Comprehensive Study Notes on the Anatomy and Functional Localization of the Cerebrum

General Overview of the Cerebrum

The cerebrum is the largest part of the brain and is situated within the anterior and middle cranial fossae of the skull. It is structurally divided into two primary parts: the diencephalon, which forms the central core, and the telencephalon, which forms the cerebral hemispheres. These cerebral hemispheres represent the largest part of the brain total. They are separated by a deep midline sagittal fissure known as the longitudinal cerebral fissure. This fissure contains the sickle-shaped fold of dura mater called the falx cerebri, as well as the anterior cerebral arteries.

A second horizontal fold of dura mater, the tentorium cerebelli, separates the cerebral hemispheres from the cerebellum located inferiorly. Within the depths of the longitudinal fissure lies the great commissure, the corpus callosum, which serves to connect the two hemispheres across the midline. The surface of each cerebral hemisphere is characterized by numerous folds or gyri, which are separated from one another by sulci or fissures. This folding arrangement is designed to significantly increase the surface area of the cerebral cortex.

Major Sulci and the Identification of Cerebral Lobes

The various sulci are utilized as anatomical boundaries to divide each hemisphere into specific lobes. The primary sulci used for this division include the central sulcus, the lateral sulcus, the parieto-occipital sulci, and the calcarine sulci. Based on these divisions, the brain is categorized into the frontal, parietal, temporal, and occipital lobes.

The central sulcus is of great clinical and functional importance. It indents the superior medial border of the hemisphere approximately $0.4$ inch ($1\,cm$) behind the midpoint. It runs downward and forward across the lateral surface of the hemisphere. Its lower end is separated from the posterior ramus of the lateral sulcus by a narrow bridge of cortex. The gyrus situated anterior to the central sulcus contains motor cells that initiate movements on the opposite side of the body, while the gyrus posterior to it contains the general sensory cortex which receives sensory information from the opposite side of the body.

The lateral sulcus is a deep cleft located on the inferior and lateral surfaces of the cerebral hemisphere. It consists of a short stem that divides into three rami upon reaching the lateral surface: the anterior horizontal ramus, the anterior ascending ramus, and the posterior ramus. At the bottom of this deep sulcus lies the insula, an area of cortex that cannot be seen from the surface unless the lips of the lateral sulcus are separated. While its fiber connections are not completely understood, the insula is believed to be important for planning or coordinating the articulatory movements necessary for speech.

Medial and Occipital Sulci

The parieto-occipital sulcus begins on the superior medial margin of the hemisphere, approximately $5\,cm$ anterior to the occipital pole. It passes downward and anteriorly on the medial surface until it meets the calcarine sulcus. The calcarine sulcus itself is found on the medial surface of the hemisphere, commencing under the posterior end of the corpus callosum and arching upward and backward to reach the occipital pole. It is joined at an acute angle by the parieto-occipital sulcus at approximately the halfway point of its length.

Surface Anatomy of the Cerebral Hemispheres

The superolateral surface of the hemisphere contains well-defined lobes. The frontal lobe is located anterior to the central sulcus and superior to the lateral sulcus. Its superolateral surface is divided by three sulci—the precentral, superior frontal, and inferior frontal sulci—into four gyri: the precentral, superior frontal, middle frontal, and inferior frontal gyri. The inferior frontal gyrus is notably invaded by the anterior and ascending rami of the lateral sulcus.

The parietal lobe is situated posterior to the central sulcus and superior to the lateral sulcus, extending posteriorly to the parieto-occipital sulcus. Its lateral surface is divided by the postcentral and intraparietal sulci into three gyri: the postcentral gyrus, the superior parietal lobule (gyrus), and the inferior parietal lobule (gyrus). The temporal lobe lies inferior to the lateral sulcus and is divided by the superior and middle temporal sulci into the superior, middle, and inferior temporal gyri. The occipital lobe occupies the small area behind the parieto-occipital sulcus.

On the medial and inferior surfaces, the lobes are not as clearly defined. Important recognized areas include the septum pellucidum, a thin vertical sheet of nervous tissue consisting of white and gray matter covered by ependyma. It separates the anterior horns of the left and right lateral ventricles and is essentially a double membrane with a closed, slit-like cavity. The cingulate gyrus begins beneath the anterior end of the corpus callosum and continues above it, separated by the callosal sulcus and bounded superiorly by the cingulate sulcus. The precuneus is an area of cortex bounded anteriorly by the posterior end of the cingulate sulcus and posteriorly by the parieto-occipital sulcus. The cuneus is a triangular area bounded by the parieto-occipital sulcus above and the calcarine sulcus below. Additional gyri include the lingual gyrus (between the collateral and calcarine sulci), the parahippocampal gyrus (terminating anteriorly as the hooklike uncus), and the medial occipitotemporal gyrus. The inferior surface of the frontal lobe contains the olfactory sulcus, overseen by the olfactory bulb and tract; the gyrus rectus lies medial to this sulcus, while several orbital gyri lie lateral to it.

Internal Structure and the Basal Nuclei

The cerebral hemispheres are covered with an outer layer of gray matter known as the cerebral cortex. The interior contains the lateral ventricles, masses of gray matter called basal nuclei, and nerve fibers embedded in neuroglia that constitute the white matter. There are two lateral ventricles, one per hemisphere. These C-shaped cavities are lined with ependyma and filled with cerebrospinal fluid. Each consists of a body (parietal lobe) and anterior (frontal), posterior (occipital), and inferior (temporal) horns. They communicate with the third ventricle through the interventricular foramen (foramina of Monro), which is bounded anteriorly by the anterior column of the fornix and posteriorly by the anterior end of the thalamus.

The basal nuclei are masses of gray matter situated within the hemisphere, including the corpus striatum, the amygdaloid nucleus, and the claustrum. The corpus striatum is located lateral to the thalamus and is divided by the internal capsule into the caudate nucleus and the lentiform nucleus. The caudate nucleus is a large C-shaped mass closely related to the lateral ventricle. The lentiform nucleus is situated deep in the white matter, related medially to the internal capsule and laterally to the external capsule. The external capsule is a thin sheet of white matter that separates the lentiform nucleus from the claustrum, which in turn separates the external capsule from the subcortical white matter of the insula.

White Matter and Fiber Classifications

The white matter of the cerebral hemisphere is composed of myelinated nerve fibers supported by neuroglia, categorized into three groups based on their connections. Commissural fibers connect corresponding regions of the two hemispheres. The largest is the corpus callosum, divided into the rostrum (continuous with the lamina terminalis), genu (fibers form the forceps minor), body (fibers form the radiation of the corpus callosum and the tapetum), and splenium (fibers form the forceps major). Other commissures include the anterior commissure (connecting temporal lobes and olfactory tracts), posterior commissure (involved in the pupillary light reflex), habenular commissure, and the commissure of the fornix which connects the hippocampal formations.

Association fibers connect various cortical regions within the same hemisphere and are divided into short groups (connecting adjacent gyri) and long groups. Long association fibers include the uncinated fasciculus (motor speech to temporal pole), cingulum (frontal/parietal to parahippocampal), superior longitudinal fasciculus (largest bundle, connecting frontal to occipital/temporal), inferior longitudinal fasciculus (occipital to temporal), and fronto-occipital fasciculus.

Projection fibers, specifically within the internal capsule, transmit afferent and efferent fibers to and from the brainstem and the entire cerebral cortex. The internal capsule is bent into an anterior limb and a posterior limb, meeting at the genu. It is bounded medially by the caudate nucleus and thalamus, and laterally by the lentiform nucleus. Radiating fibers from the internal capsule form the corona radiata. Fibers in the most posterior part of the internal capsule's posterior limb radiate toward the calcarine sulcus as the optic radiation.

The Diencephalon

The diencephalon is a midline structure with symmetrical right and left halves that form the boundaries of the third ventricle. It extends from the interventricular foramina anteriorly to the point where the third ventricle becomes continuous with the cerebral aqueduct posteriorly. The inferior surface is formed by hypothalamic structures, including the optic chiasma, optic tracts, infundibulum (with tuber cinereum), and mammillary bodies. The superior surface is concealed by the fornix, with the actual wall formed by the roof of the third ventricle (ependyma and tela choroidea). The lateral surface is bounded by the internal capsule, while the medial surface is formed by the thalamus and hypothalamus, separated by the hypothalamic sulcus.

The diencephalon consists of four major parts: the thalamus, epithalamus, subthalamus, and hypothalamus. The thalamus is a large ovoid mass of gray matter serving as a relay and integrative station for all main sensory systems except olfaction. Its posterior end is expanded into the pulvinar, which overhangs the superior colliculus. Its medial surface is often connected to the opposite thalamus by the interthalamic connection. Thalamic nuclei include the anterior nucleus (AN), dorsomedial nucleus (DM), ventrolateral nucleus (VL), ventral posteromedial (VPM), and ventral posterolateral (VPL), among others.

Epithalamus, Pineal Gland, and Subthalamus

The epithalamus consists of the habenular nuclei and the pineal gland. The habenular nuclei are centers for integrating olfactory, visceral, and somatic afferent pathways. The pineal gland is a small conical structure attached to the diencephalon. It contains pinealocytes and glial cells but no nerve cells; it is innervated by sympathetic fibers from the superior cervical ganglia. It accumulates "brain sand" (calcified material) with age. Functionally, it is an endocrine gland secreting melatonin, which increases during darkness to regulate circadian rhythms and inhibit the release of gonadotrophic hormones from the pituitary gland. It lacks a blood-brain barrier.

The subthalamus, shaped like a biconvex lens, lies inferior to the thalamus and contains the cranial ends of the red nuclei and substantia nigra. It is involved in muscle activity control and contains tracts such as the medial, spinal, and trigeminal lemnisci passing to the thalamic nuclei.

The Hypothalamus and Third Ventricle

The hypothalamus lies below the thalamus and forms the floor and lower lateral walls of the third ventricle. It extends from the optic chiasma to the mammillary bodies. It controls the autonomic nervous system and endocrine systems, regulating body temperature, fluids, hunger, thirst, sexual behavior, and emotion. Key structures include the optic chiasma (where nasal retinal fibers cross), the tuber cinereum (which becomes the infundibulum and then the neurohypophysis), and the mammillary bodies. The area anterior to the hypothalamus is the preoptic area.

The third ventricle is a slit-like cleft between the thalami. Its anterior wall is formed by the lamina terminalis and anterior commissure. The posterior wall is formed by the cerebral aqueduct opening, the posterior commissure, and the pineal and habenular recesses. The lateral walls are formed by the thalamus and hypothalamus, joined by the interthalamic connection. The superior wall (roof) consists of ependyma and vascular tela choroidea, which forms the choroid plexuses. The inferior wall (floor) includes the optic chiasma, tuber cinereum, infundibulum, and mammillary bodies.

Cellular and Laminar Structure of the Cerebral Cortex

The cerebral cortex is composed of gray matter containing approximately $10$ billion neurons, with a thickness ranging from $1.5$ to $4.5\,mm$ (thickest at the crests of gyri). There are five main nerve cell types:

1. Pyramidal cells: Size $10-50\,\mu m$, though giant Betz cells reach $120\,\mu m$.
2. Stellate (granule) cells: Small, polygonal cells about $8\,\mu m$ in diameter.
3. Fusiform cells: Vertical axis, situated deep in the cortex.
4. Horizontal cells of Cajal: Parallel to the surface, located superficially.
5. Cells of Martinotti: Small multipolar cells with axons perpendicular to the surface.

The cortex is organized into six layers:

1. Molecular layer (plexiform): Most superficial, contains Cajal cells.
2. External granular layer: Small pyramidal and stellate cells.
3. External pyramidal layer: Intermediate pyramidal and stellate cells.
4. Internal granular layer: Stellate cells; contains the external band of Baillarger.
5. Ganglionic layer (internal pyramidal): Large pyramidal (including Betz cells), stellate, and Martinotti cells; contains the inner band of Baillarger.
6. Multiform layer: Fusiform and modified pyramidal cells.

Cortical structure can be Homotypical (possessing all six layers) or Heterotypical. Heterotypical cortex includes the Granular type (well-developed layers 2 and 4; found in the postcentral gyrus) and the Agranular type (poorly developed layers 2 and 4, dominant layers 3 and 5; found in the precentral gyrus).

Functional Localization of the Cortex

Cortical functions are assigned according to Brodmann numbers. In the frontal lobe, the Primary Motor Cortex (Area 4) in the precentral gyrus controls contralateral limb movements; lesions here cause flaccid paralysis. The Premotor Cortex (Area 6) controls proximal and axial muscles. The Supplementary Motor Cortex (Area 6, medial surface) is involved in complex movement programming; combined lesions of Areas 4 and 6 result in spastic paralysis. The Frontal Eye Field (Areas 6, 8, 9) controls conjugate eye movements; lesions cause eye deviation toward the side of the lesion. Broca’s Motor Speech Area (Areas 44, 45) is located in the inferior frontal gyrus; lesions cause expressive aphasia. The Prefrontal Cortex (Areas 9, 10, 11, 12) regulates personality and depth of feeling.

In the parietal lobe, the Primary Somatic Sensory Cortex (Areas 3, 1, 2) is organized as a sensory homunculus; lesions cause contralateral sensory disturbances. The Somesthetic Association Area (Areas 5, 7) integrates sensory modalities; lesions lead to astereognosis. In the occipital lobe, the Primary Visual Area (Area 17) is located around the calcarine fissure. The right visual cortex represents the left field of vision. Lesions result in contralateral homonymous hemianopia with macular sparing. Specific lesions can cause quadrantic hemianopia or central scotomas. The Secondary Visual Area (Areas 18, 19) relates visual data to past experiences.

In the temporal lobe, the Primary Auditory Area (Areas 41, 42) is located in Heschl's gyrus. Lesions cause bilateral partial deafness and inability to locate sound sources. The Secondary Auditory Area (Area 22) interprets sounds; lesions cause word deafness. Wernicke’s Sensory Speech Area (Area 22, superior temporal gyrus) permits language understanding; lesions cause receptive aphasia (fluent but nonsensical speech). Other areas include the Taste Area (Area 43, lower postcentral gyrus) and the Vestibular Area (near face-related gyri).

Cerebral dominance signifies that certain activities are predominantly performed by one hemisphere. In over 90% of adults, the left hemisphere is dominant, controlling language and speech. The non-dominant hemisphere interprets spatial perception, faces, and music. Lesions in the dominant angular gyrus result in alexia (inability to read) and agraphia (inability to write).