Brain is the body’s chief control center, containing billions of interconnected nerve cells. It receives, collects, processes, and stores information, and controls the body’s responses.
The brain is enclosed by the cranium and meninges and is bathed in cerebrospinal fluid. The tremendous metabolic rate of the brain makes it highly susceptible to oxygen deprivation .Also the brain is the main part of the central nervous system, which also includes the spinal cord.
The brain has a tremendous metabolic rate and therefore needs a continuous supply of oxygen and nutrients. Although it accounts for only 2% of a person’s body weight, the brain receives approximately 20% of the total resting cardiac output. This amounts to a flow of about 750 ml of blood per minute. The volume remains relatively constant even with changes in physical or mental activity. This continuous flow is so crucial that a failure of cerebral circulation for as short an interval as 10 seconds causes unconsciousness.
The brain, like the spinal cord, is composed of gray and white matter. Gray matter the site of the neuron cell bodies, dendrites, and synapses forms a surface layer called the cortex over the cerebrum and cerebellum, and deeper masses called nuclei surrounded by white matter (see fig.B1).
Figure B1. Ventricles of the Brain.
The white matter thus lies deep to the cortical gray matter in most of the brain, opposite from the relationship of gray and white matter in the spinal cord. As in the spinal cord, the white matter is composed of tracts, or bundles of axons, which here connect one part of the brain to another. It gets its bright white color from myelin.
Like the spinal cord, the brain is enveloped in connective tissue membranes, the meninges, which lie between the nervous tissue and bone. The meninges of the brain are basically the same as those of the spinal cord dura mater, arachnoid mater, and pia mater although there are some differences in the dura mater (fig. B2.).
Fig B2.The Meninges of the Brain. Frontal section of the head.
In the cranial cavity, the dura consists of two layers an outer periosteal layer, equivalent to the periosteum of the cranial bone, and an inner meningeal layer. Only the meningeal layer continues into the vertebral canal. The cranial dura mater lies closely against the cranial bone, with no intervening epidural space like the one around the spinal cord. It is attached to the cranial bone only in limited places-around the foramen magnum, the sella turcica, the crista galli, and the sutural lines of the skull.
Ventricles and Cerebrospinal Fluid
The brain has four internal chambers called ventricles. The largest are the two lateral ventricles, which form an arc in each cerebral hemisphere (fig. B1).
Through a pore called the interventricular foramen, each lateral ventricle is connected to the third ventricle, a narrow medial space inferior to the corpus callosum. From here, a canal called the cerebral aqueduct passes down the core of the midbrain and leads to the fourth ventricle, a small triangular chamber between the pons and cerebellum. Caudally, this space narrows and forms a central canal that extends through the medulla oblongata into the spinal cord. On the floor or wall of each ventricle, there is a spongy mass of blood capillaries called a choroid plexus (fig. B1), named for its histological resemblance to the chorion of a fetus. Ependymal cells, a type of neuroglia, cover each choroid plexus and the entire interior surface of the ventricles and canals of the brain and spinal cord. The choroid plexuses produce some of the cerebrospinal fluid.
Cerebrospinal fluid (CSF) is a clear, colorless liquid that fills the ventricles and canals of the CNS and bathes its external surface. The brain produces about 500 mL of CSF per day, but the fluid is constantly reabsorbed at the same rate and only 100 to 160 mL are present at one time. About 40% of it is formed in the subarachnoid space external to the brain, 30% by the general ependymal lining of the brain ventricles, and 30% by the choroid plexuses. Cerebrospinal fluid forms partly by the filtration of blood plasma through the choroid plexuses and other capillaries of the brain. The ependymal cells chemically modify the filtrate as it passes through them into the ventricles and subarachnoid space. The Cerebrospinal fluid is not a stationary fluid but continually flows through and around the Cranial Nervous System, driven partly by its own pressure and partly by rhythmic pulsations of the brain produced by each heartbeat. The Cerebrospinal fluid secreted in the lateral ventricles flows through the interventricular foramina into the third ventricle (fig. B3), then down the cerebral aqueduct to the fourth ventricle. The third and fourth ventricles and their choroid plexuses add more Cerebrospinal fluid along the way.
Figure B3.The Flow of Cerebrospinal Fluid.
According to the figure above.
(1) Cerebrospinal fluid is secreted by choroid plexus in each lateral ventricle.
(2) Cerebrospinal fluid flows through interventricular foramina into third ventricle.
(3) Choroid plexus in third ventricle adds more Cerebrospinal fluid.
(4)Cerebrospinal fluid flows down cerebral aqueduct to fourth ventricle.
(5) Choroid plexus in fourth ventricle adds more Cerebrospinal fluid.
(6) Cerebrospinal fluid flows out two lateral apertures and one median aperture.
(7) Cerebrospinal fluid fills subarachnoid space and bathes external surfaces of brain and spinal cord.
(8) At arachnoid villi, Cerebrospinal fluid is resorbed into venous blood of dural venous sinuses.
Cerebrospinal fluid serves three purposes:
- Buoyancy. Because the brain and Cerebrospinal fluid are very similar in density, the brain neither sinks nor floats in the CSF. It hangs from delicate specialized fibroblasts of the arachnoid meninx. A human brain removed from the body weighs about 1,500 g, but when suspended in Cerebrospinal fluid its effective weight is only about 50 g. By analogy, consider how much easier it is to lift another person when you are standing in a lake than it is on land. This buoyancy effect allows the brain to attain considerable size without being impaired by its own weight. If the brain rested heavily on the floor of the cranium, the pressure would kill the nervous tissue.
- Protection. Cerebrospinal fluid also protects the brain from striking the cranium when the head is jolted. If the jolt is severe, however, the brain still may strike the inside of the cranium or suffer shearing injury from contact with the angular surfaces of the cranial floor. This is one of the common findings in child abuse (shaken child syndrome) and in head injuries (concussions) from auto accidents, boxing, and the like.
- Environmental stability. Cerebrospinal fluid transports nutrients and chemicals to the brain and removes waste products from the brain. Additionally, Cerebrospinal fluid protects nervous tissue from chemical fluctuations that would disrupt neuron function. The waste products and excess CSF are eventually transported into the venous circulation, where they are filtered from the blood and secreted in urine in the urinary system. Slight changes in Cerebrospinal fluid composition can cause malfunctions of the nervous system. For example, a high glycine concentration disrupts the control of temperature and blood pressure, and a high pH causes dizziness and fainting.
Blood Supply and the Brain Barrier System
Although the brain constitutes only 2% of the adult body weight, it receives 15% of the blood (about 750 mL/min) and consumes 20% of the oxygen and glucose. But despite its critical importance to the brain, blood is also a source of agents such as bacterial toxins that can harm the brain tissue. Consequently, there is a brain barrier system that strictly regulates what substances get from the bloodstream into the tissue fluid of the brain. One component of this system is the blood-brain barrier (BBB), which seals nearly all of the blood capillaries throughout the brain tissue. In the developing brain, astrocytes reach out and contact the capillaries with their perivascular feet. They do not fully surround the capillary, but stimulate the formation of tight junctions between the endothelial cells that line it. Anything passing from the blood into the tissue fluid has to pass through the endothelial cells themselves, which are more selective than gaps between the cells.
At the choroid plexuses, there is a similar blood-CSF barrier, composed of ependymal cells joined by tight junctions. Tight junctions are absent from ependymal cells elsewhere, because it is important to allow exchanges between the brain tissue and CSF. That is, there is no brain-CSF barrier. The brain barrier system (BBS) is highly permeable to water, glucose, and lipid-soluble substances such as oxygen and carbon dioxide, and to drugs such as alcohol, caffeine, nicotine, and anesthetics. While the BBS is an important protective device, it is an obstacle to the delivery of drugs such as antibiotics and cancer drugs, and thus complicates the treatment of brain diseases. The BBB is absent from patches called circumventricular organs (CVOs) on the walls of the third and fourth ventricles. Here, the blood has direct access to the brain tissue, enabling the brain to monitor and respond to fluctuations in blood chemistry. Unfortunately, the CVOs also afford a route for the human immunodeficiency virus (HIV) to invade the brain.
The brain is divided into three main regions:
- The forebrain, where memory, the mind, and intelligence are based. This is also involved in body-part movements, receiving sensations, speech, hearing, and sight.
- The midbrain, which works mainly as a relay station for messages to and from the brain. Eye movements are controlled here.
- The hindbrain, which coordinates complex body movements, especially of the arms and legs.
Arising from the brainstem (a slim stalk at the top of the spinal cord), the brain spreads out to fill the space inside the skull. The forebrain, or cerebrum, consists of two cerebral hemispheres . Within the cerebrum lie the corpus callosum (linking the two brain hemispheres), the thalamus, and the hypothalamus. The thalamus and hypothalamus are also part of the region within the cerebrum called the diencephalon.The embryonic diencephalon has three major derivatives which are,the thalamus, hypothalamus, and epithalamus. These structures surround the third ventricle of the brain.
The thalamus is superior to the hypothalamus and inferior to the cerebrum (See Figure B4). The third ventricle is a narrow cavity that passes through both the thalamus and hypothalamus. Many of the functions of the thalamus are concerned with sensation. Sensory impulses to the brain (except those for the sense of smell) follow neuron pathways that first enter the thalamus, which groups the impulses before relaying them to the cerebrum, where sensations are felt. For example, holding a cup of hot coffee generates impulses for heat, touch and texture, and the shape of the cup (muscle sense), but we do not experience these as separate sensations. The thalamus integrates the impulses from the cutaneous receptors and from the cerebellum, that is, puts them together in a sort of electrochemical package, so that the cerebrum feels the whole and is able to interpret the sensation quickly. Some sensations, especially unpleasant ones such as pain, are believed to be felt by the thalamus. However, the thalamus cannot localize the sensation; that is, it does not know where the painful sensation is. The sensory areas of the cerebrum are required for localization and precise awareness. The thalamus may also suppress unimportant sensations. If you are reading an enjoyable book, you may not notice someone coming into the room. By temporarily blocking minor sensations, the thalamus permits the cerebrum to concentrate on important tasks. Parts of the thalamus are also involved in alertness and awareness, and others contribute to memory. For these functions, as for others, the thalamus works very closely with the cerebrum.
Figure B4.The human brain.
The hypothalamus (fig B4.) forms part of the walls and floor of the third ventricle. It extends anteriorly to the optic chiasm, an X-shaped crossing of the optic nerves, and posteriorly to a pair of humps called the mammillary bodies. The mammillary bodies are composed of nuclei belonging to the hypothalamus and limbic system (part of the forebrain to be discussed later); they are the primary route of sensory input to the hypothalamus, and their output goes also to the thalamus and lower brainstem. The pituitary gland is attached to the hypothalamus by a stalk between the optic chiasm and mammillary bodies. The hypothalamus is the major control center of the autonomic nervous system and endocrine system and plays an essential role in the homeostatic regulation of nearly all organs of the body. Its nuclei include centers concerned with a wide variety of visceral functions as follows.
- Hormone secretion. The hypothalamus secretes hormones that control the anterior pituitary gland. Acting through the pituitary, it regulates growth, metabolism, reproduction, and stress responses. It also produces two hormones that are stored in the posterior pituitary gland, oxytocin concerned with labor contractions and lactation, and antidiuretic hormone concerned with water conservation and it sends nerve signals to the posterior pituitary to stimulate release of these hormones at appropriate times.
- Autonomic effects. The hypothalamus is a major integrating center for the autonomic nervous system. It sends descending fibers to nuclei lower in the brainstem that influence heart rate, blood pressure, pupillary diameter, and gastrointestinal secretion and motility, among other functions.
- Thermoregulation. The hypothalamic thermostat is a nucleus that monitors blood temperature. When the temperature becomes too high or low, the thermostat signals other hypothalamic nuclei the heat-losing center or heat-producing center, respectively which control cutaneous vasodilation and vasoconstriction, sweating, shivering, and piloerection. These reactions usually return the body temperature to normal.
- Food and water intake. Neurons of the hunger and satiety centers monitor blood glucose and amino acid levels and produce sensations of hunger and satisfaction of the appetite. Hypothalamic neurons called osmoreceptors monitor the osmolarity of the blood and stimulate the hypothalamic thirst center when the body is dehydrated. Thus, our drives to eat and drink are under hypothalamic control.
- Sleep and circadian rhythms. The caudal part of the hypothalamus is part of the reticular formation. In this region are nuclei that regulate falling asleep and waking. Superior to the optic chiasm, the hypothalamus contains a suprachiasmatic nucleus that controls our circadian rhythm (24-hour cycle of activity).
- Emotional responses. The hypothalamus contains nuclei for a variety of emotional responses including anger, aggression, fear, pleasure, rage and contentment; and for sexual drive, copulation, and orgasm. The mammillary bodies provide a pathway by which emotional states can affect visceral function for example, when anxiety accelerates the heart or upsets the stomach.
- Memory. In addition to their role in emotional circuits, the mammillary bodies lie in the pathway from the hippocampus to the thalamus. The hippocampus is a center for the creation of new memories the cerebrum’s “teacher” so as intermediaries between the hippocampus and cerebral cortex, the mammillary bodies are essential for the acquisition of new memories.
- Master control of the autonomic nervous system. The hypothalamus is a major autonomic integration center. In essence, it is the “president” of the corporation known as the autonomic nervous system. It projects descending.
- Regulation of body temperature. The body’s thermostat is located within the hypothalamus. Neurons in the preoptic area detect altered blood temperatures and signal other hypothalamic nuclei, which control the mechanisms that heat or cool the body (shivering and sweating, respectively).
The epithalamus consists mainly of the pineal gland, the habenula and a thin roof over the third ventricle.
This is the largest part of the human brain which consists of two hemispheres separated by the longitudinal fissure. The cerebrum is the last center to receive sensory input and carry out integration before commanding voluntary motor responses. It communicates with and coordinates the activities of the other parts of the brain. The cerebrum carries out the higher thought processes required for learning and memory and for language and speech.
The cerebrum located in the region of the telencephalon, is the largest and most obvious portion of the brain. It accounts for about 80% of the mass of the brain and is responsible for the higher mental functions, including memory and reason. The cerebrum consists of the right and left hemispheres, which are incompletely separated by a longitudinal cerebral fissure. Portions of the two hemispheres are connected internally by the corpus callosum , a large tract of white matter (see fig. B1.). A portion of the meninges called the falx (falks) cerebri extends into the longitudinal fissure. Each cerebral hemisphere contains a central cavity, the lateral ventricle (fig. B4), which is lined with ependymal cells and filled with cerebrospinal fluid
The cerebrum consists of two layers. The surface layer, referred to as thecerebral cortex, is composed of gray matter that is 2-4 mm (0.08-0.16 in.) thick (fig. B1). Beneath the cerebral cortex is the thickwhite matter of the cerebrum, which constitutes the second layer.Also the white matter is the most volume of the cerebrum. This is composed of glia and myelinated nerve fibers that transmit signals from one region of the cerebrum to another and between the cerebrum and lower brain centers. These fibers travel in bundles called tracts. There are three types of cerebral tracts (fig. B5).
Figure B5. Tracts of Cerebral White Matter.
(a) Left lateral aspect, showing association tracts. (b) Frontal section, showing commissural and projection tracts.
- Projection tracts extend vertically between higher and lower brain or spinal cord centers and carry information between the cerebrum and the rest of the body. The corticospinal tracts, for example, carry motor signals from the cerebrum to the brainstem and spinal cord. Other projection tracts carry signals upward to the cerebral cortex. Superior to the brainstem, such tracts form a dense band called the internal capsule between the thalamus and basal nuclei (described shortly), then radiate in a diverging, fanlike array (the corona radiata) to specific areas of the cortex.
- Commissural tracts cross from one cerebral hemisphere to the other through bridges called commissures. The great majority of commissural fibers pass through the corpus callosum. A few tracts pass through the much smaller anterior and posterior commissures (fig.B6). Commissural tracts enable the two sides of the cerebrum to communicate with each other.
Figure. B6. Medial Aspect of the Brain. Median section, left lateral view.
- Association tracts connect different regions of the same hemisphere. Long association fibers connect different lobes to each other, whereas short association fibers connect different gyri within a single lobe.
The cerebral cortex is characterized by numerous folds and grooves called convolutions. Convolutions form during early fetal development, when brain size increases rapidly and the cortex enlarges out of proportion to the underlying white matter. The elevated folds of the convolutions are the cerebral gyri and the depressed grooves are the cerebral sulci. The convolutions effectively triple the area of the gray matter, which is composed of nerve cell bodies.
In the human brain the cerebral cortex is folded extensively. The folds are called convolutions or gyri and the grooves between them are fissures or sulci (you can see the folding of the cortex in the frontal section of the brain in Fig. B7).
Figure B7. Frontal section of the brain in anterior view.
This folding permits the presence of millions more neurons in the cerebral cortex. The cerebral cortex of an animal such as a dog or cat does not have this extensive folding. This difference enables us to read, speak, do long division, write poetry and songs, and do so many other “human” things that dogs and cats cannot do.
The cerebral cortex is divided into lobes that have the same names as the cranial bones external to them. These lobes have been mapped; that is, certain areas are known to be associated with specific functions. Each cerebral hemisphere is subdivided into five lobes by deep sulci or fissures. Four of these lobes appear on the surface of the cerebrum and are named according to the overlying cranial bones (fig. B8.).
The frontal lobe forms the anterior portion of each cerebral hemisphere (fig. B8.). A prominent deep furrow called the central sulcus separates the frontal lobe from the parietal lobe. The central sulcus extends at right angles from the longitudinal fissure to the lateral sulcus. The lateral sulcus extends laterally from the inferior surface of the cerebrum to separate the frontal and temporal lobes. The precentral gyrus (see fig B8.), an important motor area, is positioned immediately in front of the central sulcus. The frontal lobe’s functions include initiating voluntary motor impulses for the movement of skeletal muscles, analyzing sensory experiences, and providing responses relating to personality. The frontal lobes also mediate responses related to memory, emotions, reasoning, judgment, planning, and verbal communication. Processing area in the frontal lobe, receives information from the other association areas and uses this information to reason and plan our actions. Integration in this area accounts for our most cherished human abilities to think critically and to formulate appropriate behaviors.
Motor and Sensory Areas of the Cortex .The primary motor area is in the frontal lobe just anterior to the central sulcus. Voluntary commands to skeletal muscles begin in the primary motor area, and each part of the body is controlled by a certain section. The primary somatosensory area is just posterior to the central sulcus in the parietal lobe. Sensory information from the skin and skeletal muscles arrives here, where each part of the body is sequentially represented.
The general sensory areas in the parietal lobes receive impulses from receptors in the skin and feel and interpret the cutaneous sensations. The left area is for the right side of the body and vice versa. These areas also receive impulses from stretch receptors in muscles for conscious muscle sense. The largest portions of these areas are for sensation in the hands and face, those parts of the body with the most cutaneous receptors and the most muscle receptors. The taste areas, which overlap the parietal and temporal lobes, receive impulses from taste buds on the tongue and elsewhere in the oral cavity. A primary taste area, also in the parietal lobe, accounts for taste sensations. A primary visual area in the occipital lobe receives information from our eyes, and a primary auditory area in the temporal lobe receives information from our ears.
The temporal lobe
Temporal lobe lies inferior to the lateral sulcus and underlies the temporal bone. This lobe is involved with hearing and smell.
The occipital lobe.
The occipital lobe forms the posterior portion of the cerebrum and is not distinctly separated from the temporal and parietal lobes (see fig. B8.). It lies superior to the cerebellum and is separated from it by an infolding of the meningeal layer called the tentorium cerebelli . The principal functions of the occipital lobe concern vision. It integrates eye movements by directing and focusing the eye. It is also responsible for visual association correlating visual images with previous visual experiences and other sensory stimuli.The occipital lobe is at the rear of the head, caudal to the parieto-occipital sulcus and underlying the occipital bone. It is the principal visual center of the brain.
The insula is a small mass of cortex deep to the lateral sulcus, made visible only by retracting or cutting away some of the overlying cerebrum. It is less understood than the other lobes but apparently plays roles in taste, hearing, and visceral sensation.
Figure B8.The Lateral view of the Cerebrum.
The cerebrum is composed of two halves, called the left and right cerebral hemispheres (figure B9.). The paired cerebral hemispheres are separated by a deep longitudinal fissure that extends along the where bundles of axons called tracts form white matter regions that allow for communication between them. The largest of these white matter tracts, the corpus callosum, connects the hemispheres. The corpus callosum provides the main communications link between these hemispheres.
Figure B9.Cerebral Hemisphere.
Superior views comparing an illustration and a cadaver photo show the cerebral hemispheres, where our conscious activities, memories, behaviors, plans, and ideas are initiated and controlled.
Three points should be kept in mind with respect to the cerebral hemispheres.
- In most cases, it is difficult to assign a precise function to a specific region of the cerebral cortex. Considerable overlap and indistinct boundaries permit a single region of the cortex to exhibit several different functions. Additionally, some aspects of cortical function, such as memory or consciousness, cannot easily be assigned to any single region.
- With few exceptions, both cerebral hemispheres receive their sensory information from and project motor commands to the opposite side of the body. The right cerebral hemisphere controls the left side of the body, and vice versa.
- The two hemispheres appear as anatomic mirror images, but they display some functional differences, termed hemisphere lateralization. For example, the portions of the brain that are responsible for controlling speech and understanding verbalization are frequently located in the left hemisphere. These differences primarily affect higher-order functions
Functions of fore brain.
The special senses are the senses of taste, smell, hearing, equilibrium, and vision, mediated by relatively complex sense organs in the head. Signals from these sense organs are routed to areas of primary sensory cortex in widely separated regions of the cerebrum. The following are the regions of cerebral cortex concerned with each of these senses (fig. 10):
Figure. B10. Some Functional Regions of the Cerebral Cortex. Left hemisphere.
1) Vision. Thus, the visual process begins with a comparison of the amount of light striking any small region of the retina and the amount of light around it. Located in the occipital lobe, the primary visual cortex two millimeters thick and densely packed with cells in many layers receives messages from the lateral geniculate. In the middle layer, which receives input from the lateral geniculate, scientists found patterns of responsiveness similar to those observed in the retina and lateral geniculate cells. Cells above and below this layer responded differently. They preferred stimuli in the shape of bars or edges. Further studies showed that different cells preferred edges at particular angles, edges that moved or edges moving in a particular direction.
2) Hearing. Auditory signals are received by the primary auditory cortex in the superior region of the temporal lobe and in the nearby insula. The auditory association area occupies areas of temporal lobe inferior to the primary auditory cortex and deep within the lateral sulcus.
3) Equilibrium. Signals from the inner ear for equilibrium (balance and the sense of motion) project mainly to the cerebellum and several brainstem nuclei concerned with head and eye movements and visceral functions. Some fibers of this system, however, are routed through the thalamus to areas of association cortex in the roof of the lateral sulcus and near the lower end of the central sulcus. This is the seat of consciousness of our body movements and orientation in space.
4) Taste. Gustatory signals are received by the primary gustatory cortex in the inferior end of the postcentral gyrus of the parietal lobe and an anterior region of the insula. Tastes are detected by taste buds, special structures of which every human has some 5,000. Taste buds are embedded within papillae, or protuberances, located mainly on the tongue, with others found in the back of the mouth and on the palate. Taste substances stimulate hairs projecting from the sensory cells. Each taste bud consists of 50 to 100 sensory cells that respond to salts, acidity, sweet substances and bitter compounds. Some researchers add a fifth category named umami, for the taste of monosodium glutamate and related substances. Taste signals in the sensory cells are transferred by synapses to the ends of nerve fibers, which send impulses along cranial nerves to taste centers in the brain. From here, the impulses are relayed to other brain stem centers responsible for the basic responses of acceptance or rejection of the tastes, and to the thalamus and on to the cerebral cortex for conscious perception of taste. Specialized smell receptor cells are located in a small patch of mucus membrane lining the roof of the nose. Axons of these sensory cells pass through perforations in the overlying bone and enter two elongated olfactory bulbs lying on top of the bone. The portion of the sensory cell that is exposed to odors possesses hair-like cilia. These cilia contain the receptor sites that are stimulated by odors carried by airborne molecules. The odor molecules dissolve in the mucus lining in order to stimulate receptor molecules in the cilia to start the smell response. An odor molecule acts on many receptors to different degrees. Similarly, a receptor interacts with many different odor molecules to different degrees.
5) Smell. Olfactory signals are the only sensory signals that can reach the cortex without going through the thalamus. The primary olfactory cortex lies in the medial surface of the temporal lobe and inferior surface of the frontal lobe. The orbitofrontal cortex (see fig. B11.).Is an association area that integrates gustatory, olfactory, and visual information to create a sense of the overall flavor and desirability (or rejection) of food.
Figure B11. The Limbic System.
The primary motor cortex is in the precentral gyrus, immediately anterior to the central sulcus (figure B11). This gyrus forms the posterior margin of the frontal lobe. Like the primary somesthetic cortex, it exhibits a somatotopic, upside-down map of the contralateral side of the body, but its map represents muscle control rather than sensation. Thus, the primary motor cortex deep in the longitudinal fissure controls muscles of the foot and leg; as we progress up the gyrus toward the crown of the head, we find cortex that controls muscles of the hip and trunk; and descending the gyrus on the lateral surface of the brain, we find cortex in control of the upper limb, then face, and the tongue and pharynx. Like the sensory homunculus, the motor homunculus in (figure B12.) is distorted because the amount of cortex dedicated to a particular body region is proportional to the number of muscles and motor units in that region, not the size of the region. Thus, it requires more motor cortex to control a hand than it does to control the muscles of the trunk.
Figure. B12.The Primary Motor Area . Motor homunculus.
Motor homunculus drawn so that body parts are in proportion to the amount of primary motor cortex dedicated to their control.
Language includes several abilities like reading, writing, speaking, and understanding spoken and printed words assigned to different regions of cerebral cortex. The Wernicke area is responsible for the recognition of spoken and written language. It lies just posterior to the lateral sulcus, usually in the left hemisphere, at the “crossroads” between visual, auditory, and somesthetic areas of the cortex (see fig. B10). It is a sensory association area that receives input from all these neighboring regions of primary sensory cortex. The emotional aspect of language is controlled by regions in the opposite hemisphere that mirror the Wernicke and Broca areas. Opposite the Broca area is the affective language area. Lesions to this area result in aprosodia flat, emotionless speech. The cortex opposite the Wernicke area is concerned with recognizing the emotional content of another person’s speech. Lesions here can result in such problems as the inability to understand a joke. Lesions in the language areas of the brain tend to produce a variety of language deficits called aphasias.
Emotional feelings and memories are not exclusively cerebral functions, but rather result from an interaction between areas of the prefrontal cortex and the diencephalon. In the diencephalon, the hypothalamus and amygdala play especially important roles in emotion. Experiments by Swiss physiologist Walter Hess, leading to a 1949 Nobel Prize, showed that stimulation of various nuclei of the hypothalamus in cats could induce rage, attack, and other emotional responses. Nuclei involved in the senses of reward and punishment have been identified in the hypothalamus of cats, rats, monkeys, and other animals. The amygdala, one of the most important centers of human emotion, is a major component of the limbic system described earlier. It receives processed information from the senses of vision, hearing, taste, smell, and general somesthetic and visceral senses. Thus, it is able to mediate emotional responses to such stimuli as a disgusting odor, a foul taste, a beautiful image, pleasant music, or a stomach ache. It is especially important in the sense of fear. Output from the amygdala goes in two directions of special interest:
Some output projects to the hypothalamus and lower brainstem, and thus influences the somatic and visceral motor systems. An emotional response to a sight or sound may, through these connections,make one’s heart race, make the hair stand on end, or induce vomiting.
Other output projects to areas of the prefrontal cortex that mediate conscious control and expression of the emotions, such as our ability to express love or control anger.
Many important aspects of personality depend on an intact, functional amygdala and hypothalamus. When specific regions of the amygdala or hypothalamus are destroyed or artificially stimulated, we asa humans beings we exhibit blunted or exaggerated expressions of anger, fear, aggression, self-defense, pleasure, pain, love, sexuality, and parental affection, as well as abnormalities in learning, memory, and motivation.
Cognition is the range of mental processes by which we acquire and use knowledge such as sensory perception, thought, reasoning, judgment, memory, imagination, and intuition. Cognitive abilities of various kinds are widely distributed through the association areas of the cerebral cortex. This is the most difficult area of brain research, and the most incompletely understood area of cerebral function. Much of what we know about cognitive functions of the brain has come from studies of patients with brain lesions areas of tissue destruction resulting from cancer, stroke, and trauma.
Attention to objects in the environment is based in the parietal lobe on the side opposite the Broca and Wernicke speech centers. Lesions here can produce contralateral neglect syndrome, in which a patient seems unaware of objects on one side of the body, ignores all words on the left side of a page of reading, or fails even to recognize, dress, and take care of the left side of his or her own body. Such patients are also unable to find their way around say, to describe the route from home to work, or navigate within a familiar building.
The prefrontal cortex is concerned with many of our most distinctive abilities, such as abstract thought, foresight, judgment, responsibility, a sense of purpose, and a sense of socially appropriate behavior. Lesions here tend to render a person easily distracted from a task, irresponsible, exceedingly stubborn, unable to anticipate future events, and incapable of any ambition or planning for the future.
Memory is one of the cognitive functions, but warrants special attention. There are two kinds of memory which are procedural memory which deals with the retention of motor skills such as how to tie one’s shoes, play the violin, or type on a keyboard; and declarative memory which deals with the retention of events and facts that one can put into words, such as names, dates, or facts important to an upcoming examination. At the cellular level, both forms of memory probably involve the same processes: the creation of new synaptic contacts and physiological changes that make synaptic transmission more efficient along certain pathways. The limbic system has important roles in the establishment of memories. The amygdala creates emotional memories, such as the chilling fear of being stung when a wasp alights on the skin. The hippocampus (see fig. B11.).Is critical to the creation of long-term declarative memories. It does not store memories, but organizes sensory and cognitive experiences into a unified long-term memory. The hippocampus learns from sensory input while an experience is happening, but it has a short memory. Later, perhaps when one is sleeping, it plays this memory repeatedly to the cerebral cortex, which is a “slow.
A type of transient memory that enables us to retain what someone has said just long enough to reply, depends in part on the prefrontal cortex. Researchers discovered that certain neurons in this area are influenced by neurons releasing dopamine and other neurons releasing glutamate. While much is unknown about learning and memory, scientists can recognize certain pieces of the process. For example, the brain appears to process different kinds of information in separate ways and then store it differently. Procedural knowledge, the knowledge of how to do something, is expressed in skilled behavior and learned habits. Declarative knowledge provides an explicit, consciously accessible record of individual previous experiences and a sense of familiarity about those experiences. Declarative knowledge requires processing in the medial temporal region and parts of the thalamus, while procedural knowledge requires processing by the basal ganglia. Other kinds of memory depend on the emotional aspects of memory and the cerebellum
An important factor that influences what is stored and how strongly it is stored is whether the action is followed by rewarding or punishing consequences. This is an important principle in determining what behaviors an organism will learn and remember. The amygdala appears to play an important role in these memory events. How exactly does memory occur? After years of study, there is much support for the idea that memory involves a persistent change in the relationship between neurons. In animal studies, scientists found that this occurs through biochemical events in the short term that affects the strength of the relevant synapses. The stability of long-term memory is conferred by structural modifications within neurons that change the strength and number of synapses. For example, researchers can correlate specific chemical and structural changes in the relevant cells with several simple forms of behavioral change exhibited by the sea slug Aplysia.
Another important model for the study of memory is the phenomenon of long-term potentiation (LTP), a long-lasting increase in the strength of a synaptic response following stimulation. LTP occurs prominently in the hippocampus, as well as in other brain areas. Studies of rats suggest LTP occurs by changes in synaptic strength at contacts involving NMDA receptors. It is now possible to study LTP and learning in genetically modified mice that have abnormalities of specific genes. Abnormal gene expression can be limited to particular brain areas and time periods, such as during learning.
Scientists believe that no single brain center stores memory. It most likely is stored in the same, distributed collection of cortical processing systems involved in the perception, processing and analysis of the material being learned. In short, each part of the brain most likely contributes differently to permanent memory storage.
Figure. B13.The Human Brain.
Structures believed to be important for various kinds of learning and memory include the cerebral cortex, amygdala, hippocampus, cerebellum and basal ganglia. Areas of the left hemisphere (inset) are known to be active in speech and language. The form and meaning of an utterance is believed to arise in Wernicke’s area and then Broca’s area, which is related to vocalization. Wernicke’s area is also important for language comprehension.
The stuff of sleep
Sleep appears to be a passive and restful time when the brain is less active. In fact, this state actually involves a highly active and well-scripted interplay of brain circuits to produce the stages of sleeping. The stages of sleep were discovered in the 1950s in experiments examining the human brain waves or electroencephalogram (EEG) during sleep. Researchers also measured movements of the eyes and the limbs during sleep. They found that over the course of the first hour or so of sleep each night, the brain progresses through a series of stages during which the brain waves progressively slow down. The period of slow wave sleep is accompanied by relaxation of the muscles and the eyes. Heart rate, blood pressure and body temperature all fall. If awakened at this time, most people recall only a feeling or image, not an active dream.
During a night of sleep, the brain waves of a young adult recorded by the electroencephalogram (EEG) gradually slow down and become larger as the individual passes into deeper stages of slow wave sleep. After about an hour, the brain re-emerges through the same series of stages, and there is usually a brief period of REM sleep (on dark areas of graph), during which the EEG is similar to wakefulness. The body is completely relaxed, the person is deeply unresponsive and usually is dreaming. The cycle repeats over the course of the night, with more REM sleep, and less time spent in the deeper stages of slow wave sleep as the night progresses.
How is sleep regulated?
During wakefulness, the brain is kept in an alert state by the interactions of two major systems of nerve cells. Nerve cells in the upper part of the pons and in the midbrain, which make acetylcholine as their neurotransmitter, send inputs to the thalamus, to activate it. When the thalamus is activated, it in turn activates the cerebral cortex, and produces a waking EEG pattern. However, that is not all there is to wakefulness. As during REM sleep, the cholinergic nerve cells and the thalamus and cortex are in a condition similar to wakefulness, but the brain is in REM sleep, and is not very responsive to external stimuli. The difference is supplied by three sets of nerve cells in the upper part of the brainstem: nerve cells in the locus coeruleus that contain the neurotransmitter norepinephrine; in the dorsal and median raphe groups that contain serotonin; and in the tuberomammillary cell group that contains histamine.
These monoamine neurons fire most rapidly during wakefulness, but they slow down during slow wave sleep, and they stop during REM sleep. The brainstem cell groups that control arousal are in turn regulated by two groups of nerve cells in the hypothalamus, part of the brain that controls basic body cycles. One group of nerve cells, in the ventrolateral preoptic nucleus, contain inhibitory neurotransmitters, galanin and GABA. When the ventrolateral preoptic neurons fire, they are thought to turn of the arousal systems, causing sleep. Damage to the ventrolateral preoptic nucleus produces irreversible insomnia.
A second group of nerve cells in the lateral hypothalamus act as an activating switch. They contain the neurotransmitters orexin and dynorphin, which provide an excitatory signal to the arousal system, particularly to the monoamine neurons. In experiments in which the gene for the neurotransmitter orexin was experimentally removed in mice, the animals became narcoleptic. Similarly, in two dog strains with naturally occurring narcolepsy, an abnormality was discovered in the gene for the type 2 orexin receptor.
Recent studies show that in humans with narcolepsy, the orexin levels in the brain and spinal fluid are abnormally low. Thus, orexin appears to play a critical role in activating the monoamine system, and preventing abnormal transitions, particularly into REM sleep. Two main signals control this circuitry. First, there is homeostasis, or the body’s need to seek a natural equilibrium. There is an intrinsic need for a certain amount of sleep each day. The mechanism for accumulating sleep need is not yet clear. Some people think that a chemical called adenosine may accumulate in the brain during prolonged wakefulness, and that it may drive sleep homeostasis. Interestingly, the drug caffeine, which is widely used to prevent sleepiness, acts as an adenosine blocker, to prevent its effects. If an individual does not get enough sleep, the sleep debt progressively accumulates, and leads to a degradation of mental function. When the opportunity comes to sleep again, the individual will sleep much more, to “repay” the debt, and the slow wave sleep debt is usually “paid of” first.
The other major influence on sleep cycles is the body’s circadian clock, the suprachiasmatic nucleus. This small group of nerve cells in the hypothalamus contains clock genes, which go through a biochemical cycle of almost exactly 24 hours, setting the pace for daily cycles of activity, sleep, hormones and other bodily functions. The suprachiasmatic nucleus also receives an input directly from the retina, and the clock can be reset by light, so that it remains linked to the outside world’s day-night cycle. The suprachiasmatic nucleus provides a signal to the ventrolateral preoptic nucleus and probably the orexin neurons.
The two cerebral hemispheres look identical at a glance, but close examination reveals a number of differences. For example, in women the left temporal lobe is longer than the right. In left handed people, the left frontal, parietal, and occipital lobes are usually wider than those on the right. The two hemispheres also differ in some of their functions. Neither hemisphere is “dominant,” but each is specialized for certain tasks. This difference in function is called cerebral lateralization. One hemisphere, usually the left, is called the categorical hemisphere. It is specialized for spoken and written language and for the sequential and analytical reasoning employed in such fields as science and mathematics. This hemisphere seems to break information into fragments and analyze it in a linear way. The other hemisphere, usually the right, is called the representational hemisphere. It perceives information in a more integrated, holistic way. It is the seat of imagination and insight, musical and artistic skill, perception of patterns and spatial relationships, and comparison of sights, sounds, smells, and tastes
The midbrain is the shortest and highest section of the brainstem.The midbrain is a relay station between the reticular system lower down the brainstem and the forebrain above. The midbrain extends from the pons to the hypothalamus and encloses the cerebral aqueduct, a tunnel that connects the third and fourth ventricles. Several different kinds of reflexes are integrated in the midbrain, including visual and auditory reflexes. For example, if you see a wasp flying toward you, you automatically duck or twist away; this is a visual reflex, as is the coordinated movement of the eyeballs. Turning your head (ear) to a sound is an example of an auditory reflex. The midbrain is also concerned with what are called righting reflexes, those that keep the head upright and maintain balance or equilibrium. The midbrain is also involved in controlling the movement of the eyes and the size of their pupils.
Figure. B14.Cross section of the Middle Brain.
This involves all the main structures beneath the midbrain and includes the pons, medulla oblongata, and cerebellum. Pons and medulla oblongata form most of the brainstem, which is fused with the spinal cord. The medulla oblongata is involved in controlling breathing, heartbeat, and other vital processes. The pons is the connection between the cerebellum and the rest of the brain. The cerebellum coordinates body movements.
The pons is a bulging region on the anterior part of the brainstem that forms from part of the metencephalon (figure B4 and B5). The pons can be observed as a rounded bulge on the inferior surface of the brain, between the midbrain and the medulla oblongata (figure B15.). It consists of white fiber tracts that course in two principal directions. The surface fibers extend transversely to connect with the cerebellum through the middle cerebellar peduncles. The deeper longitudinal fibers are part of the motor and sensory tracts that connect the medulla oblongata with the tracts of the midbrain.
Figure B15.Nuclei within the pons and medulla oblongata that constitute the respiratory center.
It measures about 2.5 cm long from its caudal junction with the medulla to its rostral junction with the midbrain. Its white matter includes tracts that conduct signals from the cerebrum down to the cerebellum and medulla; tracts that carry sensory signals up to the thalamus; and tracts that cross the pons horizontally and connect the right and left hemispheres of the cerebellum. In addition, the middle cerebellar peduncles are transverse groups of fibers that connect the pons to the cerebellum. The pons also houses two autonomic respiratory centers: the pneumotaxic center and the apneustic center. These centers regulate the rate and depth of breathing.
The cerebellum is the second largest part of the brain, and it develops from the metencephalon. Is the largest part of the hindbrain (fig. B16). It is not usually considered part of the brainstem, but we consider it because of its developmental association with the pons. Its primary function is motor coordination.
Figure B16. The Cerebellum. (a) Median section, showing relationship to the brainstem. (b) Superior aspect.
The cerebellum has a complex, highly convoluted surface covered by a layer of cerebellar cortex. The folds of the cerebellar cortex are called folia (figure B17.).
The cerebellum is composed of left and right cerebellar hemispheres. Each hemisphere consists of two lobes, the anterior lobe and the posterior lobe, which are separated by the primary fissure . Along the midline, a narrow band of cortex known as the vermis separates the left and right cerebellar hemispheres. The vermis receives sensory input reporting torso position and balance. Its output to the vestibular nucleus helps maintain balance. Slender flocculonodular lobes lie anterior and inferior to each cerebellar hemisphere. The cerebellum is partitioned internally into three regions: an outer gray matter layer of cortex, an internal region of white matter, and the deepest gray matter layer, which is composed of cerebellar nuclei. The white matter of the cerebellum is called the arbor vitae because its distribution pattern resembles the branches of a tree. The cerebellum seems also to be involved in certain sensory functions. For example, if you close your eyes and someone places a tennis ball in one hand and a baseball in the other, could you tell which was which? Certainly you could, by the “feel” of each: the texture and the weight or heft. If you pick up a plastic container of coffee could you tell if the cup is full, half-full, or empty? Again, you certainly could. Do you have to think about it? No. The cerebellum is, in part, responsible for this ability.
The principal function of the cerebellum is coordinating skeletal muscle contractions by recruiting precise motor units within the muscles. This function is performed indirectly, by regulating activity along both the voluntary and involuntary motor pathways at the cerebral cortex, cerebral nuclei, and motor centers in the brainstem.
The cerebrum initiates a movement and sends a “rough draft” of the movement to the cerebellum, which then coordinates and fine-tunes it. For example, the controlled, precise movements a pianist makes when playing a concerto are due to fine-tuning by the cerebellum.
Impulses for voluntary muscular movement originate in the cerebral cortex and are coordinated by the cerebellum. The cerebellum constantly initiates impulses to selective motor units for maintaining posture and muscle tone. The cerebellum also processes incoming impulses from proprioceptors within muscles, tendons, joints, and special sense organs to refine learned movement patterns. A proprioceptor is a sensory nerve ending that is sensitive to changes in the tension of a muscle or tendon.
To regulate equilibrium, the cerebellum (and midbrain) uses information about gravity and movement provided by receptors in the inner ears.
Without the cerebellum, the pianist’s movements would be choppy and sloppy, as in banging an entire hand across the keyboard. In addition, the cerebellum has several other functions. It adjusts skeletal muscle activity to maintain equilibrium and posture. It also receives proprioceptive (sensory) information from the muscles and joints and uses this information to regulate the body’s position. For example, you are able to balance on one foot because the cerebellum takes the proprioceptive information from the body joints and “maps out” a muscle tone plan to keep the body upright. Finally, because proprioceptive information from the body’s muscles and joints is sent to the cerebellum, the cerebrum knows the position of each body joint and its muscle tone, even if the person is not looking at the joint. For example, if you close your eyes, you are still aware of which body joints are flexed and which are extended because the cerebrum gives you this awareness.
Like the cerebrum, the cerebellum has a thin outer layer of gray matter, the cerebellar cortex, and a thick, deeper layer of white matter. The cerebellum is convoluted into a series of parallel slender folia. The tracts of white matter within the cerebellum have a distinctive branching pattern called the arbor vitae that can be seen in a sagittal view (fig. B18.). Three paired bundles of nerve fibers called cerebellar peduncles support the cerebellum and provide it with tracts for communicating with the rest of the brain (fig. B19.). Following is a description of the cerebellar peduncles.
Figure B18.The structure of the cerebellum. a sagittal view.
Figure B19. The cerebellar peduncles
The cerebellar pedunclescan be seen when the cerebellar hemisphere has been removed from its attachment to the brain stem.
- Superior cerebellar peduncles connect the cerebellum with the midbrain. The fibers within these peduncles originate primarily from specialized dentate nuclei within the cerebellum and pass through the red nucleus to the thalamus, and then to the motor areas of the cerebral cortex. Impulses through the fibers of these peduncles provide feedback to the cerebrum.
- Middle cerebellar peduncles convey impulses of voluntary movement from the cerebrum through the pons and to the cerebellum.
- Inferior cerebellar peduncles connect the cerebellum with the medulla oblongata and the spinal cord. They contain both incoming vestibular and proprioceptive fibers and outgoing motor fibers.
The medulla oblongata or simply the medulla, is a part of hind brain which formed from the myelencephalon. The medulla oblongata of the brain stem is the structure that most directly controls the activity of the Autonomic Nervous System. Almost all autonomic responses can be elicited by experimental stimulation of the medulla oblongata, which contains centers for the control of thecirculatory, respiratory, urinary, reproductive, and digestive systems. Much of the sensory input to these centers travels through the sensory neurons of the vagus nerves. The medulla oblongata is a bulbous structure about 3 cm (1 in.) long, and it is the most inferior structure of the brain stem. It is continuous with the pons anteriorly and the spinal cord posteriorly at the level of the foramen magnum (see fig. B20.). Externally, the medulla oblongata resembles the spinal cord, except for two triangular elevations called pyramids on the inferior side and an oval enlargement called the olive(see fig. B20.)
On each lateral surface. The fourth ventricle, the space within the medulla oblongata, is continuous posteriorly with the central canal of the spinal cord and anteriorly with the mesencephalic aqueduct (see fig. B21).
FIGURE B21. The flow of cerebrospinal fluid.
Cerebrospinal fluid is secreted by choroid plexuses in the ventricular walls. The fluid circulates through the ventricles and central canal, enters the subarachnoid space, and is reabsorbed into the blood of the dural sinuses through the arachnoid villi.
It is the most inferior part of the brainstem and is continuous with the spinal cord inferiorly. The posterior portion of the medulla resembles the spinal cord with its flattened, round shape and narrow central canal. As the central canal extends anteriorly toward the pons, it enlarges and becomes the fourth ventricle. All communication between the brain and spinal cord involves tracts that ascend or descend through the medulla oblongata (figure B22.). Several external landmarks are visible on the medulla oblongata. The anterior surface exhibits two longitudinal ridges called the pyramids which house the motor projection tracts called the corticospinal (pyramidal) tracts.
In figure B22 above, a cross section illustrates important internal structures and decussations of the pyramids.
In the posterior region of the medulla, most of these axons cross to the opposite side of the brain at a point called the decussation of the pyramids .As a result of the crossover, each cerebral hemisphere controls the voluntary movements of the opposite side of the body. Immediately lateral to each pyramid is a distinct bulge, called the olive, which contains a large fold of gray matter called the inferior olivary nucleus. The inferior olivary nuclei relay ascending sensory impulses, especially proprioceptive information, to the cerebellar cortex. Additionally, paired inferior cerebellar peduncles are tracts that connect the medulla oblongata to the cerebellum. Within the medulla oblongata are additional nuclei that have various functions. The cranial nerve nuclei are associated with the vestibulocochlear (CN VIII), glossopharyngeal (CN IX), vagus (CN X), accessory (CN XI), and hypoglossal (CN XII) cranial nerves. In addition, the medulla oblongata contains the paired nucleus cuneatus and the nucleus gracilis , which relay somatic sensory information to the thalamus. The nucleus cuneatus receives posterior root fibers corresponding to sensory innervation from the arm and hand of the same side. The nucleus gracilis receives posterior root fibers carrying sensory information from the leg and lower limbs of the same side. Bands of myelinated fibers composing a medial lemniscus exit these nuclei and decussate in the inferior region of the medulla oblongata. The medial lemniscus projects through the brainstem to the ventral posterior nucleus of the thalamus. Finally, the medulla oblongata contains several autonomic nuclei, which regulate functions vital for life. Autonomic nuclei group together to form centers in the medulla oblongata. Following are the most important autonomic centers in the medulla oblongata and their functions:
1. Cardiac center. Both inhibitory and accelerator fibers arise from nuclei of the cardiac center. Inhibitory impulses constantly travel through the vagus nerves to slow the heartbeat. Accelerator impulses travel through the spinal cord and eventually innervate the heart through fibers within spinal nerves T1-T5.
2. Vasomotor center. Nuclei of the vasomotor center send impulses via the spinal cord and spinal nerves to the smooth muscles of arteriole walls, causing them to constrict and elevate arterial blood pressure.
3. Respiratory center. The respiratory center of the medulla oblongata controls the rate and depth of breathing and functions in conjunction with the respiratory nuclei of the pons to produce rhythmic breathing.
Other nuclei of the medulla oblongata function as centers for reflexes involved in sneezing, coughing, swallowing, and vomiting. Some of these activities (swallowing, for example) may be initiated voluntarily, but once they progress to a certain point they become involuntary and cannot be stopped.
The medulla oblongata is composed of vital nuclei and white matter that form all the descending and ascending tracts communicating between the spinal cord and various parts of the brain. Most of the fibers within these tracts cross over to the opposite side through the pyramidal region of the medulla oblongata, permitting one side of the brain to receive information from and send information to the opposite side of the body. The gray matter of the medulla oblongata consists of several important nuclei for the cranial nerves and sensory relay.
Three to four weeks after conception, one of the two cell layers of the gelatin-like human embryo, now about one-tenth of an inch long, starts to thicken and build up along the middle. As this flat neural plate grows, parallel ridges, similar to the creases in a paper airplane, rise across its surface. Within a few days, the ridges fold in toward each other and fuse to form the hollow neural tube. The top of the tube thickens into three bulges that form the hindbrain, midbrain and forebrain. The first signs of the eyes and then the hemispheres of the brain appear later. How does all this happen? Although many of the mechanisms of human brain development remain secrets, neuroscientists are beginning to uncover some of these complex steps through studies of the roundworm, fruit fly, frog, zebrafish, mouse, rat, chicken, cat and monkey. Many initial steps in brain development are similar across species, while later steps are different. By studying these similarities and differences, scientists can learn how the human brain develops and how brain abnormalities, such as mental retardation and other brain disorders, can be prevented or treated. Neurons are initially produced along the central canal in the neural tube. These neurons then migrate from their birth place to a final destination in the brain. They collect together to form each of the various brain structures and acquire specific ways of transmitting nerve messages. Their processes, or axons, grow long distances to find and connect with appropriate partners, forming elaborate and specific circuits. Finally, sculpting action eliminates redundant or improper connections, honing the specificity of the circuits that remain. The result is the creation of a precisely elaborated adult network of 100 billion neurons capable of a body movement, a perception, an emotion or a thought.
In order to understand how the structures of the adult brain are named and connected, it is essential to know how the brain develops. In the human embryo, the brain forms from the cranial (superior) part of the neural tube, which undergoes disproportionate growth rates in different regions. By the late fourth week of development, this growth has formed three primary brain vesicles , which eventually give rise to all the different regions of the adult brain.
The names of these vesicles describe their relative positions in the developing head: The forebrain is called the prosencephalon; the midbrain is called the mesencephalon; and the hindbrain is called the rhombencephalon ( figure B23 a). By the fifth week of development, the three primary vesicles further develop into a total of five secondary brain vesicles ( figure B23 b ):
■ The telencephalon arises from the prosencephalon and eventually forms the cerebrum.
■ The diencephalon derives from the prosencephalon and eventually forms the thalamus, hypothalamus, and epithalamus.
■ The mesencephalon is the only primary vesicle that does not form a new secondary vesicle
■ The metencephalo arises from the rhombencephalon and eventually forms the pons and cerebellum.
■ The myelencephalon also derives from the rhombencephalon, and it eventually forms the medulla oblongata.As the future brain develops, its surface becomes folded, especially in the telencephalon, leading to the formation of the adult sulci and gyri (see figure B23 a ). The bends and creases that occur in the developing brain determine the boundaries of the brain’s cavities. Together, the bends, creases, and folds in the telencephalon surface are necessary in order to fit the massive amount of brain tissue within the confines of the cranial cavity. Most of the gyri and sulci develop late in the fetal period, so that by the time the fetus is born, its brain closely resembles that of an adult ( figure B23 c – e ).
Figure B23. Structural Changes in the Developing Brain.
( a ) As early as 4 weeks, the growing brain is bent because of space restrictions in the developing head. ( b ) At 5 weeks, the secondary brain vesicles appear. ( c ) By 13 weeks, the telencephalon grows rapidly and envelops the diencephalon. ( d ) Some major sulci and gyri are present by 26 weeks. ( e ) The features of an adult brain are present at birth.
Following the period of growth, the network is pared back to create a more sturdy system. Only about one-half of the neurons generated during development survive to function in the adult. Entire populations of neurons are removed through internal suicide programs initiated in the cells. The programs are activated if a neuron loses its battle with other neurons to receive life-sustaining nutrients called trophic factors. These factors are produced in limited quantities by target tissues. Each type of trophic factor supports the survival of a distinct group of neurons. For example, nerve growth factor is important for sensory neuron survival. It has recently become clear that the internal suicide program is maintained into adulthood, and constantly held in check. Based on this idea, researchers have found that injuries and some neurodegenerative diseases kill neurons not directly by the damage they inflict, but rather by activating the death program. This discovery, and its implication that death need not inevitably follow insult, have led to new avenues for therapy. Brain cells also form too many connections at first. For example, in primates, the projection from the two eyes to the brain initially overlaps, and then sorts out to separate territories devoted only to one or the other eye. Furthermore, in the young primate cerebral cortex, the connections between neurons are greater in number and twice as dense as an adult primate. Communication between neurons with chemical and electrical signals is necessary to weed out the connections. The connections that are active and generating electrical currents survive while those with little or no activity are lost.
Organization of Neural Tissue Areas in the Brain
Two distinct tissue areas are recognized within the brain and spinal cord: gray matter and white matter. The gray matter houses motor neuron and interneuron cell bodies, dendrites, telodendria, and unmyelinated axons. The white matter derives its color from the myelin in the myelinated axons. During brain development, an outer, superficial region of gray matter forms from migrating peripheral neurons. As a result, the external layer of gray matter, called the cerebral cortex, covers the surface of most of the adult brain (the cerebrum and the cerebellum). The white matter lies deep to the gray matter of the cortex. Finally, within the masses of white matter, the brain also contains discrete internal clusters of gray matter called cerebral nuclei , which are oval, spherical, or sometimes irregularly shaped clusters of neuron cell bodies.
The brain’s refining and building of the network in humans, continues after birth. An organism’s interactions with its surroundings fine-tune connections. Changes occur during critical periods. These are windows of time during development when the nervous system must obtain certain critical experiences, such as sensory, movement or emotional input, to develop properly. Following a critical period, connections become diminished in number and less subject to change, but the ones that remain are stronger, more reliable and more precise. Injury, sensory or social deprivation occurring at a certain stage of postnatal life may affect one aspect of development, while the same injury at a different period may affect another aspect. In one example, a monkey is raised from birth up to six months of age with one eyelid closed. As a result of diminished use, the animal permanently loses useful vision in that eye. This gives cellular meaning to the saying “use it or lose it.” Loss of vision is caused by the actual loss of functional connections between that eye and neurons in the visual cortex. This finding has led to earlier and better treatment of the eye disorders congenital cataracts and “crossed-eyes” in children. Research also shows that enriched environments can bolster brain development during postnatal life. For example, studies show that animals brought up in toy-filled surroundings have more branches on their neurons and more connections than isolated animals. In one recent study, scientists found enriched environments resulted in more neurons in a brain area involved in memory. Scientists hope that new insights on development will lead to treatments for those with learning disabilities, brain damage and even neurodegenerative disorders or aging.
THE LIMBIC SYSTEM
The limbic system is one of the brain’s most important centers of emotion and learning. It was originally described in the 1850s as a ring of structures on the medial side of the cerebral hemisphere, encircling the corpus callosum and thalamus. Its most anatomically prominent components are the cingulate gyrus that arches over the top of the corpus callosum in the frontal and parietal lobes, the hippocampus in the medial temporal lobe (fig. B24), and the amygdala immediately rostral to the hippocampus, also in the temporal lobe. There are still differences of opinion on what structures to consider as parts of the limbic system, but these three are agreed upon; other components include the mammillary bodies and other hypothalamic nuclei, some thalamic nuclei, parts of the basal nuclei, and parts of the prefrontal cortex.
Limbic system components are interconnected through a complex loop of fiber tracts allowing for somewhat circular patterns of feedback among its nuclei and cortical neurons. All of these structures are bilaterally paired; there is a limbic system in each cerebral hemisphere. The limbic system was long thought to be associated with smell because of its close association with olfactory pathways, but beginning in the early 1900s and continuing even now, experiments have abundantly demonstrated more significant roles in emotion and memory. Most limbic system structures have centers for both gratification and aversion .Stimulation of a gratification center produces a sense of pleasure or reward; stimulation of an aversion center produces unpleasant sensations such as fear or sorrow. Gratification centers dominate some limbic structures, such as the nucleus accumbens (not illustrated), while aversion centers dominate others such as the amygdala.The roles of the amygdala in emotion and the hippocampus in memory are described in the next section, on higher forebrain functions.
Support and Protection of the Brain
The brain is protected and isolated by multiple structures. The bony cranium provides rigid support, while protective connective tissue membranes called meninges surround, support, stabilize, and partition portions of the brain. Cerebrospinal fluid (CSF) acts as a cushioning fluid. Finally, the brain has a blood-brain barrier to prevent harmful materials from entering the bloodstream.
The cranial meninges are three connective tissue layers that separate the soft tissue of the brain from the bones of the cranium, enclose and protect blood vessels that supply the brain, and contain and circulate cerebrospinal fluid. In addition, some parts of the cranial meninges form some of the veins that drain blood from the brain. From deep (closest to the brain) to superficial (farthest away from the brain), the cranial meninges are the pia mater, the arachnoid, and the dura mater ( figure B25).
The pia mater is the innermost of the cranial meninges. It is a thin layer of delicate areolar connective tissue that is highly vascularized
and tightly adheres to the brain, following every contour of the surface. Arachnoid The arachnoid , also called the arachnoid mater or the arachnoid membrane , lies external to the pia mater ( figure B25). The term arachnoidmeans “resembling a spider web,” and this meninx is so named because it is partially composed of a delicate web of collagen and elastic fibers, termed the arachnoid trabeculae . Immediately deep to the arachnoid is the subarachnoid space . The arachnoid trabeculae extend through this space from the arachnoid to the underlying pia mater. Between the arachnoid and the overlying dura mater is a potential space, the subdural space . The subdural space becomes an actual space if blood or fluid accumulates there, a condition called a subdural hematoma.
The dura mater is an external tough, dense irregular connective tissue layer composed of two fibrous layers. As its Latin name indicates, it is the strongest of the meninges. Within the cranium, the dura mater is composed of two layers. The meningeal layer lies deep to the periosteal layer. The periosteal layer, the more superficial layer, forms the periosteum on the internal surface of the cranial bones. The meningeal layer is usually fused to the periosteal layer, except in specific areas where the two layers separate to form large, blood-filled spaces called dural venous sinuses. Dural venous sinuses are typically triangular in cross section, and unlike most other veins, they do not have valves to regulate venous blood flow. The dural venous sinuses are, in essence, large veins that drain blood from the brain and transport this blood to the internal jugular veins that help drain blood circulation to the head.
The dura mater and the bones of the skull may be separated by the potential epidural space, which contains the arteries and veins that nourish the meninges and bones of the cranium. Under normal (healthy) conditions, the potential space is not a space at all. However, it has the potential to become a real space and fill with fluid or blood as a result of trauma or disease
Cranial Dural Septa
The meningeal layer of the dura mater extends as flat partitions (septa) into the cranial cavity at four locations. Collectively, these double layers of dura mater are called cranial dural septa. These membranous partitions separate specific parts of the brain and provide additional stabilization and support to the entire brain. There are four cranial dural septa: the falx cerebri, tentorium cerebelli, falx cerebelli, and diaphragma sellae (figure B26 ).
Figure. B26. Cranial Dural Septa.
An illustration and a cadaver photo of a midsagittal section of the skull show the orientation of the falx cerebri, falx cerebelli, tentorium cerebelli, and diaphragma sellae.
The falx is the largest of the four dural septa. This large, sickle-shaped vertical fold of dura mater, located in the midsagittal plane, projects into the longitudinal fissure between the left and right cerebral hemispheres. Anteriorly, its inferior portion attaches to the crista galli of the ethmoid bone; posteriorly, its inferior portion attaches to the internal occipital crest. Running within the margins of this dural septa are two dural venous sinuses: the superior sagittal sinus and the inferior sagittal sinus. The tentorium cerebelli is a horizontally oriented fold of dura mater that separates the occipital and temporal lobes of the cerebrum from the cerebellum. It is named for the fact that it forms a dural “tent” over the cerebellum. The transverse sinuses run within its posterior border. The anterior surface of the tentorium cerebelli has a gap or opening, called the tentorial notch, to allow for the passage of the brainstem. Extending into the midsagittal line inferior to the tentorium cerebelli is the falx cerebelli , a sickle-shaped vertical partition that divides the left and right cerebellar hemispheres. A tiny occipital sinus (another dural venous sinus) runs in its posterior vertical border. The smallest of the dural septa is the diaphragma sellae which forms a “roof” over the sella turcica of the sphenoid bone. A small opening within it allows for the passage of a thin stalk, called the infundibulum that attaches the pituitary gland to the base of the hypothalamus.
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