Cerebral Cortex

Keywords

neocortex, granular and agranular areas, primary, unimodal, and multimodal areas, agnosia, apraxia, aphasia, neglect and disconnection syndromes, ascending reticular activating system, Wernicke area, Broca area

Chapter Outline
Most Cerebral Cortex Is Neocortex, 143
- Different Neocortical Layers Have Distinctive Connections, 143
- Neocortex Also Has a Columnar Organization, 144
Neocortical Areas Are Specialized for Different Functions, 144
- There Are Sensory, Motor, Association, and Limbic Areas, 144
The Corpus Callosum Unites the Two Cerebral Hemispheres, 147
- Disconnection Syndromes Can Result From White Matter Damage, 147
Consciousness and Sleep Are Active Processes, 147
- There Are Two Forms of Sleep, 148

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. 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 pyramidal neurons project to the striatum, brainstem, and spinal cord, and layer VI pyramidal neurons 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 parietal lobes project through the posterior half of the body and the occipital lobes and parts of the temporal lobes project through the enlarged splenium .

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. There is a posterior commissure located just above the superior colliculus, but it is very small and short, with simple connections for the bilateral pupillary light reflex.

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; 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. 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. Important Brodmann numbers are indicated in parentheses in the figures in this chapter.

There Are Sensory, Motor, Association, and Limbic Areas

Key Concepts

Primary somatosensory cortex is in the parietal lobe.
Primary visual cortex is in the occipital lobe.
Primary auditory cortex is in the temporal lobe.
There are primary vestibular, gustatory, and olfactory areas.
Most motor areas are in the frontal lobe.
Association areas mediate higher mental functions.
Parietal association cortex mediates spatial orientation.

Primary areas ( Fig. 22.1 ) of cortex are those most directly linked to the rest of the world, either through inputs from thalamic sensory relay nuclei or through outputs to the brainstem and spinal cord. Primary motor cortex occupies part of the precentral gyrus, primary somatosensory cortex the postcentral gyrus, primary auditory cortex the transverse temporal gyri, and primary visual cortex the banks of the calcarine sulcus. These primary areas contain precise but distorted somatotopic , tonotopic , or retinotopic maps with large representations of functionally important areas like the fingers, speech frequencies, and the fovea. There is also a primary gustatory area in the anterior insula and a primary vestibular area in the posterior insula, but relatively less is known about them. Primary olfactory cortex (see Chapter 13 ), in and near the anterior temporal lobe called piriform cortex , is different in not being neocortical and not receiving its input from the thalamus.

Adjacent to each of the primary cortical areas are areas that are involved in the same function and that receive projections from the primary area (and usually from the appropriate thalamic relay nucleus as well). They have less precise somatotopic, tonotopic, and retinotopic maps than the primary areas, but their cells have more complex response properties. These are referred to as unimodal (i.e., single-function) association areas ( Fig. 22.2 ). Corresponding to the great importance of vision for primates, there is a particularly large expanse of visual association cortex . Damage to unimodal areas can cause different kinds of sensory-specific agnosia (from a Greek word meaning “lack of knowledge”), in which someone is unable to recognize objects or some of their properties using a particular sensory modality even though basic sensation using that modality is normal; loss of the ability to recognize faces or colors are two examples. Premotor cortex and the supplementary motor area are comparable unimodal areas adjacent to primary motor cortex.

The single-function, unimodal association areas send converging outputs to two large expanses of more complex association cortex ( Fig. 22.3 ). The first is a parietal-occipital-temporal region surrounded by sensory areas; it receives thalamic inputs from the pulvinar (which also projects to unimodal sensory association areas). The second is anterior to premotor cortex and is called prefrontal cortex ; it receives thalamic inputs from the dorsomedial nucleus . Neurons in these areas have still more complex properties; they may respond to multiple kinds of stimuli and may only respond under particular behavioral conditions. Lesions in these multimodal association areas can cause deficits more complex than simple weakness or diminished sensation. Apraxia refers to a condition in which someone is unable to perform a skilled movement in spite of wanting to move and not being weak or uncoordinated, and most often follows left parietal damage. Neglect syndromes in a way are sensory analogs of apraxia. These are conditions in which someone is unable to direct attention to one side and may be totally unaware of one entire side of his or her body. Contralateral neglect most often follows damage to the right parietal or temporal lobe with neglect often to the left side.