Cerebral Hemispheres

The cerebral hemisphere has four lobes: frontal, parietal, occipital, and temporal. The frontal lobe is separated from the parietal lobe by the central (Rolandic) sulcus, the parietal lobe is separated from the occipital lobe by the parieto-occipital sulcus, and the temporal lobe is separated from the frontal and parietal lobes by the sylvian (lateral) fissure.

The main named areas of the frontal lobe are the precentral gyrus (the primary motor strip of the cerebral cortex) and the three frontal gyri anterior to the motor strip: the superior, middle, and inferior frontal gyri (Fig. 2-1). On the medial surface one finds the cingulate gyrus just superior to and bounding the corpus callosum, and the gyrus rectus extending along the medial basal surface of the anterior cranial fossa (Fig. 2-2).

Figure 2-1 Surface anatomy of the brain from a lateral view. Gyri are labeled in this figure.

From Nieuwenhuys R, Voogd J, van Huijen C: The human central nervous system: a synopsis and atlas, rev ed 1, Berlin, Springer-Verlag, 1988.

Figure 2-2 Medial surface of brain. Midsagittal view of the brain.

From Burt AM: Textbook of neuroanatomy. Philadelphia, WB Saunders, 1993, p. 159.

The parietal lobe contains the postcentral gyrus (the center for somatic sensation), the supramarginal gyrus just above the temporal lobe, and the angular gyrus near the apex of the temporal lobe (see Fig. 2-1). Two superficial gyri of note are the superior and inferior parietal lobules, which are separated by an interparietal sulcus. On its medial side the precuneate gyrus is present in front of the parieto-occipital fissure, with the cuneate gyrus posteriorly in the occipital lobe (see Fig. 2-2).

The temporal lobe contains the brain-functioning elements of speech, memory, and hearing. Superior (auditory), middle, and inferior temporal gyri are seen on the superficial aspect of the brain (see Fig. 2-1). Deep to the sylvian fissure is the insula, or isle of Reil, which is bounded laterally by the opercular regions. The frontal operculum (superior to the sylvian fissure and in the frontal lobe) contains portions of Broca’s motor speech area. The inferior part of the insula near the sylvian fissure is called the limen of the insula. The inferior and medial surface of the temporal lobe reveals the parahippocampal gyrus with the hippocampus just superior to it (Fig. 2-3). Anteriorly, the almond-shaped amygdala dominates. If you walked along the cortex of the parahippocampal-hippocampal region of the inferomedial temporal lobe, you would get dizzy because it simulates a spiral staircase. But if you started at the right collateral sulcus (in the coronal plane) just inferior to the parahippocampus and traveled northward, you would first hit the entorhinal cortex, then turn at the parasubiculum, pass along the subiculum proper, and continue laterally to the presubiculum. All of these represent parahippocampal structures. You would then curl in a spiral into the hippocampus’ cornu ammonis and dentate gyrus with the fimbria found superomedially and the alveus on top of the cornu ammonis. The cornu itself has four zones of granular cells: CA1 (Sommer sector) is lateral, CA2 (dorsal resistant zone) is superior, CA3 (resistant Spielmeyer sector) is superomedial, and CA4 (end folium) is inferomedial. CA1 is also called the vulnerable sector because it is the most sensitive area of the brain (with the globus pallidus) to anoxia. CA3 is resistant to anoxic damage. Sclerosis of CA1 is the etiology of mesial temporal sclerosis or hippocampal atrophy and has been linked to febrile seizures.

Figure 2-3 Hippocampal anatomy. Arrow indicates the hippocampal sulcus (superficial part). 1, cornu ammonis (Ammon’s horn); 2, gyrus dentatus; 3, hippocampal sulcus (deep or vestigial part); 4, fimbria; 5, prosubiculum; 6, subiculum proper; 7, presubiculum; 8, parasubiculum; 9, entorhinal area; 10, parahippocampal gyrus; 11, collateral sulcus; 12, collateral eminence; 13, temporal (inferior) horn of the lateral ventricle; 14, tail of the caudate nucleus; 15, stria terminalis; 16, choroid fissure and choroid plexuses; 17, lateral geniculate body; 18, lateral part of the transverse fissure (wing of ambient cistern); 19, ambient cistern; 20, mesencephalon; 21, pons; 22, tentorium cerebelli.

Modified after Williams, 1995. From Duvernoy HM: The human hippocampus. New York, Springer-Verlag, 1998, p. 18. Used with permission.

The occipital lobe is the lobe most commonly associated with visual function. At its apex is the calcarine sulcus, with the cuneate gyrus just above it (posteroinferior to the parieto-occipital fissure) and the calcarine gyrus just below it (see Figs. 2-1 and 2-2).

The diencephalon contains the thalamus and hypothalamus. The diencephalic syndrome refers to visual changes, euphoria, increased appetite, emaciation, amnesia, with alertness, usually caused by a hypothalamic astrocytoma (or adolesence). The thalamus has many nuclei, the most important of which are the medial and lateral geniculate nuclei, associated with auditory and visual functions, respectively (see the following sections). The thalamus is found on either side of the third ventricle and connects across the midline by the massa intermedia. Its other functions include motor relays, limbic outputs, and coordination of movement. Portions of the thalamus also subserve pain, cognition, and emotions. The hypothalamus is located at the floor of the third ventricle, above the optic chiasm and suprasellar cistern. The hypothalamus is connected to the posterior pituitary via the infundibulum, or stalk, through which hormonal information to the pituitary gland is transmitted. The hypothalamus is critical to the autonomic functions of the body.

Brain Stem

The mesencephalon differentiates into the midbrain. The midbrain is the site of origin of the third and fourth cranial nerves (CN III and IV). Additionally, the midbrain contains the red nucleus, substantia nigra, and cerebral aqueduct, or aqueduct of Sylvius (Fig. 2-4). White matter tracts conducting the motor and sensory commands pass through the midbrain. The midbrain is also separated into the tegmentum and tectum, which refer to portions of the midbrain anterior and posterior to the cerebral aqueduct, respectively. The tectum, or roof, consists of the quadrigeminal plate (corpora quadrigemina), which houses the superior and inferior colliculi. The tegmentum contains the fiber tracts, red nuclei, nuclei of cranial nerves III and IV, and periaqueductal gray matter. The substantia nigra is the anterior border of the tegmentum. Anterior to the tegmentum are the cerebral peduncles.

Figure 2-4 Midbrain anatomy. This constructive interference steady state image shows both oculomotor nerves in their cisternal portions, leading to the cavernous sinus (long white arrows), the left trochlear nerve (double arrows) emanating from the posterior midbrain and coursing in the ambient cistern, and the right trochlear nerve decussating posteriorly in the midline (small black arrow). Can you see the optic nerves in the optic canals bilaterally (arrowheads)?

The metencephalon develops into the pons and cerebellum (see following section). The pons contains the nuclei for cranial nerves V, VI, VII, and VIII (Figs. 2-5 and 2-6). Pontine white matter tracts transmit sensory and motor fibers to the face and body (see Fig. 2-5). The pons also houses major connections of the reticular activating system for vital functions. One identifies the pons on the sagittal scan by its “pregnant belly.”

Figure 2-5 Pontine anatomy. A, Axial T2-weighted image (T2WI) shows cranial nerve V exiting the pons (black arrows). Note the superior cerebellar peduncles (arrowheads), fourth ventricle (4), medial longitudinal fasciculus (MLF) (asterisk), and basilar artery (white arrow). B, Pontine anatomy at the level of the superior cerebellar peduncle (25) shows several descending and ascending tracts. C, Facial colliculi (black arrows) are clearly seen on this axial T2WI. The middle cerebellar peduncle (P) is the dominant structure leading to the cerebellum. Also shown are the basilar artery (white arrow), cerebellar pontine angle cistern (C), and nodulus (N). D, At the facial colliculus (27) one finds numerous cranial nerve nuclei and traversing lemnisci. B and D,

From Kretschmann H-J, Weinrich W: Cranial neuroimaging and clinical neuroanatomy: magnetic resonance imaging and computed tomography, rev 2nd ed, New York, Thieme, 1993, pp. 139 and 137, respectively.

Figure 2-6 Pons anatomy. The left abducens nerve is denoted by the white arrow, whereas the cochlear (more anterior) and inferior vestibular nerve (more posterior) are seen bilaterally in the cerebellopontine angle cistern (black arrows). The cochlea (C) and vestibule (V) are obvious and the inferior cerebellar peduncle has been labeled P.

The myelencephalon becomes the medulla. The medulla contains the nuclei for cranial nerves IX, X, XI, and XII. Again, the sensory and motor tracts to and from the face and brain are transmitted through the medulla. Other named portions of the medulla include the pyramids, an anterior paramedian collection of fibers transmitting motor function, and the olivary nucleus in the midmedulla (Fig. 2-7).

Figure 2-7 Medulla anatomy. A, The segment reveals the junction of the vertebral arteries to the basilar artery. The roots of the abducens nerve arise at the border between the medulla oblongata and pons. The upper part of the inferior olivary nucleus is positioned in the medulla oblongata. B, Axial T2-weighted image shows the preolivary sulcus (short arrows), the olivary sulcus (open arrows), pyramidal tract (long black arrows), and the inferior cerebellar peduncle (i), hypoglossal nuclei (squiggly arrows), and nerve complex (cranial nerves IX and X) (arrowhead). The olive (o), “which is stirred and not shaken,” is well demonstrated. C, White arrows point out hypoglossal nerves coursing to hypoglossal canals (HC). On either side of the midline posterior cleft are the gracile nuclei (black arrows). Lateral to them will be the cuneate nuclei. D, The mandibular nerve lies just below the foramen ovale. The roots of the vagus nerves branch off the medulla oblongata. E, The caudal portion of the medulla oblongata, the rootlets of the hypoglossal nerves, and the hypoglossal canal are included.

B–E from Kretschmann H-J, Weinrich W: Cranial neuroimaging and clinical neuroanatomy: magnetic resonance imaging and computed tomography, rev ed 2, New York, Thieme, 1993, pp. 133, 131, 129, 127, respectively.

Cerebellum

The cerebellum is located in the infratentorial space posterior to the brain stem. The anatomy of the cerebellum is complex, with many named areas. For simplicity’s sake, most people separate the cerebellum into the superior and inferior vermis and reserve the term cerebellar hemispheres for the rest of the lateral and central portions of the cerebellum. For those interested in details, the superior vermis has a central lobule and lingula visible anteriorly, and the inferior vermis has a nodulus, uvula, pyramid, and tuber on its inferior surface (Fig. 2-8). The superior surface provides a view of the culmen, declive, and folium of the superior vermis. Superolaterally, there is a bump called the flocculus, which may extend toward the cerebellopontine angle cistern. This is a potential “pseudotumor,” often misidentified as a vestibular schwannoma. The tonsils are located inferolaterally and are the structures that herniate downward through the foramen magnum in Chiari malformations.

Figure 2-8 Cerebellar anatomy. A, The parts of the cerebellar vermis. Diagram of a median section. B, Coronal diagram of the cerebellar lobes and their lobules. C, Sagittal magnetic resonance image of cerebellum.

A and B from Putz R, Pabst R [eds]: Sobotta atlas of human anatomy, ed 13, Philadelphia, Lippincott Williams & Wilkins, 1996, pp. 292, 293.

Gray matter masses in the cerebellum include the fastigial, globose, emboliform, and dentate nuclei; the dentate nuclei are seen well on T1-weighted images (T1WI), whereas the fastigial, globose, and emboliform nuclei cannot be discerned. The dentate nuclei are situated laterally in the white matter of the cerebellum, and can be seen on computed tomography (CT) because they may calcify in later life.

Three major white matter tracts connect the cerebellum to the brain stem (Fig. 2-9). The superior cerebellar peduncle (brachium conjunctivum) connects midbrain structures to the cerebellum, the middle cerebellar peduncle connects the pons to the cerebellum, and the inferior cerebellar peduncle (restiform body) connects the medulla to the cerebellum.

Figure 2-9 Cerebellar pathways. The afferent systems of the cerebellum (lateral view). The left half of the anterior lobe of the cerebellum was removed. The archicerebellum was separated and removed caudally from the middle cerebellar peduncle.

From Kretschmann H-J, Weinrich W: Cranial neuroimaging and clinical neuroanatomy: magnetic resonance imaging and computed tomography, rev ed 2, New York, Thieme, 1993, p. 326.

The functional divisions of the cerebellum are discussed in the following section. The flocculonodular lobe, fastigial nucleus, and uvula of the inferior vermis receive input from vestibular nerves and are thought to be involved primarily with maintaining equilibrium. Lesions of this part of the cerebellum, the archicerebellum, cause wide-based gait and dysequilibrium.

The superior vermis, most of the inferior vermis, and globose and emboliform nuclei receive spinocerebellar sensory information. Muscle tone information, postural tone, and coordination of locomotion appear to be influenced by these sites and by their effect on brain stem fibers, the red nuclei, and vestibular nuclei. The hemispheric portions of the cerebellum receive information from the pons and help to control coordination of voluntary movements. Abnormalities of the hemisphere (or the neocerebellum) include dysmetria, dysdiadochokinesis, intention tremors, nystagmus, and j^er_ky ataxia.

Corpus Callosum

The medial surface of the brain in the midline is dominated by the corpus callosum. This is the large white matter tract that spans the two hemispheres. Its named parts include the rostrum (its tapered anteroinferior portion just above the anterior commissure), the genu (the anterior wide sweep over the third ventricle), the body or trunk (the superiormost aspect), and the splenium (the posteriormost aspect) (see Fig. 2-2). Embryologically, the genu and body form first, then comes the splenium, and finally the rostrum. This is useful in understanding partial agenesis of the corpus callosum.

Other white matter tracts that cross the midline include the anterior commissure, located at the inferior aspect of the corpus callosum just above the lamina terminalis, and the posterior commissure, just anterior to the pineal gland near the habenula. The anterior commissure transmits tracts from the amygdala and temporal lobe to the contralateral side. The habenula and hippocampal commissures cross-connect the two hemispheres and thalami.

Deep Gray Nuclei

The basal ganglia are known by a number of names in the neuroanatomic literature. These gray matter structures lie between the insula and midline. The globus pallidus is the medial gray matter structure identified just lateral to the genu of the internal capsule (Fig. 2-10). Superficial to it lies the putamen. The caudate nucleus head indents the frontal horns of the lateral ventricle and is anterior to the globus pallidus; however, the body of the caudate courses over the globus pallidus, paralleling the lateral ventricle and ending in a tail of tissue near the amygdala.

Figure 2-10 Deep gray matter anatomy. A, Axial scan shows the caudate (C), putamen (P), and globus pallidus (G), as well as the anterior limb (long black arrow) and posterior limb (short black arrow) of the internal capsule. White matter tracts pass between the basal ganglia. The thalamus and periaqueductal gray matter line the third ventricle. The tiny dots of the fornix anteriorly (asterisk) and posterior commissure posteriorly (white arrow), as well as pulvinar thalamic gray matter (Pu), are also evident. B, Under the thalami (T), one can visualize the subthalamic nucleus (black arrow) and the substantia nigra (white arrow). The hippocampus (H) is present further laterally. The thalami are joined in the midline at the massa intermedia, and one can also see the forniceal columns (below asterisk) projecting above the thalami.

Additional terms used to refer to the various portions of the basal ganglia include the corpus striatum (all three structures and the amygdala) and the lentiform or lenticular nuclei (the globus pallidus and putamen).

The basal ganglia receive fibers from the sensorimotor cortex, thalamus, and substantia nigra, as well as from each other. Efferents go to the same locations and to the hypothalamus. The main function of the basal ganglia appears to be coordination of smooth movement.

The other deep gray matter structures of interest in the supratentorial space are the thalami, which sit on either side of the third ventricle. The thalamus is subdivided into many different nuclei by white matter striae. As will be discussed, the medial and lateral geniculate nuclei, located along the posterior aspect of the thalamus, are the most significant nuclei within the thalamic deep gray structures, because they serve as relay stations for visual and auditory function. The pulvinar is the posterior expansion of the thalamus. Behind the pulvinar are the wings of the ambient cistern. The massa intermedia connects the thalami across the third ventricle.

In the infratentorial space, the dentate nucleus, the largest deep gray matter structure, has connections to the red nuclei and to the thalami. The deep gray matter structures are most important in coordinative movements of the limbs and the trunk. Other central nuclei within the cerebellum include the emboliform, globose, and fastigial nuclei.

Ventricular System

The normal volume of cerebrospinal fluid (CSF) in the entire CNS is approximately 150 mL, with 75 mL distributed around the spinal cord, 25 mL within the ventricular system, and 50 mL surrounding the cortical sulci and in the cisterns at the base of the brain. In elderly persons, the intracranial CSF volume increases from 75 mL to a mean of approximately 150 mL in women and 190 mL in men (a statistically significant difference), a further indication of women’s phylogenetic superiority over men—less water, more brain. The normal production of CSF has been estimated to be approximately 450 mL/day, thereby replenishing the amount of CSF two to three times a day. As one might expect, the reabsorption of CSF is critical in this instance, and the arachnoid villi are the major sites where CSF is resorbed into the intravascular system from the extracellular fluid.

The flow of CSF runs from the choroid plexi in the floor of the lateral ventricles via the foramina of Monro, to the third ventricle, out the cerebral aqueduct of Sylvius, and into the fourth ventricle (Fig. 2-11). Each ventricle’s choroid plexus contributes to CSF production. After leaving the foramina of Magendie (medially) and Luschka (laterally) of the fourth ventricle, CSF flows into the cisterns of the brain and the cervical subarachnoid space and then down the intrathecal spinal compartment. The CSF ultimately percolates back up over the convexities of the hemispheres, where it is resorbed by the arachnoid villi into the intravascular space.

Figure 2-11 Ventricular system. Three-dimensional reconstruction from segmented images demonstrates the ventricular system derived from a magnetic resonance imaging data set.

There are several named cisterns around the brain. The names, locations, and the structures traversing these cisterns are summarized in Table 2-1.

Table 2-1 Cisterns of the Brain

Physiologic Calcifications

The pineal gland calcifies with age. A small percentage of children (2% of those younger than age 8 years and 10% of adolescents) show calcification of the pineal gland. By age 30 years, most people have calcified pineal glands. Anterior to the pineal gland, one often sees the habenular commissure as a calcified curvilinear structure.

The choroid plexus is calcified in about 5% of children by age 15 and in most adults by age 40.

The dura of the falx or tentorium is virtually never calcified in children and should be viewed as suspicious for basal cell nevus syndrome in that setting. The dura shows higher rates of calcification in patients who have had shunts placed or who have been irradiated.

Basal ganglia calcification is also rarely observed in individuals younger than age 30 years and should provoke a search for metabolic disorders or a past history of perinatal infections if seen in youngsters. (See the Appendix for causes of basal ganglia calcification).

Brodmann Areas

The functional units of the cerebral hemispheres have been separated into what are called Brodmann areas. These numbered areas correspond to different gyri that subserve various functions. The Brodmann areas are the currency with which fMRI scientists transact business and are therefore important to learn. In addition, knowing which gyri are responsible for which properties can be critical to predicting deficits in patients with strokes. This knowledge will also direct YOUR attention as the reader of images with a given clinical history to a specific site where subtle pathology may reside. Neuroradiologists on occasion receive requisitions for studies in which only the patient’s symptoms or neurologic signs are given (if they are lucky!). Thus, you may need to trace the pathway for that particular symptom or sign and localize the lesion better. It is only by knowing the pathway that might account for the patient’s symptom that you develop a well-trained eye for detecting disease. You see what you know.

Although this is by no means an exhaustive review, Table 2-2 will help you define structure and function in Brodmann’s terms.

Table 2-2 Functional Anatomy by Brodmann Areas

Talairach and Tournoux Space

The output of fMRI is often transposed onto a standard Talairach and Tournoux space that every neuroscientist can use to determine the location of gyri. This means that people’s brains become warped—to a standard space defined by Madame Talairach, the wife of a 20th century neuroanatomist. Hers has become the standard by which all brains are judged. The use of Talairach space also allows “summing of data” of many brains to one agreed-upon standard.

The anterior commissure to posterior commissure (AC-PC) line is frequently used to select axial sections in a reproducible fashion and is also the basis for Talairach space. The Talairach coordinates in X (right to left), Y (anterior to posterior), and Z (superior to inferior) axes are used in fMRI to identify the location of gyri. A spot that has a negative X location is to the right, positive X is to the left, positive Y is anterior to the midpoint of the AC-PC line, negative Y is posterior to the midpoint of the AC-PC line, negative Z is below the plane of the AC-PC line, and positive Z is above the AC-PC line.

Motor System

“I’ve got to scratch that itch!” What brain connections will this simple task require?

The primary origin of the stimulus for motor function is the precentral gyrus of the frontal lobe, which receives input from many sensory areas (Fig. 2-12; Table 2-3). Stimulation of the motor area of one precentral gyrus causes contraction of muscles on the opposite side of the body. The motor cortex, like the sensory area, is arranged such that the lower extremity is located superomedially along the paracentral lobule in the midline, whereas the upper extremity is located inferolaterally. The cells innervating the hip are at the top of the precentral sulcus; the leg is draped over medially along the interhemispheric fissure. The face (especially the tongue and mouth) has an inordinately large area of motor and sensory representation along the inferiormost aspect of the precentral motor strip on the surface of the brain, just above the sylvian fissure. This picture of the homunculus, with leg hanging over the vertex and with an enormous mouth and tongue, reminds a Baby Boomer of Mick Jagger (Chris Rock for Generation X), with mouth open, jabbering away. The motor contribution to speech is located at the inferior frontal gyrus (frontal operculum regions).

Figure 2-12 A, Corticospinal tracts. Fibers from the precentral gyrus and other nearby cortical areas descend through the cerebral peduncles, pons, and medullary pyramids; most cross in the pyramidal decussation to form the lateral corticospinal tract. Those that do not cross in the pyramidal decussation form the anterior corticospinal tract; most of these fibers cross in the anterior white commissure before ending in the spinal gray matter. Most corticospinal fibers do not synapse directly on the motor neurons. They are drawn that way here for simplicity. Primary somatic sensory (B) and motor (C) areas of the cortex. The body parts illustrated here show which parts of the body are “mapped” to correlates in each cortical area. The exaggerated face indicates that more cortical area is devoted to processing information to and from the many receptors and motor units than for the leg or arm, for example.

A from Nolte J: The human brain: an introduction to its functional anatomy, ed 4, St. Louis, Mosby, 1999, p. 249. B and C from Thibodeau GA, Patton KT: Anatomy and physiology, ed 4, St. Louis, Mosby, 1999, p. 394.

Table 2-3 Motor Pathways

Sometimes finding the central sulcus can be a bear (Fig. 2-13). This is necessary for discriminating motor from sensory areas, particularly when surgery to resect a perirolandic tumor is contemplated. Retaining motor function is desired. Consult Box 2-1 for some clues on how to identify the central sulcus.

Figure 2-13 Central sulcus. A, Note the shape of the medial end of the postcentral sulcus, the bifid y (between black and white arrows in A), and how the superior frontal sulcus (arrowheads) terminates in the precentral sulcus (asterisk). B, The central sulcus is the next sulcus posterior to the precentral sulcus. Note that the thickness of the cortical gray matter of the precentral gyrus (black arrow) is greater than that of the postcentral gyrus (white arrow). Usually PRE/POST thickness ≥1.5.

From the motor cortex of the frontal lobe, the white matter fibers pass into the corticospinal tract, which extends through the white matter of the centrum semiovale to the posterior limb of the internal capsule. From the posterior portion of the posterior limb of the internal capsule, the corticospinal tract continues through the central portion of the cerebral peduncle in the anterior portion of the midbrain. These fibers continue in the anterior portion of the pons to the pyramids of the medulla, where most of them decussate (in the pyramidal decussation) and proceed inferiorly in the lateral corticospinal tract of the spinal cord. Fifteen percent of fibers do not decussate in the medulla. These fibers pass into the anterior funiculus along the anterior median fissure of the spinal cord as the anterior corticospinal tract. The fibers of the pyramidal tract, which include both the lateral and anterior corticospinal tract, synapse with the anterior horn cell spinal cord nuclei.

Motor supply to the face travels from the cortex, through the corona radiata, into the genu of the internal capsule, via the corticobulbar tract. The corticobulbar fibers are located more anteromedially in the cerebral peduncles and have connections to the brain stem nuclei as they descend. Most of the connections to the various cranial nerve nuclei are contralateral to the corticobulbar tract; however, some ipsilateral fibers are present as well.

The pyramidal tract is responsible for voluntary movement and contains the corticospinal and corticobulbar fibers. The extrapyramidal system includes the corpus striatum, which receives fibers from the cerebral cortex, the thalamus, and the substantia nigra, with connections to the caudate nucleus and putamen. These fibers originate from the cerebral cortex but pass through the internal and external capsule to reach the basal ganglia. The dentate nuclei, found in the cerebellar hemispheres, also send tracts to the thalamus and motor areas of the frontal cortex. The red nucleus of the midbrain receives fibers from the cortical motor area and transmits fibers via the rubrospinal tract to the spinal cord, which also regulates motion.

Abnormalities of the pyramidal system mainly produce weakness, paralysis, or spasticity of voluntary motor function. Extrapyramidal system abnormalities often produce involuntary movement disorders, including tremors, choreiform (jerking) movements, athetoid (slow sinuous) movements, hemiballismic (flailing) motions, and muscular rigidity (pyramidal, paralysis; extrapyramidal, extremity excesses).