cerebral cortex

cerebral cortex Seen from the outside, the most obvious component of the human brain is the intricately folded cerebral cortex that covers the pair of cerebral hemispheres, which conceal most of the rest of the brain. The convolutions, or gyri, of the cortex, and the fissures or sulci that separate them, vary enormously from brain to brain, and from one hemisphere to the other in each individual. True cerebral cortex is found in the brains of fishes, reptiles, and birds, but is a major feature of the brain in all mammals. In humans it is relatively enormous in size, even compared with our closest relatives, chimpanzees and gorillas. The dominance of the cortex in the human brain led Thomas Willis, the eminent Oxford physician, to propose, in 1664, that it is the seat of ‘higher’ mental processes, such as perception, memory, and will.

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 connections

Cortical 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 areas

Korbinian 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 cortex

An 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 cortex

An 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.

Laurence Garey

See nervous system.See also brain; central nervous system; vision.