brain

brain The brain is a pinkish-grey, wrinkled organ that fills the skull — looking, for all the world, like a huge walnut. It is hard to believe, from its appearance, that this ugly lump of jelly contains the mechanisms of thought, perception, will, and consciousness, that it is the seat of our personality. Its 100 000 million nerve cells, each with an average of 100 00 connections from other neurons, makes the human brain the most complicated and least understood object in the known universe.

The brain and the spinal cord constitute the central nervous system. In the human embryo the brain grows from three swellings in the head end of a tube of developing nervous tissue. The frontmost swelling differentiates into the cerebral hemispheres, consisting mainly of the thalamus, the hypothalamus, the corpus striatum (involved in the control of movement) and the cerebral cortex. The two rear swellings form the brain stem (midbrain, pons, and medulla) and the cerebellum. When we look at the outside of the human brain we see little more than the cerebral cortex, which is very enlarged in humans compared with other mammals.

The brain is surrounded by protective membranes, the meninges, continuous with those covering the spinal cord. The outermost layer, the dura mater, is tough and protects the brain physically. Beneath the dura is the arachnoid mater, through which cerebral arteries and veins penetrate to reach the brain. The surface of the brain is intimately covered by the innermost layer, the pia mater, from which tiny blood vessels plunge into the cortex. A clear fluid, cerebrospinal fluid (CSF), which is secreted inside cavities called cerebral ventricles, within the brain, circulates in the subarachnoid space between the arachnoid and the pia. CSF protects the brain, both physically and chemically. The brain, hungry for oxygen and glucose, receives its blood through a rich system of arteries derived from two major sources, the internal carotid arteries and the vertebral arteries. Sudden blockage or haemorrhage in an artery (a stroke) can have catastrophic consequences, including almost immediate loss of consciousness or function, and even death.

The adult human brain weights about 1400 g, but there is much individual variation. The side view of the brain is dominated by the highly convoluted cerebral hemispheres, with the brain stem protruding from below, bearing the cerebellum on its back. The axis of each cerebral hemisphere, from the frontal pole, back to the occipital pole, and then down and around to the temporal pole, forms a C-shape — a reminder of the folding process that occurs during embryological development. Each hemisphere is divided into four lobes. On the surface of the lobes are variously named convolutions or gyri, with fissures, or sulci, some of them very deep, separating the gyri (see Fig. 1). The exact pattern of fissures varies enormously from brain to brain, and even between the two hemispheres, but some are very distinctive. The lateral sulcus, one of the first to appear in the embryo, divides the frontal from the temporal lobe. Likewise, the central sulcus divides the frontal from the parietal lobe. The rearmost of the four lobes is the occipital lobe, but there is no sulcus to define its limit on the lateral surface. The two hemispheres, roughly mirror-images of each other, are separated by the huge Sylvian fissure described in 1660 by Franciscus Sylvius, a physician and anatomist in Leyden.

If a cut is made into the depth of the Sylvian fissure, dividing the brain in two, a complex series of structures is revealed on the inner surface of the hemisphere (Fig. 1b). Most apparent is the corpus callosum (Latin: ‘beam-like body’), a massive tract of nerve fibres (axons) connecting the 2 hemispheres. Like the side view of the entire hemisphere, the cut corpus callosum appears as an upside-down C-shape. So too does a smaller longitudinal fibre tract below it called the fornix (Latin for ‘arch’ — Roman prostitutes fornicated beneath the arches!). Just below this is a hole, leading into the cerebral ventricles. This interventricular foramen communicates between the third ventricle (a midline cavity with the thalamus in its wall) and the lateral ventricle, deep within the hemisphere. The third ventricle dips down between the hypothalamus of each side, below which we can see the pituitary gland. The thalamus joins to the brain stem below. The intricate folded pattern of the cerebellum fills most of the space between the bottom of the occipital lobe, above, and the upper surface of the brain stem, below. Beneath the cerebellum the tent-shaped fourth ventricle is visible, communicating at this level with the subarachnoid space around the brain. The fourth ventricle communicates with the third ventricle via a narrow tube, the cerebral aqueduct, which runs up through the midbrain.

The cerebral hemispheres consist of a thin outer rind of grey matter, containing mainly the bodies of nerve cells (neurons), surrounding a core of white matter (named after the whitish colour of the axons of the neurons). Deep within the hemispheres are a number of important cell groups (nuclei), as well as the ventricular system. Axons arising in the cerebral cortex and those running to it traverse the internal capsule, a thick band of white matter in each hemisphere. The largest of the deep nuclei is the corpus striatum (named because of its striped appearance when cut), which is of vital importance in integration of muscular action. Another mass of grey matter behind the corpus striatum is the thalamus, which lies in the walls of the third ventricle. It is a relay station for sensory and motor pathways on their way to the cerebral cortex. Just below the thalamus is the hypothalamus. Although small, it is one of the most important parts of the brain, for it participates in a number of vital activities. It regulates a variety of hormonal functions by direct action on the pituitary gland, and exerts control over the autonomic nervous system, the ‘vegetative’ part of the nervous system, which controls the involuntary activity of, for example, our gastrointestinal tract, heart, and blood vessels.

The hypothalamus is also an integral part of the limbic system (‘limbus’ is Latin for a border, and the limbic system forms an almost circular boundary to the inner surface of the cerebral hemisphere). The limbic system is involved in vital cyclical activity — including appetites and sexual cycles, and emotions such as fear, anger, and aggression — and in all-important short-term memory. It involves not only the hypothalamus but also the thalamus, part of the cerebral cortex called the hippocampus (Latin for ‘sea-horse’, because of its shape), and their interconnections. The hippocampus sends its axons backwards in the fornix, which then curves forward, like an arch, to meet the fornix of the other side, ending in the mamillary bodies of the hypothalamus. A tract then conveys axons up to the thalamus, which then sends fibres indirectly to the hippocampus again. So the circuit is completed.

The corpus striatum receives information from the cerebral cortex, the thalamus, and a nucleus called the substantia nigra (‘black substance’), in the midbrain. In Parkinson's disease, the cells in the substantial nigra that project to the corpus striatum degenerate and this leads to problems with motor control and co-ordination (muscle rigidity and tremor).

The cerebral cortex is one of the major features of the mammalian brain, and especially in humans it reaches a very high level of development. It is responsible for the initiation of movements, and for interpreting input from all our sensory systems, as well as for integrating motor and sensory activity necessary for speech and other cognitive functions. It is the seat of our very thoughts, personality, and character.

The cerebellum has on its surface a series of tight folds, called folia, similar to, but narrower than, the gyri of the cerebral cortex. The cerebellum consists mainly of two hemispheres that receive their major input from the spinal cord and the cerebral cortex. However, a small, but important, part receives information from the vestibular system, the apparatus in the inner ear that signals information about our position in space and, therefore, helps us balance ourselves. The cerebellum is responsible for unconscious control of motor activity. Although voluntary movement is thought to be initiated in the cerebral cortex, the cerebellum guides such movements. Further, it is involved in learning new skills of movement, often a painfully frustrating business. For instance, when we learn to drive a car, our initial attempts are clumsy and full of errors. We have to learn to co-ordinate movements of hand, eye, and foot in order to turn the key and to control gears, brake lever and accelerator, and clutch and brake peddles so that the vehicle is set in motion and safely stopped again. At first, the whole process demands huge mental effort, as if we were using our cerebral cortex consciously to call up the various movements and muscle groups we need. However, after many attempts, our efforts become smoother and less laborious, and we find that we are achieving the desired results with much less stalling of the motor or threat to the bodywork. Later still, we discover that we can drive around without really thinking about it much, and we are sometimes surprised, if distracted by other preoccupations, to realize that we are on the road and driving safely without clear memories of starting the vehicle and getting under way. We have successfully completed a motor ‘apprenticeship’, with the cerebellum taking over the routine management of the task from the cerebral cortex. It is as if the cerebellum were a programmable computer controlling the output of the motor system, and its programs have been slowly improved to take more and more change of the operation. Think of learning to play a sport or a musical instrument; but think also of walking, talking, and writing. In all these, and many more activities, we can look upon the first, hesitant steps as being essentially cortical, while the final, polished result is more cerebellar.

The brain stem extends between the thalamus and the spinal cord, gradually decreasing in size and in the complexity of its internal structure. It is divided, from top to bottom, into the midbrain, the pons (bridge), and the medulla oblongata (usually simply referred to as the medulla). The entire brain stem is largely hidden from view by the highly developed masses of the cerebral and cerebellar hemispheres. The midbrain is attached to the base of the cerebral hemispheres by the cerebral peduncles, two massive, flattened bundles of nerve fibres. The longitudinal orientation of the cerebral peduncles is abruptly interrupted by the pons, which gives the impression of a giant ring, slipped on to the brain stem between the peduncles and the medulla. The medulla merges gradually with the spinal cord.

The brain stem contains much white matter, with ascending and descending tracts that can be traced in continuity with those of the spinal cord, including various sensory pathways from the skin and organs, and the corticospinal or pyramidal tract, conveying motor information from the cerebral cortex down to the spinal cord. There are also various groups of neurons (nuclei) within the brain stem. Several of these give rise to the cranial nerves, through which the brain sends and receives information to and from the head and the organs of the trunk. Other groups of brain stem neurons are vital to the life of the body and to the conscious function of the brain: they generate the rhythmic nerve impulses that maintain breathing, regulate the heart and circulation, and activate the cerebral cortex itself.

Investigation of brain function

Compared with the pulsating heart or blood-filled liver, the brain looks rather unimpressive. No wonder, then, that many ancient cultures chose those other organs as their assumed seat of the mind or soul. Now that we generally accept that the brain is responsible for action, perception, and understanding, one of the greatest scientific challenges is to explain how it works.

The clues to the functions of the brain were once provided only by ‘nature's experiments’: the consequences of damage caused by disease or injury. The advent of anaesthesia allowed investigation of the effects in animals of more precisely localized damage and of the responses to electrical stimulation at particular sites. The development of microelectrode techniques made it possible to record the electrical activity of individual neurons in the brains of anaesthetized animals, or in isolated slices of brain tissue. The human brain has been stimulated during neurosurgery under local anaesthetic, and the resulting movements of the body and sensations described by the patient have identified particular regions concerned with motor and sensory function. Electrical activity can also be recorded from the human brain through electrodes on the scalp (electroencephography). Finally, new technologies developed in the twentieth century provided ways of ‘mapping’, non-invasively, the function of the living human brain. These imaging techniques can show, for example, the regional distribution of blood flow or metabolic activity, reflecting neuronal activity in the various parts of the brain during different actions or sensations. Thus they are assisting in the understanding of healthy function, as well as in the diagnostic localization of abnormalities.

Laurence Garey

See nervous system.See also brain stem; central nervous system; cerebral cortex; cerebral ventricles; cerebrospinal fluid; hypothalamus; imaging techniques; magnetic brain stimulation; thalamus.