2. The Cerebral Hemispheres
Revised August 6, 2007
The objectives of this chapter are to identify:
- The prominent external features of the telencephalon (cerebrum).
- The major brain divisions and the major structures seen on a midsagittal section of the brain.
- The vascular territories of the cerebral arteries.
As viewed from above, the most prominent parts of the brain are the cerebral hemispheres, which are separated by the interhemispheric fissure (= longitudinal fissure) (#8431). The hemispheres consist of a superficial sheet of gray matter that is thrown into folds. The surface irregularities of the cerebrum are the sulci and gyri (#51113). A gyrus is the elevated portion between two sulci (furrows). Regardless of first impressions, the gyri and sulci form certain patterns that are generally constant from brain to brain, depending on the species. To be sure, variations do occur. Some animal brains (e.g., rat, opossum, rabbit) have few gyri and have "smooth" brains. The developing human brain is initially smooth (#4095); however, the adult brain has numerous gyri.
Each cerebral hemisphere is divided into lobes (fig 2a).
A lobe is roughly that part of the hemisphere covered by one of the calvarial bones. The frontal lobe lies under the frontal bone and the parietal lobe under the parietal bone. The lobes are defined by certain sulci. Two sulci that are standard reference points occur on the lateral surface of the hemisphere. The more distinctive one is deep and usually termed a "fissure" instead of a "sulcus." This is the lateral fissure (Sylvian fissure) (#4331).
The other standard reference sulcus is the central sulcus (Rolandic fissure) (#4207) that runs downward and forward from its origin about midway along the superior border of the hemisphere. This sulcus, as with the sulci and gyri in general, is easier to find if you first remove the overlying meninges and blood vessels. Then find the most anterior gyrus that extends without interruption from the superior margin of the hemisphere to the lateral fissure. The sulcus behind this gyrus is the central sulcus. The cortex above the lateral fissure and in front of the central sulcus forms the frontal lobe (#4216). The cortex below the lateral fissure comprises the temporal lobe (#4209).
Two additional landmarks need to be identified to define the remaining lobes. The occipital lobe (#4210) is the cortex caudal to an imaginary line drawn from the top of the parietooccipital fissure to the preoccipital notch. The cortex above the lateral fissure caudal to the central sulcus but rostral to the occipital cortex is the parietal lobe (#4206). These descriptive terms are not used only by anatomists but have a wider application, as witness the terms "temporal lobe epilepsy" and "frontal lobe syndrome."
Deep within the lateral fissure is the insula, an island (really, a peninsula) of cortex that is buried by the overgrowth of the frontal, parietal and temporal lobes. The lips of the lateral fissure have to be spread apart to see the insula (#8434).
The two cerebral hemispheres appear to be identical, and a description of one serves as well for the other. Functionally, however, they are different. This is referred to as the functional asymmetry of the hemispheres. The "dominant," usually left, hemisphere functions in language. Discrete lesions of this hemisphere produce aphasia.
The "nondominant", usually right, hemisphere is important in understanding spatial relationships, such as the recognition of patterns and forms. There is evidence that it also has special functions relative to musical expression. Presumably the two hemispheres, which are united by the corpus callosum (#4850) and the anterior commissure (#5398) (vide infra), normally function as a unit.
A. The frontal lobe
The important gyri to be identified are the precentral and inferior frontal gyri (fig 2b). The precentral gyrus (#4225) is anterior to the central sulcus. It is the primary motor cortex. The inferior frontal gyrus (#4344) borders the lateral fissure in front of the precentral gyrus. In the dominant hemisphere, the posterior two-thirds of this gyrus is Broca's area (#4077), the motor speech area. Lesions here produce motor (expressive) aphasia where patients have a deficit in speech production.
B. The parietal lobe
The postcentral gyrus (#4319) is posterior to the central sulcus (fig 2b). It is the primary cortical region for the sensations of touch and pressure and is commonly referred to as primary somatosensory (somesthetic) cortex. The parietal lobe contains two other intriguing gyri, the supramarginal and angular gyri. The supramarginal gyrus (green) is draped over the dorsal tip of the lateral fissure. The angular gyrus (#4218) bends over the superior temporal sulcus as though to prevent this crevice from extending into the parietal lobe. In the dominant hemisphere, these two gyri function in speech, reading, and the use of numbers (calculation).
C. The temporal lobe
On its lateral surface, the temporal lobe has three distinct gyri, which, after a fashion, parallel the lateral fissure. They are the superior (#4340), middle (#4341), and inferior temporal (#4342) gyri.
Gently spread apart the lips of the lateral fissure to see the two gyri on the superior surface of the temporal lobe. These are the transverse temporal gyri (Heschl's gyri) (#8437). Both are auditory cortex. The rostral one is the primary auditory cortex. In the dominant hemisphere, the cortex adjacent to the auditory cortex in the posterior third of the superior temporal gyrus is Wernicke's area, the sensory speech area. Lesions here produce receptive aphasia where patients cannot comprehend speech.
Several longitudinally running gyri occur on the ventral surface of the temporal lobe. The medial one is the parahippocampal gyrus (#5361), which covers the most ancient part of cortex, the hippocampus. Because it is deep to the parahippocampal gyrus, the hippocampus cannot be seen without performing a special dissection. The hippocampus is of interest because of its role in memory.
D. The occipital lobe
Identify the medial portions of the five major brain divisions in a midsagittal section of the brain (#4281): diencephalon (#4279), mesencephalon (#4278), pons and cerebellum (#4282), and medulla (#4276). Some of the important midsagittal structures can be seen in fig 2c.
A. The telencephalon (cerebrum)
One of the most prominent structures seen in a midsagittal section (fig 2c) is the corpus callosum (#4850). Its rostral part does a knee bend and is known as the genu ("knee") (#4834). Extending ventrally from the genu is a thin portion of the callosum, the rostrum (#4831). The thick, even part of the callosum extending posteriorly from the genu is the body (#4850), which ends as the splenium (#4833). The cingulate gyrus (#4274) is immediately above the corpus callosum. The cortex that surrounds the medial extension of the central sulcus is the paracentral lobule, a medial continuation of primary somatosensory cortex and primary motor cortex.
The bundle of myelinated axons that arches forward from under the splenium is the fornix ("arch") (#3994, #4592). The thin sheet that stretches between the fornix and corpus callosum is the septum pellucidum (#4597). It separates the ventricular cavity (lateral ventricle) of one hemisphere from that of the opposite hemisphere (#4265, partially exposed and colored pink). Occasionally there is a definite space within the septum; this is the cavum septum pellucidum (#8569). (It is not a part of the brain ventricular system.) The most rostral part of the fornix forms the anterior margin of the interventricular foramen (foramen of Monro). Through this opening, the paired lateral ventricles communicate with the midline third ventricle (outlined in blue, #4318) of the diencephalon. Below and rostral to the interventricular foramen is another telencephalic commissure, the anterior commissure (#4596 green dot).
B. The diencephalon
The sidewalls, roof, and floor of the third ventricle are formed by the diencephalon, whose major divisions are the thalamus (#7921, #4838), hypothalamus (#4270, #4848), epithalamus (pineal body) (#4839) and the subthalamus, which is deep to the surface and can only be seen on brain sections. The faint groove extending back from the interventricular foramen is the hypothalamic sulcus (#11874). It separates the thalamus above from the hypothalamus below. Recall that the tuber cinereum (#11880) and mammillary bodies (#4847) are part of the hypothalamus. The anterior commissure (#4596) and the optic chiasm (#4846) mark the rostral extent of the diencephalon. Its caudal extent is indicated by a line connecting the mammillary bodies and the pineal body (pineal gland).
C. The mesencephalon
Caudally, the third ventricle narrows into the aqueduct (of Sylvius) (#4262). The aqueduct identifies the mesencephalon, whose dorsal surface has two pairs of elevations. Hence, the term quadrigeminal plate ("quadruplets") or tectum ("roof") (#8972). The rostral pair of elevations forms the superior colliculi (colliculus means "little hill " or "mound") (#4913) and the caudal pair, the inferior colliculi (#4912). The trochlear nerve (IV) (#11703) emerges from the caudal border of the inferior colliculus and curls laterally around the cerebral peduncle (#5641).
D. The rhombencephalon
The aqueduct drains into the fourth ventricle (#4261), which is dorsal to the pons and medulla. This ventricle is covered by the cerebellum. Ventricle IV, in turn, narrows to become the central canal of the spinal cord. (The central canal is not patent in either the newborn or adult.) In addition, ventricle IV empties into the subarachnoid space through the left and right lateral apertures of Luschka and the median aperture of Magendie (#4261).
The white matter of the cerebellum is centrally located and has the appearance of a tree: the arbor vitae ("tree of life") (#7210). The gray matter of the cerebellar cortex is elaborately, yet evenly, folded into folia ("leaves"). The folia are grouped into lobules, which are separated by cerebellar fissures.
III. Vascular Territories of the Cerebral Arteries
Review the lobes of the cerebral hemispheres in association with their vascular supply. The cerebral cortex is supplied by the anterior, middle, and posterior cerebral arteries. Fig 2d and fig 2e indicate the cortical areas vascularized by each artery. The pattern is not difficult to see. The anterior cerebral artery sends branches to the medial surface of the frontal and parietal lobes (#7981, #52081).
Most of the occipital lobe and most of the ventral part of the temporal lobe are supplied by the posterior cerebral artery (fig 2e). Not all of the lateral surface of the hemisphere is nourished by the middle cerebral artery; the "peripheral" portions are vascularized by branches of the anterior (#3989) and posterior cerebral arteries (#3990).
Using any information that you might have gleaned, predict what sensory loss would result from interruption of each cerebral artery. Occlusion of which artery near its origin would have the most devastating effect?
Click for the Syllabus Quiz for Chapter 2
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