Each nerve cell in the brain can fire a very small electrical pulse, and does so according to inputs received from other nerve cells. Individual pulses are weak -- the entire brain only creates about 20 watts of power -- but each firing has to be sufficient to energize the next cell, or group of cells. Signal transmission from one cell to another is achieved chemically rather than electrically, with a chemical known as a neurotransmitter released at each nerve ending. The transmitter crosses the synapse (gap) to the next neuron and becomes lodged in a specific receptor site. This then activates a chain of events in the second neuron which causes it to fire and thus pass the impulse along. The passing of an impulse enables us to remember faces, think consciously and behave with intelligence.

Learning to walk not only requires appropriate nerve connections and musculature, but also balance. The organ of balance -- the vestibular apparatus or labyrinth -- exists within the inner ear. It consists of two distinct systems. Firstly, there are three semi-circular canals, each filled with fluid and each set in a different plane. All three act as a receptor system, with the contained fluid accelerating or decelerating, enabling the brain to detect and record head movement. Secondly, there are the saccule and the utricle. These also work in a mechanical fashion. They possess chalky particles (otoliths or ear dust) which respond to gravity. The particles come to rest upon sensitive hairs, enabling the organ's owner to know which way is up and which is down. These two systems permit balance to be achieved.

Teeth are formed from the skin, the enamel originating from the outer layer or epidermis and the inside of the tooth from the dermis. The milk (baby) teeth begin to form in the foetus at around seven weeks post-conception. Ten weeks later they are fully d eveloped in the jaw, and the buds of the permanent teeth have started to form behind them. The buds for the permanent teeth lie dormant until they start to grow in the sixth 

The coronary arteries are susceptible to the build-up of fatty deposits, a process that can start in childhood but becomes significant in middle and old age. Age also causes the artery walls to become thicker and less elastic -- a process known as arteriosclerosis. The fatty deposits accumulate cells and connective tissue and consolidate into hard plaques called atheroma, which partially block the artery. The combination of these two conditions is termed atherosclerosis, and is the precursor to heart attack and angina. Once the arteries around the heart narrow, there is a danger that they will become blocked. A piece of plaque is dislodged, further obstructing the flow, and a blood clot forms around it. Such a blockage is known medically as a myocardial infarction: in common terms, a heart attack.

Death and renewal are kept in balance by genetic instructions governing cell behavior. In cancer, the system of regulation breaks down, and cells begin to divide uncontrollably. Sometimes the process is rapid, and sometimes it may take 10, 20 or 30 years, but eventually the cancerous cells build up to form a lump known as a tumour or neoplasm (meaning 'new growth'). As the tumour grows, capillaries form around it to supply it with blood, probably in response to chemicals secreted by the tumour cells. Instead of performing a useful function, the cancer tissue greedily absorbs nutrients at the expense of the neighboring tissues. If the tumour is on a vital organ, disruption of the organ's functions can lead to death.

At six-weeks-old, all of the embryo's major organs are now present in rudimentary form. Its fingers are separated, but its toes are still only outlines. It has marked skeletal growth, but its skeleton is still cartilage, not bone. The embryo's complete intestinal tube is formed, and male/female differences are becoming visible.

For girls, by far the most significant event of puberty is menstruation, the rhythmic pattern of events that controls female fertility. At its root is the monthly ripening and release of an egg from the ovary. Ovulation is caused by a surge in the production of luteinising hormone (LH) by the pituitary gland in the brain. Levels of LH and follicle stimulating hormone (FSH) begin rising from the first day of menstruation. FSH causes a number of egg follicles to begin ripening, and in turn the ovary steps up its secretion of the hormone oestrogen. Two days before ovulation, the pituitary steps up it hormone output, and the ovary responds by switching some of its secretion from oestrogen to progesterone. At the same time, the most mature of the ripening egg follicles ruptures to release its ovum, while the rest of them die off.

Once in the vicinity of the egg, the sperm are chemically attracted to it. When they reach their destination, they must first penetrate the corona radiata before the outer membranes of their heads bind to special receptors in the egg's thick outer covering. Each sperm's store of protein-dissolving enzymes is then released, allowing its head to burrow downwards towards the egg cell itself, a journey taking about 30 minutes. Many sperm take part in this disrobing process, helping the eventual victor on its way. When the first sperm encounters the egg, the outer membranes of the sperm and egg fuse, and chemical changes in the egg rapidly prevent any more sperm from entering.

Linked to the production of adult odors at puberty is a change in the pattern of sweating. Humans have two types of sweat glands, each with very different functions. The eccrine glands are found all over the body and produce a faintly acid, watery fluid containing less than 2% solids. The evaporation of this fluid from the skin helps to keep the body cool. In contrast, the apocrine glands are located in the armpits, naval, ears, nipples and pubic zone. They secrete a thick, waxy, grayish substance that is diluted by fluid from other glands nearby. The apocrine glands do not become fully functional until puberty. They have no role in temperature regulation but are related to the scent glands of animals, and are thought to function solely for odor secretion. They are not stimulated by heat but by stress and emotion, including sexual excitement.

Humans and all mammals have what is known as double circulation, meaning that blood passes through the heart twice. On the first pass it is sent to the lungs to collect oxygen and get rid of carbon dioxide. From there it returns to the heart and is pumped at high pressure out into the arterial system. The arteries are elastic in order to cope with the surge of blood. As we age they become stiffer, causing the blood pressure to rise and making the heart work harder.
Brain PowerBrain Power

It is the human brain that has got our species where it is today. Together with a clever hand and the ability to speak, both of which need a good brain, it was the dramatic evolution of humanity's cerebral hemispheres that permitted this single species to become the most accommodating, wide-ranging, intelligent, vociferous, creative and potentially destructive species the world has ever known.

Three million years ago the biggest brain among our ancestors measured 450 cubic centimeters (equivalent to that of modern chimpanzees and gorillas). Average modern brains are closer to 1,400 cc for males and 1,250 cc for females. In proportional terms the smaller female brain forms 2.5 percent of the body, while the bigger male brain makes up 2 percent. Many animals have even better body/brain ratios: In some monkeys, it is closer to 5 percent. And various animals have bigger brains than humans — elephants (four times as big) and sperm whales (six times larger), for example.

But brain function is not dictated solely by size. Some intelligent humans have brains twice the weight of other intelligent humans. The human brain is still a fairly diminutive part of the body, despite its massive size increase over the course of evolution.

Size (and weight) of brain tissue is less important than the degree of convolution. Such folding, causing ridges and grooves, leads to a greater surface area, and this is highly relevant to the brain's ability. So too is neural organization, the interconnections between the millions of nerve cells within the 1.4 kilograms of human brain. These cannot be mapped in a living brain, and there is no way — yet — of discovering even in a dead brain where its former owner would have rated on the IQ scale.

The brain is divided into distinct structures, all with different roles. At the base of the brain, above the spinal cord, is the brain stem. This area controls involuntary movement, posture and all the subconscious tasks required for the general maintenance of life. Above the brain stem is the cerebrum, or forebrain, which includes the highly enlarged cerebral hemispheres.

These hemispheres are, in essence, where Homo sapiens resides. Most of the brain's appetite for oxygen is generated here: The hemispheres are 16 times more demanding, by weight, than any other tissue. They are separated from each other by the longitudinal fissure, linked only by a thick bundle of nerve fibers running from one side to the other. The cerebrum, in particular the cerebral hemispheres, is the part of the brain that most distinguishes humans from animals. It was this portion that enlarged so dramatically from early to modern humans. 
 

• have sex over 3,000 times, with perhaps eight different partners.
• dribble 154 quarts of saliva before its first birthday.
• crawl 93 miles before age 2.
• after age 2, learn a new word every two hours for the next 10 years.
• utter its first words at about 12 months after birth
• by age 6, master the essentials of language.
• by age 21, breathe over 3 1/2 million balloons full of air.
• fall in love twice.
• spend two weeks kissing.
• have two children and four grandchildren.
• be able to put names to around 2,000 faces, but only consider about 150 as friends.
• be able to distinguish over one million colors.
• produce 200 billion new red blood cells every day.
• grow until it contains ten thousand billion cells, nearly all of which will undergo a constant cycle of death and renewal.
• spend five solid years eating and drinking to supply fuel to its body.
• walk 14,000 miles.
• spend a year traveling in a car.
• spend nine years working for pay.
• spend 22 years sleeping.
• spend 12 years talking.
• blink 415 million times.
• produce 42,400 quarts of urine.
• spend over 12 years watching TV.
• spend 2 1/2 years on the telephone.
• grow 30 yards of fingernails.
• shed 40 pounds of dead skin. 

We all know we face this fate, but why? Why, when evolution has adapted us so perfectly for surviving and reproducing, do we suddenly find things beginning to go wrong? 

Most researchers would now argue that aging is not part of the evolutionary pattern at all, but completely outside it. Evolution is driven by natural selection, in which individuals whose genes give them some sort of advantage pass on those genes. The human race has never cultivated genes endowing health and strength in old age because it has never needed them. 

Evidence from the skeletal remains of ancient man, and in more recent times from historical records, suggests that for the overwhelming majority of human existence, life has been brutally short. We are the pioneers of old age, and nothing in our genes prepares us for it. Once they have seen us through development and reproduction, our genes abandon us to our fate.

Theories

Science has not yet come up with a universally accepted theory of aging, but with the increasingly rapid growth in our knowledge of molecular biology, it is not surprising that the search has become concentrated at the cellular level. 

Each cell carries the body's full genetic code in the DNA stored in its nucleus, and the DNA controls the cell's activities and responses. Every time a cell divides, the DNA must replicate itself to produce a perfect copy. To prevent errors — known as mutations — the DNA is constantly checked over by enzymes within the cell nucleus. When they encounter a mistake in the code, the checking enzymes summon repair enzymes, which cut out and replace the faulty portion. This system is efficient but not foolproof: Some mutations go undetected and are passed on when the cell divides. The damage repair system also becomes less effective as we grow older. The DNA being passed on to new cells contains more and more errors. Those errors are manifested in the macroscopic world as the effects of aging.

One of the major causes of DNA damage is free radicals, highly reactive molecules produced during normal metabolic reactions involving oxygen. To neutralize them, the cell contains various compounds known as antioxidants, among them the vitamins C and E and various chemicals found in fruits and vegetables. Nevertheless, free radical damage does occur and is thought to be one of the mechanisms of aging. It has been found that humans, the longest-lived of all animals, have much higher levels of antioxidants than shorter-lived species. 

The End of Life 

If asked how we would like to die, many of us would reply "from old age." Instead of suffering disease prior to death, we would like to remain reasonably healthy until one day our body stops working, preferably in our sleep. But does anyone really die of old age, or is there always an underlying disease? 

At the cellular level, death is integral to life. Every day 2 percent of our blood cells die and are replaced by fresh ones. The cells lining the gut die off and are renewed every third day, and we are constantly shedding dead skin cells. Bone cells are in perpetual turnover, so that our whole skeleton is replaced every seven years.

Cells also die without being replaced. By the second half of our lives we are losing thousands of brain cells every day, the brain losing around 2 percent of its weight every decade after 50. Cells in the cardiac muscle also die off, and the sino-atrial node, the heart's pacemaker, can lose 90 percent of its cells by age 75. Therefore, to some extent death is a process, with the final death of the whole as its culmination. 

Oxygen — the vital ingredient

The single crucial factor in all death, whatever the malfunction that leads to it, is lack of oxygen. Whether the lungs fail to obtain it, the heart stops pumping the blood that carries it, or the brain can no longer coordinate its delivery, if our tissues stop receiving oxygen we are dead. As circulation, respiration and neuronal control are intimately linked, failure in one quickly sets up a vicious circle that drags down the others. 

Diagnosing Death

Death is the ultimate finality, yet it has no simple definition or diagnosis. Until quite recently someone was pronounced dead once they seemed to have stopped breathing. When the stethoscope was invented the emphasis switched to the stopping of the heartbeat, but the diagnosis of death was still not foolproof. Conditions such as hypothermia, some drug overdoses, metabolic disturbances and coma can mimic death, and even today stories surface from time to time of mortuary attendants noticing a twitch from a supposed cadaver. The majority of deaths though are plain to see and involve no doubt. 

The Final Moments

When death is very sudden, the body may be thrown in a matter of seconds from excellent health to a state in which life can no longer be sustained. However, most deaths come more slowly and involve a gradual failure of bodily systems. Close to death the breathing becomes irregular, with very quick, deep breaths being followed by very slow ones. This is called Cheyne-Stokes respiration and is caused by the receptors near the heart and in the brain stem responding sluggishly to changes in oxygen and carbon dioxide in the blood. 

Breaths may be noisy because the person is too weak to cough and clear the mucus from the throat and airways. High blood pressure can force fluid from the blood through the capillary walls of the lungs, which become congested. The last few breaths at the moment of death often feature a rasping sound known as the death rattle, caused by spasms in the muscles of the vocal cords. Occasionally a violent tightening of the laryngeal muscles will produce a sudden barking sound. 

Immediately before death there is often a short period termed the agonal phase (from the Greek agon, meaning struggle). The muscles may jerk in spasms caused by blood acidity, and sometimes there will be a brief convulsion or heaving of the chest or shoulders. Breaths may come in enormous gasps. Death agonies are distressing for observers, but the dying person is usually too ill to be aware of them. 

At death the pupils become dilated as the muscles that control the iris relax for the last time. Occasionally the eye sheds a terminal tear, known as the lacrima mortis. 

Glimpses Beyond Death

No one can know what it is like to die, but a few individuals claim to have caught a premature glimpse. "Near death experiences," a term coined in 1975, are one result of the modern ability to snatch patients back from the brink of what, until recently, would have been death. Today, some 30 to 40 percent of those "brought back to life" by medical intervention have reported NDEs. 

Their most common experience is a sense of peace while traveling down a tunnel toward a bright light. This joy is coupled with a choice: either return to ordinary life or stay in the lovely world to which they are heading. 

There have been too many such recollections with too many similarities for them to be dismissed out of hand. There does appear to be a consistent set of events perceived by some people who come close to death; the debate is over what causes them. Many who have had NDEs believe that they prove the existence of some kind of life after death, of a continuing consciousness that is independent of the body. There is often a profound change in survivors' values and beliefs, and sometimes a religious conversion. The scientific explanation is that NDEs are the brain's response to the physiological crisis at hand. 

Mankind has many cultural and religious beliefs about an immortal "soul," which is either reincarnated or journeys to an afterlife. The only certain form of immortality is the genetic inheritance that we received from our ancestors and may or may not pass on to descendants. 
 

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Thanx: Discovery.com 
Copyright ©1999 Discovery Communications Inc.
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