Chapter 14 The Brain And Cranial Nerves

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Rostral and caudal

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Two directional terms used in descriptions of CNS anatomy are rostral and caudal. Rostral means “toward the nose” and caudal means “toward the tail.” In the spinal cord and brainstem, which are vertically oriented, rostral means “higher” and caudal means “lower.”

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cerebrum (seh-REE-brum or SER-eh-brum)

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The cerebrum (seh-REE-brum or SER-eh-brum) is about 83% of the brain’s volume and consists of a pair of half globes called the cerebral hemispheres. Each hemisphere is marked by thick folds called gyri (JY-rye; singular, gyrus) separated by shallow grooves called sulci (SUL-sye; singular, sulcus). A very deep median groove, the longitudinal fissure, separates the right and left hemispheres from each other. At the bottom of this fissure, the hemispheres are connected by a thick bundle of nerve fibers called the corpus callosum—a prominent landmark for anatomical description with a distinctive C shape in sagittal section.

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The cerebellum (SER-eh-BEL-um)

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occupies the posterior cranial fossa inferior to the cerebrum, separated from it by the transverse cerebral fissure. It is also marked by gyri, sulci, and fissures. The cerebellum is the second-largest region of the brain, constituting about 10% of its volume but containing over 50% of its neurons.

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The brainstem

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is defined differently by various authorities. The original definition, adopted here, is that it is all of the brain except the cerebrum and cerebellum. Its major components, from rostral to caudal, are the diencephalon, midbrain, pons, and medulla oblongata. Many authorities, however, exclude the diencephalon, since it can also be classified with the cerebrum as part of the forebrain.

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Gray matter

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the seat of the neurosomas, dendrites, and synapses—forms a surface layer called the cortex over the cerebrum and cerebellum, and deeper masses called nuclei surrounded by white matter.

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White matter

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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, white matter is composed of tracts, or bundles of axons, which here connect one part of the brain to another and to the spinal cord.

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neural tube

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The neural folds roll toward each other and fuse, somewhat like a closing zipper, beginning in the cervical region and progressing both caudally and rostrally. By day 26, this process creates a hollow channel called the neural tube.

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Neural crest

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As the neural tube develops, some ectodermal cells that originally lay along the margin of the groove separate from the rest and form a longitudinal column on each side called the neural crest. Neural crest cells give rise to the two inner meninges (arachnoid mater and pia mater); most of the peripheral nervous system, including the sensory and autonomic nerves and ganglia and Schwann cells; and some other structures of the skeletal, integumentary, and endocrine systems.

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Forebrain

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The forebrain divides into two of them, the telencephalon (TEL-en-SEFF-uh-lon) and diencephalon- (DY-en-SEFF-uh-lon) A portion of the brain between the midbrain and corpus callosum; composed of the thalamus, epithalamus, and hypothalamus.

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Midbrain

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midbrain remains undivided and retains the name mesencephalon

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Hindbrain

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hindbrain divides into two vesicles, the metencephalon (MET-en-SEF-uh-lon) and myelencephalon (MY-el-en-SEF-uh-lon)

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dural sinuses

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the two layers of dura are separated by dural sinuses, spaces that collect blood that has circulated through the brain.

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superior sagittal sinus and transverse sinus

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Two major, superficial ones are the superior sagittal sinus, found just under the cranium along the median line, and the transverse sinus, which runs horizontally from the rear of the head toward each ear. These sinuses meet like an inverted T at the back of the brain and ultimately empty into the internal jugular veins of the neck.

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ventricles

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The brain has four internal chambers called ventricles (fluid filled): The largest and most rostral ones are the two lateral ventricles, which form an arc in each cerebral hemisphere. Through a tiny pore called the interventricular foramen, each lateral ventricle is connected to the third ventricle, a narrow median 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.

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choroid (CO-royd) plexus

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On the floor or wall of each ventricle is a spongy mass of blood capillaries called a choroid (CO-royd) plexus, named for its histological resemblance to a fetal membrane called the chorion. Ependyma, a type of neuroglia that resembles a cuboidal epithelium, lines the ventricles and canals and covers the choroid plexuses. It produces cerebrospinal fluid.

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Cerebrospinal fluid (CSF)

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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 is normally present at one time

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arachnoid granulations

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CSF is reabsorbed by arachnoid granulations, extensions of the arachnoid meninx shaped like little sprigs of cauliflower, protruding through the dura mater into the superior sagittal sinus. CSF penetrates the walls of the granulations and mixes with blood in the sinus.

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Cerebrospinal fluid serves three purposes 1 of 3:

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Buoyancy. Because the brain and CSF are 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 CSF 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 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.

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Cerebrospinal fluid serves three purposes 2 of 3:

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Protection. CSF 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.

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Cerebrospinal fluid serves three purposes 3 of 3:

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Chemical stability. CSF rinses metabolic wastes from the nervous tissue and regulates its chemical environment. Slight changes in CSF composition can cause malfunctions of the nervous system. For example, a high glycine concentration disrupts the control of body temperature and blood pressure, and a high pH causes dizziness and fainting.

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brain barrier system

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that strictly regulates what can get from the bloodstream into the tissue fluid of the brain.

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blood–brain barrier (BBB)

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which consists of tight junctions between the endothelial cells that form the capillary walls. In the developing brain, astrocytes reach out and contact the capillaries with their perivascular feet, inducing endothelial cells to form tight junctions that completely seal off the gaps between them. This ensures that anything leaving the blood must pass through the cells and not between them. The endothelial cells are more selective than gaps between them would be, and can exclude harmful substances from the brain tissue while allowing necessary ones to pass through. At the choroid plexuses, the brain is protected by a similar blood–CSF barrier formed by tight junctions between the ependymal cells. 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.

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circumventricular organs (CVOs)

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Trauma and inflammation sometimes damage the BBS and allow pathogens to enter the brain tissue. Furthermore, there are places called circumventricular organs (CVOs) in the third and fourth ventricles where the barrier is absent and the blood has direct access to brain neurons. These enable the brain to monitor and respond to fluctuations in blood glucose, pH, osmolarity, and other variables. Unfortunately, CVOs also afford a route of invasion by the human immunodeficiency virus (HIV).

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medulla oblongata (OB-long-GAH-ta)

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The most caudal part of the brainstem, immediately superior to the foramen magnum of the skull, connecting the spinal cord to the rest of the brain. begins at the foramen magnum of the skull and extends for about 3 cm rostrally, ending at a groove between the medulla and pons. It looks superficially like an extension of the spinal cord, but slightly wider.

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medulla oblongata: Pyramids, olive, gracile and cuneate fasciculi

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Externally, the anterior surface features a pair of ridges called the pyramids. Resembling side-by-side baseball bats, these are wider at the rostral end, taper caudally, and are separated by a groove, the anterior median fissure continuous with that of the spinal cord. Lateral to each pyramid is a prominent bulge called the olive. Posteriorly, the gracile and cuneate fasciculi of the spinal cord continue as two pairs of ridges on the medulla.

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Medula Ascending Nerves

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ascending fibers include first-order sensory fibers of the gracile and cuneate fasciculi, which end in the gracile and cuneate nuclei, a cross section of the medulla. Here, they synapse with second-order fibers that decussate and form the ribbonlike medial lemniscus on each side. The second-order fibers rise to the thalamus, synapsing there with third-order fibers that complete the path to the cerebral cortex

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Medula Descending Nerves

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The largest group of descending fibers is the pair of corticospinal tracts filling the pyramids on the anterior surface. These carry motor signals from the cerebral cortex on the way to the spinal cord, ultimately to stimulate the skeletal muscles. Any time you carry out a body movement below the neck, the signals en route to your muscles pass through here. About 90% of these fibers cross over at the pyramidal decussation, an externally visible point near the caudal end of the pyramids. As a result, muscles below the neck are controlled by the contralateral side of the brain. A smaller tectospinal tract controls the neck muscles.

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Medula sensory and motor functions

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he former include the senses of hearing, equilibrium, touch, pressure, temperature, taste, and pain; the latter include chewing, salivation, swallowing, gagging, vomiting, respiration, speech, coughing, sneezing, sweating, cardiovascular and gastrointestinal control, and head, neck, and shoulder movements. Signals for these functions enter and leave the medulla not only by way of the spinal cord, but also by four pairs of cranial nerves that begin or end here: cranial nerves VIII (in part), IX, X, and XII.

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inferior olivary nucleus (Medula)

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a major relay center for signals going from many levels of the brain and spinal cord to the cerebellum. The reticular formation, detailed later, is a loose network of nuclei extending throughout the medulla, pons, and midbrain

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cardiac center

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which regulates the rate and force of the heartbeat

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vasomotor center

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which regulates blood pressure and flow by dilating and constricting blood vessels;

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respiratory centers

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two respiratory centers, which regulate the rhythm and depth of breathing; and other nuclei involved in the aforementioned motor functions.

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pons

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measures about 2.5 cm long. Most of it appears as a broad anterior bulge rostral to the medulla. Posteriorly, it consists mainly of two pairs of thick stalks called cerebellar peduncles, the cut edges in the upper half of figure. They connect the cerebellum to the pons and midbrain and will be discussed with the cerebellum.

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midbrain

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a short segment of brainstem that connects the hindbrain and forebrain. It contains the cerebral aqueduct, continuations of the medial lemniscus and reticular formation, and the motor nuclei of two cranial nerves that control eye movements: cranial nerves III (oculomotor) and IV (trochlear).

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Tectum

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The part of the midbrain posterior to the cerebral aqueduct is a rooflike tectum. It exhibits four bulges, the corpora quadrigemina. The upper pair, called the superior colliculi (col-LIC-you-lye), functions in visual attention, visually tracking moving objects, and such reflexes as blinking, focusing, pupillary dilation and constriction, and turning the eyes and head in response to a visual stimulus (for example, to look at something that you catch sight of in your peripheral vision). The lower pair, called the inferior colliculi, receives signals from the inner ear and relays them to other parts of the brain, especially the thalamus. Among other functions, they mediate the reflexive turning of the head in response to a sound, and one’s tendency to jump when startled by a sudden noise.

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cerebral peduncles

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midbrain consists mainly of the cerebral peduncles—two stalks that anchor the cerebrum to the brainstem. Each peduncle has three main components: tegmentum, substantia nigra, and cerebral crus. The tegmentum is dominated by the red nucleus, named for a pink color imparted by its high density of blood vessels. Fibers from the red nucleus form the rubrospinal tract in most mammals, but in humans its connections go mainly to and from the cerebellum, with which it collaborates in fine motor control. The substantia nigra (sub-STAN-she-uh NY-gruh) is a dark gray to black nucleus pigmented with melanin. It is a motor center that relays inhibitory signals to the thalamus and basal nuclei, preventing unwanted body movement. Degeneration of the neurons in the substantia nigra leads to the muscle tremors of Parkinson disease. The cerebral crus (pronounced “cruss”; plural, crura) is a bundle of nerve fibers that connect the cerebrum to the pons and carry the corticospinal nerve tracts.

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central (periaqueductal) gray matter

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This is involved with the reticulospinal tracts in controlling awareness of pain

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reticular formation

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The reticular formation is a loose web of gray matter that runs vertically through all levels of the brainstem, appearing at all three levels of figure. It occupies much of the space between the white fiber tracts and the more anatomically distinct brainstem nuclei, and has connections with many areas of the cerebrum. It consists of more than 100 small neural networks defined less by anatomical boundaries than by their use of different neurotransmitters.

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reticular formation functions 1:

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Somatic motor control. Some motor neurons of the cerebral cortex send their axons to reticular formation nuclei, which then give rise to the reticulospinal tracts of the spinal cord. These tracts adjust muscle tension to maintain tone, balance, and posture, especially during body movements. The reticular formation also relays signals from the eyes and ears to the cerebellum so the cerebellum can integrate visual, auditory, and vestibular (balance and motion) stimuli into its role in motor coordination. Other motor nuclei include gaze centers, which enable the eyes to track and fixate objects, and central pattern generators—neural pools that produce rhythmic signals to the muscles of breathing and swallowing.

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reticular formation functions 2:

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Cardiovascular control. The reticular formation includes the previously mentioned cardiac and vasomotor centers of the medulla oblongata.

Pain modulation. The reticular formation is one route by which pain signals from the lower body reach the cerebral cortex. It is also the origin of the descending analgesic pathways mentioned in the description of the reticulospinal tracts. Under certain circumstances, the nerve fibers in these pathways act in the spinal cord to deaden one’s awareness of pain

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reticular formation functions 3:

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Sleep and consciousness. The reticular formation has projections to the thalamus and cerebral cortex that allow it some control over what sensory signals reach the cerebrum and come to our conscious attention. It plays a central role in states of consciousness such as alertness and sleep. Injury to the reticular formation can result in irreversible coma.

Habituation. This is a process in which the brain learns to ignore repetitive, inconsequential stimuli while remaining sensitive to others. In a noisy city, for example, a person can sleep through traffic sounds but wake promptly to the sound of an alarm clock or a crying baby. Reticular formation nuclei that modulate activity of the cerebral cortex are called the reticular activating system or extrathalamic cortical modulatory system.

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cerebellum

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is the largest part of the hindbrain and second-largest part of the brain as a whole. It consists of right and left cerebellar hemispheres connected by a narrow wormlike bridge called the vermis. Each hemisphere exhibits slender, transverse, parallel folds called folia separated by shallow sulci. The cerebellum has a surface cortex of gray matter and a deeper layer of white matter. In a sagittal section, the white matter exhibits a branching, fernlike pattern called the arbor vitae. Each hemisphere has four masses of gray matter called deep nuclei embedded in the white matter. All input to the cerebellum goes to the cortex and all of its output comes from the deep nuclei.

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Cerebellum nuerons

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Although the cerebellum is only about 10% of the mass of the brain, it has about 60% as much surface area as the cerebral cortex and it contains more than half of all brain neurons—about 100 billion of them. Its tiny, densely spaced granule cells are the most abundant type of neuron in the entire brain. Its most distinctive neurons, however, are the unusually large, globose Purkinje (pur-KIN-jee) cells. These have a tremendous profusion of dendrites compressed into a single plane like a flat tree. The Purkinje cells are arranged in a single file, with these thick dendritic planes parallel to each other like books on a shelf. Their axons travel to the deep nuclei, where they synapse on output neurons that issue fibers to the brainstem.

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Cerebellum Cerebellar Peduncles

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The cerebellum is connected to the brainstem by three pairs of stalks called cerebellar peduncles (peh-DUN-culs): a pair of inferior peduncles connected to the medulla oblongata, a pair of middle peduncles to the pons, and a pair of superior peduncles to the midbrain. These consist of thick bundles of nerve fibers that carry signals to and from the cerebellum. Most spinal input enters the cerebellum by way of the inferior peduncles; most input from the rest of the brain enters by way of the middle peduncles; and cerebellar output travels mainly by way of the superior peduncles.

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thalamus

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Each side of the brain has a thalamus, an ovoid mass perched at the superior end of the brainstem beneath the cerebral hemisphere. The two thalami form about four-fifths of the diencephalon. Laterally, they protrude into the lateral ventricles. Medially, they protrude into the third ventricle and are joined to each other by a narrow intermediate mass in about 70% of people. The thalamus consists of at least 23 nuclei, most of which fall into five groups: anterior, posterior, medial, lateral, and ventral. The thalamus also serves in motor control by relaying signals from the cerebellum to the cerebrum and providing feedback loops between the cerebral cortex and the basal nuclei (deep cerebral motor centers). Finally, the thalamus is involved in the memory and emotional functions of the limbic system, a complex of structures that include some cerebral cortex of the temporal and frontal lobes and some of the anterior thalamic nuclei.

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hypothalamus

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forms the floor and part of the walls of the third ventricle. It extends anteriorly to the optic chiasm (ky-AZ-um), where the optic nerves meet, and posteriorly to a pair of humps called the mammillary bodies. Each mammillary body contains three to four mammillary nuclei. Their primary function is to relay signals from the limbic system to the thalamus. The pituitary gland is attached to the hypothalamus by a stalk (infundibulum) between the optic chiasm and mammillary bodies.The hypothalamus is the major control center of the endocrine and autonomic nervous systems.

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hypothalamus functions:

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Hormone secretion. The hypothalamus secretes hormones that control the anterior pituitary gland, thereby regulating growth, metabolism, reproduction, and stress responses. The hypothalamus also produces two hormones that are stored in the posterior pituitary gland, concerned with labor contractions, lactation, and water conservation.

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hypothalamus functions:

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Autonomic effects. The hypothalamus is a major integrating center for the autonomic nervous system. It sends descending fibers to lower brainstem nuclei that influence heart rate, blood pressure, gastrointestinal secretion and motility, and pupillary diameter, among other functions.

Thermoregulation. The hypothalamic thermostat consists of a collection of neurons, concentrated especially in the preoptic nucleus, that monitor body temperature. When the temperature deviates too much from its set point, this center activates mechanisms for lowering or raising the body temperature

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hypothalamus functions:

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Food and water intake. The hypothalamus regulates sensations of hunger and satiety. One nucleus in particular, the arcuate nucleus, contains receptors for hormones that increase hunger and energy expenditure, other hormones that reduce both, and hormones that exert long-term control over body mass. Hypothalamic neurons called osmoreceptors monitor blood osmolarity and stimulate water-seeking and drinking behavior when the body is dehydrated. Dehydration also stimulates the hypothalamus to produce antidiuretic hormone, which conserves water by reducing urine output.

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hypothalamus functions:

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Sleep and circadian rhythms. The caudal part of the hypothalamus is part of the reticular formation. It contains nuclei that regulate the rhythm of sleep and waking. Superior to the optic chiasm, the hypothalamus contains a suprachiasmatic nucleus that controls our 24-hour (circadian) rhythm of activity.

Memory. The mammillary nuclei lie in the pathway of signals traveling from the hippocampus, an important memory center of the brain, to the thalamus. Thus, they are important in memory, and lesions to the mammillary nuclei cause memory deficits.

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hypothalamus functions:

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Emotional behavior and sexual response. Hypothalamic centers are involved in a variety of emotional responses including anger, aggression, fear, pleasure, and contentment; and in sexual drive, copulation, and orgasm.

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epithalamus

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is a very small mass of tissue composed mainly of the pineal gland, the habenula (a relay from the limbic system to the midbrain), and a thin roof over the third ventricle

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frontal lobe (lobes of cerebrum)

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lies immediately behind the frontal bone, superior to the eyes. From the forehead, it extends caudally to a wavy vertical groove, the central sulcus. It is chiefly concerned with voluntary motor functions, motivation, foresight, planning, memory, mood, emotion, social judgment, and aggression.

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parietal lobe (lobes of cerebrum)

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forms the uppermost part of the brain and underlies the parietal bone. Starting at the central sulcus, it extends caudally to the parieto–occipital sulcus, visible on the medial surface of each hemisphere. It is the primary site for receiving and interpreting signals of the general senses described later; for taste (one of the special senses); and for some visual processing.

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occipital lobe (lobes of cerebrum)

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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.

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temporal lobe (lobes of cerebrum)

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is a lateral, horizontal lobe deep to the temporal bone, separated from the parietal lobe above it by a deep lateral sulcus. It is concerned with hearing, smell, learning, memory, and some aspects of vision and emotion.

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insula (lobes of cerebrum)

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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 because it is less accessible to testing in living subjects, but it apparently plays roles in understanding spoken language, in taste, and in integrating information from visceral receptors.

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Cerebral White Matter

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1. Projection tracts extend vertically between higher and lower brain and 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 broad, dense sheet 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.

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Cerebral White Matter

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2. Commissural tracts cross from one cerebral hemisphere to the other through bridges called commissures (COM-ih-shurs). The great majority of commissural tracts pass through the large corpus callosum. A few tracts pass through the much smaller anterior and posterior commissures. Commissural tracts enable the two sides of the cerebrum to communicate with each other.

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Cerebral White Matter

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3. Association tracts connect different regions within the same cerebral hemisphere. Long association fibers connect different lobes of a hemisphere to each other, whereas short association fibers connect different gyri within a single lobe. Among their roles, association tracts link perceptual and memory centers of the brain; for example, they enable you to see a rose, name it, and imagine its scent.

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cerebral cortex

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a layer covering the surface of the hemispheres. Even though it is only 2 to 3 mm thick, the cortex constitutes about 40% of the mass of the brain and contains 14 to 16 billion neurons. It possesses two principal types of neurons called stellate cells and pyramidal cells. Stellate cells have spheroidal somas with dendrites projecting for short distances in all directions. They are concerned largely with receiving sensory input and processing information on a local level. Pyramidal cells are tall and conical. Their apex points toward the brain surface and has a thick dendrite with many branches and small, knobby dendritic spines. The base gives rise to horizontally oriented dendrites and an axon that passes into the white matter. Pyramidal cells include the output neurons of the cerebrum—the only cerebral neurons whose fibers leave the cortex and connect with other parts of the CNS. Pyramidal cell axons have collaterals that synapse with other neurons in the cortex or in deeper regions of the brain.

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neocortex (cerebral cortex)

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About 90% of the human cerebral cortex is a six-layered tissue called neocortex. The six layers of neocortex, vary from one part of the cerebrum to another in relative thickness, cellular composition, synaptic connections, size of the neurons, and destination of their axons. Layer IV is thickest in sensory regions and layer V in motor regions, for example. All axons that leave the cortex and enter the white matter arise from layers III, V, and VI.

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limbic system

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is an important center of emotion and learning. It is 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 (SING-you-let) gyrus, which arches over the top of the corpus callosum in the frontal and parietal lobes; the hippocampus in the medial temporal lobe; and the amygdala (ah-MIG-da-luh) immediately rostral to the hippocampus, also in the temporal lobe.

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The Basal Nuclei

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The basal nuclei are masses of cerebral gray matter buried deep in the white matter, lateral to the thalamus. Neuroanatomists disagree on how many brain centers to classify as basal nuclei, but agree on at least three: the caudate nucleus, putamen (pyu-TAY-men), and globus pallidus. These three are collectively called the corpus striatum because of their striped appearance. The putamen and globus pallidus together are also called the lentiform45 nucleus, because they form a lens-shaped body. They are involved in motor control

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electroencephalogram (EEG)

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is useful in studying normal brain functions such as sleep and consciousness, and in diagnosing degenerative brain diseases, metabolic abnormalities, brain tumors, trauma, and so forth. States of consciousness ranging from high alertness to deep sleep are correlated with changes in the EEG. The complete and persistent absence of brain waves is a common clinical and legal criterion of brain death.

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Alpha (α) waves

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have a frequency of 8 to 13 Hz and are recorded especially in the parieto–occipital area. They dominate the EEG when a person is awake and resting, with the eyes closed and the mind wandering. They are suppressed when a person opens the eyes, receives specific sensory stimulation, or engages in a mental task such as performing mathematical calculations. They are absent during deep sleep.

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Beta (β) waves

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have a frequency of 14 to 30 Hz and occur in the frontal to parietal region. They are accentuated during mental activity and sensory stimulation.

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Theta (θ) waves

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have a frequency of 4 to 7 Hz. They are normal in children and in drowsy or sleeping adults, but a predominance of theta waves in awake adults suggests emotional stress or brain disorders.

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Delta (δ) waves

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are high-amplitude “slow waves” with a frequency of less than 3.5 Hz. Infants exhibit delta waves when awake, and adults exhibit them in deep sleep. A predominance of delta waves in awake adults indicates serious brain damage.

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Sleep

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can be defined as a temporary state of unconsciousness from which one can awaken when stimulated. It is one of many bodily functions that occur in cycles called circadian (sur-CAY-dee-an) rhythms, so named because they are marked by events that reoccur at intervals of about 24 hours. Sleep is characterized by a stereotyped posture (usually lying down with the eyes closed) and inhibition of muscular activity (sleep paralysis). It superficially resembles other states of prolonged unconsciousness such as coma and animal hibernation, except that individuals cannot be aroused from those states by sensory stimulation.

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Sleep Stages

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• Stage 1. One feels drowsy, closes the eyes, and begins to relax. Thoughts come and go, often accompanied by a drifting sensation. One awakens easily if stimulated. The EEG is dominated by alpha waves.

• Stage 2. One passes into light sleep. The EEG declines in frequency but increases in amplitude. Occasionally it exhibits 1 or 2 seconds of sleep spindles, high spikes resulting from interactions between neurons of the thalamus and cerebral cortex.

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Sleep Stages

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• Stage 3. This is moderate to deep sleep, typically beginning about 20 minutes after stage 1. Sleep spindles occur less often, and theta and delta waves appear. The muscles relax, and the vital signs (body temperature, blood pressure, and heart and respiratory rates) fall.

• Stage 4. This is also called slow-wave sleep (SWS), because the EEG is dominated by low-frequency, high-amplitude delta waves. The muscles are now very relaxed, vital signs are at their lowest levels, and one becomes difficult to awaken.

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rapid eye movement (REM) sleep

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About five times a night, a sleeper backtracks from stage 3 or 4 to stage 2 and exhibits bouts of rapid eye movement (REM) sleep. This is so named because the eyes oscillate back and forth as if watching a movie. It is also called paradoxical sleep because the EEG resembles the waking state, yet the sleeper is harder to arouse than in any other stage. The vital signs increase and the brain consumes even more oxygen than when awake. Sleep paralysis, other than in the muscles of eye movement, is especially strong during REM sleep. Paralysis may serve to prevent the sleeper from acting out his or her dreams and may have prevented our tree-dwelling ancestors from falling during their sleep.

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suprachiasmatic (SOO-pra-KY-az-MAT-ic) nucleus (SCN)

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The cycle of sleep and waking is controlled by a complex interaction between the cerebral cortex, thalamus, hypothalamus, and reticular formation. One of the control centers for sleep is the suprachiasmatic (SOO-pra-KY-az-MAT-ic) nucleus (SCN), located just above the optic chiasm in the anterior hypothalamus. The SCN uses this input to synchronize multiple body rhythms with the external rhythm of night and day—including not just sleep but also body temperature, urine production, hormone secretion, and other functions. It does not in itself induce sleep or waking, but regulates the time of day that a person sleeps.

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orexins

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Two related brain neuropeptides called orexins act as an important “sleep switch.” Produced by the lateral and posterior hypothalamus, orexins strongly stimulate wakefulness and elevate the metabolic rate.

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narcolepsy

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Blocking orexin receptors induces sleep, and orexin levels are low or absent in a disorder called narcolepsy, in which a person experiences excessive daytime sleepiness and fatigue and may often fall asleep at work or school, with abnormally quick onset of REM sleep. Narcolepsy seems to be an autoimmune disease caused by antibody-mediated destruction of the orexin-producing neurons.

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Cognition

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is the range of mental processes by which we acquire and use knowledge—sensory perception, thought, reasoning, judgment, memory, imagination, and intuition.

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association areas

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Such functions are widely distributed over regions of cerebral cortex called association areas, which constitute about 75% of all brain tissue. This is the most difficult area of brain research and the most incompletely understood aspect of cerebral function. Much of what we know about it has come from studies of patients with brain lesions—areas of tissue destruction resulting from cancer, stroke, and trauma.

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prefrontal cortex (frontal association area)

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the most rostral part of the frontal lobe, is well developed only in primates, especially humans. It integrates information from sensory and motor regions of the cortex and from other association areas. It gives us a sense of our relationship to the rest of the world, enabling us to think about it and to plan and execute appropriate behavior. It is responsible for giving appropriate expression to our emotions. Lesions here may produce profound personality disorders and socially inappropriate behaviors.

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Memory

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Information management by the brain entails learning (acquiring new information), memory proper (information storage and retrieval), and forgetting (eliminating trivial information)

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anterograde amnesia and retrograde amnesia

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brain-injured people are either unable to store new information (anterograde amnesia) or to recall things they knew before the injury (retrograde amnesia). Amnesia refers to defects in declarative memory (such as the ability to describe past events), not procedural memory (such as the ability to tie your shoes)

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hippocampus

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of the limbic system is an important memory-forming center. 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 learner” but forms longer-lasting memories

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memory consolidation

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This process of “teaching the cerebral cortex” until a long-term memory is established is called memory consolidation. Long-term memories are held in various areas of cortex. One’s vocabulary and memory of faces and familiar objects, for example, reside in the superior temporal lobe, and memories of one’s plans and social roles are in the prefrontal cortex.

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primary sensory cortex

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Regions called primary sensory cortex are the sites where sensory input is first received and one becomes conscious of a stimulus. Adjacent to these are association areas where this sensory information is interpreted.

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special senses

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are limited to the head, and some employ relatively complex sense organs. They are vision, hearing, equilibrium, taste, and smell.

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Vision

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Visual signals are received by the primary visual cortex in the far posterior region of the occipital lobe. This is bordered anteriorly by the visual association area, which includes all the remainder of the occipital lobe, some of the posterior parietal lobe (concerned with spatial perception), and much of the inferior temporal lobe, where we recognize faces and other familiar objects.

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Hearing

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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. This is where we become capable of recognizing spoken words, a familiar piece of music, or a voice on the telephone.

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Equilibrium

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Signals from the inner ear for equilibrium 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.

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Taste and smell

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Gustatory (taste) signals are received by the primary gustatory cortex in the inferior end of the postcentral gyrus of the parietal lobe (discussed shortly) and an anterior region of the insula. Olfactory (smell) signals are received by the primary olfactory cortex in the medial surface of the temporal lobe and inferior surface of the frontal lobe. The orbitofrontal cortex mentioned earlier serves as a multimodal association area for both of these senses.

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general (somatosensory, somesthetic, or somatic) senses

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are distributed over the entire body and employ relatively simple receptors. They include such senses as touch, pressure, stretch, movement, heat, cold, and pain. Coming from the head, such signals reach the brain by way of certain cranial nerves, especially the trigeminal nerve; from the rest of the body, they ascend sensory tracts of the spinal cord such as the spinothalamic tract. In both routes, they decussate to the contralateral thalamus.

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postcentral gyrus

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The thalamus processes the input and selectively relays signals to the postcentral gyrus. This is a fold of the cerebrum that lies immediately caudal to the central sulcus and thus forms the rostral border of the parietal lobe. We can trace it from just above the lateral sulcus to the crown of the head and then downward into the longitudinal fissure.

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primary somatosensory cortex

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We can trace it from just above the lateral sulcus to the crown of the head and then downward into the longitudinal fissure. Its cortex is called the primary somatosensory cortex. Adjacent to it is a somatosensory association area, caudal to the gyrus and in the roof of the lateral sulcus. Awareness of stimulation occurs in the primary somatosensory cortex, but making cognitive sense of it is a function of the association area.

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somatotopy

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As the diagram shows, receptors in the lower limb project to superior and medial parts of the gyrus, and receptors in the face project to the inferior and lateral parts. Such point-for-point correspondence between an area of the body and an area of the CNS is called somatotopy.

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Motor Control

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The intention to contract a skeletal muscle begins in the motor association (premotor) area of the frontal lobes. This is where we plan our behavior—where neurons compile a program for the degree and sequence of muscle contractions required for an action such as dancing, typing, or speaking. The program is then transmitted to neurons of the precentral gyrus (primary motor area), which is the most posterior gyrus of the frontal lobe, immediately anterior to the central sulcus. Neurons here send signals to the brainstem and spinal cord, which ultimately results in muscle contractions.

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upper motor neurons

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The pyramidal cells of the precentral gyrus are called upper motor neurons. Their fibers project caudally, with about 19 million fibers ending in nuclei of the brainstem and 1 million forming the corticospinal tracts. Most of these fibers decussate in the lower medulla oblongata (at the pyramidal decussation) and form the lateral corticospinal tract on each side of the spinal cord. A smaller number of fibers pass through the medulla without decussation and form the anterior corticospinal tracts, which cross over lower in the spinal cord.

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lower motor neurons

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In the brainstem or spinal cord, the fibers from the upper motor neurons synapse with lower motor neurons whose axons innervate the skeletal muscles

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dyskinesias

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Lesions of the basal nuclei cause movement disorders called dyskinesias. These are sometimes characterized by abnormal difficulty initiating movement, such as rising from a chair or beginning to walk, and by a slow shuffling walk. Such motor dysfunctions are seen in Parkinson disease. Smooth, easy movements require the excitation of agonistic muscles and inhibition of their antagonists. In Parkinson disease, the antagonists are not inhibited. Therefore, opposing muscles at a joint fight each other, making it a struggle to move as one wishes. Other dyskinesias are characterized by exaggerated or unwanted movements, such as flailing of the limbs (ballismus) in Huntington disease.

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Language

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includes several abilities—reading, writing, speaking, signing, and understanding words—assigned to different regions of cerebral cortex

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Wernicke (WUR-ni-keh) area

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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 crossroad between visual, auditory, and somatosensory areas of cortex, receiving input from all these neighboring regions. The angular gyrus, part of the parietal lobe just caudal and superior to the Wernicke area, is important in the ability to read and write.

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Broca area

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When we intend to speak, the Wernicke area formulates phrases according to learned rules of grammar and transmits a plan of speech to the Broca area, located in the inferior prefrontal cortex of the same hemisphere. PET scans show a rise in the metabolic activity of the Broca area as one prepares to speak or sign. This area generates a motor program for the muscles of the larynx, tongue, cheeks, and lips to produce speech, as well as for the hand motions of signing. It transmits this program to the primary motor cortex, which executes it—that is, it issues commands to the lower motor neurons that supply the relevant muscles.

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Aphasia (ah-FAY-zee-uh)

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is any language deficit resulting from lesions in the hemisphere (usually the left) containing the Wernicke and Broca areas. The many forms of aphasia are difficult to classify. Nonfluent (Broca) aphasia, due to a lesion in the Broca area, results in slow speech, difficulty in choosing words, or use of words that only approximate the correct word. For example, a person may say “tssair” when asked to identify a picture of a chair.

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cerebral lateralization

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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.

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cranial nerves

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To be functional, the brain must communicate with the rest of the body. Most of its input and output travels by way of the spinal cord, but it also communicates by way of 12 pairs of cranial nerves. These arise primarily from the base of the brain, exit the cranium through its foramina, and lead to muscles and sense organs located mainly in the head and neck. The cranial nerves are numbered I to XII starting with the most rostral pair. Each nerve also has a descriptive name such as optic nerve and vagus nerve.

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Cranial Nerves Classification

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Cranial nerves are traditionally classified as sensory (I, II, and VIII), motor (III, IV, VI, XI, and XII), or mixed (V, VII, IX, and X). In reality, only cranial nerves I and II (for smell and vision) are purely sensory, whereas all of the rest contain both afferent and efferent fibers and are therefore mixed nerves. Those traditionally classified as motor not only stimulate muscle contractions but also contain sensory fibers of proprioception, which provide the brain with feedback for controlling muscle action and make one aware of such things as the position of the tongue and orientation of the head. Cranial nerve VIII, concerned with hearing and equilibrium, is traditionally classified as sensory, but it also has motor fibers that return signals to the inner ear and “tune” it to sharpen the sense of hearing. The nerves traditionally classified as mixed have sensory functions quite unrelated to their motor functions.