Cerebral Cortex

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22 Cerebral Cortex

Chapter Outline

Most Cerebral Cortex Is Neocortex

Different Neocortical Layers Have Distinctive Connections

The Corpus Callosum and Anterior Commissure Interconnect the Two Cerebral Hemispheres

Association Bundles Interconnect Areas within Each Cerebral Hemisphere

Neocortex Also Has a Columnar Organization

Neocortical Areas Are Specialized for Different Functions

There Are Sensory, Motor, Association, and Limbic Areas

Language Areas Border the Lateral Sulcus, Usually on the Left

The Right and Left Cerebral Hemispheres Are Specialized for Different Functions

Prefrontal Cortex Mediates Working Memory and Decision Making

The Corpus Callosum Unites the Two Cerebral Hemispheres

Disconnection Syndromes Can Result from White Matter Damage

Consciousness and Sleep Are Active Processes

There Are Two Forms of Sleep

Both Brainstem and Forebrain Mechanisms Regulate Sleep-Wake Transitions

The cerebral cortex is ultimately the part of the CNS that makes us human. Other parts of the CNS like sensory pathways bring in raw data, the reticular activating system adjusts levels of excitability, but the cortex is where events are analyzed, plans are hatched, and responses are formulated. The cerebral cortex is a big sheet of repeated functional modules, with the operations of different arrays of modules corresponding to progressively more complex mental functions.

Most Cerebral Cortex Is Neocortex

Key Concepts

Pyramidal cells are the most numerous neocortical neurons.

Neocortex has six layers.

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

The layering of neocortex is a mechanism for sorting its inputs and outputs. Afferents from other cortical areas (by far the majority), from the thalamus, and from modulatory nuclei in the brainstem and elsewhere distribute themselves in distinctive patterns among the various layers. Similarly, the pyramidal cells of any given layer have preferred targets; for example, layer V pyramids project to the striatum, brainstem, and spinal cord, and layer VI pyramids project to the thalamus.

The Corpus Callosum and Anterior Commissure Interconnect the Two Cerebral Hemispheres

One source of afferents from other cortical areas is the contralateral hemisphere. The corpus callosum is a bundle of several hundred million axons that interconnect the two cerebral hemispheres. Many of these fibers project from sites in the frontal, parietal, or occipital cortex on one side to the mirror-image sites on the other side. However, others interconnect areas that are functionally related to each other but not mirror images. Projections from the frontal lobes fill the genu and the anterior half of the body of the corpus callosum. The occipital lobes and parts of the temporal lobes project through the enlarged splenium, and the parietal lobes through the posterior half of the body (THB6 Figure 22-28, p. 570).

The anterior commissure contains similar fibers that interconnect the rest of the temporal lobes, as well as other fibers that interconnect components of the olfactory system.

Association Bundles Interconnect Areas within Each Cerebral Hemisphere

The second general source of corticocortical afferents is other areas in the same hemisphere. Many of these travel in well-defined association bundles, but they are intermingled with other fiber bundles in the white matter of the cerebral hemisphere and not as obvious as the corpus callosum and anterior commissure. One functionally important association bundle is the arcuate fasciculus, which travels above the insula (THB6 Figure 22-9, p. 548); as described a little later, it interconnects two prominent language areas.

Neocortex Also Has a Columnar Organization

Despite the horizontal layering of neocortex, other data indicate that in a functional sense it is organized into columns on the order of 100 µm wide and oriented perpendicular to the cortical surface. Endings in one cortical area from either the thalamus or another cortical area are often organized into such columns, separated by other columns that do not receive such endings. Visual cortex is organized into columns of cells having similar response properties, with adjacent columns differing in some parameter (THB6 Figure 17-35, pp. 449 and 450). Likewise, somatosensory cortex is organized into columns of neurons that respond best to a particular kind of stimulus.

Neocortical Areas Are Specialized for Different Functions

Key Concept

Different neocortical areas have subtly different structures.

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.

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