The cerebral cortex is the thin outer cloak of grey matter that covers the external surface of the cerebral hemispheres, like the ‘bark’ of a tree, which is indeed what its Latin root refers to. The total area of cortex in man is estimated at nearly a square meter; it is about 4 mm thick, and it contains 10 000 million or more nerve cells (neurons). The number of synapses (connections between nerve fibres and other neurons) is even more staggering — there are, on average, around 10 000 synapses on every cortical neuron.
When thin sections are viewed under a microscope, with appropriate staining of cell bodies, the cortex is seen to consist of several distinct layers. The almost universally adopted layering scheme is that proposed by the late nineteenth-century German anatomist Korbinian Brodmann. Most of the cortex, the so-called neocortex, has a 6-layered structure, but some areas of the hemisphere are covered with simpler cortex with fewer layers, which is believed to be representative of a relatively primitive stage of brain development.
Cortical neurons and their connectionsCortical neurons are basically classified as pyramidal and non-pyramidal cells. A pyramidal cell can be recognized by a single, fairly thick process, the apical dendrite, which sprouts out of the top of the cell body and extends up toward the cortical surface. The other dendrites (branches of the cell body that receive incoming information from the fibres of other neurons), called basal dendrites, form a skirt around the lower part of the cell body. All dendrites bear large numbers of spines, small excrescences on which incoming nerve fibres terminate to form synapses. Pyramidal cells receive incoming nerve fibres from the thalamus and from other areas of cortex, as well as from nearby neurons. The axon of a pyramidal cell (the process that conveys impulses away from the neuron to other nerve cells) can extend a long way, for example more than a meter down to the spinal cord, as well as sending branches to other cortical neurons.
The axon of a typical pyramidal cell can make thousands of synapses on other neurons. When the cell fires an impulse it sweeps along all the branches of the axon to reach the synapses at the terminals, where it triggers the release of the excitatory neurotransmitter glutamate. This affects receptor molecules in the membrane of the target neuron in such a way that it becomes more permeable to sodium ions, which rush into the cell, making the interior more positively charged. This depolarization increases the probability that the target cell will itself fire off an impulse.
If the target cell of the axon of a pyramidal neuron lies below the cerebral hemispheres, in the brain stem or spinal cord, it is termed a projection axon. If it goes to other cortical areas in the same hemisphere, it is an association axon, while if it innervates cells in the cortex of the opposite hemisphere it is called commissural.
Other neurons in the cortex are all non-pyramidal cells — a very heterogeneous group. They are called interneurons because they have relatively short axons that make local connections and do not leave the cortex itself. Non-pyramidal cells can be either excitatory or inhibitory. If the latter, they commonly use the important inhibitory substance GABA (gamma-aminobutyric acid) as the transmitter at their synapses. Non-pyramidal cells, like pyramidal cells, receive axons from the thalamus, from other cortical areas, and from other local interneurons.
The outermost layer of the neocortex, layer I, consists of a mesh of axons and dendrites with very few cell bodies. The other layers consist of pyramidal and non-pyramidal neurons in varying proportions. Layer IV contains a high proportion of non-pyramidal cells, and receives most of the incoming fibres from the thalamus. Layers V and VI have particularly large pyramidal cells that project to subcortical centres, such as the spinal cord and thalamus.
Specialized cortical areasKorbinian Brodmann also recognized that the cortex is divided into a large number of fields or areas, distinguished by slight differences in the appearance of the layers. He suggested that each anatomically distinctive area is specialized for a particular sensory, motor, or associative function.
Each hemisphere is mainly concerned with the control of muscles and with sensory input from the opposite side of the body, and also with visual and auditory information from the opposite side of the outside world. Hence damage of one hemisphere tends to affect sensation and movement on the opposite side. The cerebral cortex can be seen as the terminus of all the sensory pathways of the nervous system, in the sense that the cortex seems to be needed for conscious perception. Only when information originating in the eyes, the ears, the skin, or any other sensory organ finally reaches the cortex can it then be felt as a subjective experience. Equally, the cortex is the origin of our intended actions. From the motor cortex, axons, especially those from the very large ‘Betz cells’ in layer V, project all the way to the spinal cord to contact motor neurons, which relay the signals out to the muscles. But the cortex is also an integrative organ. Large areas of it are associative in function, meaning that they bring together activity from different sensory and motor systems to make higher-level functions possible, such as speech, memory, and thought. We know from studies of patients who have suffered damage to various parts of the cortex that some of this association cortex is intimately related to our character or personality.
The visual cortexAn area of the neocortex that has been particularly well studied is the primary visual cortex (area 17 of Brodmann), found at the back of the occipital lobe, mainly on the banks of a deep sulcus. Both eyes send signals, via the thalamus, to the visual cortex of both hemispheres, in such a way that each hemisphere receives information about the opposite half of the visual field. Thalamic fibres carrying information from the right eye and the left eye are segregated from each other as they enter the cortex and they form alternating patches of right-eye and left-eye input, about 0.3 mm across, to the cells of layer IV. Since connections between cortical cells mainly run up and down, this has the effect of imposing a pattern of functional ‘columns’ on the cortex, the neurons below any particular point on the cortical surface tending to be dominated functionally by either the right or the left eye. Such ‘columnar’ organization is a characteristic feature of the cerebral cortex, reflecting the segregation of different classes of incoming nerve fibres and the arrangement of connections between cortical neurons.
Area 17 is responsible for basic visual feature detection, but there exist dozens of other, interconnected visual areas in the occipital lobe and even in temporal and parietal lobes. Some are concerned with colour discrimination, or complex pattern recognition, certain cells even responding when the eyes view a stimulus as complex as a face. These areas belong to the association cortex, mentioned above, which is a striking feature of the human brain, permitting the integration and further analysis of simple sensory information to form the basis of meaningful, conscious experience, and the accurate control of action.
The primitive cortexAn example of the more ‘primitive’ cortex described earlier is the hippocampus (Latin for ‘sea-horse’, on account of its appearance in cross-section), which is tucked underneath, on the inner aspect of the temporal lobe. It is unusual in that it has white matter on the outside, and its structure is simple compared with the neocortex, with only three layers. The hippocampus, which receives processed information from much of the association cortex, seems to be involved in short-term conscious memory. It is functionally connected with the hypothalamus and the limbic system, parts of the brain that control basic life functions such as hormonal systems and basic body rhythms and appetites.
So the cerebral cortex is important for an amazing range of functions, from basic drives for self-preservation to the highest levels of consciousness.
See nervous system.See also brain; central nervous system; vision.
The cerebral cortex is the large overgrowth of the mammalian forebrain. It is best developed in primates and especially in humans, where it makes up a thin sheet, about 3 mm thick and 1600 cm2 in area, folded into intricate convolutions to fit in the skull. Most of the cortex is buried in the banks and depths of elongated crevices called sulci. At a gross level the cerebral cortex is divided into four lobes. In the human brain the frontal lobe is located behind the forehead and above the eyes; the occipital lobe is found at the back of the head. Between them are the parietal lobe, near the top of the head, and the temporal lobe, along the sides of the head, behind the ears (see Figure 1). These lobes are easiest to locate in the brains of humans and other primates, where the sulci divide the cortex into lobes, and the lobes into smaller units, called lobules.
Even at the gross level of lobes, each region of anatomy in the cerebral cortex entails a distinct physiology. Thus, within the frontal lobes there are motor regions devoted to the planning and execution of movements; within the parietal lobe, regions devoted to the sense of touch; within the occipital lobe, regions devoted to vision; and, within the temporal lobe, regions devoted to the sense of hearing (see Figure 1). Each lobe also includes regions, neither sensory nor motor, called association regions, which further analyze sensory information or combine information from two or more senses or from several brain regions. For example, in the frontal lobe there is a region called the prefrontal cortex, which is important for planning and keeping track of sequential tasks such as cooking an elaborate meal or remembering driving directions long enough to get to a destination. Specialized parts of the prefrontal cortex behind the eyes are associated with control of social behavior, sorting appropriate and inappropriate responses demanded by various situations (Damasio, 1994; Fuster, 1997).
The cerebral cortex of the human is highly specialized. For example, one region at the junction of the frontal and temporal lobes, Broca's area, coordinates the movements of the mouth and tongue to produce speech ("motor speech"), while a second region at the junction of the parietal and temporal lobes, Wernicke's area, is necessary for understanding the meaning of spoken and written words ("sensory speech"). Humans with damage in Broca's area are unable to articulate grammatically correct speech, while those with damage in Wernicke's area are capable of fluent, though nonsensical, speech (Geschwind, 1979).
All cortical lobes are further divided into anatomically and physiologically distinct areas. Each cortical area is defined by the unique organization and physiology of the cells within it and by the set of connections the area has with other parts of the brain. There are nearly one hundred known cortical areas, which are usually designated by a numbering scheme, or code. For example, the primary area of cortex devoted to vision is designated area 17, or V1 (visual area 1), and the primary area for movement is area 4, or M1 (see Figure 1).
The cells of the cortex make up the outer gray matter of the brain, and their axons leave the cortex and connect with other cortical areas and other parts of the brain. The cortex also receives information from other brain regions. The white matter just beneath the cortex is made up of axons entering or leaving the cortex. These connections render the cortex a massive communication system. For example, there is two-way communication between the cortex and a region beneath the cortex, the thalamus (Steriade, Jones, and McCormick, 1997). This reciprocal link is very precise, so that one area of the cortex communicates mainly with one or two of the many groups of cells, or nuclei, in the thalamus. In addition, each cortical area on one side of the brain has some connections with its reciprocal area and a few neighboring areas on the opposite side. The connections to the opposite side are made by axons of cortical cells, which form bundles called the corpus callosum and the anterior commissure. The corpus callosum is a major highway linking most of the cortical areas between the two brain halves, or hemispheres. The anterior commissure is a smaller pathway, connecting mostly temporal areas across the hemispheres.
There are other connections between cortical areas on the same side of the brain. Each of the precise connections with the thalamus, opposite cortex, and cortex on the same side contributes to the unique physiological characteristics of a cortical area. More widespread connections that go to all areas of cortex come from cells beneath the cortex and use the chemicals dopamine, norepinephrine, serotonin, acetylcholine, or histamine as neurotransmitters. These widespread connections appear responsible for setting the overall level of activity in the cortex (Foote and Morrison, 1987; Kandel et al., 2000).
A typical area of the cerebral cortex is divided horizontally into six layers. The cells in each layer have similar structure and connections and are segregated from cells in other layers with different properties (see Figure 2, top left panel). Superimposed on the horizontal organization is a vertical grouping of cells across layers, forming cortical columns, or modules. The key to the cortical column is that the cells within a single column are interconnected such that all its cells exhibit common physiological properties, while cells in neighboring columns exhibit different properties. Columns or modules of cells with similar properties are the fundamental units of organization and function in the cerebral cortex and are found throughout the cortex, including areas involved in vision, hearing, movement, or high-order cognitive processes and memory (Mountcastle, 1998). For example, in the visual area of the primate cortex, cells that analyze signals from one eye make up one set of columns, each 0.5 to 1.0 millimeter wide, and these interdigitate with a second set of columns containing cells that analyze signals from the other eye. Thus, cells in different columns are dominated by one eye or the other (Hubel and Wiesel, 1977).
The correlation of function with structure prevails among single cells as well. There are two general categories of cells in the cortex. One, called a pyramidal cell, has a triangular cell body and is excitatory (see Figure 2, top panels). Pyramidal cells are the principal source of axons that leave an area of cortex, carrying information to other cortical areas or to regions outside the cortex. The second category includes cells with a rounded cell body; called nonpyramidal, they play a role in local communication.
Nonpyramidal cells are a heterogeneous group and include one or two types of excitatory cells and many types of inhibitory cells. The latter have different shapes and colorful names, such as chandelier cells, basket cells, and double-bouquet cells (see Figure 2, bottom panels). Inhibitory cells, which use gamma-aminobutyric acid (GABA) as a neurotransmitter, are important in controlling the activity of other neurons in the cortex (Somogyi et al, 1998). Excitatory cells in the cortex use glutamate as a neuro-transmitter. From these excitatory and inhibitory cells and the interconnections they give rise to physiological properties that are unique to the cerebral cortex (Peters and Jones, 1984).
Although extraordinarily complex, the cerebral cortex has a general plan of organization. The great diversity of function of the cerebral cortex is built up from the physiology of single cells to progressively larger compartments that link neurons within modules, layers, areas, and lobes.
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