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In most animals, the brain is located in the head. In vertebrates, the brain is protected by the bones of the skull. The meninges are the system of membranes that separate the skull from the brain. Although the brain is usually referred to as the supervisory center of vertebrates, the same term can also be used for the invertebrate central nervous system.
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2 Function 3 Study of the brain 4 Related topics |
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The vertebrate brain can be subdivided as follows:
The cerebrum and the cerebellum consist each of two halves (hemispheres). The corpus callosum connects the two hemispheres of the cerebrum.
In most vertebrates the metencephalon is the highest integration center in the brain, whereas in mammals this role has been adopted by the telencephalon. The cerebrum is the largest section of the mammalian brain and in humans, its surface has many deep fissures (sulci) and convolutions (gyri), giving a wrinkled appearance to the brain.
The adult human brain usually weighs about 1 - 1.5 kilograms in an average volume of 1,600 cubic centimetres.
The blood supply to the brain involves several arteries that enter the brain and communicate in a circle called the circle of Willis. Blood is then drained 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; a brain that weights 1500gms in air weighs only 50gms when suspended in CSF. (Livingston, 1965). The brain is easily damaged by compression, so the fluid surrounding the central nervous system must be maintained at a constant volume. 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 injuries and a pediatric disease know as hydrocephelus. Increased fluid pressure can result in permanant brain injury and death.
Fluid movement within the brain is limited by the blood-brain barrier, brain-cerebrospinal fluid barrier and the blood-cerebrospinal fluid barrier. See cephalic disorders for information on congenital brain disorders.
Vertebrate brains receive signals through nerves arriving from most portions of a body, interpret those signals and formulate reactions based on prior experiences and on physical needs. A similarly extensive nerve network delivers signals from a brain to control muscles throughout a body. Brains also produce hormones that can influence organs elsewhere in a body and brains react to hormones produced elsewhere in the body.
The human brain is the seat of cognitive processes related to perception, interpretation, imagination and memory, of which a person might or might not be aware. Older species with less complex brains also rely on brains for cognition. Beyond cognitive functions, the brain regulates autonomic processes related to essential body functions such as respiration and heartbeat.
Brains of complex species including humans typically receive signals at a brain stem where nerves arrive from throughout the body. From synapses in the brain stem, nerve signals are routed through a mid-brain section to appropriate areas of a cerebral cortex. Cognitive processing occurs largely in the cerebral cortex but relies on mid-brain and limbic functions as well, especially those of the thalamus and hippocampus.
The brain appears to have no homunculus, but instead derives consciousness from interaction among numerous systems within the brain. Executive decision-making functions rely on cerebral activities, especially those of the frontal lobes, but redundant and complementary processes within the brain result in a diffuse assignment of executive control that can be difficult to attribute to any single locale.
Mid-brain functions include routing, selecting, mapping and cataloguing information, including information perceived from the environment and information that is remembered and processed throughout the cerebral cortex. Endrocrine functions housed in the mid-brain play a leading role in modulating arousal of the cortex and of autonomic systems. Nerves from mid-brain areas regulating autonomic functions join nerves routing messages to and from the cerebrum in a bundle that passes through the spinal column to related parts of a vertebrate body. A few nerves, especially those inervating parts of the head, follow pathways outside the spinal cord.
Voluntary muscular movements are transmitted through neural networks of the cerebrum to basal ganglia, as part of a complex feedback system that regulates movement. Complex muscle movements such as a series of notes played on a keyboard or a series of footsteps can be initiated as a packaged concept from the cognitive functions of the cerebral cortex to be executed as a series of steps with the help of the cerebellum.
Hormones, sensory information, autonomic processes, and cognitive processes alike can exert strong influence on the regulation of brain activities. Stimulus from any source can trigger a general arousal process that decreases reliance on cortical processes, or that enhances and focuses cortical processes. Such diverse causes as hunger, fatigue, beliefs, unfamiliar information or actual threats can trigger an alert response that can exert control over cognitive priorities. Practice, planning and experience can all play strong roles in controlling cognitive and executive activities of the brain, among humans and among other species.
During many past millennia, the function of the brain was unknown. Ancient Egyptians threw the brain away prior to the process of mummification. Ancient thinkers such as Aristotle imagined that mental activity took place in the heart. The Alexandrian biologists Herophilus and Erasistratus were among the first to conclude that the brain was the seat of intelligence. Galen's theory that the brain's cavities, or ventricles, were the sites of thought and emotion prevailed until the work of the Renaissance anatomist Vesalius.
The brain is now studied in neurology and psychiatry, and known to be the organ responsible for the phenomena of consciousness, thought and emotion.
Studies of brain damage from accidents led to the identification of specialized areas of the brain devoted to functions such as the processing of seeing and hearing.
Brain imaging has allowed the function of the living brain to be studied without damaging the brain. For the first time, this has allowed the study of the neurophysiology of the brain to be studied in detail in a wide range of psychological tests. Functional neuroimaging (fMRI) allows researchers to monitor activities in a brain while they are happening.
A new discipline of cognitive science has started to fuse the results of these investigations with observations from psychology, philosophy and computer science.Function
Study of the brain