Overview of the Central Nervous System (Gross Anatomy of the Brain) Part 2

Parietal Lobe

The parietal lobe houses the functions that perceive and process somatosensory events. It extends posteriorly from the central sulcus to its border with the occipital lobe (Fig. 1-2). The parietal lobe contains the postcentral gyri, which has the central sulcus as its anterior border and the postcentral sulcus as its posterior border. The postcentral gyrus is the primary receiving area for somesthetic (i.e., kinesthetic and tactile) information from the periphery (trunk and extremities). Here, one side of the cerebral cortex receives information from the opposite side of the body. Like the motor cortex, the postcentral gyrus is somatotopically organized and can be depicted as having a sensory homunculus, which parallels that of the motor cortex.

The remainder of the parietal lobe can be divided roughly into two regions, a superior and an inferior parietal lobule, separated by an interparietal sulcus. The inferior parietal lobule consists of two gyri: the supramarginal and angular gyri. The supramarginal gyrus is just superior to the posterior extent of the lateral sulcus, and the angular gyrus is immediately posterior to the supramarginal gyrus and is often associated with the posterior extent of the superior temporal sulcus (Fig. 1-2). These regions receive input from auditory and visual cortices and are believed to perform complex perceptual discriminations and integrations. At the ventral aspect of these gyri and extending onto the adjoining part of the superior temporal gyrus is Wernicke’s area. This region is essential for comprehension of spoken language. Lesions of this region produce another form of aphasia, Wernicke’s aphasia (or sensory aphasia), which is characterized by impairment of comprehension and repetition, although speech remains fluent. The superior parietal lobule integrates sensory and motor functions and aids in programming complex motor functions associated with the premotor cortex. Damage to this region produces CNS disturbances, such as apraxia of movement and sensory neglect.


Occipital Lobe

Although a part of the occipital lobe lies on the lateral surface of the cortex, the larger component occupies a more prominent position on the medial surface of the hemisphere.

Temporal Lobe

One of the most important functions of the temporal lobe is the perception of auditory signals. Situated inferior to the lateral sulcus, the temporal lobe consists of superior, middle, and inferior temporal gyri. On the inner aspect of the superior surface of the superior temporal gyrus lie the transverse gyri of Heschl (not shown in Fig. 1-2), which constitute the primary auditory receiving area.

Medial Surface of the Brain

The principal structures on the medial aspect of the brain can be seen clearly after the hemispheres are divided in the midsagittal plane (Fig. 1-3). On the medial aspect of the cerebral cortex, the occipital lobe can be seen most clearly. It contains the primary visual receiving area, the visual cortex. The primary visual cortex is located inferior and superior to the calcarine sulcus (calcarine fissure), a prominent sulcus formed on the medial surface that runs perpendicular into the parieto-occipital sulcus, which divides the occipital lobe from the parietal lobe (Fig. 1-3).

Located more rostrally from the occipital lobe and situated immediately inferior to the precentral, postcentral, and premotor cortices is the cingulate gyrus. Its ventral border is the corpus callosum. The cingulate gyrus is generally considered part of the brain’s limbic system, which is associated with emotional behavior, regulation of visceral processes, and learning.

Another prominent medial structure is the corpus cal-losum, a massive fiber pathway that permits communication between equivalent regions of the two hemispheres. The septum pellucidum lies immediately ventral to the corpus callosum and is most prominent anteriorly. It consists of two thin-walled membranes separated by a narrow cleft, forming a small cavity (cavum of septum pelluci-dum). It forms the medial walls of the lateral ventricles. The septum pellucidum is attached at its ventral border to the fornix.

The fornix is the major fiber system arising from the hippocampal formation, which lies deep within the medial aspect of the temporal lobe. It emerges from the hippocampal formation posteriorly and passes dorso-medially around the thalamus to occupy a medial position inferior to the corpus callosum but immediately superior to the thalamus (see Fig. 1-6).

Inferior surface of the brain showing the principal gyri and sulci of the cerebral cortex. On the inferior surface, the midbrain, pons, parts of the cerebellum, and the medulla can be clearly identified.

FIGURE 1-4 Inferior surface of the brain showing the principal gyri and sulci of the cerebral cortex. On the inferior surface, the midbrain, pons, parts of the cerebellum, and the medulla can be clearly identified.

A basic function of the fornix is to transmit information from the hippocampal formation to the septal area and hypothalamus. The diencephalon lies below the fornix and has two parts (Fig. 1-3). The thalamus is larger and is responsible for relaying and integrating information to different regions of the cerebral cortex from a variety of structures associated with sensory, motor, autonomic, and emotional processes. The hypothalamus, the smaller structure, lies ventral and slightly anterior to the thalamus. Its roles include the regulation of a host of visceral functions, such as temperature; endocrine functions; and feeding, drinking, emotional, and sexual behaviors. The ventral aspect of the hypothalamus forms the base of the brain to which the pituitary gland is attached.

Inferior (Ventral) Surface of the Cerebral Cortex

As part of our task in understanding the anatomical organization of the brain, it is useful to examine its arrangement from the inferior view.

The medial aspect of the anterior part of the prefrontal cortex contains a region called the gyrus rectus (Fig. 1-4). Lateral to the gyrus rectus lies a structure called the olfactory bulb, a brain structure that appears as a primitive form of cortex consisting of neuronal cell bodies, axons, and synaptic connections. The olfactory bulb receives information from the first (olfactory) cranial nerve and gives rise to a pathway called the olfactory tract. These fibers then divide into the medial and lateral olfactory branches (called striae). The lateral pathway conveys olfactory information to the temporal lobe and underlying limbic structures, whereas the medial olfactory stria projects to medial limbic structures and contralateral olfactory structures.

Posterior Aspect of the Cerebral Cortex: Temporal and Occipital Lobes

The occipitotemporal gyrus lies medial to the inferior temporal gyrus and is bound medially by the collateral sul-cus. The parahippocampal gyrus lies medial to the collateral sulcus. There is a medial extension of the anterior end of the parahippocampal gyrus called the uncus. The hip-pocampal formation and amygdala (described below) are situated deep to the cortex of the parahippocampal gyrus and uncus (Figs. 1-4, 1-5, and 1-6). These structures have a very low threshold for induction of seizure activity and are commonly the focus of seizures in temporal lobe epilepsy.

Forebrain Structures Visible in Horizontal and Frontal Sections of the Brain

Ventricles

As shown in horizontal and frontal sections of the brain (Fig. 1-5), cavities present within each hemisphere are called ventricles and contain cerebrospinal fluid.In brief, CSF is secreted primarily from specialized epithelial cells found mainly on the roofs of the ventricles called the choroid plexus. CSF serves the CNS as a source of electrolytes, as a protective and supportive medium, and as a conduit for neuroactive and metabolic products. It also helps remove neuronal metabolic products from the brain.

The lateral ventricle is the cavity found throughout much of each cerebral hemisphere (Fig. 1-5). It consists of several continuous parts: an anterior horn, which is present at rostral levels deep in the frontal lobe; a posterior horn, which extends into the occipital lobe; an interconnecting body, which extends from the level of the interventricular foramen to the posterior horn; and, at the junction of the body and posterior horn, the inferior horn, which extends in ventral and anterior directions deep into the temporal lobe, ending near the amygdala (also referred to as amygdaloid complex) (Figs. 1-5 and 1-6).

Lateral view of the positions and relationships of the ventricles of the brain. Note that the lateral ventricles are quite extensive, with different components (i.e., posterior, inferior, and anterior horns). The medial and lateral apertures represent the channels by which cerebrospinal fluid can exit the brain.

FIGURE 1-5 Lateral view of the positions and relationships of the ventricles of the brain. Note that the lateral ventricles are quite extensive, with different components (i.e., posterior, inferior, and anterior horns). The medial and lateral apertures represent the channels by which cerebrospinal fluid can exit the brain.

Horizontal section depicting internal forebrain structures after parts of the cerebral cortex have been dissected away. Visible are the caudate nucleus, thalamus, fornix, hippocampus, and amygdala. Note the shape and orientation of the hip-pocampal formation and its relationship to the amygdala as well as the positions occupied by the globus pallidus and putamen, which lie lateral to the internal capsule (label shown in Figure 1-7), and the thalamus, which lies medial to the internal capsule.

FIGURE 1-6 Horizontal section depicting internal forebrain structures after parts of the cerebral cortex have been dissected away. Visible are the caudate nucleus, thalamus, fornix, hippocampus, and amygdala. Note the shape and orientation of the hip-pocampal formation and its relationship to the amygdala as well as the positions occupied by the globus pallidus and putamen, which lie lateral to the internal capsule (label shown in Figure 1-7), and the thalamus, which lies medial to the internal capsule.

Within the diencephalon, another cavity, called the third ventricle, can be identified. It lies along the mid-line of the diencephalon, and the walls are formed by the thalamus (dorsally) and the hypothalamus (ventrally). The third ventricle extends throughout the diencephalon and communicates anteriorly with the left and right lateral ventricles through the interventricular foramen. Posteriorly, at the level of the diencephalic-midbrain border, it is continuous with the cerebral aqueduct, which allows CSF to flow from the third ventricle to the fourth ventricle (Fig. 1-5), where it will exit the ventricular system through the lateral and median apertures into the subarachnoid space.

Next post:

Previous post: