HOW TO WORK HUMAN BRAIN
Posted June 12, 2009on:
The human brain is the center of the human nervous system and is a highly complex organ. It has the same general structure as the brains of other mammals, but is over five times as large as the “average brain” of a mammal with the same body size. Most of the expansion comes from the cerebral cortex, a convoluted layer of neural tissue that covers the surface of the forebrain. Especially expanded are the frontal lobes, which are involved in executive functions such as self-control, planning, reasoning, and abstract thought. The portion of the brain devoted to vision is also greatly enlarged in humans.
Brain evolution, from the earliest shrewlike mammals through primates to hominids, is marked by a steady increase in encephalization, or the ratio of brain to body size. The human brain has been estimated to contain 50–100 billion (1011) neurons, of which about 10 billion (1010) are cortical pyramidal cells. These cells pass signals to each other via around 100 trillion (1014) synaptic connections.
In spite of the fact that it is protected by the thick bones of the skull, suspended in cerebrospinal fluid, and isolated from the bloodstream by the blood-brain barrier, the delicate nature of the human brain makes it susceptible to many types of damage and disease. The most common forms of physical damage are closed head injuries such as a blow to the head, a stroke, or poisoning by a wide variety of chemicals that can act as neurotoxins. Infection of the brain is rare because of the barriers that protect it, but is very serious when it occurs. More common are genetically based diseases, such as Parkinson’s disease, multiple sclerosis, and many others. A number of psychiatric conditions, such as schizophrenia and depression, are widely thought to be caused at least partially by brain dysfunctions, although the nature of the brain anomalies is not very well understood.
The adult human brain weighs on average about 3 lb (1.5 kg) with a size of around 1130 cubic centimetres (cc) in women and 1260 cc in men, although there is substantial individual variation. The brain is very soft, having a consistency similar to tofu. When alive, it is tan-gray on the outside and mostly yellow-white on the inside, with subtle variations in color. The photo on the right shows a horizontal slice of the head of an adult man, from the National Library of Medicine‘s Visible Human Project. In this project, two human cadavers (from a man and a woman) were frozen and then sliced into thin sections, which were individually photographed and digitized. The slice here is taken from a small distance below the top of the brain, and shows the cerebral cortex (the convoluted cellular layer on the outside) and the underlying white matter, which consists of myelinated fiber tracts traveling to and from the cerebral cortex. At the age of 20, a man has around 176,000 km and a woman, about 149,000 km of myelinated axons in their brains.