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Your amazing body

The brain

The brain is the most complex organ in the human body. It is the control centre of life. It not only affects and governs everything you do, how you think, feel and act, but what kind of person you are.

The brain is also our most mysterious organ. The centre of human consciousness, and of subconsciousness, there is still much about it that baffles doctors, scientists and philosophers.

Proportionate to body weight, humans have the largest brains among mammals. No animal brain approaches the human brain's capacity for learning, language and thought. But the human brain is not just a super-computer. No computer can dream, fall in love or get bored.

• How the brain is constructed
• What the brain does
• How the body cares for the brain
• What can go wrong with the brain
• Looking after your brain

How the brain is constructed

The brain is far more complicated than anything ever made by humans even though it's only about the size and shape of an oval grapefruit.

Weighing a mere 1.4 kilos (three pounds), the brain can consume up to a third of the body's energy, depending on the demand you put upon it. At the same time, it demands the same proportion of the body's blood supply. That is why a mentally challenging office job can be just as tiring as physical work. Think hard and you could lose weight!

The brain is very soft, rather like an infinitely wrinkled jelly. It consists of an amazing mass of 100 billion nerve cells, known as neurons, and there are one million billion connections between them, as well as to muscles and glands and other tissue.

Neurons vary greatly but share three major features. Each has a centre (nucleus), an axon which transmits nerve impulses electrochemically to other cells and branches (dendrites) which receive them.

The brain is shaped like two fists side by side on a single wrist. The 'fists' are the left and right hemispheres of the largest part of the brain, the forebrain, or cerebrum. The 'wrist' is the brainstem, which connects the brain to the spinal column. At the back of the brain, below the cerebrum, is the cerebellum.

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Each hemisphere has an outer layer consisting of six layers of cells called the grey matter and known as the cerebral cortex. It is only millimetres thick but accounts for 40 percent of the entire brain mass because of the way the brain tissue is folded.

Beneath the cortex are tracts of nerve fibres forming the remaining 60 percent of the brain mass, this is known as the white matter. These tracts connect areas of the cortex to each other and to nerve centres in the centre of the forebrain and brainstem.

What the brain does

Although the cerebrum looks symmetrical, the two hemispheres differ in size, shape and function. The right hemisphere controls the left side of the body, and vice-versa. In most people, the left brain seems dominant in its control of language and logic, the right hemisphere in spatial perception, art, music and creative thought. The two hemispheres communicate via dense bundles of nerve fibres called the corpus callosum.

Deep inside the hemispheres are:

• basal ganglia which are the grey matter that signals movements to the muscles
• the thalamus acts as a relay centre for incoming sensory information
• the cherry-size hypothalamus controls heart beat, body temperature, thirst, appetite and aspects of the nervous system
• the pea-sized pituitary gland produces hormones that govern growth, sexual development and many other processes
• encircling the thalamuses are a number of nerve centres called the limbic system - this is thought to be involved in the handling of anxiety, fear and other emotions, some memory functions and the sense of smell

The surface of each hemisphere consists of a number of lobes. As the diagram tries to show, there are four main types:

• the frontal lobe is involved in intellect, organising, behaviour and emotions
• the occipital lobe at the back of the brain processes visual information
• two parietal lobes at the sides control touch, pressure and fine sensation such as judgment of texture, weight, size and shape
• two temporal lobes near the base distinguish smells and sounds, sort new information and are thought to be responsible for short-term memory

There are also 12 pairs of nerves which connect the cerebrum and the brainstem directly. These are the cranial nerves and they govern smell, vision, taste, hearing, breathing, the tongue and movements of the neck and back muscles.

The brainstem controls functions essential for survival such as breathing, digestion, heart rate, blood pressure, arousal and alertness. The cerebellum helps co-ordinate movement, muscles, balance and posture. Both brain stem and cerebellum work by reflex action - no thinking is involved. They receive sensory information such as heat or cold, pressure or pain, from receptors all over the body and transmit the appropriate physical responses. So blinking and standing up, for example, don't require conscious thought.

How the body cares for the brain

The brain is formidably protected in three key ways. First, of course, is the skull. Secondly, the brain is encased in three layers of membranes, the meninges. Cerebrospinal fluid circulating within them both nourishes the brain and cushions it against sudden movements or blows to the head.

Thirdly, the so-called blood-brain barrier, a membrane that separates the brain tissue from the blood whilst letting needed substances and oxygen through, ensures a controlled, consistent environment for the brain. This is why you can be anaesthetised or get drunk.

What can go wrong with the brain

Defects and disorders of the brain have much the same causes as disease in other body organs - for example infection, tumours and degeneration. But because the brain is so tightly packed inside the skull, anything which takes up space, like an abscess, a tumour or a blood clot raises the pressure inside the skull that impairs its function. Also, unlike other organs, brain cells lost through injury or disease cannot be replaced, so loss of function is harder or impossible to reverse.

The most common examples of brain problems include:

• congenital defects which can result in mental handicap, such as Down's syndrome
• impaired blood/oxygen supply (cerebral palsy and stroke)
• head injury (amnesia, coma, haemorrhage)
• infection (encephalitis, meningitis)
• tumours (malignant and non-malignant growths)
• degeneration (multiple sclerosis, Alzheimer's disease, Parkinson's disease)
• other disorders (migraine, epilepsy)
• psychiatric conditions (clinical depression, schizophrenia)

Looking after your brain

Like other organs, the brain does best if well nourished. Diets high in animal fats and low in fresh fruit and vegetables can lead to hardened arteries and strokes just as they can lead to heart attacks.

Exercise is important too. As with muscles, so with the brain: if you don't use it, you may lose it. And, of course, it suffers if poisoned so it is important not to smoke.

A good night's sleep is also essential. During sleep, the brain continues working, as we all know from dreams, but at the same time recharges and renews itself.

July 2001

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