From the brain, and from the brain only, arise our pleasures, joys, laughter and jests, as well as our sorrows, pains, griefs, and tears.
Hippocrates (460?-377? BC), The Sacred Disease
The brain regulates and coordinates many critical functions, from thought processes to bodily movements. For this reason, it is important to identify the anatomy of the brain (Fig. 3.1).
FIG. 3.1 Axial, T2-weighted MRI of brain with intraparenchymal hematoma in left basal nuclei.
OBJECTIVES
• Describe the meninges.
• Identify the structures of the limbic system, and cerebrospinal fluid.
• Describe the production and absorption of describe their function.
• Identify the major arteries of the cerebrum, and list the
• Identify the components of the ventricular system. structures they supply.
• Identify the basal cisterns.
• List the arteries that constitute the circle of Willis.
• List the structures of the diencephalon.
• Identify the superficial cortical veins, deep veins, and of the cerebrum, brainstem, and cerebellum.
• Describe the location and function of the components dural sinuses of the cerebrum.
• Identify the function and course of the cranial nerves.
MENINGES
The brain is a delicate organ that is surrounded and protected by three membranes called the meninges (Fig. 3.2). The outermost membrane, the dura mater (tough mother), is the strongest. This double-layered membrane is continuous with the periosteum of the cranium. Located between the dura mater and the cranium are the meningeal vessels, which supply blood to the cranium and meninges. There is also a potential space between the dura mater and the cranium called the epidural (extradural) space. Located between the two layers of dura mater are the dural sinuses, which provide venous drainage from the brain. Folds of dura mater help to separate the structures of the brain and provide additional cushioning and support. The dural folds include the falx cerebri, tentorium cerebelli, and the falx cerebelli. The falx cerebri separates the cerebral hemispheres, whereas the tentorium cerebelli, which spreads out like a tent, forms a partition between the cerebrum and cerebellum. Lesions located above the tentorium cerebelli are considered supratentorial, and if they are located below, they are called infratentorial. An oval opening in the tentorium cerebelli forms the tentorial notch (incisura), which surrounds the midbrain and provides the only communication between the supratentorial and infratentorial spaces within the brain. The falx cerebelli separates the two cerebellar hemispheres (Figs. 3.3-3.6). The middle membrane, known as the arachnoid membrane (spiderlike), is a delicate, transparent membrane that is separated from the dura mater by a potential space called the subdural space. The arachnoid membrane follows the contour of the dura mater. The inner layer, or pia mater (delicate, tender mother), is a highly vascular layer that adheres closely to the contours of the brain. The subarachnoid space separates the pia mater from the arachnoid mater. This space contains cerebrospinal fluid that circulates around the brain and spinal cord and provides further protection to the central nervous system (CNS) (Fig. 3.2).
Transtentorial herniation is the protrusion of brain tissue through the tentorial notch. It can occur as a result of increased intracranial pressure resulting from edema, hemorrhage, or tumor.
Skull fractures with rupture of the meningeal arteries can cause a life-threatening condition known as an epidural hematoma (EDH), which causes accumulation of blood in the epidural space between the dura and cranium. A subdural hematoma (SDH) is a collection of blood from ruptured vessels located in the subdural space.
VENTRICULAR SYSTEM
Ventricles
The ventricular system provides a pathway for the circulation of the cerebral spinal fluid (CSF) throughout the CNS. A major portion of the ventricular system is composed of four fluid-filled cavities (ventricles) located deep within the brain (Figs. 3.7-3.9). The two most superior cavities are the right and left lateral ventricles. These ventricles lie within each cerebral hemisphere and are separated at the midline by a thin membrane known as the septum pellucidum (Figs. 3.10 and 3.11). The lateral ventricles consist of a central portion called the body and three extensions: the frontal (anterior), occipital (posterior), and temporal (inferior) horns (Figs. 3.7-3.16). The junction of the body and the occipital and temporal horns form a triangular area termed the trigone (atria). The lateral ventricles communicate inferiorly with the third ventricle via the paired interventricular foramen (foramen of Monro) (Figs. 3.7, 3.8, and 3.10). The third ventricle is a thin, slitlike structure, located midline just inferior to the lateral ventricles (Figs. 3.7-3.11). The anterior wall of the third ventricle is formed by a thin membrane termed the lamina terminalis, and the lateral walls are formed by the thalamus. The third ventricle communicates with the fourth ventricle via a long, narrow passageway termed the cerebral aqueduct (aqueduct of Sylvius). The cerebral aqueduct reaches the fourth ventricle by traversing the posterior portion of the midbrain (Figs. 3.7, 3.8 and 3.13). The fourth ventricle is a diamond-shaped cavity located anterior to the cerebellum and posterior to the pons (Figs. 3.7, 3.8, and 3.12-3.16). Separating the fourth ventricle from the cerebellum is a thin membrane forming the superior and inferior medullary velum (Fig. 3.13). CSF exits the ventricular system through foramina in the fourth ventricle to communicate with the subarachnoid space within the basal cisterns. The major exit route is the median aperture (foramen of Magendie), located on the posterior wall of the fourth ventricle, which communicates with the cisterna magna (Figs. 3.7 and 3.17). There are two lateral apertures, termed the foramen of Luschka, which communicate with the cerebellopontine angle cistern. From the fourth ventricle, CSF continues into the spinal cord via the central canal (Figs. 3.8 and 3.17).
The septum pellucidum is frequently used as a landmark to determine if the midline of the brain has shifted as a result of trauma or increased cranial pressure.
Located within the ventricular system is a network of blood vessels and nerve cells termed the choroid plexus, which produces CSF. The choroid plexus lines the floor of the lateral ventricles, roof of the third ventricle, and inferior medullary velum of the fourth ventricle (Fig. 3.17). Frequently, the choroid plexus is partially calcified, making it more noticeable on computed tomography (CT) images (Figs. 3.18 and 3.19). There exists a continuous circulation of CSF in and around the brain. Excess CSF is reabsorbed in the dural sinuses by way of arachnoid villi. These villi are berry-like projections of arachnoid tissue that penetrate the dura mater (Figs. 3.2 and 3.17). Enlargements of the arachnoid villi are termed granulations. Within the cranium, these granulations can cause pitting or depressions, which are variations of normal anatomy.
Cisterns
The subarachnoid space is a relatively narrow, fluid- filled space surrounding the brain and spinal cord. There are locations, primarily around the base of the brain, where the subarachnoid space becomes widened (Fig. 3.17). The combined term for these widened areas or pools of CSF is the basal (subarachnoid) cisterns (Fig. 3.20). Each cistern is generally named after the brain structure it borders.
One of the largest cisterns is the cisterna magna. It is located in the lower posterior fossa bordered by the medulla oblongata, cerebellar hemispheres, and occipital bone. It is continuous with the subarachnoid space of the spinal canal (Figs. 3.13, 3.14, and 3.17). The interpeduncular cistern is located between the cerebral peduncles of the midbrain and communicates inferiorly with the prepontine cistern (Figs. 3.20-3.22). The prepontine cistern is located just anterior and inferior to the pons and communicates laterally with the cerebellopontine angle (CPA) cistern (Figs. 3.17, 3.23, and 3.24). The CPA cistern is located at the junction of the pons and cerebellum. It contains important structures, including CNs V, VII, and VIII and the superior and anterior inferior cerebellar arteries. The ambient cistern courses around the lateral surface of the midbrain, connecting the interpeduncular cistern with the quadrigeminal (superior) cistern (Figs. 3.20-3.22). The quadrigeminal cistern lies between the splenium of the corpus callosum and the superior surface of the cerebellum just posterior to the colliculi of the midbrain or the tectum (quadrigeminal plate) (Figs. 3.20 and 3.21). Located above the sella turcica is the suprasellar (chiasmatic) cistern, which contains the optic chiasm and the circle of Willis (Figs. 3.13, 3.17, and 3.20-3.22).
Bleeding within the subarachnoid space is called a subarachnoid hemorrhage (SAH). The most common cause of an SAH is a ruptured aneurysm. Patients with this condition will commonly present to the emergency department complaining of the worst headache of their lives. Blood within the subarachnoid space acts as a chemical irritant to the brain and causes an increase in intracranial pressure.
CEREBRUM
The cerebrum is the largest portion of the brain and is divided into left and right cerebral hemispheres. Each hemisphere contains neural tissue arranged in numerous folds called gyri. The gyri are separated by shallow grooves called sulci and by deeper grooves called fissures. The main sulcus that can be identified on CT and magnetic resonance images (MRIs) of the brain is the central sulcus, which divides the precentral gyrus of the frontal lobe and postcentral gyrus of the parietal lobe (Figs. 3.25 and 3.26). These gyri are important to identify because the precentral gyrus is considered the motor strip of the brain and the postcentral gyrus is considered the sensory strip of the brain. Other gyri important for imaging include the cingulate, parahippocampal, and superior temporal gyrus (see limbic system and temporal lobe). The two main fissures of the cerebrum are the longitudinal fissure and the lateral (Sylvian) fissure (Figs. 3.27 and 3.28). The longitudinal fissure is a long, deep furrow that divides the left and right cerebral hemispheres. Located in this fissure are the falx cerebri and superior and inferior sagittal sinuses. The lateral fissure is a deep furrow that separates the frontal and parietal lobes from the temporal lobe. Numerous blood vessels, primarily branches of the middle cerebral artery, follow the course of the lateral fissure (Figs. 3.21, 3.22, and 3.25-3.28).
Gray and White Matter Organization
The cerebrum as a whole has many critically important functions, including thought, judgment, memory, and discrimination. The cerebrum consists of gray matter (neuron cell bodies) and white matter (myelinated axons) (Figs. 3.27 and 3.28). The cerebral cortex, the outermost portion of the cerebrum, is composed of gray matter approximately 3 to 5 mm thick. The cortex not only receives sensory input but also sends instructions to the muscles and glands for control of body movement and activity. Deep in the cortex is the white matter, which contains fibers that create pathways for the transmission of nerve impulses to and from the cortex. The largest and densest bundle of white matter fibers within the cerebrum is the corpus callosum. This midline structure forms the roof of the lateral ventricles and connects the right and left cerebral hemispheres. The four parts of the corpus callosum, from anteroinferior to posterior, are the rostrum, genu, body, and splenium (Figs. 3.29-3.32).
Two other important bundles of white matter fibers are the anterior and posterior commissures (Figs. 3.29 and 3.30). The anterior commissure crosses the midline within the lamina terminalis and connects the anterior portions of each temporal lobe (Figs. 3.29, 3.30, 3.33, and 3.34). The posterior commissure is a pathway made of several fibers that transmit nerve impulses for pupillary (consensual) light reflexes. This pathway crosses the midline posterior to the third ventricle, immediately above the cerebral aqueduct and inferior to the pineal gland (Figs. 3.29, 3.30, and 3.34).
Cerebral Lobes
The cerebral cortex of each hemisphere can be divided into four individual lobes: frontal, parietal, occipital, and temporal (Fig. 3.35). These four lobes correspond in location to the cranial bones with the same name. Each lobe has critical regions that are associated with specific functions. The frontal lobe is the most anterior lobe of the brain. The boundaries of the frontal lobe are the central sulcus, which separates it from the parietal lobe, and the lateral fissure, which separates it from the temporal lobe (Figs. 3.25, 3.26, 3.35, and 3.36). The frontal lobe mediates a wide variety of functions, such as reasoning, judgment, emotional response, planning and execution of complex actions, and control of voluntary muscle movement. The frontal lobe is also involved in speech production and contains the motor speech (language) center, Broca’s area. Broca’s area lies unilaterally on the inferior surface of the frontal lobe dominant for language, typically in the left inferior frontal gyrus (Fig. 3.35). This area is involved in the coordination or programming of motor movements for the production of speech sounds. The parietal lobe is located in the middle portion of each cerebral hemisphere just posterior to the central sulcus.
The horizontal portion of the lateral fissure separates the parietal lobe from the temporal lobe (Figs. 3.25, 3.26, 3.28, and 3.37). The parietal lobe is associated with the perception of temperature, touch, pressure, vibration, pain, and taste and is involved in writing and in some aspects of reading. The most posterior lobe, the occipital lobe, is separated from the parietal lobe by the parieto-occipital fissure. This lobe is involved in the conscious perception of visual stimuli. The primary visual area receives input from the optic tract via the optic radiations extending from the thalamus (Fig. 3.36). The temporal lobe is anterior to the occipital lobe and is separated from the parietal lobe by the lateral fissure (Fig. 3.37). Conscious perceptions of auditory and olfactory stimuli and dominance for language are functions of the temporal lobe. Memory processing occurs via the amygdala and hippocampus, clusters of gray matter located in the parahippocampal gyrus of the temporal lobe (Figs. 3.27 and 3.28). Located on the superior temporal gyrus is the auditory cortex, which can be divided into primary and secondary auditory areas (Figs. 3.27, 3.32, and 3.35). The primary auditory area, Heschl’s gyrus, receives the major auditory sensory information from the bilateral cochlea, whereas the secondary auditory area, Wernicke’s area, is the center for comprehension and formulation of speech (Fig. 3.35). Deep in the temporal lobe is another area of cortical gray matter termed the insula (island of Reil), often referred to as the fifth lobe. The insula is separated from the temporal lobe by the lateral fissure and is thought to mediate motor and sensory functions of the viscera (Figs. 3.27, 3.28, and 3.31-3.38).
Studies in neuroimaging have found correlations between structural abnormalities and decreased gray matter volume of the superior temporal gyrus in individuals with schizophrenia and autism.
Basal Nuclei
The basal nuclei (ganglia) are a collection of subcortical gray matter consisting of the caudate nucleus, lentiform nucleus, and claustrum (Figs. 3.27, 3.39, and 3.40). Collectively, they contribute to the planning and programming of muscle action and movement. The largest basal nuclei are the caudate nucleus and lentiform nucleus. Both nuclei serve as relay stations between the thalamus and the cerebral cortex of the same side. The caudate nucleus parallels the lateral ventricle and consists of a head, body, and tail. The head causes an indentation to the frontal horns of the lateral ventricles, and the tail terminates at the amygdala in the temporal lobe (Figs. 3.40-3.44). The lentiform nucleus is a biconvex lens-shaped mass of gray matter located among the insula, caudate nucleus, and thalamus. The lentiform nucleus can be further divided into the globus pallidus and the putamen (Figs. 3.41 and 3.42). The claustrum is a thin linear layer of gray matter lying between the insula and the lentiform nucleus and is thought to be involved in the mediation of visual attention (Figs. 3.41-3.44).
Basal nuclei disease or dysfunction causes symptoms of difficulty starting, stopping, or sustaining movement and problems with memory and thought processes. Brain disorders associated with basal nuclei dysfunction include Huntington disease, Wilson disease, dystonia, and Parkinson disease. Injuries to the brain that can also cause damage to the basal nuclei are many and include stroke, tumors, carbon monoxide poisoning, liver disease, infection, and drug overdose.
Three large tracts of white matter, the internal, external, and extreme capsules, separate the basal nuclei and transmit electrical impulses throughout the brain. The internal capsule is shaped like a boomerang and separates the thalamus and caudate nucleus from the lentiform nucleus. The external capsule is a thin layer of white matter that separates the claustrum from the lentiform nucleus. Another thin layer of white matter located between the claustrum and insular cortex is the extreme capsule (Figs. 3.40-3.44).
The basal nuclei allow for the unconscious coordination of swinging our arms in rhythm with our legs as we walk.
DIENCEPHALON
The diencephalon is a complex of structures within the brain; its major components are the thalamus and hypothalamus. The diencephalon functions as a relay station for sensory information and as an interactive site between the central nervous and endocrine systems, and it is closely associated with the limbic system.
Thalamus
The thalamus consists of a pair of large oval gray masses that are interconnected with most regions of the brain and spinal cord via a vast number of fiber tracts. The thalamus makes up a portion of the walls of the third ventricle and connects through the middle of the third ventricle by adhesions known as the massa intermedia (Figs. 3.45 and 3.46). The thalamus serves as a relay station to and from the cerebral cortex for all sensory stimuli, with the exception of the olfactory nerves (Figs. 3.27-3.31 and 3.39-3.43).
FIG. 3.45 Sagittal view of hypothalamus and hypothalamic nuclei. Box indicates close-up view of the hypothalamic nuclei.
FIG. 3.46 Midsagittal, Tl-weighted MRI of brainstem.
Hypothalamus
The hypothalamus consists of a cluster of small but critical nuclei located below the thalamus just posterior to the optic chiasm, forming the floor of the third ventricle. Anatomically, it includes the optic chiasm, mamillary bodies, and infundibulum, and it is functionally related to the pituitary gland (Figs. 3.45 and 3.46). The hypothalamus functions in integrating the activities of the autonomic, endocrine, and limbic systems by helping to maintain homeostasis as it controls regulation of temperature, appetite, sexual drive, and sleep patterns. In addition, the hypothalamus modulates the activities of the anterior and posterior lobes of the pituitary gland via the release of neurohormones, which stimulate or inhibit the release of pituitary hormones.
Pituitary Gland
The pituitary gland (hypophysis) is an endocrine gland connected to the hypothalamus by the infundibulum.
The infundibulum is a slender stalk located between the optic chiasm and the mamillary bodies (Figs. 3.45 and 3.46). The pituitary gland is located in the sella turcica at the base of the brain (Figs. 3.45-3.49). The protected location of this gland suggests its importance. It is sometimes called the master gland because it controls and regulates the functions of many other glands through the action of its six major types of hormones. The hypothalamus sends signals to the pituitary gland to stimulate or inhibit hormone production. The pituitary gland can be broken down into an anterior lobe (adenohypophysis) and a posterior lobe (neurohypophysis) (Fig. 3.46). The anterior lobe synthesizes and releases six hormones: growth hormone (GH), prolactin (PRL), follicle-stimulating hormone (FSH), luteinizing hormone (LH), adrenocorticotropic hormone (ACTH), and thyroid-stimulating hormone (TSH). These six hormones help to regulate the function of other endocrine glands that influence growth, blood pressure, metabolism, temperature regulation, the reproductive glands, and response to stress. The posterior lobe does not produce hormones directly but releases into circulation two hormones that are synthesized in the hypothalamus. These hormones are antidiuretic hormone (ADH), which is commonly called vasopressin, and oxytocin.
Epithalamus
The epithalamus is the most posterior portion of the diencephalon and comprises the posterior commissure and pineal gland. The pineal gland, an endocrine structure, secretes the hormone melatonin, a serotonin-derived hormone that aids in the regulation of circadian rhythms. Melatonin also helps regulate the reproductive hormones LH and FSH. The pineal gland sits on the roof of the midbrain just posterior to the third ventricle and below the splenium of the corpus callosum. It is sometimes calcified, which aids in its detection on CT scans and lateral radiographs of the cranium (Figs. 3.29, 3.30, 3.45, 3.46, 3.50, and 3.51).
LIMBIC SYSTEM
The limbic system is a complex group of interconnected brain structures and fiber tracts located within and adjacent to the medial surface of the temporal lobes. These structures contain critical connecting pathways that extend to other areas deep within the midbrain, basal nuclei, and cerebral hemispheres (Figs. 3.52-3.57). They have a common functional role in the emotional aspects of behavior. Particularly, the limbic system is involved in aggression, submissive and sexual behavior, memory, learning, and general emotional responses. Structures of the limbic system include the hippocampus, amygdala, olfactory tracts, fornix, cingulate gyrus, and mamillary bodies. The parahippocampal gyrus is the in-rolled medial border of the temporal lobe and resembles the shape of a seahorse when viewed in the coronal plane (Fig. 3.57). Contained within this gyrus are the hippocampus and amygdala, prominent structures involved with memory and emotion. The hippocampus is an important structure that has a strong role in the transition of short-term memory to long-term memory. The amygdala is an almond-shaped mass of gray matter located deep within the parahippocampal gyrus anterior to the hippocampus (Figs. 3.40 and 3.52-3.54). The amygdala coordinates the actions of the autonomic and endocrine systems and is concerned with decision making, emotional processing, and aggressive and sexual behavior. The olfactory tracts run underneath the frontal lobes and connect to the amygdala to bring information from the sense of smell to the limbic system (Figs. 3.52 and 3.56). The limbic system is integrated with other important structures of the brain via limbic fiber tracts. The most frequently identified limbic tract is the fornix. The fornix is an arch-shaped structure that lies below the splenium of the corpus callosum and makes up the inferior margin of the septum pellucidum. It serves specifically to integrate the hippocampus with other functional areas of the brain (Figs. 3.45, 3.46, 3.52, 3.55, and 3.57). The cingulate gyrus is a prominent gyrus located on the medial border of each cerebral hemisphere just superior to the corpus callosum (Figs. 3.52, 3.55, and 3.57). This area is considered to be the brain’s emotional control center, so it plays an important role in the limbic system.
The mamillary bodies are two small rounded bodies in the floor of the posterior hypothalamus responsible for memory and motivation. They receive direct input from the hippocampus via the fornix and give rise to fibers that terminate in the anterior thalamus and the periaqueductal gray matter of the midbrain (Figs. 3.45, 3.46, 3.52, and 3.53).
Damage to the hippocampus may result in the loss of memory. High-resolution magnetic resonance images (MRIs) of the hippocampus are useful in evaluating patients with dementia or seizures associated with hippocampal sclerosis.
The amygdala is involved in learning and helps establish whether environmental cues and experiences are rewarding or dangerous. Abnormalities of the amygdala can profoundly influence behavior and have been linked to numerous neuropsychiatric and neurodevelopmental disorders. Amygdala dysregulation may cause increased risk taking, inappropriate social behavior, and elevated anxiety.
BRAINSTEM
The brainstem is a relatively small mass of tissue packed with motor and sensory nuclei, making it vital for normal brain function. Of the 12 cranial nerves, 10 originate from nuclei located in the brainstem. Its major segments are the midbrain, pons, and medulla oblongata (Figs. 3.45, 3.46, 3.58, and 3.59). Located within the central portion of the brainstem and common to all three segments is the tegmentum, an area that provides integrative functions, such as complex motor patterns, aspects of respiratory and cardiovascular activity, and regulation of consciousness (Fig. 3.60). The central core of the tegmentum contains the reticular formation, an area containing the cranial nerve nuclei and ascending and descending tracts to and from the brain. The brainstem as a whole acts as a conduit among the cerebral cortex, cerebellum, and spinal cord (Fig. 3.61).
Midbrain
The midbrain (mesencephalon), which is located above the pons at the junction of the middle and posterior cranial fossae, is the smallest portion of the brainstem. The midbrain is primarily composed of massive bundles of nerve fiber tracts and can be divided into two major segments: cerebral peduncles and the tectum (quadrigeminal plate) (Figs. 3.58-3.60). The midbrain surrounds the cerebral aqueduct, which contains CSF and connects the third and fourth ventricles. Posterior to the cerebral aqueduct is the tectum, which makes up the roof or dorsal surface of the midbrain (Figs. 3.58-3.60).
The tectum consists of four rounded protuberances termed colliculi. The upper pair, the superior colliculi, is a center for visual reflexes that coordinates movements of the eyes with those of the head and neck. The lower pair, the inferior colliculi, acts as a relay station for the auditory pathway, providing auditory information to the thalamus (Figs. 3.46 and 3.59-3.65). Anterior to the cerebral aqueduct are the two large cerebral peduncles (Figs. 3.46, 3.58, 3.60, and 3.62-3.65). These ropelike bundles, composed predominantly of axons that are a direct extension of the fibers of the internal capsule, extend from the cerebral cortex to the spinal cord (Figs. 3.46, 3.60, and 3.63-3.65).
The cerebral peduncles are made more noticeable by the presence of the darkly pigmented substantia nigra, a broad layer of cells that contain melanin (Figs. 3.63 and 3.64). The substantia nigra is involved with the production of dopamine, a neurotransmitter in the brain that functions in facilitating movement and controlling the brain’s reward system. Within the tegmentum of the midbrain, at the level of the superior colliculi, is the red nucleus. The red nucleus is composed of a tract of motor nerve fibers and serves as a relay station between the cerebellum and the cerebral hemispheres (Figs. 3.63 and 3.64). The red nucleus contributes to the coordination of movements and the sense of balance. Another portion of the tegmentum is the periaqueductal gray matter, which surrounds the cerebral aqueduct. This area receives sensory input that conveys pain and temperature information to the brain (Figs. 3.60, 3.63, and 3.64).
Parkinson disease, a neurodegenerative condition causing tremor and motor impairment, is caused by a loss of dopamine-secreting neurons in an area of the midbrain called the substantia nigra.
Dopamine is a neurotransmitter that plays a major role in reward and motivation behavior. Most rewards (e.g., food, sex, drugs of abuse, etc.) are capable of stimulating the release of dopamine in the brain. Dopamine may help with depression as well as focus/motivation; thus when dopamine levels are either elevated or low, resulting difficulties in focusing and staying on task can occur.
Pons
The pons is a large, oval-shaped expansion of the brainstem centrally located between the midbrain and medulla oblongata. The pons creates a prominent bulge as it lies just posterior to the clivus and anterior to the cerebellum. The term pons literally means “bridge.” This definition is appropriate because the pontine fibers relay signals between the spinal cord and the cerebral and cerebellar cortices (Figs. 3.46, 3.58, 3.60, 3.61, and 3.66-3.69).
Medulla Oblongata
The medulla oblongata extends from the pons to the foramen magnum, where it continues as the spinal cord (Figs. 3.45 and 3.46). The medulla oblongata contains all fiber tracts between the brain and spinal cord, as well as vital centers that regulate internal activities of the body. These centers are involved in the control of heart rate, respiratory rhythm, and blood pressure. The center of the anterior and posterior surfaces of the medulla oblongata is marked by the anterior and posterior median fissures (Figs. 3.58 and 3.59). These two fissures divide the medulla oblongata into two symmetric halves. Located on either side of the anterior median fissure are two bundles of nerve fibers called medullary pyramids (Figs. 3.58, 3.70, and 3.71). The pyramids contain nerve tracts that contribute to voluntary motor control. At the lower end of the pyramids, some of the nerve tracts cross over (decussate) to the opposite side (Fig. 3.58). This decussation in part accounts for the fact that each half of the brain controls the opposite half of the body. On each lateral surface of the medulla oblongata is a rounded oval prominence called the olive. The olives consist of nuclei that are involved in coordination, balance, and modulation of sound impulses from the inner ear (Figs. 3.58 and 3.70-3.72).
CEREBELLUM
The cerebellum, which is referred to as the “little brain,” attaches posteriorly to the brainstem and occupies the posterior cranial fossa (Fig. 3.73). The cerebellum is the coordination center for motor functions. Although the cerebellum does not initiate actual motor functions, it uses the brainstem to connect with the cerebrum to execute a variety of movements, including maintenance of muscle tone, posture, balance, and coordination of movement. The cerebellum consists of two cerebellar hemispheres. These hemispheres have an interesting appearance because the folds of gray matter resemble cauliflower. A midline structure called the vermis connects the two cerebellar hemispheres (Figs. 3.53, 3.54, 3.73, and 3.74). On the inferior surface of the cerebellar hemispheres are two rounded prominences called the cerebellar tonsils (Figs. 3.75 and 3.76).
Three pairs of nerve fiber tracts, the cerebellar peduncles, connect the cerebellum to the brainstem (Fig. 3.59). The superior cerebellar peduncles connect the cerebellum to the midbrain. The middle cerebellar peduncles serve as attachments to the pons, and the inferior cerebellar peduncles attach to the medulla oblongata (Figs. 3.74 and 3.77-3.79). All information traveling to and from the cerebellum is routed through the cerebellar peduncles.
Deep within the center of each cerebellar hemisphere is a collection of nuclei called the dentate nucleus, the largest and most lateral of the deep cerebellar nuclei (Figs. 3.73, 3.74, and 3.76). Fibers of the dentate nucleus project to the thalamus via the superior cerebellar peduncles. From there, the fibers travel to the motor areas of the cerebral cortex, namely, the precentral gyrus, thus influencing motor control.
A defect involving downward displacement or herniation of the brainstem and cerebellum through the foramen magnum is termed Arnold-Chiari malformation (deformity), or tonsillar herniation.
CEREBRAL VASCULAR SYSTEM
The vascular supply to the brain is unique. In comparison with the arteries in the body, the walls of the arteries in the brain are thin and weak, causing them to be susceptible to aneurysms and strokes. The veins of the brain do not contain valves. This lack of valves allows the blood to flow in either direction, creating a route for blood-borne pathogens to pass from the body to the head and vice versa. The capillaries of the brain are unlike those elsewhere in the body in that they do not allow movement of certain molecules from their vascular compartment into the surrounding brain tissue. This unique quality of impermeability is termed the blood-brain barrier (BBB). The presence of a normal BBB prevents large amounts of contrast medium from entering the brain. Pathologic conditions can disrupt the integrity of the BBB, allowing contrast to escape from the vessel into the surrounding tissues. However, there are some structures located within the brain that do not have a BBB, so they will naturally enhance when contrast media is used. It is normal for the pituitary gland, infundibulum, pineal gland, choroid plexus, mucosal surfaces of the nasopharynx and sinuses, venous structures, and meninges to be enhanced to varying degrees after contrast administration.
Arterial Supply
The brain receives arterial blood from two main pairs of vessels and their branches, the internal carotid arteries and the vertebral arteries, which make up the anterior and posterior circulation, respectively (Figs. 3.80 and 3.92A). Many normal variations of the arterial blood supply exist. This section focuses on the most common anatomic findings visualized in cross-section (Figs. 3.80-3.103).
Internal Carotid Arteries. The internal carotid arteries supply the frontal, parietal, and temporal lobes of the brain and orbital structures. These arteries arise from the bifurcation of the carotid arteries in the neck and can be divided into seven segments (Table 3.1 and Figs. 3.85 and 3.86). They ascend through the base of the skull and enter the carotid canals of the temporal bones (Figs. 3.80, 3.82, and 3.83). The internal carotid artery (ICA) then turns forward within the cavernous sinus, then up and backward through the dura mater, forming an S shape (which is referred to as the carotid siphon) before it reaches the base of the brain (Figs. 3.80 and 3.853.87). As the ICA exits the cavernous sinus, it branches into the ophthalmic artery just inferior to the anterior clinoid process (Figs. 3.80 and 3.85). The ICA then runs lateral to the optic chiasm and branches into the anterior cerebral artery and the larger middle cerebral artery (Tables 3.1 and 3.2 and Figs. 3.80, 3.81, and 3.84-3.88).
The anterior cerebral artery and its branches supply the anterior frontal lobe and the medial aspect of the parietal lobe (Fig. 3.84). The main segments and branches of the anterior cerebral artery are the horizontal (A1) segment, the vertical (A2) segment, and the distal (A3) segment (Figs. 3.84-3.88). The horizontal segment extends from the ICA bifurcation to the anterior communicating artery. The anterior communicating artery joins the two anterior cerebral arteries just anterior to the optic chiasm (Figs. 3.87-3.89). The vertical segment, an extension of the horizontal segment, courses superiorly toward the rostrum of the corpus callosum. The major branches of the vertical segment are the orbitofrontal, frontopolar, callosomarginal, and splenial arteries (Figs. 3.84 and 3.85). The distal segment curves around the genu of the corpus callosum and continues as the pericallosal artery (Tables 3.1 and 3.2 and Figs. 3.80 and 3.84)
TABLE 3.1 Segments of Internal Carotid, Anterior Cerebral, and Middle Cerebral Arteries
|
Artery |
Segments |
Location |
|
Internal carotid artery |
Cervical (C1) |
Bifurcation of common carotid artery to carotid canal of temporal bone |
|
(ICA) |
Petrous (C2) |
Carotid canal to foramen lacerum within the petrous portion of temporal bone |
|
Lacerum (C3) |
Extends above foramen lacerum to curve toward cavernous sinus |
|
|
Cavernous (C4) |
Cavernous sinus |
|
|
Clinoid (C5) |
Exits the cavernous sinus to enter the subarachnoid space near the anterior clinoid process of sphenoid bone |
|
|
Ophthalmic (supraclinoid) (C6) |
Extends from clinoid segment to the origin of the posterior communicating artery (PCoA) |
|
|
Communicating (terminal) (C7) |
Origin of the PCoA to the bifurcation of the ICA into the anterior and middle cerebral arteries |
|
|
Anterior cerebral artery |
Horizontal (precommunicating) (A1) |
Termination of ICA to junction with anterior communicating artery (ACoA) |
|
(ACA) |
Vertical (postcommunicating) (A2) |
From junction with ACoA, superiorly through longitudinal fissure, to origin or callosomarginal artery |
|
Distal (A3) |
Continues from callosomarginal artery origin as pericallosal artery |
|
|
Middle cerebral artery |
Horizontal (M1) |
ICA bifurcation to lateral fissure |
|
(MCA) |
Insular (M2) |
Courses superiorly within the lateral fissure to the insula |
|
Opercular (M3) |
Courses inferolaterally through lateral fissure |
|
|
Cortical (M4) |
Exits lateral fissure to cortex |
TABLE 3.2 Internal Carotid Artery Branches
|
Artery Region Supplied |
|
|
Ophthalmic artery |
Globe, orbit, frontal scalp, and frontal and ethmoid sinuses |
|
Anterior cerebral artery (ACA) |
Anterior frontal lobe and medial aspect of parietal lobe, head of caudate nucleus, anterior limb of the internal capsule, and anterior globus pallidus |
|
Middle cerebral artery (MCA) |
Lateral surface of the cerebrum, insula, anterior and lateral aspects of temporal lobe, nearly all the basal nuclei, and posterior and anterior internal capsules |
The middle cerebral artery is by far the largest of the cerebral arteries and is considered a direct continuation of the internal carotid artery. The middle cerebral artery gives off many branches as it supplies much of the lateral surface of the cerebrum, insula, and anterior and lateral aspects of the temporal lobe; nearly all the basal nuclei; and the posterior and anterior internal capsule (Figs. 3.80 and 3.81). The four major segments of the middle cerebral artery are the horizontal (M1), insular (M2), opercular (M3), and cortical (M4) (Tables 3.1 and 3.2 and Figs. 3.81 and 3.85-3.91). The horizontal segment courses from the origin at the ICA bifurcation laterally toward the insula and branches into the lateral lenticulostriate arteries, which supply to the lentiform nucleus, parts of the internal capsule, and caudate nucleus (Fig. 3.81). The insular segment courses along the insula, continuing as the opercular segment that emerges from the lateral fissure. Upon exiting the lateral fissure, the opercular segment becomes the cortical segment, which splits into the superior and inferior groups of cortical branches that supply nearly the entire surface of the cerebral hemispheres.
A lacunar stroke or infarct is caused by occlusion of an artery that supplies the deep structures of the brain, such as the basal nuclei and internal capsule. The lenticulostri- ate branches of the middle cerebral artery supply blood to this area. Occlusion of one of these vessels will result in a lacunar infarct, causing this area to be the most frequent site of strokes.
TABLE 3.3 Segments of Vertebral and Posterior Cerebral Arteries
|
Artery |
Segment |
Location |
|
Vertebral artery (VA) |
Extraosseous (V1) |
Subclavian artery to C6 |
|
Foraminal (V2) |
C6 to C1 |
|
|
Extraspinal (V3) |
C1 to foramen magnum |
|
|
Intradural (V4) |
Courses superomedially behind clivus, joins with contralateral VA to form basilar artery |
|
|
Posterior cerebral artery (PCA) |
Precommunicating (P1) |
Extends from basilar artery bifurcation to posterior communicating artery |
|
Ambient (P2) |
Around cerebral peduncle within ambient cistern |
|
|
Quadrigeminal (P3) |
Quadrigeminal cistern |
|
|
Calcarine (P4) |
In calcarine fissure on medial aspect of occipital lobe |
Vertebral Arteries. The vertebral arteries begin in the neck at the subclavian artery and ascend vertically through the transverse foramina of the cervical spine. They can be divided into four segments (see Table 3.3). The vertebral arteries curve around the atlanto- occipital joints to enter the cranium through the foramen magnum (Fig. 3.92). The two vertebral arteries course along the medulla oblongata and unite ventral to the pons, forming the basilar artery (Figs. 3.80, 3.82-3.87, and 3.92-3.100). The vertebral and basilar arteries give rise to several pairs of smaller arteries that supply the cerebellum, pons, and inferior and medial surfaces of the temporal and occipital lobes. The four major pairs of arteries are listed in order from inferior to superior: posterior inferior cerebellar (PICA), anterior inferior cerebellar (AICA), superior cerebellar (SCA), and posterior cerebral (PCA). Located between the anterior inferior cerebellar artery and superior cerebellar artery are many tiny perforating pontine vessels (Figs. 3.92B and 3.943.97). The posterior cerebral arteries can be divided into four major segments: precommunicating or peduncular (P1), ambient (P2), quadrigeminal (P3), and calcarine (P4) (Fig. 3.94). The precommunicating segment is a short segment that extends laterally from the basilar bifurcation to the posterior communicating artery (Fig. 3.92C). The posterior communicating artery forms a connection between the posterior cerebral artery and the ICA (Figs. 3.92A and C, 3.94, 3.99, and 3.100). The ambient segment courses posteriorly in the ambient cistern around the midbrain and then continues as the quadrigeminal segment located within the quadrigeminal cistern. The calcarine segment is located on the medial surface of the occipital lobe. The distal posterior cerebral artery frequently divides into many branches, including several temporal and occipital arteries (Tables 3.3 and 3.4 and Figs. 3.94, 3.101, and 3.102).
Arteriovenous malformations (AVMs) are the most common type of congenital vascular malformation. They consist of a tangle of dilated arteries and veins, usually accompanied by arteriovenous shunting. Approximately 40% of individuals with AVMs will bleed by the age of 40 years.
Pathology involving the cerebrovascular system is a common cause of cranial neurologic deficits. The brain needs a constant source of oxygen and glucose and is dependent on the vascular system to provide a steady supply. Any injury or disease affecting the cerebrovascular system can result in vascular insufficiency. Vascular interruptions lasting more than a few minutes will result in necrosis of adjacent brain tissue.
Circle of Willis. The cerebral arterial circle, or circle of Willis, is a critically important anastomosis among the four major arteries (two vertebral and two internal carotid) feeding the brain. The circle of Willis is formed by the anterior and posterior cerebral, anterior and posterior communicating, and the internal carotid arteries. The circle is located mainly in the suprasellar cistern at the base of the brain. Many normal variations of this circle may occur in individuals. The circle of Willis functions as a means of collateral blood flow between cerebral hemispheres in the event of blockage (Figs. 3.94 and 3.98-3.103).
TABLE 3.4 Vertebral and Basilar Artery Branches
|
Artery Region Supplied |
|
|
Posterior inferior cerebellar (PICA) |
Inferior cerebellum |
|
Anterior inferior cerebellar (AICA) |
Anterior and inferior cerebellum |
|
Pontine vessels |
Pons |
|
Superior cerebellar (SCA) |
Superior cerebellum, portions of midbrain, and pons |
|
Posterior cerebral (PCA) |
Occipital and temporal lobes |
Venous Drainage
The venous system of the brain and its coverings are primarily composed of the dural sinuses, superficial cortical veins, and deep veins of the cerebrum.
Dural Sinuses. The dural sinuses are very large veins located within the dura mater of the brain. All the veins of the head drain into the dural sinuses and ultimately into the internal jugular veins of the neck. The seven major dural sinuses are the superior and inferior sagittal, straight, transverse, sigmoid, cavernous, and petrosal (Figs. 3.104 and 3.105). The superior sagittal sinus lies in the longitudinal fissure between the falx cerebri and the cranium. It begins at the crista galli, runs the entire length of the falx cerebri, and ends at the internal occipital protuberance of the occipital bone (Figs. 3.3 and 3.104-3.108).
The inferior sagittal sinus, which is much smaller than the superior sagittal sinus, runs posteriorly just under the free edge of the falx cerebri within the longitudinal fissure (Figs. 3.3, 3.104, 3.105, and 3.107). The inferior sagittal sinus converges into the great cerebral vein to form the straight sinus. The straight sinus extends along the length of the junction of the falx cerebri and the tentorium cerebelli (Figs. 3.3, 3.104, 3.105, and 3.107-3.109). The junction of the superior sagittal, transverse, and straight sinuses creates the large confluence of sinuses or the torcular herophili (Figs. 3.3, 3.104, 3.105, 3.109, and 3.110). The transverse sinuses extend from the confluence of sinuses between the attachment of the tentorium and the cranium (Figs. 3.3 and 3.110-3.112). As the transverse sinuses pass through the tentorium cerebelli, they become the sigmoid sinuses. The S-shaped sigmoid sinuses continue in the posterior cranial fossa to join the jugular bulbs of the internal jugular veins (Figs. 3.104, 3.105, 3.110, and 3.112).
FIG. 3.110 Coronal MRI with transverse and sigmoid sinuses.
FIG. 3.115 Axial CT of cavernous sinus with contrast enhancement.
The cavernous sinuses, located on each side of the sella turcica and body of the sphenoid bone, are formed by numerous interconnected venous channels. They envelop the internal carotid arteries and third through sixth cranial nerves. Each cavernous sinus receives blood from the superior and inferior ophthalmic veins and communicates with the transverse sinuses via the petrosal sinuses (Figs. 3.104, 3.112, and 3.113-3.117).
Superficial Cortical and Deep Veins. The superficial cortical veins are located along the surface of each cerebral hemisphere and are responsible for draining the cerebral cortex and portions of the white matter. The veins drain into the dural sinuses with numerous anastomoses between the superficial and deep veins (Fig. 3.118).
The deep veins of the cerebrum drain the white matter and include the thalamostriate, septal, internal cerebral, basal (vein of Rosenthal), and great cerebral vein (vein of Galen) (Figs. 3.104-3.123).
The thalamostriate vein runs in a groove between the thalamus and caudate nucleus, where it drains both structures. The septal vein runs posteriorly across the septum pellucidum and joins with the thalamostriate veins to create the paired internal cerebral veins at the inferior aspect of the interventricular foramen (Figs. 3.104, 3.105, and 3.122). The basal vein of Rosenthal drains the medial temporal lobe and basal nuclei as it curves posteriorly around the cerebral peduncle and quadrigeminal plate to join the great cerebral vein. Each internal cerebral vein runs posteriorly beneath the third ventricle to meet with the paired basal veins beneath the corpus callosum to form a short trunk, the great cerebral vein. The unpaired great cerebral vein (vein of Galen) is a short, midline vessel running between the splenium of the corpus callosum and pineal gland, where it joins with the inferior sagittal sinus to form the straight sinus. All cerebral venous output will eventually drain into one of the dural sinuses and ultimately into the internal jugular veins (Figs. 3.104, 3.105, 3.107-3.109, and 3.118-3.123).
CRANIAL NERVES
There are 12 cranial nerves (CNs), numbered from anterior to posterior according to their attachment to the brain. All but the first and second CNs arise from the brainstem (Figs. 3.58, 3.59, and 3.124). Each of these nerves corresponds to a specific function of the body (Table 3.5). It is important to recognize the adjacent brain structures that act as anatomic landmarks to localize the course of the CNs in the head.
TABLE 3.5 Cranial Nerves
|
Cranial Nerves Type Foramen Function |
|||
|
Olfactory (I) |
Sensory |
Olfactory foramina in cribriform plate of ethmoid bone |
Smell |
|
Optic (II) |
Sensory |
Optic foramen |
Vision |
|
Oculomotor (III) |
Motor |
Superior orbital fissure |
Movement of superior, inferior, and medial rectus; inferior oblique; and levator palpebrae muscles |
|
Trochlear (IV) |
Motor |
Superior orbital fissure |
Movement of superior oblique muscle |
|
Trigeminal (V) |
Mixed |
Meckel's cave |
Sensory from face and head and movement of muscles of mastication and suprahyoid muscles |
|
Ophthalmic (V1) |
Sensory |
Superior orbital fissure |
Sensation from cornea, iris, scalp, eyelids, lacrimal apparatus, nasal cavity, forehead, ethmoid and frontal sinuses, nose |
|
Maxillary (V2) |
Sensory |
Foramen rotundum |
Sensation from upper lip, upper jaw and teeth, maxillary sinuses, palate |
|
Mandibular (V3) |
Mixed |
Foramen ovale |
Movement of muscles of mastication and suprahyoid muscles Sensation from lower jaw and teeth, TMJ, parotid and sublingual glands, anterior 2/3 of tongue |
|
Abducens (VI) |
Motor |
Superior orbital fissure |
Movement of lateral rectus muscle |
|
Facial (VII) Branches: -Temporal -Zygomatic -Buccal -Marginal mandibular -Cervical |
Mixed |
Internal auditory canal, facial canal, stylomastoid foramen |
Movement of the muscles of facial expression Taste from anterior 2/3 of tongue, floor of mouth, and palate Sensation from external auditory meatus (EAM); lacrimal, parotid, sublingual, and submandibular glands |
|
Vestibulocochlear (VIII) Vestibular branch Cochlear branch |
Sensory |
Internal auditory canal |
Sensation from vestibular structures for equilibrium Sensation from cochlea for interpretation of sound |
|
Glossopharyngeal (IX) |
Mixed |
Jugular foramen |
Movement of muscle for swallowing |
|
Group 1 |
Taste from posterior 1/3 of tongue |
||
|
Group 2 Group 3 |
Sensory input on pain and temperature from middle ear Sensory input from carotid sinus and carotid body |
||
|
Vagus (X) |
Mixed |
Jugular foramen |
Movement of pharyngeal and laryngeal muscles Movement of smooth muscle in trachea, bronchi, digestive tract; moderates cardiac pacemaker and vasoconstriction of coronary arteries Sensation from EAM and dura mater of posterior cranial fossa |
|
Accessory (XI) |
Motor |
Jugular foramen |
|
|
Cranial root |
Movement of pharynx and palate |
||
|
Spinal root |
Movement of sternocleidomastoid (SCM) and trapezius muscles |
||
|
Hypoglossal (XII) |
Motor |
Hypoglossal canal |
Movement of tongue muscles |
Olfactory Nerve (CN I)
The olfactory nerve is the nerve of smell. The olfactory neurosensory cells are located in the covering of the superior nasal concha and the superior part of the nasal septum. The axons of these cells unite to form 18 to 20 small nerve bundles that are known collectively as olfactory nerve fibers. The nerve fibers pass through the olfactory foramina in the cribriform plate of the ethmoid bone to synapse with the olfactory bulb in the anterior cranial fossa. The right and left olfactory tracts extend from the olfactory bulbs and run along the inferior surface of the frontal lobes to pass to the lateral hippocampal gyrus and interact with the limbic system (Figs. 3.124-3.127). Each olfactory nerve is surrounded by the three layers of the cranial meninges.
Optic Nerve (CN II)
The optic nerve is the nerve of sight. Sensory nerve cells arise from the retina and converge toward the posterior aspect of the eye (Figs. 3.129 and 3.130). These fibers unite to form the large optic nerve that passes posteromedially through the optic canal into the middle cranial fossa to join its partner at the optic chiasm just anterior to the infundibulum (Figs. 3.44, 3.58, 3.128, and 3.129). In the optic chiasm, the fibers from the medial side of the retina cross to the opposite side, and the fibers from the lateral aspect remain on the same side (Fig. 3.130).
This decussation of the medial fibers allows for binocular vision. Posterior to the optic chiasm, the optic nerve extends as optic tracts, which continue around the midbrain and terminate in the posterolateral thalamus (Figs. 3.15, 3.53, and 3.130). The optic pathway continues posteriorly from the thalamus as nerve axons forming optic radiations that are relayed to the visual cortex located in the occipital lobe (Figs. 3.130 and 3.131).
Damage to the visual system will result in visual losses related to the location of the damage. If the optic nerve is damaged anterior to the optic chiasm, the result will be loss of vision in that eye. At the optic chiasm, damage on the medial aspect will result in loss of peripheral vision, whereas damage on the lateral aspect results in loss of the ipsilateral central (nasal) visual field. If damage occurs posterior to the optic chiasm, the result will be loss of input from the contralateral visual fields of both eyes.
Oculomotor Nerve (CN III)
The oculomotor nerve moves the eye by supplying fibers to all extraocular muscles of the eye except the superior oblique and lateral rectus muscles (Figs. 3.132 and 3.133). This nerve emerges from the midbrain and passes anteriorly into the interpeduncular cistern. It runs lateral to the posterior communicating artery through the roof of the cavernous sinus and travels in the lateral wall superolateral to the ICA (Fig. 3.134). The nerve enters the orbit through the superior orbital fissure and then breaks into superior and inferior branches that innervate the superior, medial, and inferior rectus muscles, as well as the inferior oblique and levator palpebrae muscles (Figs. 3.132-3.135 and 3.137).
FIG. 3.136 Axial, T2-weighted MRI of trochlear nerve (CN IV).
Trochlear Nerve (CN IV)
The trochlear nerve innervates only the superior oblique muscle of the eye. It is the only cranial nerve that emerges from the posterior surface of the brainstem (Figs. 3.59, 3.132, and 3.136). The nerve originates in the tegmentum and exits the posterior surface of the midbrain. It travels around the brainstem to enter the cavernous sinus just below the oculomotor nerve. This nerve enters the orbit through the superior orbital fissure, where it finally reaches the superior oblique muscle (Figs. 3.132, 3.134, 3.136, and 3.137).
Trigeminal Nerve (CN V)
The trigeminal nerve, the largest of the cranial nerves, has three major divisions: ophthalmic, maxillary, and mandibular (Fig. 3.138). It is the major sensory nerve of the face and contains motor fibers for the muscles of mastication and sensory fibers from the head. The nerve exits the brain between the pons and the middle cerebellar peduncles (Fig. 3.58). Before trifurcating into three branches, the nerve enters Meckel’s cave and forms the trigeminal ganglion, where it is covered in dura, resulting in a CSF-filled subarachnoid space referred to as the trigeminal cistern (Figs. 3.138-3.141). The ophthalmic branch (Vj) runs through the lateral wall of the cavernous sinus and enters the orbit through the superior orbital fissure, where it branches again to provide sensation to the lacrimal apparatus, cornea, iris, forehead, ethmoid and frontal sinuses, and nose (Figs. 3.134 and 3.138). The maxillary branch (V2) courses in the lateral wall of the cavernous sinus and exits the skull through the foramen rotundum (Figs. 3.134 and 3.138). Branches of the maxillary nerve continue through the inferior orbital fissure and infraorbital foramen. This branch provides sensation to the cheek, palate, sides of the nose and upper jaw and teeth, and maxillary sinuses. The mandibular branch (V3) is considered a “motor” nerve and exits the skull through the foramen ovale. It innervates the muscles of mastication, ear canal, lower jaw and teeth, parotid and sublingual glands, and anterior two-thirds of the tongue (Figs. 3.138-3.141).
Trigeminal neuralgia or tic douloureux is a neurologic syndrome involving one or more branches of the trigeminal nerve. The syndrome is characterized by sudden, severe attacks of excruciating pain which can last from a few seconds to several minutes. The painful attacks may be triggered by talking, eating, drinking, or a simple touch to the face. The most common cause is thought to be a result of compression of the nerve by an adjacent artery or vein, causing irritation. Other causes may include tumors located within the CPA cistern, trauma, infections, and multiple sclerosis. Treatment options include medication to control pain, nerve injections, and surgical or rhizotomy procedures.
Abducens Nerve (CN VI)
The abducens nerve supplies motor impulses to the lateral rectus muscle of the eye. It originates near the midline of the lower portion of the pons and ascends through the prepontine cistern to the cavernous sinus. Of all the CNs within the cavernous sinus, the abducens nerve courses most medial. It exits the skull through the superior orbital fissure, where it meets up with the lateral rectus muscle (Figs. 3.132-3.134, 3.137, and 3.142).
Facial Nerve (CN VII)
The facial nerve emerges as two distinct roots from the lower portion of the pons in a recess between the olive and inferior cerebellar peduncle and enters the internal auditory canal of the temporal bone, just anterior to CN VIII (Figs. 3.58 and 3.143). After passing through the temporal bone, the nerve continues along the facial canal, where it finally emerges from the skull through the stylomastoid foramen and runs through the parotid gland (see Chapter 2, temporal bone). While in the parotid gland, the nerve splits into five terminal branches: temporal, zygomatic, buccal, marginal mandibular, and cervical. The facial nerve innervates the facial muscles, lacrimal gland, and parotid, sublingual, and submandibular glands. In addition, it provides taste sensation to the anterior two- thirds of the tongue (Figs. 3.143 and 3.145-3.147).
A type of temporary facial nerve paralysis is called Bell’s palsy. It is believed that a viral infection may cause the facial nerve to become inflamed and swell, causing the resultant paralysis. Symptoms include mild weakness, twitching, a drooping eyelid or corner of the mouth, excessive tearing, and drooling.
FIG. 3.142 Axial, T2-weighted MRI of abducens nerve (CN VI)
Vestibulocochlear Nerve (CN VIII)
The vestibulocochlear nerve exits the brainstem at the pontomedullary junction and enters the internal auditory canal posterior to the facial nerve (Figs. 3.58 and 3.144). The vestibulocochlear nerve has two distinct components, vestibular and cochlear. The vestibular branch picks up impulses from the semicircular canals that aid in the maintenance of equilibrium. The cochlear branch receives impulses from the cochlea and separates these impulses into high and low frequencies for the interpretation of sound (Figs. 3.144-3.146).
Glossopharyngeal Nerve (CN IX)
The glossopharyngeal nerve supplies motor impulses to the muscles involved in swallowing. In addition, its sensory component can be divided into three groups: group 1 innervates the posterior third of the tongue, group 2 provides sensory input on pain and temperature from the middle ear, and group 3 gathers sensory input from the carotid sinus and carotid body. The carotid sinus is a dilatation at the origin of the ICA that contains baroreceptors, which react to changes in arterial blood pressure. The carotid body, a small neurovascular structure located at the bifurcation of the common carotid artery, acts as a chemoreceptor, which senses changes in the chemical composition of blood (see Chapter 5, carotid arteries). The glossopharyngeal nerve emerges as a series of rootlets from the medulla oblongata between the olive and inferior cerebellar peduncles (Fig. 3.58). It exits the cranium through the jugular foramen and courses to the root of the tongue (Figs. 3.148-3.150).
Vagus Nerve (CN X)
In Latin, vagus means “wandering,” which the vagus nerve does as it “wanders” inferiorly from the brainstem to cover an extensive course as it innervates areas of the neck, thorax, and abdomen. The vagus nerve arises from the medulla oblongata as 8 to 10 rootlets between the inferior cerebellar peduncle and the olive, eventually converging into two roots that exit the skull through the jugular foramen (Fig. 3.58). It descends through the carotid sheath while in the neck and continues inferiorly to the thorax and abdomen. At the neck, it passes through the superior thoracic aperture between the subclavian artery and brachiocephalic vein, where it continues its course toward the diaphragm behind the respective main bronchi. There are many branches of the vagus nerve that supply such structures as the dura of the posterior fossa, auricle, external auditory meatus, pharynx, soft palate, larynx, heart, stomach, liver, duodenum, and pancreas (Figs. 3.148-3.151).
The vagus nerve (CNX) provides the bulk of the parasympathetic input to the gastrointestinal system, lungs, and heart. The vagus nerve triggers the release of the neurotransmitter acetylcholine and hormones such as prolactin, vasopressin, and oxytocin that influence digestion, metabolism, and the relaxation response and may also slow the immune response. People with a stronger vagus response may recover more quickly after stress, injury, or illness.
Activation of the vagus nerve typically leads to a reduction in heart rate and/or blood pressure. This occurs commonly in the setting of gastrointestinal illness or in response to other stimuli that may include emotional stress. When the circulatory changes are great enough, vasovagal syncope results from a sudden drop in cardiac output, causing cerebral hypoperfusion.
Accessory Nerve (CN XI)
The accessory nerve has both cranial and spinal roots. These two roots form a common stem before their exit through the jugular foramen. The cranial root, an accessory to the vagus nerve, emerges from a series of rootlets arising from the medulla oblongata (Fig. 3.58). These fibers supply the skeletal muscles of the pharynx and palate. The spinal root arises from a series of rootlets from the lateral cervical cord to innervate the sternomastoid and trapezius muscles in the neck and back (Figs. 3.149, 3.151, and 3.152).
Hypoglossal Nerve (CN XII)
All of the muscles of the tongue with the exception of the palatoglossus are supplied by the hypoglossal nerve. Several rootlets arise from the medulla oblongata between the olive and the medullary pyramids (Fig. 3.58). The rootlets unite to form a trunk that passes posterior to the vertebral artery to exit the cranium through the hypoglossal canal of the occipital bone (Fig. 3.149). Inferior to the skull, the hypoglossal nerve crosses lateral to the bifurcation of the common carotid artery to enter the floor of the mouth and innervate the muscles of the tongue (Figs. 3.151-3.153).
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