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The human brain is all white blood cell or just brain cells controls the central nervous system (CNS), by way of the cranial nerves and spinal cord, the peripheral nervous system (PNS) and regulates virtually all human activity. Involuntary, or "lower," actions, such as heart rate, respiration, and digestion, are unconsciously governed by the brain, specifically through the autonomic nervous system.
Complex, or "higher," mental activity, such as thought, reason, and abstraction, is consciously controlled.
The human brain is the most complex organ in the human body. It contains some one hundred billion neurons, which are capable of electrical and chemical communication with tens of thousands of other nerve cells. Nerve cells in turn rely on some quadrillion (1015) synaptic connections for their communications.
Anatomically, the brain can be divided into three parts: the forebrain, midbrain, and hindbrain; the forebrain includes the several lobes of the cerebral cortex that control higher functions, while the mid- and hindbrain are more involved with unconscious, autonomic functions. During encephalization, human brain mass increased beyond that of other species relative to body mass. This process was especially pronounced in the neocortex, a section of the brain involved with language and consciousness. The neocortex accounts for about 76% of the mass of the human brain; with a neocortex much larger than other animals, humans enjoy unique mental capacities despite having a neuroarchitecture similar to that of more primitive species. Basic systems that alert humans to stimuli, sense events in the environment, and maintain homeostasis are similar to those of basic vertebrates. Human consciousness is founded upon the extended capacity of the modern neocortex, as well as the greatly developed structures of the brain stem.
Sagittal slice from a MRI scan of a human brain. See an animated sequence of slices.
The normal adult human brain typically weighs between 1 and 1.5 kg (3 lb) and has an average volume of 1.6 litres. The mature human brain consumes some 20-25% of the energy used by the body, while the developing brain of an infant consumes around 60%. Such heavy energy usage generates large quantities of heat, which must be continually removed to prevent brain damage. Both sexes have similar brain weight to body weight ratios, though the differences in weight persist even when adjusted for relative body mass. Although no functional inference can be made from this statement, it does provide scientific proof that the weight for the brain of human males is larger and heavier than that of human females, usually by about 200 gramme (7 ounces). The human brain varies in many interesting ways between the sexes and may be an example of sexual dimorphism within the species (see also Sex and intelligence). It should also be noted that these differences in brain development and function may underlie the difference observed for weight and brain matter, and as such may not pertain directly to executive function since this faculty occurs by way of disparate neurophysiology for males and females.
The bulbous cerebral cortex is composed of convoluted grey matter internally supported by deep brain white matter. The two hemispheres of the brain are separated by a prominent central fissure and connect to each other at the corpus callosum. A well-developed cerebellum is found at the back of the brain. Brain stem structures are almost completely enveloped by the cerebellum and telencephalon, with the medulla oblongata projecting through the foramen magnum to merge with the spinal cord.
The blood supply to the brain involves the paired carotid arteries that enter the brain and communicate in the circle of Willis before branching out to their destinations. Further blood supply comes via the vertebral arteries. Blood drains from the brain through a network of sinuses that drain into the right and left internal jugular veins.
The brain is suspended in cerebrospinal fluid (CSF), which also fills spaces called ventricles inside it. The dense fluid protects the brain and spinal cord from shock. Fluid movement within the brain is limited by the blood-brain barrier and the blood-cerebrospinal fluid barrier.
The brain is easily damaged by compression, so the fluid surrounding the central nervous system must be maintained at constant pressure. Humans are estimated to produce about 500 ml or more of cerebrospinal fluid each day, with only about 15 percent of the body's estimated 150 ml of CSF at any given time located in the ventricles of the brain. The remainder fills the subarachnoid space, which separates the soft tissues of the brain and spinal cord from the hard surrounding bones (skull and vertebrae). Elevated levels of CSF are associated with traumatic brain injury and hydrocephalus. Increased fluid pressure can result in permanent brain injury and death.
Anatomical restraints prevent the human brain from getting even bigger. At birth, an infant's skull is as large as it can be without imperiling the lives of mothers and infants during childbirth. The difficulty experienced by humans in giving birth is nearly unique in the animal kingdom, requiring the head of the emerging infant to be rotated as it passes through the birth canal. Female humans possess large pelvic openings to accommodate the birth of large-headed offspring, but at the cost of thinning of the pelvic bones. Bones too thin can no longer bear the mother's weight or the mechanical stresses of walking and running, and a compromised ability to flee dangers may prevent the female from coming of childbearing age.
At birth, the human skull is rather soft, and it deforms somewhat during its passage through the birth canal, then recovers its shape. This allows it to expand to make room for the brain, which continues to grow, at the same rate as that of an unborn fetus, for an additional year. In all other animals the growth rate of the brain slows significantly at birth.
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