Monday, December 25, 2006

87)The seat of sequential, discursive intellect in man.

Dec 19th 2006
From The Economist print edition

A history and geography of the brain

THE reason that people have brains is that they are worms. This is not a value judgment but a biological observation. Some animals, such as jellyfish and sea urchins, are radially symmetrical. Others are bilaterally symmetrical, which means they are long, thin and have heads.

Headless animals have no need for brains. But in those with a head the nerve cells responsible for it—and thus for sensing and feeding—tend to boss the others around. That still happens even when a long, thin animal evolves limbs and a skeleton. Bilateralism equals braininess.

A healthy human brain contains about 100 billion nerve cells. What makes nerve cells special is that they have long filamentary projections called axons and dendrites which carry information around in the form of electrical pulses. Dendrites carry signals into the cell. Axons carry signals to other cells. The junction between an axon and a dendrite is called a synapse.

Information is carried across synapses not by electrical pulses but by chemical messengers called neurotransmitters. One way of classifying nerve cells is by the neurotransmitters they employ. Workaday nerve cells use molecules called glutamic acid and gamma aminobutyric acid. More specialised cells use dopamine, serotonin, acetylcholine and a variety of other molecules. Dopamine cells, for example, are involved in the brain's reward systems, generating feelings of pleasure.

Many brain drugs, both therapeutic and recreational, work either by mimicking neurotransmitters or altering their activity. Heroin mimics a group of molecules called endogenous opioids. Nicotine mimics acetylcholine. Prozac promotes the activity of serotonin. And cocaine boosts the effect of dopamine, which is one reason why it is so addictive.

Apart from specialised nerve cells, there is a lot of anatomical specialisation in the brain itself. Three large structures stand out: the cerebrum, the cerebellum and the brain stem. In addition, there is a cluster of smaller structures in the middle. These are loosely grouped into the limbic system and the basal ganglia, although not everyone agrees what is what.

Most brain structures, reflecting the bilateral nature of brainy organisms, are paired. In particular, the cerebrum is divided into two hemispheres whose only direct connection is through three bundles of nerves, the most important of which is called the corpus callosum. (Many parts of the brain have obscure Latin names.)

This anatomical division of the brain reflects its evolutionary history. The brains of reptiles correspond more or less to the structures known in mammals as the brain stem and the cerebellum. In mammals, the brain stem is specialised for keeping the heart and lungs working. The cerebellum is for movement, posture and learning processes associated with these two things. It is the limbic system, basal ganglia and cerebrum that do the interesting stuff that distinguishes mammalian brains from those of their reptilian ancestors.

The limbic system is itself divided. Some of the main parts are the hippocampus, the amygdala, the thalamus and the hypothalamus. The largest of the basal ganglia is the caudate. The pineal gland, which lies behind the limbic system, is the only brain structure that does not come in pairs. The 17th-century French philosopher René Descartes thought it was the seat of the human soul.

Descartes, however, was wrong. It is in fact the cerebrum's outer layer, the cerebral cortex, that is man's true distinguishing feature. The cerebral cortex forms 80% of the mass of a human brain, compared with 30% of a rat's. It is divided into lobes, four on each side. The rearmost one, called the occipital, handles vision. Then come the parietal and temporal lobes, which deal with the other senses and with movement. At the front, as you would expect, is the frontal lobe.

This is humanity's “killer app”, containing many of the cognitive functions associated with human-ness (although that most characteristic human function, language, is located in the temporal and parietal lobes, and only on one side, usually the left). Man's huge frontal lobes are the reason for the species' peculiarly shaped head. No wonder that in English-speaking countries the brainiest of the species are known as “highbrow”.


Islam, eminently logical, placing the greatest emphasis on knowledge, purports to understand God's creation:Aga Khan 4.
The God of the Quran is the One whose Ayats(Signs) are the Universe in which we live, move and have our being:Aga Khan 3