In animals, the brain, or encephalon (Greek for “in the head”), is the control center of the central nervous system. In most animals, the brain is located in the head, protected by the skull and close to the primary sensory apparatus of vision, hearing, taste and olfaction. In humans, it is an organ of thought. While all vertebrates have a brain, invertebrates have either a centralized brain or collections of individual ganglia. Brains can be extremely complex. For example, the human brain contains more than 100 billion neurons, each linked to as many as 10,000 others.
Most brains exhibit a substantial distinction between grey matter and white matter. Gray matter consists of the cell bodies of the neurons, while white matter consists of the fibers (axons) that connect neurons. The axons are surrounded by a fatty insulating sheath called myelin, giving the white matter its distinctive color. The outer layer of the brain is gray matter called cerebral cortex. Deep in the brain, compartments of white matter (fasciculi, fiber tracts), gray matter (nuclei) and spaces filled with cerebrospinal fluid (ventricles) are found.
The brain innervates the head through cranial nerves, and it communicates with the spinal cord, which innervates the body through spinal nerves. Nervous fibers transmitting signal from the brain are called efferent fibers. The fibers transmitting signals to the brain are called afferent (or sensory) fibs. Nerves can be afferent, efferent or mixed (i.e., containing both types of fibers).
The brain controls a wide variety of functions. It is the site of reason and intelligence, which include such components as cognition, perception, attention, memory and emotion. The brain is also responsible for control of posture and movements. It makes possible cognitive, motor and other forms of learning. The brain can perform a variety of functions automatically, without the need for conscious awareness, such as coordination of sensory systems (eg. sensory gating and multisensory integration), walking, and homeostatic body functions such as heart rate, blood pressure, fluid balance, and body temperature.
Many functions are controlled by coordinated activity of the brain and spinal cord. Moreover, some behaviors such as simple reflexes and basic locomotion, can be executed under spinal cord control alone.
The Brain and Sleep
The brain undergoes transitions from wakefulness to sleep (and subtypes of these states). These state transitions are crucially important for proper brain functioning. (For example, it is believed that sleep is important for knowledge consolidation). Each brain state is associated with characteristic brain waves.
Neurons are electrically active brain cells that process information, whereas Glial cells perform supporting function. Brain cell metabolism consumes considerable amounts of energy. In addition to being electrically active, neurons constantly synthesise neurotransmitters. Neurons modify their properties (guided by gene expression) under the influence of their input signals. This plasticity underlies learning and adaptation.
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