HUMAN BRAIN

The human brain is the center of the human nervous system. Enclosed in the cranium, the human brain has the same general structure as that of other mammals, but is over three times larger than the brain of a typical mammal with an equivalent body size.[1] Most of the spatial expansion comes from the cerebral cortex, a convoluted layer of neural tissue which covers the surface of the forebrain. Especially expanded are the frontal lobes, which are associated with executive functions such as self-control, planning, reasoning, and abstract thought. The portion of the brain devoted to vision, the occipital lobe, is also greatly enlarged in human beings.

Brain evolution, from the earliest shrew-like mammals through primates to hominids, is marked by a steady increase in encephalization, or the ratio of brain to body size. Estimates vary for the number of neuronal and non-neuronal cells contained in the brain, ranging from 80 or 90 billion (~85 109) non-neuronal cells (glial cells) and an approximately equal number of (~86 109) neurons,[2] of which about 10 billion (1010) are cortical pyramidal cells, to over 120 billion neuronal cells, with an approximately equal number of non-neuronal cells[3]. These cells pass signals to each other via as many as 1000 trillion (1015, 1 quadrillion) synaptic connections.[4] Due to evolution and synaptic pruning, however, the modern human brain has been shrinking over the past 28,000 years.[5][6]

The brain monitors and regulates the body's actions and reactions. It continuously receives sensory information, and rapidly analyzes this data and then responds accordingly by controlling bodily actions and functions. The brainstem controls breathing, heart rate, and other autonomic processes that are independent of conscious brain functions. The neocortex is the center of higher-order thinking, learning, and memory. The cerebellum is responsible for the body's balance, posture, and the coordination of movement.

Despite being protected by the thick bones of the skull, suspended in cerebrospinal fluid, and isolated from the bloodstream by the blood-brain barrier, the human brain is 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, though serious, is rare due to the biological barriers which protect it. The human brain is also susceptible to degenerative disorders, such as Parkinson's disease, multiple sclerosis, and Alzheimer's disease. A number of psychiatric conditions, such as schizophrenia and depression, are widely thought to be associated with brain dysfunctions, although the nature of such brain anomalies is not well understood.