brain

brain,

the supervisory center of the nervous systemnervous system,
network of specialized tissue that controls actions and reactions of the body and its adjustment to the environment. Virtually all members of the animal kingdom have at least a rudimentary nervous system.
..... Click the link for more information. in all vertebrates. It also serves as the site of emotions, memory, self-awareness, and thought.

Anatomy and Function

Occupying the skullskull,
the skeletal structure of the head, composed of the facial and cranial bones. The skull houses and protects the brain and most of the chief sense organs; i.e., the eyes, ears, nose, and tongue.
..... Click the link for more information. cavity (cranium), the adult human brain normally weighs from 2 1-4 to 3 1-4 lb (1–1.5 kg). Differences in weight and size do not correlate with differences in mental ability; an elephant's brain weighs more than four times that of a human. In invertebrates a group of ganglia or even a single ganglion may serve as a rudimentary brain.

By means of electrochemical impulses the brain directly controls conscious or voluntary behavior, such as walking and thinking. It also monitors, through feedback circuitry, most involuntary behavior—connections with the autonomic nervous system enable the brain to adjust heartbeat, blood pressure, fluid balance, posture, and other functions—and influences automatic activities of the internal organs. There are no pain receptors in brain tissue. A headache is felt because of sensory impulses coming chiefly from the meninges or scalp.

Anatomically the brain has three major parts, the hindbrain (including the cerebellumcerebellum
, portion of the brain that coordinates movements of voluntary (skeletal) muscles. It contains about half of the brain's neurons, but these particular nerve cells are so small that the cerebellum accounts for only 10% of the brain's total weight.
..... Click the link for more information. and the brain stembrain stem,
lower part of the brain, adjoining and structurally continuous with the spinal cord. The upper segment of the human brain stem, the pons, contains nerve fibers that connect the two halves of the cerebellum.
..... Click the link for more information. ), the midbrain, and the forebrain (including the diencephalon and the cerebrum). Every brain area has an associated function, although many functions may involve a number of different areas. The cerebellum coordinates muscular movements and, along with the midbrain, monitors posture. The brain stem, which incorporates the medulla and the pons, monitors involuntary activities such as breathing and vomiting.

The thalamusthalamus
, mass of nerve cells centrally located in the brain just below the cerebrum and resembling a large egg in size and shape. The thalamus is a routing station for all incoming sensory impulses except those of smell, transmitting them to higher (cerebral) nerve centers.
..... Click the link for more information. , which forms the major part of the diencephalon, receives incoming sensory impulses and routes them to the appropriate higher centers. The hypothalamushypothalamus
, an important supervisory center in the brain, rich in ganglia, nerve fibers, and synaptic connections. It is composed of several sections called nuclei, each of which controls a specific function.
..... Click the link for more information. , occupying the rest of the diencephalon, regulates heartbeat, body temperature, and fluid balance. Above the thalamus extends the corpus callosum, a neuron-rich membrane connecting the two hemispheres of the cerebrum.

The cerebrum, occupying the topmost portion of the skull, is by far the largest sector of the brain. Split vertically into left and right hemispheres, it appears deeply fissured and grooved. Its upper surface, the cerebral cortex, contains most of the master controls of the body. In the cortex ultimate analysis of sensory data occurs, and motor impulses originate that initiate, reinforce, or inhibit the entire spectrum of muscle and gland activity. The parts of the cerebrum intercommunicate through association tracts consisting of connector neurons. Association neurons account for approximately half of the total number of nerve cells in the brain. The tracts are believed to be involved with reasoning, learning, and memory. The left half of the cerebrum controls the right side of the body; the right half controls the left side.

Other important parts of the brain include the pituitary glandpituitary gland,
small oval endocrine gland that lies at the base of the brain. It is sometimes called the master gland of the body because all the other endocrine glands depend on its secretions for stimulation (see endocrine system).
..... Click the link for more information. , the basal ganglia, and the reticular activating system (RAS). The pituitary participates in growth regulation. The basal ganglia, located just above the diencephalon in each cerebral hemisphere, handle coordination and habitual but acquired skills like chewing and playing the piano. The RAS forms a special system of nerve cells linking the medulla, pons, midbrain, and cerebral cortex. The RAS functions as a sentry. In a noisy crowd, for example, the RAS alerts a person when a friend speaks and enables that person to ignore other sounds.

Nerve fibers in the brain are sheathed in a near-white substance called myelin and form the white matter of the brain. Nerve cell bodies, which are not covered by myelin sheaths, form the gray matter. The billions of nerve cells in the brain are structurally supported by the hairlike filaments of glial cells. Smaller than nerve cells and ten times as numerous, the glia account for an estimated half of the brain's weight. Cranial blood vessels in the brain have certain selective permeability characteristics that largely constitute the "blood-brain barrier." The entire brain is enveloped in three protective sheets known as the meningesmeninges
, three membranous layers of connective tissue that envelop the brain and spinal cord (see nervous system). The outermost layer, or dura mater, is extremely tough and is fused with the membranous lining of the skull.
..... Click the link for more information. , continuations of the membranes that wrap the spinal cordspinal cord,
the part of the nervous system occupying the hollow interior (vertebral canal) of the series of vertebrae that form the spinal column, technically known as the vertebral column.
..... Click the link for more information. . The two inner sheets enclose a shock-absorbing cushion of cerebrospinal fluid.

Neural Pathways

Sensory nerve cells feed information to the brain from every part of the body, external and internal. The brain evaluates the data, then sends directives through the motor nerve cells to muscles and glands, causing them to take suitable action. Alternatively, the brain may inhibit action, as when a person tries not to laugh or cry, or it may simply store the information for later use. Both incoming information and outgoing commands traverse the brain and the rest of the nervous system in the form of electrochemical impulses.

The human brain consists of some 10 billion interconnected nerve cells with innumerable extensions. This interlacing of nerve fibers and their junctions allows a nerve impulse to follow any of a virtually unlimited number of pathways. The effect is to give humans a seemingly infinite variety of responses to sensory input, which may depend upon experience, mood, or any of numerous other factors. During both sleep and consciousness, the ceaseless electrochemical activity in the brain generates brain waves that can be electronically detected and recorded (see electroencephalographyelectroencephalography
, science of recording and analyzing the electrical activity of the brain. Electrodes, placed on or just under the scalp, are linked to an electroencephalograph, which is an amplifier connected to a mechanism that converts electrical impulses into the
..... Click the link for more information. ).

Research

Brain research, now often referred to as a part of neuropsychology, cognitive science, psychobiology, or other similar fields, has become much more active in recent years. Aided largely by advanced new imaging techniques such as MRI (magnetic resonancemagnetic resonance,
in physics and chemistry, phenomenon produced by simultaneously applying a steady magnetic field and electromagnetic radiation (usually radio waves) to a sample of atoms and then adjusting the frequency of the radiation and the strength of the magnetic field
..... Click the link for more information. imaging) and the PET scanPET scan
or positron emission tomography
, a medical imaging technique that monitors metabolic, or biochemical, activity in the brain and other organs by tracking the movement and concentration of a radioactive tracer injected into the bloodstream.
..... Click the link for more information. , neuroscientists have been better able to localize specific functions involving thought, language, perceiving, mental imaging, memory, and other abilities. Much more has been learned about the roles of neurotransmittersneurotransmitter,
chemical that transmits information across the junction (synapse) that separates one nerve cell (neuron) from another nerve cell or a muscle. Neurotransmitters are stored in the nerve cell's bulbous end (axon).
..... Click the link for more information. as well. New life has been given to the traditional philosophical debate on how to reconcile the seeming contradiction between the richness of subjective experience, including self-awareness, with purely scientific explanations of brain function.

Bibliography

See D. Dennett, Consciousness Explained (1991); J. A. Hobson, The Chemistry of Conscious States (1994); S. A. Greenfield, The Human Brain (1997); M. R. W. Dawson, Understanding Cognitive Science (1998); J. M. Allman, Evolving Brains (1999); V. X. Ramachandran, The Tell-Tale Brain (2011); R. Desalle and I. Tattersall, The Brain (2012); M. R. Trimble, The Soul in the Brain (2013); A. Jasanoff, The Biological Mind (2018).

Brain

A collection of specialized cells (neurons) in the head that regulates behavior as well as sensory and motor functions. The three main parts of the brain in vertebrates are the cerebrum, the cerebellum, and the brainstem that connects them with each other and with the spinal cord (see illustration). The two cerebral hemispheres are separated by a midline fissure that is bridged by a massive bundle of axons running in both directions, the corpus callosum. Each hemisphere has a core of groups of neurons (the basal ganglia); an outer shell of neurons in layers (the cerebral cortex); and massive bundles of axons for communication within the cerebrum and with the rest of the brain. These bundles are called white matter because of the waxy myelin sheaths surrounding the axons.

Midsagittal (midline, medial) section through the human brain

The basal ganglia comprises three main groups. (1) The thalamus receives axons from all sensory systems and transmits information to the cortex. It also receives feedback from cortical neurons during sensory processing. (2) The striatum, comprising bundles of axons cutting through the groups of neurons, also has two-way communication with the cortex and assists in the organization of body movement. (3) The hypothalamus receives orders from the cortex and organizes the chemical systems that support body movement. One output channel is hormonal, and controls the pituitary gland (hypophysis) which in turn controls the endocrine system. The other channel is neural, comprising axons coursing through the brainstem and spinal cord to the motor neurons of the autonomic nervous system, which regulates the heart, blood vessels, lungs, gastrointestinal tract, sex organs, and skin. The autonomic and endocrine systems are largely self regulating, but they are subject to control by the cortex through the hypothalamus. See Autonomic nervous system, Endocrine system (vertebrate), Neurobiology

The cortex is also called gray matter because it contains the axons, cell bodies, and dendrites of neurons but there is very little myelin. An index of the capacity of a brain is cortical surface area. In higher mammals, the cortical surface increases more rapidly than the volume during fetal development; as a result the surface folds, taking the form of convexities (gyri) and fissures (sulci) that vary in their details from one brain to another. However, they are sufficiently reliable to serve as landmarks on the cerebral hemisphere that it can be subdivided into lobes. Four lobes make up the shell of each hemisphere, namely the frontal, parietal, temporal, and occipital lobes. Each lobe contains a motor or sensory map (an orderly arrangement of cortical neurons associated with muscles and sensory receptors on the body surface). The central sulcus delimits the frontal and parietal lobes. The precentral gyrus contains the motor cortex whose neurons transmit signals to motor neurons in the brainstem and spinal cord which control the muscles in the feet, legs, trunk, arms, face, and tongue of the opposite side of the body. The number of neurons for each section is determined by the fineness of control, not the size of the muscle; for example, the lips and tongue have larger areas than the trunk. Within the postcentral gyrus is the primary somatosensory cortex. Sensory receptors in the skin, muscles, and joints send messages to the somatosensory cortical cells through relays in the spinal cord and the thalamus to a map of the opposite side of the body in parallel to the map in the motor cortex. The lateral fissure separates the temporal lobe from the parietal and frontal lobes. The cortex on the inferior border of the fissure receives input relayed through the thalamus from the ears to the primary auditory cortex. The occipital lobe receives thalamic input from the eyes and functions as the primary visual cortex.

In humans, the association cortex surrounds the primary sensory and motor areas that make up a small fraction of each lobe. The occipital lobe has many specialized areas for recognizing visual patterns of color, motion, and texture. The parietal cortex has areas that support perception of the body and its surrounding personal space. Its operation is manifested by the phenomenon of phantom limb, in which the perception of a missing limb persists for an amputee. Conversely, individuals with damage to these areas suffer from sensory neglect. The temporal cortex contains areas that provide recognition of faces and of rhythmic patterns, including those of speech, dance, and music. The frontal cortex provides the neural capabilities for constructing patterns of motor behavior and social behavior. It was the rapid enlargement of the frontal and temporal lobes in human evolution over the past half million years that supported the transcendence of humans over other species. This is where the capacity to create works of art, and also to anticipate pain and death, is located. Insight and foresight are both lost with bilateral frontal lobe damage, leading to reduced experience of anxiety, asocial behavior, and a disregard of consequences of actions.

A small part of frontal lobe output goes directly to motor neurons in the brainstem and spinal cord for fine control of motor activities, such as search movements by the eyes, head, and fingers, but most goes either to the striatum from which it is relayed to the thalamus and then back to the cortex, or to the brainstem from which it is sent to the cerebellum and then through the thalamus back to the cortex. In the cerebellum, the cortical messages are integrated with sensory input predominantly from the muscles, tendons, and joints, but also from the eyes and inner ears (for balance) to provide split-second timing for rapid and complex movements. The cerebellum also has a cortex and a core of nuclei to relay input and output. Their connections, along with those in the cerebral cortex, are subject to modification with learning in the formation of a working memory (the basis for learned skills). See Memory, Motor systems

The cerebellum and striatum do not set goals, initiate movements, store temporal sequences of sensory input, or provide orientation to the spatial environment. These functions are performed by parts of the cortex and striatum deep in the brain that constitute another loop, the limbic system. Its main site of entry is the entorhinal cortex, which receives input from all of the sensory cortices, including the olfactory system. The input from all the sensory cortices is combined and sent to the hippocampus, where it is integrated over time. Hippocampal output returns to the entorhinal cortex, which distributes the integrated sensory information to all of the sensory cortices, updates them, and prepares them to receive new sensory input. This new information also reaches the hypothalamus and part of the striatum (the amygdaloid nucleus) for regulating emotional behavior. Bilateral damage to the temporal lobe including the hippocampus results in loss of short-term memory. Damage to the amygdaloid nucleus can cause serious emotional impairment. The Papez circuit is formed by transmission from the hippocampus to the hypothalamus by the fornix, then to the thalamus, parietal lobe, and entorhinal cortex. The limbic system generates and issues goal-directed motor commands, with corollary discharge to the sensory systems that prepares them for the changes in sensory input caused by motor activity (for example, when one speaks and hears oneself, as distinct from another).

Each hemisphere has its own limbic, Papez, cortico-thalamic, cortico-striatal, and cortico-cerebellar loops, together with sensory and motor connections. When isolated by surgically severing the callosum, each hemisphere functions independently, as though two conscious persons occupied the same skull, but with differing levels of skills in abstract reasoning and language. The right brain (spatial)-left brain (linguistic) cognitive differences are largely due to preeminent development of the speech areas in the left hemisphere in most right- and left-handed persons. Injury to Broca's area (located in the frontal lobe) and Wernicke's area (located in the temporal lobe) leads to loss of the ability, respectively, to speak (motor aphasia) or to understand speech (sensory aphasia). Studies of blood flow show that brain activity during intellectual pursuits is scattered broadly over the four lobes in both hemispheres. See Aphasia, Central nervous system, Hemispheric laterality

Brain

the anterior section of the central nervous system in vertebrate animals and man, located in the cranial cavity. The brain is the material basis of higher nervous activity and the chief regulator of all life functions of the organism.

In invertebrate animals that have a central nervous system, brain function is performed by the cephalic ganglion, which is developed among higher insects and mollusks to such an extent that it is also called a brain.

The brain consists of the telencephalon (the cerebral hemispheres); the diencephalon, which includes the thalamus, hypothalamus, metathalamus, and epithalamus; the mesencephalon, which includes the cerebral peduncle and the corpora quadrigemina; the metencephalon, which consists of the pons and the cerebellum; and the medulla oblongata, which is a direct continuation of the spinal cord. All sections located between the spinal cord and the diencephalon form the brain stem; through it pass the afferent (centripetal, sensory) nerve fibers, which are directed from the spinal cord and the cranial nerves to the higher sections of the brain, and the efferent (centrifugal, motor) nerve fibers, which go in the reverse direction. The brain stem contains groups of specific afferent nerve cells (nuclei) that receive information from the skin and muscle receptors in the head region, as well as from other sense organs (hearing, equilibrium, taste). Also located in the brain stem are an aggregate of cells in the form of a structure known as the reticular formation, as well as a number of nerve centers that govern vitally important functions (for example, respiration, blood circulation, and digestion).

A primitive brain is already present in the lancelet. a forerunner of the vertebrates. In the vertebrate series the brain gradually becomes more complex and the divisions enumerated above are formed. The progressive complexifica-tion of the brain can be traced during embryonic development.

The brain attains its highest development in man. chiefly because of the enlargement and complexity of the structure of the two cerebral hemispheres, which are joined morphologically and functionally by a thick bundle of nerve fibers (the corpus callosum). On the average, the adult human brain weighs 1.470 g, its volume is 1,456 cm³ and its surface area 1,622 cm², so that in absolute figures the human brain is second only to those of the whale (6,000–7,000 g) and the elephant (5,700 g). The relative mass of the brain, however, according to the index of la. Ia. Roginskii, is highest in man (for man, 32; the dolphin. 16; the elephant, 10.4; and monkeys, 2–4). Enlargement of the surface of the cerebral hemisphere in man and the higher animals has occurred through an increase in the number of furrows and cerebral convolutions that form the lobes of the hemispheres (for example, the frontal, parietal, temporal, and occipital lobes, the insula, and the gyrus cinguli). The cerebral hemispheres of the brain consist of three principal elements. The first of these is a surface layer of gray matter called the cerebral cortex. The thickness of this layer in man is 1–5 mm. The number of its neurons totals approximately 14 billion; they are connected with one another and with other sections of the brain and spinal cord by afferent, efferent, and associative neural fibers. In the cortex, as well as in other brain structures, there are glial cells (neuroglia, or glia), which participate in the metabolic processes of neural tissue, perform a supportive function, and possibly play some sort of specific role in brain activity. The second major substance is the white matter, formed by neural fibers that travel to the brain from the periphery and to the periphery from the brain, as well as by fibers that connect the various areas of the cortex and both hemispheres. The third element consists of a number of subcortical ganglia (basal ganglia), embedded within the hemispheres (that is, in the midst of the white matter but consisting of gray matter); the most important of these ganglia are the corpus striatum and the globus pallidus.

The brain is covered by the dura mater, the arachnoidea encephali, and the pia mater, between which is found the cerebrospinal fluid (which also fills the cavities of the cerebral ventricles). The circulatory system of the brain and the cerebrospinal fluid serve as channels of transport for nutrients, oxygen, and other substances necessary for the life activities of the neurons. Decomposition products are removed from the brain along the same channels. The brain is extremely sensitive to oxygen insufficiency.

The brain may be considered, according to a number of anatomical and functional characteristics, as an aggregate of sensory systems. The receptors (nerve endings) of any afferent system receive stimuli, which are then sent in the form of nervous impulses along the centripetal neural pathways to the brain. Currents of nervous impulses carry information to the brain about the strength and nature of the stimuli received by the receptors of the sensory organs (for example, the eyes, ears, and skin) and all internal organs, muscles, and tendons. In the subcortical structures, then in the cortical sections of the analyzers, and ultimately by the whole cortex, this information is processed—that is, analyzed and synthesized. The brain then sends commands to the executive organs (efferent systems) about the character of responsive reaction to the stimuli.

The responsive reactions may be of two types—unconditioned reflexes or conditioned reflexes. Motor reflexes are primarily a function of the extrapyramidal system, which consists of the subcortical ganglia; the corpus striatum receives impulses from the thalamus and from the cortex and transmits them to the globus pallidus, whence they enter the nuclei of the brain stem and finally the motor neurons of the anterior horns of the spinal cord. In lower vertebrates (fishes, amphibians, and reptiles) this is the only system of motor coordination; in mammals there is, in addition, the pyramidal system, along which impulses from the cortex are transmitted directly to the motor neurons of the spinal cord. This system attains its highest level of development in the apes and man, and it provides the most complex conditioned-reflex and voluntary movements. The pyramidal system, being interconnected with the extrapyramidal, at this level of development takes over the leading role. Unconditioned autonomic reactions (for example, vascular, secretory, metabolic) are a function of the nerve centers of the thalamus, hypothalamus, and other structures of the brain stem. The cerebral cortex is connected with these structures as well; a number of autonomic conditioned reactions may therefore arise.

Normal operation of the brain is possible only at a certain level of irritability of its principal divisions. There are three paths for maintaining that level. The first is through the reticular formation of the brain stem, into which impulses enter along branches (collaterals) of the centripetal paths to the thalamus and from there to the appropriate regions of the cortex. After processing in the reticular formation, the nervous impluses lose the specific features of belonging to a particular analyzer and become nonspecific. These impulses are directed at the appropriate moment along ascending paths to all regions of the cerebral cortex, activating them and imparting a certain level of irritability (tonus). The second path for the maintenance of cortical tonus is through the sympathetic nervous system and the cerebellum. The third path is through specific pathways that lead away from the sense organs. Conditioned-reflex mechanisms may also take part in the maintenance of tonus. It is presumed that the higher vertebrates have some cortical self-regulation (including self-regulation of cortical tonus), which is especially well developed in man. Self-regulation of tonus is provided by bilateral links between the cortex and the reticular formation, and also between the sympathetic nervous system and the cerebellum. Intensive investigation is being done on the self-regulatory mechanisms of the brain that ensure those levels of higher nervous activity in man that are called thought and consciousness and are determined by the ability of the brain to receive, process, and store information and give out the results of the processing.

The limbic system plays an important role in brain activity. It is located on the interior surface of the cerebral hemispheres and deep within the lateral ventricles. It consists of the hippocampus, septum, amygdaloid bodies, pyriform and cingulate gyri, mamillary bodies, and fimbria; the thalamus and hypothalamus (and a number of other structures) are sometimes also considered part of its composition. It is postulated that the limbic system is related to certain kinds of memory and to the instinctive hereditary reactions that make for an inborn basis for the emotions. Disturbances of certain kinds of memory have been noted in humans where there is significant destruction of the hippocampus and amygdaloid nuclei. Patients remember events preceding this kind of operation, but if distracted they are unable to recall what they intended to do five or ten minutes before. Destruction of certain structures of the limbic system in animals is accompanied by the disruption of sequential activity; the animal, not yet having completed one movement, begins another. Electrical stimulation of the amygdaloid bodies, septum, and hypothalamus in monkeys induces pugnacity, aggressiveness, and increased sexual activity. Moreover, mutual relations between certain individuals in a group may change; a subordinate monkey may become dominant, and vice versa.

Despite significant progress in the study of brain function, for which science is greatly indebted to the classic works of I. M. Sechenov, I. P. Pavlov, V. M. Bekhterev, and C. Sherrington, the internal mechanisms of its integrative activity and coordination remain as yet unexplained. In this connection the structure and functions of the brain are undergoing intensive study in the laboratories and clinics of many countries by means of physiological, clinical, biochemical, biophysical, morphological, cybernetic, and other methods of investigation.

REFERENCES

Shmal’gauzen, I. I. Osnovy sravnitel’noi anatomii pozvonochnykh zhivotnykh, 4th ed. Moscow, 1947. Pages 225–76.
Orbeli, L. A. Voprosy vysshei nervnoi deiatelnosti. Moscow-Leningrad, 1949. Pages 397–419, 448–63.
Pavlov, I. P. Poln. sobr. soch., vol. 3, book 2. Moscow-Leningrad, 1951. Pages 320–44.
Bykov, K. M. “Kora golovnogo mozga i vnutrennie organy.” Izbr. proizv., vol. 2. Moscow, 1954. Pages 358–84.
Sechenov, I. M. Refleksy golovnogo mozga. Moscow, 1961.
Voronin, L. G. Kurs lektsii pofiziologii vysshei nervnoi deiatel’nosti. Moscow, 1965. Pages 225–59.
Fiziologiia cheloveka. Moscow, 1966. Chapter 15.
Prosser, C. L., and F. Brown. Sravnitel’ naia fiziologiia zhivotnykh. Moscow, 1967. (Translated from English.) Chapter 21.
Luriia, A. R. Vysshie korkovye funktsii cheloveka.... Moscow, 1969. Pages 7–80.

L. G. VORONIN

Brain

the central division of the nervous system in animals and man, which provides for the highly efficient regulation of all of the body’s vital functions (including higher nervous activity) and keeps the organism in balance with its environment. In man, the brain is the organ responsible for mental functions, including thought.

The evolution of animals has been accompanied by a concentration of nerve cells in ganglia connected by axial and transverse bridges (commissures and connectives). The diffuse neural network characteristic of coelenterates is succeeded by a ventral nerve cord (the ventral brain) in worms, mollusks, and arthropods. In many of these species, the anterior section of the cord, which is very well developed, is referred to as a brain. It consists of subesophageal and supraesophageal ganglia. In some mollusks and insects, the supraesophageal ganglion consists of several sections—the primary, secondary, and tertiary brains.

In vertebrates, including man, a distinction is made between the spinal cord, which is located in the vertebral canal, and the brain proper, which is situated in the skull. The brain proper is divided into the stem (medulla oblongata, pons, mesencephalon, diencephalon), the cerebellum, and the cerebrum (cerebral hemispheres), which is covered by the cortex.

The brain is covered by three meninges: the dura mater, the arachnoid, and the pia mater. The space between them, like the brain cavities, is filled with cerebrospinal fluid. The spinal cord is supplied with blood from the vertebral arteries and branches of the aorta; the brain receives blood from the two internal carotid and two vertebral arteries. The brain consists of gray matter (aggregations of nerve cells, or neurons) and white matter (aggregations of the neural processes). In addition to the neurons and their processes, the brain is made up of neuroglia—tissue that performs tropic, metabolic, barrier, reactive, and, possibly, secretory functions.

O. M. BENIUMOV

The evolutionary process that led to the appearance of man is very closely associated with the development of the brain. Therefore, the study of brain development in man’s extinct ancestors helps to throw light on his origin.

The various progressive structural features of the human brain were not equally pronounced in different periods of anthropogenesis. In the early stages, a comparatively large brain was found in creatures who externally resembled the higher apes. In Archanthropoi and Paleoanthropoi, the volume of the brain increased considerably. In addition, there was a marked qualitative reorganization and differentiation, especially in the associative zones—the anterior sections of the frontal lobe and the inferior parietal and posterotemporal subregions.

The progressive development of the associative zones paralleled the increasing complexity of the work done by primitive man. Natural selection preserved the most efficient combinations. The optimum combination was achieved in Neoanthropoi, in whom a relatively large brain was combined with a relatively very large telencephalon (cerebrum) dominated by a neocortex. The area of the neocortex was increased by a more complex pattern of sulci, gyri, and recesses.

The harmonious proliferation of associative zones led to the shaping of specifically human brain features. The large number of neurons, their high degree of specialization, and the complexity of their arrangement and contacts make for the brain’s reliability and high level of integration. The brain of modern man is characterized by broad intraspecific and individual variability, both in size and in the patterns of sulci and gyri. Racial differences are atypical.

M. S. VOINO

brain

[brān] (anatomy) The portion of the vertebrate central nervous system enclosed in the skull. (zoology) The enlarged anterior portion of the central nervous system in most bilaterally symmetrical animals.

brain

1. the soft convoluted mass of nervous tissue within the skull of vertebrates that is the controlling and coordinating centre of the nervous system and the seat of thought, memory, and emotion. It includes the cerebrum, brainstem, and cerebellum 2. the main neural bundle or ganglion of certain invertebrates

brain

[brān] that part of the central nervous system contained within the cranium, comprising the forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon); it develops from the embryonic neural tube. The brain is a mass of soft, spongy, pinkish gray nerve tissue that weighs about 1.2 kg in a human being. It is connected at its base with the spinal cord, which is also part of the central nervous system. Called also encephalon. (See also color plates.)
The brain is made up of billions of nerve cells, intricately connected with each other. It contains centers" >nerve centers (groups of neurons and their connections) which control many involuntary functions, such as circulation, temperature regulation, and respiration, and interpret sensory impressions received from the eyes, ears, and other sense organs. Consciousness, emotion, thought, and reasoning are functions of the brain. It also contains centers or areas for associative memory which allow for recording, recalling, and making use of past experiences.Cerebrum. The largest and main portion of the brain, the cerebrum is made up of an outer coating, or cortex" >cerebral cortex, consisting of matter" >gray matter, several cell layers deep, covering the hemispheres" >cerebral hemispheres. The cortex is the thinking and reasoning brain, the intellect, as well as the part of the brain that receives information from the senses and directs the conscious movements of the body.
In appearance the cortex is rather like a relief map, with one very deep valley (longitudinal fissure) dividing it lengthwise into symmetrical halves, and each of the halves again divided by two major valleys and many shallower folds. The longitudinal fissure runs from the brow to the back of the head, and deep within it is a bed of matted white fibers, the corpus callosum, which connects the left and right hemispheres" >cerebral hemispheres.
The major folds of the cortex divide each hemisphere into four sections or lobes: the lobe" >occipital lobe at the back of the skull, the lobe" >parietal lobe at the side, the lobe" >frontal lobe at the forehead, and the lobe" >temporal lobe at the temple. The Senses. The major senses of vision" >vision and hearing" >hearing have been well mapped in the cortex; the center for vision is at the back, in the occipital lobe, and the center for hearing is at the side, in the temporal lobe. Two other areas have been carefully explored, the sensory and motor areas for the body, which parallel each other along the fissure of Rolando. In the sensory strip are the brain cells that register all sensations, and in the motor strip are the nerves that control the voluntary muscles. In both, the parts of the body are represented in an orderly way.
It is in the sensory areas of the brain that all perception takes place. Here sweet and sour, hot and cold, and the form of an object held in the hand are recognized. Here are sorted out the sizes, colors, depth, and space relationships of what the eye sees, and the timbre, pitch, intensity, and harmony of what the ear hears. The significance of these perceptions is interpreted in the cortex and other parts of the brain. A face is not merely seen; it is recognized as familiar or interesting or attractive. Remembering takes place at the same time as perception, so that other faces seen in the past, or experiences linked to that face are called up. Emotions may also be stirred. For this type of association the cortex draws on other parts of the brain by way of the communicating network of nerves. Memory. In the temporal lobe, near the auditory area, is a center for memory" >memory. This center appears to be a storehouse where memories are filed. When this area alone is stimulated, a particular event, a piece of music, or an experience long forgotten or deeply buried is brought to the individual's mind, complete in every detail. This is a very mechanical type of memory; when the stimulation is removed the memory ends. When it is applied again, the memory begins again, not where it left off, but from the beginning. Brainstem. This is the stemlike portion of the brain connecting the cerebral hemispheres with the spinal cord, and comprising midbrain, pons, and medulla oblongata. Some consider it to include the diencephalon. Thalamus. This organ lies beneath the cortex, deep within the cerebral hemispheres. It is a relay station for body sensations, and integrates these sensations on their way to the cortex. The thalamus is an organ of crude consciousness and of sensations of rough contact and extreme temperatures, either hot or cold. It is principally here that pain is felt. In the thalamus, responses are of the all-or-nothing sort; even mild stimuli would be felt as acutely painful if they were not graded and modified by the cortex.Hypothalamus. This organ lies below the thalamus, at the base of the cerebrum. It is small (no larger than a lump of sugar), but takes part in such vital activities as the ebb and flow of the body's fluids and the regulation of metabolism, blood sugar levels, and body temperature. It directs the body's many rhythms, including those of activity and rest, appetite and digestion, sexual desire, and menstrual and reproductive cycles. The hypothalamus is also the body's emotional brain. It is the integrating center of the autonomic nervous system, with its sympathetic and parasympathetic branches, and is located close to the pituitary gland. Midbrain. Just below the thalamus is the short narrow pillar of the midbrain. This contains a center for visual reflexes, such as moving the head and eyes, as well as a sound-activated center, obsolete in humans, for pricking up the ears. Medulla Oblongata. Below the midbrain is the medulla oblongata, the continuation upward of the spinal cord. In the medulla, the great trunk nerves, both motor and sensory, cross over, left to right and right to left, producing the puzzling phenomenon by which the left cerebral hemisphere controls the right half of the body, while the right hemisphere controls the left half of the body. This portion of the brain also contains the centers that activate the heart, blood vessels, and respiratory system. Cerebellum. The cerebellum (Latin for “little brain”) is attached to the back of the brainstem, under the curve of the cerebrum. It is connected, by way of the midbrain, with the motor area of the cortex and with the spinal cord, as well as with the semicircular canals, the organs of balance. The function of the cerebellum is apparently to blend and coordinate motion of the various muscles involved in voluntary movements. It does not direct these movements; that is the function of the cortex. The cortex, however, operates in terms of movements, not of muscles. As a conscious function the cortex may, for example, direct the arm to pick up a glass of water; the cerebellum, which operates entirely below the level of consciousness, then translates this instruction into detailed actions by the 32 different muscles in the hand, plus several more in the arm and shoulder. When the cerebellum is injured, the patient's movements are jerky and uncoordinated.Cranial Nerves. These are twelve nerves that arise within the skull. All but the olfactory nerve emerge from the brainstem. Most, with the important exception of the vagus nerve, serve the head and neck. See also cranial nerves.Protection of the Brain. The brain is protected by the bony skull and by three layers of membranes, the meninges. Between the middle and inner layer is a space filled with cerebrospinal fluid, which serves as a shock absorber. The same system of membranes and fluid protects the spinal cord. The brain is protected from harmful substances in the bloodstream by a barrier called the blood-brain barrier, which keeps some of the substances out of the brain entirely and delays the entry of others for hours or even days after they have penetrated the rest of the body.

Projection areas of the brain.brain abscess a localized suppurative lesion within the intracranial cavity; most cases are secondary to middle ear infections. Other causes include compound fracture of the skull with contamination of brain tissue, sinusitis, and infections of the face, lung or heart. Symptoms include fever, malaise, irritability, severe headache, convulsions, vomiting, and other signs of intracranial hypertension. Treatment consists of surgical removal of the infected area and administration of antibiotics.brain death the irreversible cessation of all brain activity for an appropriate observation period, at least 24 hours, so that cardiopulmonary functions must be artificially maintained. A presidential commission in the USA accepted criteria for such a diagnosis, including cessation of all brain functions, including cerebral functions and brainstem (reflex) functions; irreversibility of the cessation; establishment of the cause of coma, sufficient to explain the loss of brain function; exclusion of possibility of recovery of brain function; and persistence of the cessation for an appropriate period of observation or trial of therapy. Complicating conditions must also be excluded. Called also irreversible coma.brain scanning a nuclear medicine procedure for the detection of brain tumors, areas of stroke syndrome, abscesses, hematomas, and other intracranial lesions. A radiopharmaceutical, such as ^99mTc-pertechnetate, is injected intravenously and is carried to the brain, where it localizes around any lesion that alters the blood-brain barrier. A scintillation camera makes an image of the distribution of radioactivity in which a lesion appears as a region of increased activity. tomography" >Computed tomography brain scanning is an alternative procedure, which is more effective than radionuclide scans for the detection of some lesions.brain tumor a neoplasm of the intracranial portion of the central nervous system. Any abnormal growth within the skull creates a special problem because it is in a confined space and will press on normal brain tissue and interfere with the functions of the body controlled by the affected parts. This is true whether the tumor itself is benign or malignant. Fortunately, the functions of certain areas of the brain are well known, and a disturbance of some specific function guides the clinician to the affected area. If diagnosed early, a benign tumor often can be removed surgically with a good chance of recovery. Malignant tumors are more difficult to remove. The causes of brain tumors are not known. They are not common, but they can occur at any age and in any part of the brain. Some originate in the brain itself, while others metastasize from a tumor in another part of the body.
The symptoms of brain tumor vary and depend on the location and size of the tumor. Headache together with nausea is sometimes the first sign. The headache can be generalized or localized in one part of the head, and the pain is usually intense. Vomiting can be significant if it is sudden and without nausea. Disturbances of vision, loss of coordination in movement, weakness, and stiffness on one side of the body are also possible symptoms. Loss of sight, hearing, taste, or smell may result from brain tumor. A tumor can also cause a distortion of any of these senses, such as seeing flashes at the sides of the field of vision, or smelling odors or hearing sounds that do not exist. It can affect the ability to speak clearly or to understand the speech of others. Varying degrees of weakness or paralysis in the arms or legs may appear. A tumor may cause convulsions. Changes in personality or mental ability are rare in cases of brain tumor. When such changes occur they may take the form of lapses of memory or absentmindedness, mental sluggishness, or loss of initiative.wet brain brain edema.

Brain

(brān), Walter Russell, English physician, 1895-1966. See: Brain reflex.

brain

(brān), [TA] That part of the central nervous system contained within the cranium.
See also: encephalon. Compare: cerebrum, cerebellum. [A.S. braegen]

brain

(brān)n.a. The portion of the vertebrate central nervous system that is enclosed within the cranium, continuous with the spinal cord, and composed of gray matter and white matter. It is the primary center for the regulation and control of bodily activities, receiving and interpreting sensory impulses, and transmitting information to the muscles and body organs. It is also the seat of consciousness, thought, memory, and emotion.b. A functionally similar portion of the invertebrate nervous system.

brain

The epicentre of the central nervous system, which is located within the cranial vault and divided into the right and left hemispheres. The brain functions as a primary receiver, organiser and distributor of information for the body; it is the centre of thought and emotion, co-ordinates and controls bodily activities and interprets sensory visual, auditory, olfactory, tactile and other information.

watershed infarct

Neurology Infarction of a region peripheral to 2 arteries and susceptible to ischemia; WIs are often hemorrhagic, as restoration of the circulation allows blood to flow into damaged capillaries and 'leak' into the ischemic tissue Watershed infarctions-locations Brain After internal carotid artery occlusion, causing vascular 'steal' phenomena, or between the anterior and middle cerebral arteries, which may be compromised in circle of Willis occlusions, often in a background of generalized atherosclerosis and as a possible complication of directed therapeutic embolization; cerebral perfusion may be impaired by cardiac arrest, pericardial tamponade and ex-sanguination Large intestine At either the splenic flexure, the site of anastomosis between the inferior and superior mesenteric arteries, or at the rectum, a region supplied by peripheral irrigation from the inferior mesenteric artery and the hypogastric artery

brain

(brān) [TA] That part of the central nervous system contained within the cranium.
See also: encephalon
Compare: cerebrum, cerebellum[A.S. braegen]

brain

(bran)

MAJOR ARTERIES OF THE BRAIN

BRAIN STRUCTURESA large highly-organized complex of neuron cell bodies, axons, dendrites, and glia filling the cavity inside the skull; the brain is enwrapped by fluid-filled protective membranes called meninges.

Anatomy

The brain is a soft, compact organ responsible for consciousness, planned neural programs, and cognition. The brain is continually active, and, although it is only 2% of the body’s mass, it receives 17% of the heart's output and consumes 20% of the body's oxygen supply. The brain receives its blood through four arteries: two large arteries, the right and left internal carotid arteries, run up from the chest in the front (anterior half) of the neck; and two smaller arteries, the right and left vertebral arteries, run in the back (posterior half) of the neck. The carotid arteries supply blood to about 80% of the brain, including most of the frontal, parietal, and temporal lobes of the cerebral hemispheres and the basal ganglia. The vertebral arteries supply blood to the remaining 20% of the brain, including the brainstem, cerebellum, and most of the posterior lobes of the cerebral hemispheres. See: illustration

The most basic divisions of the brain are (from rostral to caudal) the forebrain (prosenchephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon). Physically, the forebrain dominates: the rostral end of the forebrain comprises the two cerebral hemispheres, which grow over much of the remaining brain (the brainstem) and fill the skull with their wavy infolded cortices. Between the cerebral hemispheres lies the caudal portion of the forebrain, the diencephalon, which contains the thalami, collections of nuclei that are way-stations and gatekeepers for signals to and from the cerebral cortices, and the hypothalamus, a center of visceral signals and the site of the pituitary gland. Caudal to the diencephalon is the midbrain, marked by two pairs of bulges (the tectum or colliculi) on its dorsal surface. The final segment of the brainstem, the hindbrain, has a rostral division, the pontine region (metencephalon), from the dorsal side of which bulges the cerebellum. The most caudal portion of the hindbrain is the medulla oblongata (myelencephalon or, in older literature, the bulb), which smoothly grades into the spinal cord. See: fasciculus; tract; illustration

See: nucleus of the posterior commissureillustration

brain

The central organ of the body, to the maintenance, supply, transport and protection of which all the remainder of the body is dedicated. The brain contain more than 100 billion nerve cells with more than 1015 synapses. There are two main parts to the brain, the cerebrum and the cerebellum. The larger part, the cerebrum, initiates and coordinates all voluntary and most involuntary functions and is the seat of emotion, memory and intelligence. It is the medium by which all sensation, and the results of the mechanisms underlying all satisfaction, are conveyed to consciousness. It is essentially concerned with the collection, processing and storage of information, with the correlation of new data with stored data and with the organization and control of resulting responsive action. Response to stimulus is of the essence of brain function. Much is known, from the effects of disease and injury, of the localization of functions, in the brain, such as movement, sensation, vision, hearing, smell and speech. The location of areas responsible for registration and recall of memory is known, but the physical basis of memory storage remains obscure. Memory is not, like some other functions, located in a single definable area but is probably dispersed into all areas concerned with functions which may involve it. The cerebellum, the smaller part, is concerned mainly with the complex computations necessary to organize the muscle contractions needed to maintain the balance of the body and to allow walking and other movements. More than one tenth of the cardiac output is required to maintain brain function. See also BRAINSTEM.

Fig. 80 Brain . The vertebrate brain.

brain

the enlarged part of the CENTRAL NERVOUS SYSTEM beginning at the anterior end of bilaterally symmetrical animals (see BILATERAL SYMMETRY). The enlargement is associated with the aggregation of sense organs at the point which first contacts the changing environment. The brain, together with the rest of the central nervous system, coordinates the body functions. See also HEAD, CEPHALIZATION.

brain

(brān) [TA] That part of the central nervous system contained within the cranium. [A.S. braegen]

Patient discussion about brain

Q. What effect it will have in his brain………. hi all…………..whenever my bipolar son gets in to different episodes it makes me to think what effect it will have in his brain……….does it got anything to do with brain? But It didn’t strike me to discuss about this with my doctor….A. after a while without treatment it's hard to stay without any kind of brain damage... the brain is a biological material which is affected by materials that ravage it over and over again. bipolar disorder can get worse, adding hallucinations and such.

Q. Does the brain recognize pain? How does the brain recognize pain.A. First let us see the creation of the nerves which are assigned for different duties. There are roughly twenty different kinds of nerve endings in your skin that tell you if something is hot, cold, or painful. These nerve endings convert mechanical, thermal, or chemical energy into electrical signals that convey information to the brain and spinal cord - also known as the central nervous system or CNS. These signals travel to areas of your CNS where you perceive the stimuli as the painful sensations you actually feel - sensations such as searing, burning, pounding, or throbbing. Research suggests that the pain associated with fibromyalgia is caused by a "glitch" in the way the body processes pain. This glitch results in a hypersensitivity to stimuli that normally are not painful. According to the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS), research has shown that people with fibromyalgia have reduced blood flow to parts of the brain.

Q. Is surfing the internet good for your brain? I am 72 and I just discovered computers and the internet at our library. I find myself fascinated by it and I spend hours in front of the computer. Is surfing the internet good for your brain?A. This is a very current question that people ask and the answer is YES it is. A recent study showed that adults who surf the internet regularly engage larger parts of their brain when doing so compared to adults who rarely surf.

More discussions about brain

brain

brain

brain

brain

brain

Brain

brain

brain

brain

Parts of the brain

brain

brain

brain

brain

brain (someone)

brain someone

brain

brain

brain,

Anatomy and Function

Neural Pathways

Research

Bibliography

Brain

Brain

REFERENCES

Brain

brain

brain

brain

brain

Brain

brain

brain

brain

watershed infarct

brain

brain

Anatomy

brain

brain

brain

Patient discussion about brain

BRAIN

brain

Synonyms for brain

noun cerebrum

Synonyms

noun intellectual

Synonyms

noun intelligence

Synonyms

verb hit

Synonyms

Synonyms for brain

noun the seat of the faculty of intelligence and reason

Synonyms

noun the faculty of thinking, reasoning, and acquiring and applying knowledge

Synonyms

noun a person of great mental ability

Synonyms

Synonyms for brain

noun that part of the central nervous system that includes all the higher nervous centers

Synonyms

Related Words

noun mental ability

Synonyms

Related Words

noun that which is responsible for one's thoughts and feelings

Synonyms

Related Words

noun someone who has exceptional intellectual ability and originality

Synonyms

Related Words

noun the brain of certain animals used as meat

Related Words

verb hit on the head

Related Words

verb kill by smashing someone's skull

Related Words