5.24 Brain (Superior View)

The cerebrum is the largest part of the brain, and, because it so massive and overlays most of the brain structure, it is the only structure seen from a superior view. Internally, the cerebrum has two layers--an outer gray matter, the cortex, and an inner white matter, the medulla. During development there is a very rapid increase in brain size and the cortex grows faster than the underlying medulla, causing the cortex to convolute on itself. The surface folds that result are called gyri and the grooves in between are called sulci. Very deep sulci are called fissures. The most prominent fissure of all, the longitudinal fissure, separates the cerebrum into right and left halves, or hemispheres (as illustrated in images 5.24a and image 5.24b). The hemispheres are connected internally by a large bundle of transverse fibers composed of white matter called the corpus callosum. The dura mater normally covers the cerebrum and in between the hemispheres is an extension of the dura called the falx cerebri, which is normally located in the longitudinal fissure (the large horizontal groove at the center of the brain in image 5.24b).

5.25 Brain (Lateral View)

The cerebral hemisphere is further subdivided into four lobes by deep sulci or fissures. The central sulcus separates the frontal lobe (area labeled "A" in image 5.25a) from the parietal lobe (area labeled "B" in image 5.25a); the lateral sulcus separates out the temporal lobe (area labeled "C" in image 5.25a) from the parietal and frontal lobes; and the parieto-occipital sulcus is between the parietal lobe and the occipital lobe (area labeled "D" in image 5.25a). 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 involve responses that are related to memory, emotions, reasoning, judgment, planning, and verbal communication.

In image 5.25b, a major gyrus-the precentral gyrus (labeled "B" in image 5.25b), is located immediately anterior to the central sulcus which is between the precentral gyrus and postcentral gyrus (area labeled "C" in image 5.25b). The precentral gyrus is a landmark for the brain which is the primary motor area of the cerebral cortex. The premotor cortex is labeled "A" in image 5.25b. The postcentral gyrus is another major gyrus, located immediately posterior to the central sulcus, and is a landmark for the general sensory area of the cerebral cortex. The lateral cerebral sulcus separates the frontal lobe from the temporal lobe. The parieto-occipital sulcus separates the parietal lobe from the occipital lobe.

In image 5.25c, another prominent fissure, the transverse fissure, separates the cerebrum from the cerebellum. The brain stem is another structure of the brain. The frontal lobe, parietal lobe, temporal lobe, and occipital lobe are named after the bones that cover them. Image 5.25d shows Broca's area (labeled "A" in image 5.25d). This is the motor speech center that produces a detailed program for all the countless combinations of the muscular movements needed to articulate every word you have ever spoken. Image 5.25e shows Wernicke's area (labeled "A" in image 5.25e). This area also deals with speech because it receives impulses from the visual and auditory association neurons and is involved in the comprehension of written and spoken language. A fiber tract runs from Wernicke's area to Broca's, allowing the language comprehension area to communicate with the language articulation area.

5.26 Brain (Medial View)

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 the longitudinal fissure. Portions of the two hemispheres are connected internally by the corpus callosum (structure below the area labeled "A" in image 5.26a). In Image 5.26b, the corpus callosum is also identified in the illustration.

In image 5.26c, the arrows point to part of the limbic region located in the cingulate gyrus (also in image 5.26a as the area labeled "A"). The limbic system is a group of fiber tracts and nuclei that form a ring around the brain stem. It includes the cingulate gyrus of the cerebral cortex, the hypothalamus, the fornix, the hippocampus, and the amygdala. Image 5.26d show the hippocampus (arrow) and image 5.26e shows the amygdala (arrow). These structures, once called the "smell brain" because of their importance in processing of olfactory information, are now recognized as centers involved in basic emotional drives, such as anger, fear, sex, and hunger, and in short-term memory. It is our affective or feeling brain. Complex circuits between the limbic system and the lower and higher brain centers give emotional richness to our lives and support an intimate relation between our feelings and our thoughts. The hypothalamus, located in the diencephalon, is the gatekeeper of these responses that involve both autonomic and hormonal responses.

5.27 Commissural Fibers

The white matter underlying the cortex consists of myelinated axons running in three directions, association fibers (within the same hemisphere), commissural fibers (between hemispheres), and projection fibers. Projection fibers are ascending and descending tracts that transmit impulses from the cerebrum to other parts of the lower brain and spinal cord. Commissural fibers transmit impulses from one cerebral hemisphere to the opposite cerebral hemisphere. In image 5.27a, the three important groups of commissural fibers are shown; the corpus callosum, the anterior commissure, and the posterior commissure. Image 5.27b shows the corpus callosum (arrow); image 5.27c shows the anterior commissure (arrow); and image 5.27d shows the posterior commissure (arrow).

5.28 Basal Ganglia

Located at the base of each cerebral hemisphere are masses of gray matter known as the basal ganglia (arrow in image 5.28a). The ganglia, which are surrounded by white matter, are composed of groups of nerve cell bodies. Included in the basal ganglia are: the long looping caudate nucleus; the amygdaloid nucleus, which is located at the tip of the tail of the caudate nucleus; the lentiform nucleus, which is subdivided into the putamen and the globus pallidus; and the claustrum, a thin layer of gray matter just deep to the insula. The lentiform nucleus can be seen in image 5.28b (arrow). The band of white matter located between the basal ganglia and the thalamus is called the internal capsule. The internal capsule is composed of projection fibers of the major motor and sensory tracts as they pass to and from the cerebral cortex. Because of their appearance, the caudate nucleus, the internal capsule, and the lentiform nucleus are often referred to as the corpus striatum ("the striped body"). The basal ganglia are part of the extra pyramidal system, but they act principally to inhibit muscle contraction. This inhibition, combined with the stimulation of the pyramidal system, provides precision control for muscular movements. Disorders of the basal ganglia result in paralysis, tremor, and ballistic contractions of skeletal muscles. Parkinson's disease is an example of a disorder of the basal ganglia that produces muscular rigidity and persistent tremor.

5.29 Diencephalon

The brain is mushroom-shaped and divided into four principal parts: the brain stem, diencephalon, cerebrum, and cerebellum. Image 5.29a shows the diencephalon (labeled "A" in image 5.29a). The diencephalon consists primarily of the thalamus, hypothalamus, and epithalamus and sits above the brainstem, as seen in image 5.29b. The cerebrum spreads over the entire diencephalon.

In image 5.29c, the arrow points to the thalamus which contain many different nuclei, each functionally specialized. The thalamus is in contact with the entire cerebral cortex. It is a relay station for sensory information, (in fact all input, except smell), ascending to the cerebral cortex, and it sorts, selects, and transfers the information. It plays a key role in mediating cortical attention, memory, emotion, and somatic input. Image 5.29d shows a frontal section view of the thalamus.

Image 5.29e shows the hypothalamus. The hypothalamus is involved in regulating body temperature, water balance, appetite, gastrointestinal activity, sexual activity, and even emotions such as fear and rage. The hypothalamus also regulates the release of the hormones of the pituitary gland, and thus it greatly affects the endocrine system.

The epithalamus, the most dorsal portion of the diencephalon, forms a thin roof over the third ventricle. The roof has a vascular choroid plexus located on its internal surface. A small mass called the pineal gland (epiphysis) extends outward from the posterior end of the epithalamus.

Image 5.29f shows the pineal gland (labeled as "A" in image 5.29f) as a small knob between the large thalamus. The pineal gland seems to be involved in biological rhythms, such as the sleep cycle.

5.30 Brain Stem

The brain stem (image 5.30a) includes the midbrain, pons, and medulla oblongata, and it functions in the production of stereotypic, automatic actions. It also contains the sleep control centers which activate the cortex. Since it is principally made of white matter, it is the major transmission pathway for the cerebral and cerebellar fibers to connect to the spinal cord.

Image 5.30b and image 5.30e shows the midbrain (labeled "B" in image 5.30b and in image 5.30e) . The midbrain is the area where the descending voluntary motor tracts are found. The midbrain is connected to the cerebellum by the superior cerebellar peduncles. Four large nuclei, called the corpora quadrigemina, can be seen. The superior colliculi function as visual reflex centers, whereas the inferior colliculi contain auditory reflex centers. The substantia nigra and the red nucleus are centers also involved in motor control.

Image 5.30c and image 5.30e shows the pons ((labeled "C" in image 5.30c and in image 5.30e) .The pons is a major bundle of axons and dendrites, in addition to holding the sleep centers mentioned above. Other important nuclei are the higher respiratory centers which help maintain the normal rhythm of breathing. The middle cerebellar peduncles also connect the pons to the cerebellum here.

Image 5.30d and image 5.30e shows the medulla oblongata (labeled "D" in image 5.30d and in image 5.30e). The medulla contains many automatic reflex centers. The vital reflex centers include cardiac, vasomotor, and basic respiratory control. Less vital reflex centers include salivation, swallowing, sneezing, coughing, and gastric secretion, just to mention a few. The pyramids, two large motor fiber tracts, descend from the motor cortex and are visible on the ventral surface of the medulla. The inferior cerebellar peduncles connect the medulla to the cerebellum posteriorly. The olivary nuclei and the vestibular nuclei are also found in the medulla.

Image 5.30e also shows the reticular formation which is a complex network of nuclei that is located in the spinal cord and parts of the hypothalamus and thalamus (labeled "A" in image 5.30e). The principal role of the reticular activating system (RAS) is to keep the cerebrum in an alert state and to relay impulses to the cerebrum. The RAS also helps the cerebellum to maintain muscle tonus and produce smooth, coordinated contractions of skeletal muscles.

5.31 Cranial Nerves

There are 12 pairs of cranial nerves, with all but two originating from the brain stem. They are paired, and numbered from anterior to posterior with Roman numerals. Most contain both axons and dendrites and are therefore mixed nerves, but in some the sensory dendrites dominate and others are mostly motor.

The olfactory nerve (I) is indicated by the arrow in image 5.31a. The olfactory nerve is entirely sensory and conveys impulses related to smell. It originates from the olfactory mucosa of the nasal cavity, which is located at the very top of the nasal cavity. Axons from the sensory neurons pass through the cribriform plate of the ethmoid bone and synapse with other olfactory neurons in the olfactory bulb (seen as the expanded endings of the olfactory nerves). The axons of these neurons make up the olfactory tract. The fibers from the tract terminate in the primary olfactory area in the cerebral cortex.

Image 5.31b shows an example of the optic nerve (II) (arrow in image 5.31b). The optic nerve is entirely sensory and conveys impulses related to vision. After exiting the eye, the two optic nerves unite to form the optic chiasma (in center of the nerve). From the chiasma, the nerve fibers pass posteriorly to form the optic tracts. From the optic tracts, the majority of fibers terminate in the lateral geniculate body of the thalamus. They then synapse with neurons that relay signals to the visual areas of the cerebral cortex.

The other cranial nerves (CNIII - CNXII) can be viewed in image 5.31c.

• The oculomotor nerve (III) is principally a motor nerve which passes through the superior orbital fissure to the eye. It innervates four of the six extrinsic rectus muscles which move the eye in the socket and the levator palpabrae superioris muscle which raises the upper eyelid.

• The trochlear nerve (IV) is a mixed cranial nerve, but motor control predominates. It is the smallest of the 12 cranial nerves. The motor portion innervates the superior oblique muscle of the eyeball, another extrinsic rectus muscle. The sensory portion of the trochlear nerve consists of sensory fibers that run from proprioceptors of the superior oblique muscle to the midbrain.

• The trigeminal nerve (V) is a mixed cranial nerve and the largest in diameter. As indicated by its name, the trigeminal nerve has three branches: ophthalmic, maxillary, and mandibular. The trigeminal nerve originates on each side of the pons. The large sensory root has a swelling called the semilunar (Gasserian) ganglion located in a cavity of the temporal bone. From this ganglion, the ophthalmic branch enters the orbit via the superior orbital fissure, the maxillary branch passes through the foramen rotundum, and the mandibular branch passes through the foramen ovale. The motor fibers supply the muscles of mastication. These motor fibers, which control chewing movements, constitute the motor portion of the trigeminal nerve. The sensory portion of the trigeminal nerve delivers impulses related to touch, pain, and temperature.

• The abducens nerve (VI) is a mixed cranial nerve whose motor fibers extend to the lateral rectus muscle of the eyeball, an extrinsic eyeball muscle. Some sensory fibers run from proprioceptors in the lateral rectus muscle to the pons and mediate muscle sense.

• The facial nerve (VII) is also mixed cranial nerve. Its motor fibers distribute to facial, scalp, and neck muscles to cause changes in facial expression. Some motor fibers are also innervate the lacrimal, sublingual, submandibular, nasal, and palatine glands. The sensory fibers receive stimuli from the taste buds of the anterior tongue. The sensory portion of the facial nerve also relays deep general sensations from the face and motion sense of the face and scalp.

• The vestibulocochlear nerve (VIII), also known as the auditory nerve, consists of two main branches: the cochlear and the vestibular. The cochlear branch, which conveys sensory impulses associated with hearing, arises in the spiral organ in the cochlea of the internal ear. The vestibular branch arises in the semicircular canals, the saccule, and the utricle of the inner ear and transmits impulses related to equilibrium.

• The glossopharyngeal nerve (IX) is a mixed cranial nerve. The nerve originates at the medulla, and passes through the jugular foramen, where motor fibers are finally distributed to the swallowing muscles of the pharynx and to the parotid gland for triggering the secretion of saliva. The sensory fibers of this nerve supply the pharynx and taste buds of the rear of the tongue. Some sensory fibers also originate from receptors in the carotid sinus and from muscles innervated by this nerve.

• The vagus nerve (X) is a mixed, long cranial nerve that passes down the neck through the thorax and into the abdomen. Its motor fibers pass to the muscles of the pharynx, larynx, bronchi, lungs, heart, esophagus, stomach, gallbladder, small intestine, most of the colon. Sensory fibers of the vagus nerve return signals from the same organs innervated by the motor fibers.

• The accessory nerve (XI) is a mixed cranial nerve. It differs from all other cranial nerves in that it originates from both the brain stem and passes through the jugular foramen, supplying the voluntary muscles of the pharynx, larynx, and soft palate. The spinal cord portion passes through the jugular foramen to convey motor impulses to the sternocleidomastoid and trapezius muscles. The sensory fibers originate from proprioceptors in the muscles supplied by its motor component.

• The hypoglossal nerve (XII) is a mixed cranial nerve. Its motor fibers pass through the hypoglossal canal to supply the muscles of the tongue to control speech and swallowing. The sensory portion of the hypoglossal nerve consists of fibers originating from proprioceptors in the tongue muscles.

5.32 Cerebellum

The cerebellum projects outward from the dorsal surface of the brain stem (image 5.32a). It is composed of two lateral hemispheres connected in the midline by the vermis and has a surface composed of a thin cortex of gray matter. The cerebellar cortex is similar to that of the cerebrum in that it has an outer gray layer and an inner white matter medulla, but the sulci are called fissures and the gyri are called folia. The cerebellum (circled in image 5.32b) is connected to the brainstem by six paired peduncles that allow extensive neuron connections to other parts of the nervous system. The cerebellum coordinates skeletal muscle contractions and is a primary sensory integration network for signals from stretch receptors, tendon organs, joint receptors, and the inner ear. The cerebellum also receives some sensory information concerning touch, vision, and sound. When the cerebellum has been damaged, the result is muscular weakness, loss of muscle tone, and uncoordinated movements. All functions of the cerebellum are held in the unconscious. Image 5.32c shows a midsagittal section of the cerebellum.

5.33 Choroid Plexus

A tuft of capillaries covered with pia mater that evaginates from the roof of the ventricles is known as the choroid plexus (image 5.33a) located in the lateral, 3rd, and 4th ventricles. The choroid plexus is the major site for the formation of cerebrospinal fluid. Vascularized areas within the subarachnoid space are other possible secretion sites. The choroid plexus is a highly folded membrane, and its surface is covered by a specialized cuboidal layer of epithelium, which differs from the ciliated ependymal cells that line most of the ventricular surface. Image 5.33b shows the choroid plexus in a brain ventricle. The plexus itself is seen at the top center of the screen where it consists of looping capillaries which project into the lumen of the ventricle.

Image 5.33c is the choroid plexus (circled in image 5.33c) of a newborn in which the choroid plexus is immature and the cells are still migrating. The majority of the ependymal cells possess cilia which project from their apical surfaces into the ventricular cavity. The rhythmic beat of the cilia may assist in the movement of cerebrospinal fluid throughout the four communicating ventricular cavities of the brain. Image 5.33d is a closeup slide of ependymal cells (light blue). Note the elongate cilia on the apical surface and the large round nuclei (darker shade of blue) in some of the cells.

5.34 Ventricles of the Brain

The four ventricles in the brain form a connected fluid-filled cavity system. The roof of each ventricle is thin and contains no neurons. Each roof does, however, have a network of projecting capillaries called a choroid plexus. These plexuses, together with the ependymal cells that line the surface, continuously secrete cerebrospinal fluid (CSF)into the ventricles of the brain. The CSF is an excellent shock absorber and helps to remove waste products from the brain cells. The entire central nervous system contains about 125 ml of cerebrospinal fluid. It is a clear, colorless fluid resembling a partially deproteinated plasma, but its chemical composition is very complex since a major function of the choroid plexes is to protect the central nervous system neurons from the more toxic substances found in the blood.

The four ventricles can be seen in the diagram of image 5.34a. The two lateral ventricles are shown side-by-side in the cerebrum with the third ventricle in the diencephalon. Finally, the fourth ventricle is located in the brainstem at the base of the cerebellum.

Image 5.34b shows the occipital horns of the lateral ventricles. The arrow in image 5.34b points to the left lateral ventricle. They are clearly positioned inside of the cerebrum (note the cerebral cortex on the outer surface of the cerebrum). Image 5.34c shows the third ventricle (marker/star in image 5.34c). This is located in the diencephalon area of the brain. Image 5.34d shows the fourth ventricle. The arrow in image 5.34d points directly to the black-colored ventricle which is seen sandwiched between the brainstem (on the left) and the cerebellum (on the right). Image 5.34e is a see a graphical overlay of the entire ventricle system. Of course, in this saggital section the two lateral ventricles cannot be seen, but are indicated at the top of the specimen just under the corpus callosum. Note also the narrow aqueduct of Sylvius which connects the third and fourth ventricles.

5.35 Meninges

The central nervous system (CNS) is covered by protective membranes, called meninges, which consist of an outer dura mater, an middle layer arachnoid, and an innermost pia mater, as shown in image 5.35a. The cranial (or "cerebral") meninges surround the brain; the spinal meninges envelope the spinal cord and the cauda equina. The dura mater is in contact with the bone and is composed primarily of tough, white fibrous connective tissue. The cranial dura mater is a double-layered structure. Besides the major function of protection, the inner meningeal portion of the cranial dura also forms distinct septa that partition major brain structures and serve to anchor the brain inside of the cranial case.

In image 5.35b, the spinal dura mater (as shown as a thick envelope around the spinal cord) forms a tough, tubular dural sheath that continues into the vertebral canal and surrounds the spinal cord. There is no connection between the dural sheath and the vertebrae forming the vertebral canal, but instead there is a potential cavity called the epidural space. The epidural space is highly vascular and contains areolar and adipose connective tissue, which form a protective pad around the spinal cord. Image 5.35c shows the arachnoid membrane (arrow in image 5.35c) and is the middle structure of the three meninges. This delicate netlike membrane, which was named after its resemblance to a spider's web, spreads over the CNS, but generally does not extend deep into the sulci of the brain. The subarachnoid space, located between the arachnoid membrane and the deepest meninx, the pia mater, contains cerebrospinal fluid. In image 5.35d, the thin pia mater (arrow) is directly attached to the surface of the CNS and follows the irregular contours of both the brain and spinal cord. The pia mater is composed of fibrous connective tissue. But it is highly vascular and supports the blood vessels that nourish the brain and spinal cord. The pia mater and the arachnoid membrane are specialized within the ventricles to form the choroid plexuses.

Image 5.35e shows a rare but dangerous disease of the meninges, a cerebral meningioma. It accounts for about 20% of all intracranial neoplasms, with maximum frequency of occurrence in the 40's and 50's, but young adults can show this condition also. Usually surgical excision is the cure, but invasion of the tumor into bone can occur and that makes the prognosis poor. Headaches are a common symptom, often with seizures. The doubling time for this type of tumor is about every two years.

5.36 Spinal Cord

The human spinal cord extends from the foramen magnum of the skull to the level of the 1st lumbar vertebra. It varies in thickness along its length, showing both cervical and lumbar enlargements (as seen in image 5.36a) and it is covered by a spinal meninges for protection. The cord terminates in a tapered structure called the conus medullaris which sprays downward a collection of nerves called the cauda equina (horse's tail). Image 5.36b shows a cross section of the spinal cord: the dura mater (labeled "a" in image 5.36b); the gray matter (labeled "b" in image 5.36b), which is organized into a shape of the letter H; and the white matter (labeled "c"in image 5.36b) which carries the ascending and descending nerve tracts to the brain. Image 5.36c shows the spinal cord with the enveloping dura mater and image 5.36d shows the cauda equina and the conus medullaris.

5.37 Spinal Nerves

The electron microscope scan in image 5.37a shows myelinated nerve fibers. The connective tissue sheath surrounding the nerve fascicles (bundles) has been teased open and most of it has been removed. Image 5.37b shows a peripheral nerve (arrow in image 5.37b) and its fascicles (labeled "a" in image 5.37b). Image 5.37c shows a fascicle (labeled "b" in image 5.37c) surrounded by its perineurium (labeled "a" in image 5.37c). Image 5.37d shows a perineurium (arrow) surrounding a fascicle.