortical association fiber bundles that pass from one major part of the brain to another.
To the right in Figure 57-1 is shown the typical organization of nerve fibers within the different layers of the cerebral cortex. Note particularly the large number of horizontal fibers that extend between adjacent areas of the cortex, but note also the vertical fibers that extend to and from the cortex to lower areas of the brain and some all the way to the spinal cord or to distant regions of the cerebral cortex through long association bundles.
The functions of the specific layers of the cerebral cortex are discussed in Chapters 47 and 51. By way of review, let us recall that most incoming specific sensory signals from the body terminate in cortical layer IV. Most of the output signals leave the cortex through neurons located in layers V and VI; the very large fibers to the brain stem and cord arise generally in layer V; and the tremendous numbers of fibers to the thalamus arise in layer VI. Layers I, II, and III perform most of the intra-cortical association functions, with especially large numbers of neurons in layers II and III making short horizontal connections with adjacent cortical areas.
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