The Cerebral Hemispheres
The largest and most obvious parts of the human brain are the cerebral hemispheres. The cerebrum has an outer layer - the cortex, which is composed of neurons and their supporting cells, and in living brain, has a gray color thus called the gray matter. Under the gray matter is the white matter, which is composed of myelinated ascending and descending nerve fibers, and in living brain have a white color. Embedded deep within the white matter are aggregates of neurons exhibiting gray color and known as subcortical nuclei. The cerebral hemispheres are partially separate from each other along the midline by the interhemispheric fissure (deep groove) the falx cerebri (Figure 1.8A); Posteriorly, there is a transverse fissure that separates the cerebral hemisphere from the cerebellum, and contains the tentorium cerebellum. The hemispheres are connected by a large C-shaped fiber bundle, the corpus callosum, which carries information between the two hemispheres.
Figure 1.8 |
For descriptive purposes each cerebral hemisphere can be divided into six lobes. Four of these lobes are named according to the overlying bones of the skull as follows: frontal, parietal, occipital and temporal (Figures 1.8A and 1.8B), the fifth one is located internally to the lateral sulcus – the insular lobes (Figure 1.8B and 1.8D), and the sixth lobe is the limbic lobe (Figure 1.8C) which contains the limbic system nuclei. Neither the insular lobe nor the limbic lobe is a true lobe. Although the boundaries of the various lobes are somewhat arbitrary, the cortical areas in each lobe are histologically distinctive.
The surface of the cerebral cortex is highly convoluted with folds (gyri), separate from each other by elongated grooves (sulci). These convolutions allow for the expansion of the cortical surface area without increasing the size of the brain. On the lateral surface of the cerebral hemisphere there are two major deep grooves-sulci (or fissure), the lateral fissure (of Sylvian) and the central sulci (of Rolando), these sulci provide landmarks for topographical orientation (Figure 1.9A). The central sulcus separates the frontal lobe from the parietal lobe and runs from the superior margin of the hemisphere near its midpoint obliquely downward and forward until it nearly meets the lateral fissure (Figures 1.8A and 1.8B). The lateral fissure, separating the frontal and parietal lobes from the temporal lobe, begins inferiorly in the basal surface of the brain and extends laterally posteriorly and upward, separating the frontal and parietal lobes from the temporal lobe (Figure 1.9A). The frontal lobe is the portion which is rostral to the central sulcus and above the lateral fissure, and it occupies the anterior one third of the hemispheres (Figures 1.8 and 1.9). The boundaries of the parietal lobe are not precise, except for its rostral border – the central sulcus. The occipital lobe is the portion which is caudal to the parietal lobe (Figures 1.8 and 1.9). Along the lateral surface of the hemisphere, an imaginary line connecting the tip of the parietal-occipital sulcus and the preoccipital notch (Figure 1.9A), separate the parietal lobe from the occipital lobe. On the medial surface of the hemisphere (Figure 1.9B), parieto-occipital sulcus forms the rostral boundary of the parietal lobe. The temporal lobe lies ventral to the lateral sulcus, and on its lateral surface, it displays three diagonal oriented convolutions-the superior, middle, and inferior temporal gyri (Figure 1.9A). The insula lies in the depths of the lateral sulcus. It has a triangular cortical area with gyri and sulci (Figures 1.8B and 1.8D, and Figure 1.9A). The limbic lobe consists of several cortical and subcortical areas (Figure 1.9B).
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Figure 1.9 |
The cerebral cortex is a functionally organized organ. A functionally organized system is a set of neurons linked together to convey a specific type(s) of information to accomplish a particular task. It is possible to identify on the cerebral cortex primary sensory areas, secondary sensory areas, primary motor area, premotor area, supplementary motor area and association areas, which are devoted to the integration of motor and sensory information, intellectual activity, thinking and comprehension, execution of language, memory storage and recall.
The frontal lobe is the largest of the brain lobes and is comprised of four gyri, precentral gyrus that parallels the central sulcus, and three horizontal gyri: the superior, middle, and inferior frontal gyri. The inferior frontal gyrus is comprised of three parts: the orbital, the triangular and opercular. The term opercular refers to the “lips of the lateral fissure. Finally, the straight gyrus (gyrus rectus) and the orbital gyri form the base of the frontal lobe (Figure 1.9B). Four general functional areas are in the frontal lobe. They are the primary motor cortex, where all parts of the body are represented, the premotor and supplementary motor areas. A region concerned with the motor mechanisms of speech formulation comprised of the opercular and triangular parts of the inferior frontal gyrus are known as Broca’s speech area, and the remainder of the prefrontal cortex is involved in mental activity, personality insight, foresight, and reward. The orbital portion of the prefrontal cortex is important in the appropriate switching between mental sets and the regulation of emotion.
The parietal lobe is comprised of three gyri: postcentral gyrus, superior and inferior parietal gyri (Figure 1.9A). The postcentral gyrus is immediately behind the central sulcus which forms its anterior boundary. The postcentral gyrus comprises the primary somatosensory cortex which is concerned with somatosensory reception, integration and processing sensory information from the surface of the body and from the viscera, and is important for the formulation of perception. Caudal to the postcentral gyrus is the inferior parietal gyrus. The intraparietal sulcus separates the posterior parietal gyrus from the inferior parietal gyrus. The inferior parietal gyrus represents the cortical association area which integrates and processes sensory information from multiple modalities such as auditory and visual information. The inferior parietal gyrus, which is known as Wernicke's area, is also important for language and reading skills, whereas the superior parietal gyrus is concerned with body image and spatial orientations.
The temporal lobe is formed by three obliquely oriented gyri: the superior, middle, and inferior temporal gyri (Figure 1.9A). Inferomedial to the inferior temporal gyrus are the occipitotemporal and the parahippocampal gyri, which are separated by the collateral sulcus. The upper surface of the superior temporal gyrus, which extends into the lateral fissure, is called the transverse temporal gyrus (of Heschl) and is the primary auditory cortex. The caudal part of the superior temporal gyrus, which extends up to the parietal cortex, forms part of Wernicke’s area. Wernicke’s area is concerned, in part, with processing the auditory information and is important in the comprehension of language. The inferior part of the temporal lobe (i.e., the occipitotemporal gyri) is involved in visual and cognitive processing. More medially is the parahippocampal gyrus, which is involved in learning and memory. Portions of the frontal, parietal, and temporal lobes, which are adjacent to the lateral sulcus and overlie the insular cortex, are known as the operculum. The inferomedial surface of the temporal lobe is made up of the uncus and the parahippocampal gyrus medially. The inferior surface of the temporal lobe rests on the tentorium cerebelli.
The occipital lobe is the most caudal part of the brain, lies on the tentorium cerebelli (Figure 1.9A) and is comprised of several irregular lateral gyri. On its medial surface, there is a prominent fissure – the calcarine fissure and parieto-occipital sulcus. The calcarine fissure (sulcus) and the parieto-occipital sulcus also define a cortical region known as the cuneus. The cuneus sulcus divides the occipital lobe into the cuneus dorsally and ventrally into the lingual gyrus. The occipital lobe contains the primary and higher-order visual cortex.
Figure 1.10 |
The insula lobe is located deep inside the lateral fissure and can be seen only when the temporal and the frontal lobes are separated (Figures 1.8B and 1.8D). The insula is characterized by several long gyri and sulci, the gyri breves and gyri longi. There is some evidence that the insular cortical areas are involved in nociception and regulation of autonomic function.
The limbic lobe is not a true lobe and is comprised of several cortical regions such as the cingulate and parahippocampal gyri, some subcortical areas like the hippocampus, amygdala, septum, and other areas with their respective ascending and descending connections (Figures 1.8C and 1.9B). The limbic lobe is involved in memory and learning, drive related behavior, and emotional function.
There are subcortical areas in the telencephalon like the basal ganglia and the amygdaloid nucleus complex. The corpus callosum is a collection of nerve fibers that connect the two hemispheres. The corpus callosum is divided into rostrum (head), body, the most rostrally part is the genu (knee) with connecting the rostrum and the body, and the splenium at the caudal extremity (Figure 1.10). The corpus callosum plays an important role in transferring information from one hemisphere to the other.
The Diencephalon
The second major derivative of the prosencephalon is the diencephalon. The diencephalon is the most rostral structure of the brain stem; it is embedded in the inferior aspect of the cerebrum. The posterior commissure is the junctional landmark between the diencephalon and the mesencephalon. Caudally, the diencephalon is continuous with the tegmentum of the midbrain. During development the diencephalon differentiates into four regions: thalamus, hypothalamus, subthalamus and epithalamus (Figure 1.11) The epithalamus comprises the stria medullaris habenular trigone, pineal gland and the posterior commissure
Figure 1.11 |
