Which Part Of The Brain Is The "Executive Suite" That Controls Almost All Brain Activity?

Outside the specialized world of neuroanatomy and for most of the uses of daily life, the encephalon is more or less an abstruse entity. We do not experience our brain as an assembly of physical structures (nor would nosotros wish to, perhaps); if we envision it at all, we are likely to meet information technology as a big, rounded walnut, grayish in color.

This schematic image refers mainly to the cerebral cortex, the outermost layer that overlies virtually of the other brain structures like a fantastically wrinkled tissue wrapped effectually an orange. The preponderance of the cerebral cortex (which, with its supporting structures, makes up approximately fourscore percent of the brain'southward full volume) is actually a contempo development in the course of evolution. The cortex contains the physical structures responsible for most of what we phone call ''brainwork": cognition, mental imagery, the highly sophisticated processing of visual information, and the power to produce and empathise language. But underneath this layer reside many other specialized structures that are essential for movement, consciousness, sexuality, the action of our five senses, and more—all equally valuable to man being. Indeed, in strictly biological terms, these structures can claim priority over the cognitive cortex. In the growth of the individual embryo, likewise as in evolutionary history, the brain develops roughly from the base of operations of the skull up and outward. The human being brain actually has its beginnings, in the four-calendar week-onetime embryo, as a unproblematic serial of bulges at ane end of the neural tube.

Effigy ii.i.

The brain owes its outer advent of a walnut to the wrinkled and securely folded cerebral cortex, which handles the innumerable signals responsible for perception and motion and too for mental processes. Below the surface of the cortex are packed (more...)

Ventricles

The bulges in the neural tube of the embryo develop into the hindbrain, midbrain, and forebrain—divisions common to all vertebrates, from sharks to squirrels to humans. The original hollow structure is commemorated in the form of the ventricles, which are cavities containing cerebrospinal fluid. During the course of development, the three bulges become four ventricles. In the hindbrain is the fourth ventricle, continuous with the primal canal of the spinal cord. A cavity in the forebrain becomes the 3rd ventricle, which leads farther forward into the 2 lateral ventricles, one in each cerebral hemisphere.

Brainstem

The hindbrain contains several structures that regulate autonomic functions, which are essential to survival and not under our conscious control. The brainstem, at the superlative of the spinal cord, controls breathing, the beating of the heart, and the diameter of blood vessels. This region is also an important junction for the control of deliberate motion. Through the medulla, at the lower cease of the brainstem, pass all the nerves running betwixt the spinal cord and the encephalon; in the pyramids of the medulla, many of these nerve tracts for motor signals cantankerous over from one side of the body to the other. Thus, the left brain controls movement of the correct side of the body, and the right brain controls move of the left side.

In addition to being the major site of crossover for nervus tracts running to and from the brain, the medulla is the seat of several pairs of nerves for organs of the breast and belly, for movements of the shoulder and caput, for swallowing, salivation, and gustatory modality, and for hearing and equilibrium.

At the pinnacle of the brainstem is the pons—literally, a bridge—between the lower brainstem and the midbrain. Nerve impulses traversing the pons pass on to the cerebellum (or "little brain"), which is concerned primarily with the coordination of circuitous muscular movement. In addition, nervus fibers running through the pons relay sensations of touch from the spinal cord to the upper brain centers.

Many nerves for the face and caput take their origin in the pons, and these nerves regulate some movements of the eyeball, facial expression, salivation, and sense of taste. Together with nerves of the medulla, nerves from the pons likewise control breathing and the body's sense of equilibrium.

What had been the middle burl in the neural tube develops into the midbrain, which functions mainly equally a relay center for sensory and motor nerve impulses betwixt the pons and spinal string and the thalamus and cerebral cortex. Fretfulness in the midbrain also command some movements of the eyeball, student, and lens and reflexes of the eyes, caput, and trunk.

Thalamus And Hypothalamus

Deep in the core area of the brain, merely above the tiptop of the brainstem, are structures that have a great deal to practice with perception, motility, and the body's vital functions. The thalamus consists of two oval masses, each embedded in a cerebral hemisphere, that are joined by a bridge. The masses comprise nerve jail cell bodies that sort information from four of the senses—sight, hearing, taste, and bear on—and relay information technology to the cerebral cortex. (Only the sense of smell sends signals directly to the cortex, bypassing the thalamus.) Sensations of pain, temperature, and pressure are also relayed through the thalamus, equally are the nervus impulses from the cognitive hemispheres that initiate voluntary movement.

The hypothalamus, despite its relatively pocket-sized size (roughly that of a thumbnail), controls a number of drives essential for the operation of a wide-ranging omnivorous social mammal. At the autonomic level, the hypothalamus stimulates shine muscle (which lines the blood vessels, tum, and intestines) and receives sensory impulses from these areas. Thus it controls the rate of the heart, the passage of food through the alimentary canal, and contraction of the bladder.

The hypothalamus is the main point of interaction for the body's two physical control systems: the nervous organization, which transmits information in the form of minute electrical impulses, and the endocrine organisation, which brings well-nigh changes of state through the release of chemical factors. It is the hypothalamus that kickoff detects crucial changes in the torso and responds by stimulating various glands and organs to release hormones.

The hypothalamus is also the encephalon'southward intermediary for translating emotion into physical response. When strong feelings (rage, fear, pleasure, excitement) are generated in the listen, whether by external stimuli or by the action of thoughts, the cerebral cortex transmits impulses to the hypothalamus; the hypothalamus may then send signals for physiological changes through the autonomic nervous system and through the release of hormones from the pituitary. Physical signs of fear or excitement, such equally a racing heartbeat, shallow breathing, and perhaps a clenched "gut feeling," all originate here.

Also in the hypothalamus are neurons that monitor trunk temperature at the surface through nerve endings in the skin, and other neurons that monitor the blood flowing through this office of the brain itself, every bit an indicator of core body temperature. The forepart office of the hypothalamus contains neurons that human activity to lower body temperature by relaxing shine muscle in the blood vessels, which causes them to dilate and increases the rate of heat loss from the skin. Through its neurons associated with the sweat glands of the pare, the hypothalamus can as well promote heat loss by increasing the rate of perspiration. In opposite conditions, when the trunk's temperature falls beneath the (rather narrow) ideal range, a portion of the hypothalamus directs the contraction of claret vessels, slows the charge per unit of heat loss, and causes the onset of shivering (which produces a small amount of rut).

The hypothalamus is the command center for the stimuli that underlie eating and drinking. The sensations that we translate as hunger arise partly from a degree of emptiness in the breadbasket and partly from a drib in the level of 2 substances: glucose circulating in the blood and a hormone that the intestine produces presently after the intake of food. (Receptors for this hormone gauge how far digestion has proceeded since the last repast.) This organisation is not a simple "on" switch for hunger, nonetheless: another portion of the hypothalamus, when stimulated, actively inhibits eating past promoting a feeling of satiety. In experimental animals, damage to this portion of the brain is associated with continued excessive eating, eventually leading to obesity.

In addition to these numerous functions, there is evidence that the hypothalamus plays a office in the induction of sleep. For i affair, it forms part of the reticular activating system, the physical basis for that hard-to-define land known every bit consciousness (about which more later); for another, electrical stimulation of a portion of the hypothalamus has been shown to induce slumber in experimental animals, although the machinery past which this works is non even so known. In all, the hypothalamus is a richly circuitous cubic centimeter of vital connections, which will continue to advantage close report for some futurity. Because of its unique position as a midpost between thought and feeling and betwixt conscious act and autonomic function, a thorough understanding of its workings should tell u.s. much nearly the earliest history and development of the homo animal.

Pituitary And Pineal Glands

The pituitary and the pineal glands function in close association with the hypothalamus. The pituitary responds to signals from the hypothalamus by producing an array of hormones, many of which regulate the activities of other glands: thyroid-stimulating hormone, adrenocorticotropic hormone (which stimulates an outpouring of epinephrine in response to stress), prolactin (involved in the production of milk), and the sex hormones follicle-stimulating hormone and luteinizing hormone, which promote the development of eggs and sperm and regulate the timing of ovulation. The pituitary gland also produces several hormones with more general effects: human growth hormone, melanocyte-stimulating hormone (which plays a role in the pigmentation of skin), and dopamine, which inhibits the release of prolactin but is better known equally a neurotransmitter (see Chapter 5).

The pineal gland produces melatonin, the hormone associated with peel pigmentation. The secretion of melatonin varies significantly over a 24-hour wheel, from depression levels during the twenty-four hour period to a peak at nighttime, and the pineal gland has been called a "third eye" because it is controlled past neurons sensitive to low-cal, which originate in the retina of each heart and finish in the hypothalamus. In animals with a clear-cutting breeding season, the pineal gland is a link between the shifting hours of daylight and the hormonal responses of the hypothalamus, which in turn guide reproductive functions. In humans, who can conceive and give birth throughout the twelvemonth, the pineal gland plays no known role in reproduction, although there is evidence that melatonin has a share in regulating ovulation.

The "Fiddling Brain" At The Back Of The Head

While autonomic and endocrine functions are being maintained by structures deep inside the brain, another specialized expanse is sorting and processing the signals required to maintain residual and posture and to comport out coordinated movement. The cerebellum (the term in Latin means "little brain") is actually a derived form of the hindbrain—as suggested by its position at the back of the head, partly tucked under the cerebral hemispheres. In humans, with our near unlimited repertoire of movement, the cerebellum is appropriately large; in fact, information technology is the 2nd-largest portion of the brain, exceeded simply by the cerebral cortex. Its bang-up surface area is accommodated within the skull by elaborate folding, which gives it an irregular, pleated look. In relative terms, the cerebellum is actually largest in the encephalon of birds, where it is responsible for the constant streams of information between encephalon and body that are required for flying.

In humans, the cerebellum relays impulses for motility from the motor area of the cognitive cortex to the spinal cord; from there, they pass to their designated musculus groups. At the same fourth dimension, the cerebellum receives impulses from the muscles and joints that are being activated and in some sense compares them with the instructions issued from the motor cortex, so that adjustments can be made (this fourth dimension by way of the thalamus). The cerebellum thus is neither the sole initiator of motility nor a simple link in the chain of nerve impulses, but a site for the rerouting and in some cases refining of instructions for move. In that location is testify, too, that the cerebellum can store a sequence of instructions for frequently performed movements and for skilled repetitive movements—those that we call up of every bit learned "by rote."

The right and left hemispheres of the cerebellum each connect with the nervus tracts from the spinal string on the same side of the torso, and with the opposite cerebral hemisphere. For example, nervus impulses concerned with movement of the left arm originate in the correct cerebral hemisphere, and information most the orientation, speed, and forcefulness of the motility is fed back to the right cerebral hemisphere, through the left half of the cerebellum. The fretfulness responsible for movement at the ends of the arms and legs tend to have their origin near the outer edges of the cerebellum. By contrast, nerves that have their origin near the center of the cerebellum serve to monitor the body'due south overall orientation in space and to maintain upright posture, in response to information about rest that is transmitted past nerve impulses from the inner ear, amongst other sources.

Reticular Network

Some nervus fibers from the cerebellum also contribute to the reticular germination, a widespread network of neurons ("reticular" is derived from the Latin word for "net"). This formation and some neurons in the thalamus, together with others from various sensory systems of the brain, make upward the reticular activating system—the ways by which nosotros maintain consciousness. The reticular activating system as well comes into play when nosotros deliberately focus our attending, "tuning out" distractions to some caste. At the midline of the brainstem are the raphe nuclei, whose axons extend downwardly into the spinal cord and upwardly to the cerebral cortex—a attain that makes information technology possible for many areas of the nervous organisation to be contacted simultaneously. The reticular germination plays a role in movement, particularly those forms of motility that do not call for witting attention: it is also involved in transmitting or inhibiting sensations of pain, temperature, and touch. Less tangibly, the reticular activating system appears to work as a filter for the endless stimuli that can act on the nervous arrangement both from within and from outside the torso. It is this filtering of signals that allows a passenger on an airplane, for example, to doze off undisturbed by sounds of nearby conversation and steady jet engines, but to awake and become alert when the pitch of the engines changes and the plane tilts into its descent.

The "Emotional Brain"

The limbic system (from the Latin limbus, for "hem" or "edge") is another assembly of linked structures that form a loose circuit throughout the encephalon. This organisation is a fairly old part of the brain and i that humans share with many other vertebrates; in reptiles, it is known as the rhinencephalon, or "smell-brain," because information technology reacts primarily to signals of odor. In humans, of grade, the stimuli that can affect the emotional brain are only virtually limitless in their diverseness.

The limbic system is responsible for most of the basic drives and emotions and the associated involuntary behavior that are important for an creature'south survival: pain and pleasance, fearfulness, acrimony, sexual feelings, and even docility and affection. Every bit with the rhinencephalon, the sense of scent is a powerful factor. Nerves from the olfactory bulb, by which all odour is perceived, rails direct into the limbic organisation at several points and are then connected through it to other parts of the brain; hence the power of pheromones, and peradventure of other odors as well, to influence behavior in quite complex ways without necessarily reaching our witting sensation.

Also feeding into the limbic system are the thalamus and hypothalamus, also as the amygdala, a minor, almond-shaped circuitous of nerve cells that receive input from both the olfactory organization and the cerebral cortex. These connections are illustrated in an unusual fashion in the context of epilepsy. Perhaps because the amygdala is located well-nigh a mutual site of origin of epileptic seizures—that is, in the temporal lobe of the cognitive hemispheres—epileptics sometimes feel unidentifiable or unpleasant odors or changes of mood as role of the aureola preceding a seizure. The limbic system is not thought to be involved in the causes of epilepsy, but it is indirectly stimulated past the electric belch in the encephalon that sets off a seizure and gives testify of the stimulation in its own characteristic ways.

Hippocampus

The hippocampus is some other major construction of the limbic arrangement. Named for its fanciful resemblance to the shape of a sea horse, the hippocampus is located at the base of the temporal lobe near several sets of association fibers. These are bundles of nervus fibers that connect one region of the cognitive cortex with another, and so that the hippocampus, besides every bit other parts of the limbic arrangement, exchanges signals with the entire cerebral cortex. The hippocampus has been shown to be important for the consolidation of recently acquired information. (In contrast, long-term retentivity is thought to be stored throughout the cerebral cortex. The means by which short-term memory is converted into long-term memory has posed a particularly challenging riddle that only now is beginning to yield to investigation; see Affiliate 8.)

Recent work with a variety of animals has institute dense clusters of receptor sites for tetrahydrocannabinol, the active ingredient of marijuana and related drugs, in the hippocampus and other nearby structures of the limbic system. This localization helps explicate the effects of marijuana, which range from mild euphoria to wavering attending to temporarily weakened short-term memory. A loss of short-term memory is besides seen in certain syndromes of alcoholism and in Alzheimer's disease, which involves some degeneration of the hippocampus and other limbic structures.

Cerebral Cortex

The cognitive cortex occupies past far the greatest surface expanse of the human brain and presents its about striking aspect. Besides known as the neocortex, this is the most recently evolved area of the brain. In fact, the enormous expansion in the area of the cerebral cortex is hypothesized to have begun but nigh 2 1000000 years ago, in the earliest members of the genus Human being; the result today is a brain weighing approximately three times more than than would be expected for a mammal our size. The cortex is named for its resemblance to the bark of a tree, because it covers the surface of the cognitive hemispheres in a similar mode. Its wrinkled convoluted appearance is due to a growth spurt during the fourth and fifth months of embryonic development, when the gray matter of the cortex is expanding greatly as its cells grow in size. The supporting white affair, meanwhile, grows less apace; every bit a result, the brain takes on the dense folds and fissures characteristic of an object with great surface area crowded into a small-scale space.

FIGURE 2.2.

The brain is divided into a left and a right hemisphere by a deep groove that runs from the front of the head (at left) to the back (at correct). In each hemisphere, the cerebral cortex falls into four primary divisions, or lobes, set off from 1 another (more...)

Figure ii.3.

Two miniature ''maps" correspond the body on the cognitive cortex. One of these, in the motor area, assigns a specific portion of the cortex to each part of the trunk that calls for muscular command; the portions assigned to the fingers, lips, and tongue (more...)

Although the folds in the cerebral cortex appear at first to be random, they include several prominent bulges, or gyri, and grooves, or sulci, that human activity equally landmarks in what is in fact a highly ordered structure (the finer details of which are still not completely known). The deepest groove extends from the front to the back of the head, dividing the brain into the left and right hemispheres. The central sulcus, which runs from the center of the brain outward to both left and correct, and the lateral sulcus, some other left-to-right groove somewhat lower on the hemispheres and toward the back of the head, farther divide each hemisphere into four lobes: frontal, parietal, temporal, and occipital. A 5th lobe, known as the insula, is located deep within the parietal and temporal lobes and is non apparent as a divide structure on the outer surface of the cognitive hemispheres.

2 noticeable bulges, the precentral gyrus and the postcentral gyrus, are named for their positions just in front of and just backside the key sulcus, respectively. The precentral gyrus is the site of the primary motor area, responsible for witting movement. From eyebrows to toes, the movable parts of the body are "mapped" on this area of the cortex, with each musculus group or limb represented here past a population of neurons. In complementary style, the job of receiving sensations from all parts of the body is managed by the master somatosensory area, which is located in the postcentral gyrus. Here, likewise, the human form is mapped, and, every bit with the precentral gyrus, the areas devoted to the mitt and the oral cavity are disproportionately large. Their size reflects the elaborate brain circuitry that makes possible the precision grip of the man hand, the fine motor and sensory signals required for striking upward a violin arpeggio or sharpening a tool, and the coordination of the lips, tongue, and vocal apparatus to produce the highly arbitrary and significant sounds of homo language.

Close observations of animals and humans after injury to particular sites of the encephalon indicate that many areas of the cortex command quite specific functions. Additional findings have come from stimulating sites on the cortex with a small electric charge in experimental procedures or during surgery; the result might exist an activity in some part of the body (if the motor cortex is involved) or (for a sensory function) a design of electric discharges in other parts of the cortex. Careful exploration has established, for example, that the auditory expanse in the temporal lobe is made upwards of smaller regions, each attuned to different sound frequencies.

But for much of the cortex, no such straight functions take been found, and for a time these areas were known as "silent" cortex. Information technology is now clear that "association" cortex is a meliorate name for them because they fill the crucial part of making sense of received stimuli, piecing together the signals from various sensory pathways and making the synthesis available as felt experience. For example, if there is to be not only perception only conscious understanding of sounds, the auditory clan surface area (but behind the auditory surface area proper) must exist active. In the hemisphere that houses spoken communication and other verbal abilities—the left hemisphere, for near people—the auditory association surface area blends into the receptive language area (which likewise receives signals from the visual association surface area, thereby providing a neural ground for reading as well as for the comprehension of spoken language in most languages).

A large portion of the association cortex is establish in the frontal lobes, which have expanded most quickly over the past xx,000 or so generations (near 500,000 years) of human evolution. Medical imaging shows increased activity in the association cortex after other areas of the encephalon have received electrical stimulation and as well before the initiation of move. On present prove, information technology is in the association cortex that we locate long-term planning, interpretation, and the organization of ideas—perhaps the nearly recently adult elements of the modern homo brain.

Visual functions occupy the occipital lobe, the bulge at the back end of the brain. The primary area for visual perception is near surrounded by the much larger visual association expanse. Nearby, extending into the lower part of the temporal lobe, is the association expanse for visual retention—a specialized area in the cortex. Clearly, this function has been important for an omnivorous foraging primate that probably spent a long evolutionary period ranging among scattered food sources. (For an account of the intricate mechanisms that underlie depth perception and color vision, see Affiliate vii.)

A less specific kind of function has been attributed to the prefrontal cortex, located on the forward-facing role of the frontal lobes. This area is connected by association fibers with all other regions of the cortex and as well with the amygdala and the thalamus, which ways that it, besides, makes upward part of the "emotional brain," the limbic system. Injury to the prefrontal cortex or its underlying white matter results in a curious inability: the patient suffers from a reduced intensity of emotion and can no longer foretell the consequences of things that are said or done. (The injury must be bilateral to produce such an effect; if just one hemisphere is injured, the other tin can compensate and avert this foreign, potentially crippling social arrears.) Amongst its other functions, the prefrontal cortex is responsible for inhibiting inappropriate behavior, for keeping the mind focused on goals, and for providing continuity in the thought process.

Long-term memory has not still been found to reside in any exclusive part of the brain, but experimental findings indicate that the temporal lobes contribute to this function. Electrical stimulation of the cerebral cortex in this area gives rising to sensations of déjà vu ("already seen") and its opposite, jamais vu ("never seen"); it also conjures upward images of scenes witnessed or speech heard in the by. That the association areas for vision and hearing and the linguistic communication areas are all nearby may advise pathways for the storage and retrieval of memories that include several types of stimuli.

The function of language itself is housed in the left hemisphere (in nigh cases), in several discrete sites on the cortex.

The expressive linguistic communication area, responsible for the product of speech, is found toward the center of the frontal lobe; this is also chosen Broca'due south area, later the French anatomist and anthropologist of the mid-1800s who was among the first to observe differences in function betwixt the left and right hemispheres. The receptive linguistic communication area, which is located almost the junction of the parietal and temporal lobes, allows united states to comprehend both spoken and written language, as described above. This is often called Wernicke's surface area, after the German language neurologist Karl Wernicke, who in the late 1800s laid the basis for much of our electric current understanding of how the brain encodes and decodes language. A package of nerve fibers connects Wernicke's area direct to Broca'due south surface area. This tight linkage is of import, since before any spoken communication at all can be uttered, its form and appropriate words must first be assembled in Wernicke's area and so relayed to Broca's area to be mentally translated into the requisite sounds; merely so tin can it pass to the supplementary motor cortex for vocal production.

For nine of 10 right-handed people and virtually two-thirds of all left-handers, language abilities are sited in the left hemisphere. No one knows why in that location should be this asymmetrical distribution rather than an even balance or, for that matter, a consistent location of language in the left brain. What is clear is that in all cases, the hemisphere that does not contain language abilities holds the fundamental to other functions of a less distinct, more holistic nature. The appreciation of forms and textures, the recognition of the timbre of a vocalisation, and the ability to orient oneself in space all announced to lodge hither, equally practice musical talent and appreciation—a host of perceptions that do non lend themselves well to analysis in words.

The limited specialization of the two hemispheres is efficient in terms of the apply of space: it increases the functional abilities of the brain without calculation to its volume. (The skull of the man infant, it is calculated, is already every bit large as can exist accommodated through the birth canal, which in turn is constrained by the skeletal requirements for upright walking.) Moreover, the bilateral organisation allows for some flexibility if one hemisphere is injured; often the other hemisphere can recoup to some degree, depending on the age at which injury occurs (a young, still-developing brain readjusts more than readily).

The two hemispheres are connected mainly past a thick bundle of nerve fibers called the corpus callosum, or "hard torso," because of its tough consistency. A smaller packet, the anterior commissure, connects but the 2 temporal lobes. Although the corpus callosum is a good landmark for students of encephalon beefcake, its contribution to behavior has been difficult to pin down. Patients in whom the corpus callosum has been severed (a way of ameliorating epilepsy past restricting seizures to one side of the brain) go about their everyday business without impairment. Careful testing does turn up a gap between sensations processed by the correct brain and the language centers of the left encephalon—for case, a person with a severed corpus callosum is unable to name an object placed unseen in the left paw (because stimuli perceived by the left half of the torso are processed in the right hemisphere). On the whole, though, information technology appears that the massive crossing-over of nerve fibers that takes identify in the brainstem is quite adequate for about purposes, at least those related to survival.

Although the cerebral cortex is quite thin, ranging from 1.5 to 4 millimeters deep (less than iii/8 inch), information technology contains no fewer than six layers. From the outer surface inward, these are the molecular layer, fabricated up for the most part of junctures between neurons for the commutation of signals; the external granular layer, mainly interneurons, which serve as communicating nervus bodies within a region; an external pyramidal layer, with large-bodied "principal" cells whose axons extend into other regions; an internal granular layer, the primary termination point for fibers from the thalamus; a 2d, internal pyramidal layer, whose cells project their axons mostly to structures below the cortex; and a multiform layer, again containing principal cells, which in this instance projection to the thalamus. The layers vary in thickness at different sites on the cortex; for example, the granular layers (layers 2 and iv) are more prominent in the primary sensory area and less then in the principal motor area.

Edifice Blocks Of The Brain

Extensive and intricate equally the human brain is, and with the nearly limitless variation of which information technology is capable, it is built from relatively few basic units. The fundamental edifice cake of the human encephalon, like that of nervous systems throughout the brute kingdom, is the neuron, or nerve jail cell. The neuron conducts signals past means of an axon, which extends outward from the soma, or trunk of the cell, similar a single long arm. Numerous shorter arms, the dendrites ("picayune branches"), conduct signals back to the soma.

The ability of the axon to conduct nerve impulses is greatly enhanced by the myelin sheath that surrounds it, interrupted at intervals past nodes. Myelin is a fatty substance, a natural electric insulator, that protects the axon from interference by other nearby nerve impulses. The arrangement of nodes increases the speed of conductivity, so that an electrical impulse sent along the axon tin literally jump from node to node, reaching velocities every bit high as 120 meters per second.

The site of communication between whatsoever two neurons—actually not a concrete contact but an infinitesimal scissure across which signals are transmitted—is called a synapse, from the Greek word for "conjunction." An axon may extend over a variable altitude to make contact with other neurons at a synapse. The end of an axon near a synapse widens out into a bouton, or push button; the bouton contains mitochondria, which supply free energy, and a number of synaptic vesicles. It is these vesicles, each less than 200 billionths of a meter in diameter, that contain the chemical neurotransmitters to be released into the synaptic scissure. On the other side of the synapse is ordinarily a dendrite, sometimes with a dendritic spine—a modest protuberance that expands the surface expanse of the dendrite and provides a receptive site for incoming signals.

A completely different system for transmitting signals is the electrical synapse, at which the cell membranes of ii neurons are extremely close together and are linked by a bridge of tubular protein molecules. This span allows passage of water and electrically charged small molecules; any modify in electrical charge in one neuron is instantaneously transmitted to the other. Hence this machinery for relaying signals relies entirely on directly electrical coupling; an electrical synapse is about 3 nanometers (nm), or billionths of a meter, wide, equally compared with the 25-nm gap of a chemical synapse. Outside of nervous tissue, electrical synapses (and other, similar gap junctions) are the messengers of selection.

The brain is sometimes said to be total of "grey matter," which is supposed to exist the stuff of intelligence. The material referred to is really grayish pink in living encephalon, and simply gray in specimens that accept been chemically preserved; information technology consists of nerve cell bodies and dendrites and the origins and boutons of axons. It is gray matter that forms sheets of cortex on the surface of the cognitive hemispheres. White matter receives its name from the appearance of the myelin enclosing the elongated region of axons. The third main course of matter in the brain is the neuroglia, or "glue" cells. These cells do not connect the neurons, every bit their name implies; connections are already far from scarce, with the vast organization of neural soma, axons, and dendrites packed and then densely into the brain. Rather, the neuroglia provide structural back up and a source of metabolic energy for the roughly 100 billion nerve cells of the human brain.

Chemical And Electrical Signals

The actual signals transmitted throughout the brain come in two forms, electrical and chemical. The two forms are interdependent and meet at the synapse, where chemical substances can alter the electrical conditions within and outside the cell membrane.

A nerve cell at rest holds a slight negative charge (about -lxx millivolts, or thousandths of a volt, mV) with respect to the exterior; the cell membrane is said to be polarized. The negative charge, the resting potential of the membrane, arises from a very slight excess of negatively charged molecules inside the prison cell.

A membrane at rest is more or less impermeable to positively charged sodium ions (Na⁺), simply when stimulated it is transiently open to their passage. The Na⁺ions thus flow in, attracted by the negative charge inside, and the membrane temporarily reverses its polarity, with a higher positive charge within than out. This stage lasts less than a millisecond, and and so the sodium channels close again. Potassium channels (K⁺) open up, and K⁺ions move out through the membrane, reversing the menstruation of positively charged ions. (Both these channels are known as voltage-gated, meaning that they open up or close in response to changes in electrical charge occurring beyond the membrane.) Over the adjacent iii milliseconds, the membrane becomes slightly hyperpolarized, with a charge of about -80 mV, and then returns to its resting potential. During this time the sodium channels remain closed; the membrane is in a refractory phase.

An activeness potential—the very cursory pulse of positive membrane voltage—is transmitted forward along the axon; information technology is prevented from propagating astern as long as the sodium channels remain closed. After the membrane has returned to its resting potential, however, a new impulse may go far to evoke an action potential, and the cycle can brainstorm once again.

Gated channels, and the concomitant movement of ions in and out of the cell membrane, are widespread throughout the nervous system, with sodium, potassium, and chlorine being the most common ions involved. Calcium channels are too important, particularly at the presynaptic boutons of axons. When the membrane is at its resting potential, positively charged calcium ions (Caⁱⁱ⁺) exterior the jail cell far outnumber those inside. With the advent of an activeness potential, notwithstanding, calcium ions rush into the prison cell. The influx of calcium ions leads to the release of neurotransmitter into the synaptic cleft; this passes the signal to a neighboring nerve jail cell.

Having taken a close await at the electric side of the flick, nosotros are in a better position to meet where the chemical science comes in. Molecules of neurotransmitter are released into a synaptic cleft and demark to specific receptor sites on the postsynaptic side (the dendrite or dendrite spine), thereby altering the ion channels in the postsynaptic membrane. Some neurotransmitters crusade sodium channels to open, assuasive the influx of Na⁺ions and thus a lessening of negative accuse within the cell membrane. If a considerable number of these potentials are received inside a curt interval, they tin can depolarize the membrane enough to trigger an activity potential; the result is the manual of a nerve impulse. The substances that tin can cause this to occur are the excitatory neurotransmitters. By dissimilarity, other chemic compounds cause potassium channels to open, increasing the outflow of Grand⁺ions from the jail cell and making excitation less likely; the neurotransmitters that bring about this state are considered inhibitory.

A given neuron has a great quantity of sites bachelor on its dendrites and prison cell body and receives signals from many synapses simultaneously, both excitatory and inhibitory. These signals often amount to a rough residue; it is only when the net potential of the membrane in ane region shifts significantly upward or down from the resting level that a particular neurotransmitter can be said to be exerting an consequence. Interestingly, in the membrane's overall remainder sheet, the importance of a detail synapse varies with its proximity to where the axon leaves the nerve jail cell torso, and then that numerous excitatory potentials out at the ends of the dendrites may be overruled by several inhibitory potentials closer to the soma. Other kinds of synapse regulate the release of neurotransmitters into the synaptic fissure, where they go on to bear on the postsynaptic channels equally described above.

The list of known neurotransmitters, once idea to be quite brusk, continues to abound equally more than substances are found to be synthesized past neurons, independent in presynaptic boutons, and spring on the postsynaptic membrane by specific receptors. Despite stringent requirements for identifying a substance as a neurotransmitter (run into Chapter 5), well over ii dozen have been and so named, and another several dozen stiff candidates are under review.

The most brief look at the human encephalon can excite awe at its complex functions, the intricacy of its structure, and the innumerable connections all maintained on microscopic fibers a few millionths of a meter in diameter. But a slightly more intimate acquaintance with this 3-pound organ within our heads, an acquaintance that builds on observation of the brain in action and discovery of the principles past which it works, can yield something more satisfying than awe: the sense of mastery and of rewarded curiosity that comes with agreement. With the rewarding of marvel as our goal, let us take a closer await at a few aspects of the performance brain.