In the anatomy of animals, the brain, or encephalon, is the supervisory center of the nervous system. Although the brain is usually cited as the supervisory center of vertebrate nervous systems, the same term can also be used for the invertebrate central nervous system.
- For other articles about other subjects named brain see brain (disambiguation).
The chordate brain controls and coordinates most movement, behavior and homeostatic body functions such as heartbeat, blood pressure, fluid balance and body temperature. Functions of the brain are responsible for cognition, emotion, memory, motor learning and other sorts of learning.
In most animals, the brain is located in the head. In vertebrates, the brain is protected by the bones of the skull. The brains of vertebrates develop from the anterior section of a dorsal nerve cord. Small invertebrates such as insects may have a million neurons in the brain, larger vertebrate brains have as many as a million billion neurons.
Along the phylogenic scale three distinctive regions emerged in the chordate neural scheme. Sensory faculties organized around the regions. Olfactory senses were associated with the forebrain, visual senses with the midbrain, and other senses developed pathways in the hindbrain. Grey matter developed from each of the sections forming the cerebrum, the tectum, and the cerebellum.
Sometimes a gross division into three major parts is used: hindbrain (medulla oblongata and metencephalon), midbrain (mesencephalon) and forebrain (diencephalon and telencephalon). The human brain can be classified as having a cerebrum, comprising the cerebral hemispheres, a brainstem, and a hindbrain comprising the cerebellum. Varied taxonomies have been used by assorted schools at various times in history for the study of diverse species.
Vertebrate nervous systems are distinguished by encephalization and bilateral symmetry. Larger vertebrates develop a complex of layered, networked and convoluted grey matter and white matter. In modern species most closely related to the first vertebrates, brains are covered with three layers of gray matter, along with gray deep brain nucleus, supported and interconnected by deep brain white matter. Most regions of the human cerebral cortex are covered with six layers of gray fibers.
The meninges are the system of membranes that separate the skull from the brain. The three-layered covering is made of dura mater, arachnoid and pia mater. The brain is suspended in cerebrospinal fluid.
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 cerebrum and the cerebellum consist each of two halves (hemispheres). The corpus callosum connects the two hemispheres of the cerebrum.
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 produce hormones that can influence organs elsewhere in a body and brains react to hormones produced elsewhere in the body.
Brains receive signals at nerve complexes near the middle of the brain. Optic sensory pathways of vertebrates affiliate with the brain at the mesencephalon and olfactory pathways are routed to a telencephalon. From synapses in the brain stem, nerve signals from other peripheral sensory area are routed through a mid-brain section to related areas of a cerebral cortex.
Developed brains derive consciousness from interaction among numerous systems within the brain. Cognitive processing in mammals occurs in the cerebral cortex but relies on mid-brain and limbic functions as well, especially those of the thalamus and hippocampus. Among vertebrates, sensory processing involves progressively rostral regions of the brain among newer species.
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.
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