The human brain is the center of the human nervous system and is a highly complex organ. 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 human brain has been estimated to contain 50-100 billion neurons, of which about 10 billion are cortical pyramidal cells. These cells pass signals to each other via approximately 100 trillion 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 such brain anomalies is not well understood.
The adult human brain weighs on average about 3 lb (1.5 kg) with a size of around 1130 cubic centimetres (cm3) in women and 1260 cm3 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.