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The Brain

The Brain A.M.D.G 27th October 1996 The Brain By Manuel Socarrs In the central nervous system of animals, the brain is a segregated group of nerve cells, or neurones, within the cranium, or skull, in vertebrates, and within the head segment in lower forms of animals. The brain varies in size and complexity from rudimentary ganglia (a group of nerve-cell bodies) in the central nervous systems of primitive worms to the large and complex human brain. As the central control organ of the body, the brain governs the functioning of the body’s other organs. Sensory nerve cells feed external and internal information from all parts of the body to the brain. At least four medical subspecialties have a primary concern with the human brain: neurology, psychobiology, neurosurgery, and psychiatry.

The average human brain at birth weighs 390 g (14 oz); its average maximum weight, reached at age 15, is 1,315 g (46 oz). The total number of neurones in the human brain is approximately 10 billion. In contrast, the brain of a whale may weigh more than 5 kg (11 lb.), four times as much as a human brain, and the brain of a grasshopper contains no more than a few thousand neurones. A theory has been proposed which states that behavioural capacity, a broad term indicating intelligence, is related not to the size of brain but to the index of cephalisation–the amount of brain tissue in excess of that required for transmitting impulses to and from the brain. Studies have shown that a progressive evolutionary encephalisation relative to body size occurs in vertebrates and culminates in humans.

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Of equal importance to encephalisation has been the evolutionary development of the human forebrain, a greatly expanded and convoluted mantle containing neuronal centres necessary for understanding and producing language, for conceptualisation and abstraction, for judgement, and for the capacity of humans to contemplate and influence their lives. ANATOMY AND FUNCTION Localisation of function is defined by two investigative techniques: ablation and stimulation. Ablation, the removal of a small area of the brain, may result in a neurological deficit that is considered in terms of a lost function, one that is assigned to the area of the brain that was ablated. In humans, correlation of the indications of brain dysfunction during life with lesions of the brain found at autopsy has added greatly to the Human Brain The human brain has three major structural components: the large dome-shaped cerebrum (top), the smaller somewhat spherical cerebellum (lower right), and the brainstem (centre). Prominent in the brainstem are the medulla oblongata (the egg-shaped enlargement at centre); and the thalamus (between the medulla and the cerebrum).

The cerebrum is responsible for intelligence and reasoning. The cerebellum helps to maintain balance and posture. The medulla is involved in maintaining involuntary functions such as respiration, and the thalamus acts as a relay centre for electrical impulses travelling to and from the cerebral cortex. knowledge of localised functions. Movements and sensations can be produced in conscious humans by stimulating the brain electrically in appropriate areas.

A portion of the skull may be removed (a craniotomy) under local anaesthesia, giving access to the brain in a conscious patient. Gray and white matter may be cut, stimulated, or cauterised without the patient experiencing pain. Apparently no nerve endings sensitive to pain exist within the brain substance. Large blood vessels supplying the brain and certain sensory nerves attached to the brain stem are pain sensitive. Brain Stem The most notable structures of the adult human brain are the brain stem, the cerebellum, and the cerebrum (cerebral hemispheres).

The lowest, or most caudal, portion of the brain stem, the medulla oblongata, is continuous with the spinal cord. Above it, the pons bulges prominently and is continuous with the midbrain, into which the cerebral peduncles (stem-like connections) extend. The cerebral peduncles carry upper motor neurone fibres that originate in the cerebral cortex to the cell bodies of cranial nerves in the brain stem and to cells in the spinal cord, called lower motor neurones, which cause certain muscles to move. The brain stem contains all afferent and efferent nerve fibres between the spinal cord and the higher brain centres. Some upper motor neurone fibres cross in the brain stem, whereas others do not. Most cranial nerves to each side of the head connect evenly with both cerebral hemispheres. In contrast, 80 percent of the fibres in the corticospinal tract from each side cross over in the medulla oblongata, so that the affected muscles are primarily controlled by one hemisphere.

Thus a unilateral (single-sided) lesion in the cerebrum or brain stem above this crossover causes weakness or paralysis on the opposite side of the body. Cranial Nerves The human brain has 12 paired cranial nerves. The first 2, the olfactory and optic nerves, enter the brain above the brain stem and are actually extensions of the brain rather than peripheral nerves. Afferent impulses originating in peripheral sensory organs form ascending fibre tracts that cross either in the spinal cord or in the brain stem on their way to synapsing in nuclei above. The largest of these nuclei is the thalamus, whose many subdivisions act as relay stations to the cerebral cortex.

Some thalamic nuclei project to specific receiving areas of the cerebral cortex, the primary cortical areas. Different afferent tracts and thalamic Functions of the Cerebral Cortex Many motor and sensory functions have been mapped to specific areas of the cerebral cortex, some of which are indicated here. In general, these areas exist in both hemispheres of the cerebrum, each serving the opposite side of the body. Less well defined are the areas of association, located mainly in the frontal cortex, operative in functions of thought and emotion and responsible for linking input from different senses. The areas of language are an exception: both Wernickes area, concerned with the comprehension of spoken language, and Brocas area, governing the production of speech, have been pinpointed on the cortex. areas subserve sensations of pain; sensations of heat and cold; proprioception (the sensation of movement at joints); and special senses of sight, taste, and hearing.

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