Midsagittal section of the brain
The midsagittal section of the brain shows the three major parts of the brain, which are the cerebrum, cerebellum and brainstem. These brain parts are marked with visible gross features like the gyri (singular: gyrus) and sulci (singular: sulcus) of the cerebrum. They are each also divided into subparts or regions for simplified localization of structures, for example, the brainstem is composed of the midbrain, pons and medulla oblongata, while the cerebrum is divisible into lobes. Sulci are small grooves that divide and define gyri; large grooves, called fissures, divide the cerebral cortex into lobes, and also the cerebrum into the right and left cerebral hemispheres along the midsagittal plane. The fissure involved in this division is called the medial longitudinal fissure. From the midsagittal section of the brain both white and gray matter can be observed, as well as important spaces such as the third and fourth ventricles.
Diencephalon, brainstem, and related structures
The diencephalon is part of the “old brain” lying between the cerebrum and the brainstem, and is composed of the thalamus, the hypothalamus and the epithalamus. These can be seen in a midsagittal aspect upon sectioning of the third ventricle. The brainstem is the “bridge” between the cerebral hemispheres and the spinal cord. It is made up of the midbrain, pons and the medulla oblongata. They are continuous above with the cerebral hemispheres, below with the spinal cord and posteriorly with the cerebellum. The midsagittal section of the diencephalon and brainstem show some important masses of gray matter projecting onto the median plane. Some nuclei that can be observed include: the thalamic and hypothalamic nuclei bordering the third ventricle; the pineal gland, the main component of the epithalamus; the superior and inferior colliculi, two posterior bulges situated either side of the midsagittal line; and the nucleus gracilis lying immediately lateral to the posterior median fissure. Just anterior to the pituitary, and inferior to the hypothalamus, the optic chiasm passes, where nerves from each eye cross over allowing binocular vision. Vascular systems relating to the blood supply and the cerebrospinal fluid can observed progressing through the brainstem. The major blood vessel is the basilar artery, in the anterior pontine, which progresses into the posterior of the Circle of Willis. The ventricular system - excluding the lateral ventricles - can also be observed passing from the third ventricle, via the cerebral aqueduct, to the fourth ventricle and beyond to the central canal. The central canal runs through the medulla to become the central canal of the spinal cord.
The cerebellum (or small brain) weighs about 10% of the cerebral hemispheres and weighs approximately 150 g in adults. It has a superficial layer of grey matter, the cerebellar cortex, and like other parts of the brain, it is marked by numerous fissures. The cerebellum lies posterior to the brainstem, and is separated from the cerebrum by a fold of dura mater called the tentorium cerebelli.
The cerebellum consists of two hemispheres separated by the medially positioned vermis. The cerebellum also has superior and inferior surfaces. The surface of the cerebellum is marked by a series of fissures that run more or less parallel to one another. The fissures subdivide the surface of the cerebellum into narrow leaf-like bands or folia. The long axis of the majority of folia is approximately transverse. Sagittal sectioning of the cerebellum reveals the invaginations of the folia, which have a tree-like appearance and to which the term arbor-vitae (tree of life) is ascribed. The deepest fissures, such as the primary fissure (fissure prima) and the posterolateral fissure, divide the cerebellum into lobules: the anterior, posterior, and flocculonodular. The medial vermis is similarly divisible into lobules, although to a greater extent than the lateral hemispheres. The lobules of the vermis are the: lingula, central, colmen, simple, folium, tuber, pyramis, uvula, and nodule. Collectively the lateral hemispheres and vermis form the highly active relay station that is the corpus cerebella. The anterior and posterior lobes together form the corpus cerebelli.
The cerebrum is comprised of gyri and sulci, with divisions between lobes primarily identified by fissures. When observed from the medial view different aspects of the cortices can be observed as they progress into the midsagittal fissure, and indeed some cortices not otherwise observable can be seen, such as the olfactory cortex. The midsagittal aspect of the cerebrum also reveals structures spanning the fissure, such as the corpus callosum.
The corpus callosum (a group of commissural fibres) is a prominent structure on this midsagittal surface. The third ventricle of the brain, as well as some other related structures like the interventrcular foramen (of Monro) – through which the third ventricle communicates with the lateral ventricles - can also be seen on the medial surface. Other structures seen on this surface include the thalamus, hypothalamus and cerebral aqueduct (of Sylvius). Details of deepest structures seen on this surface are examined in the article “Olfactory and Limbic Regions”. The thalamus, hypothalamus and the basal ganglia are three distinct masses of gray matter like the cerebral cortex. They are located deep to the cerebral cortex and within the white matter of the cerebrum. These deep structures contribute only a small portion of the cerebrum’s gray matter, while the cerebral cortex forms the largest portion of the cerebrum’s grey matter. Surrounding the superior surfaces of the corpus callosum, sulci and gyri form medial walls of the median fissure denoting the folds of the cerebral cortex. The most prominent of the sulci is the cingulate sulcus (sulcus cinguli), which follows a curved course parallel to the upper convex margin of the corpus callosum. Anteriorly, the sulcus cinguli ends below the rostrum of the corpus callosum, and posteriorly, it turns upwards to reach the superomedial border, a little behind the upper end of the central sulcus. The area between the cingulate sulcus and the corpus callosum is the gyrus cinguli (or cingulate gyrus). Hence this gyrus is defined by the cingulate sulcus superiorly, and the corpus callosum inferiorly (specifically by the callosal sulcus which wounds around the corpus callosum).
Above the cingulate sulcus, up to the limit of the superomedial border (superomedial edge) is an area which consists of two parts. The large anterior part, called the medial frontal gyrus, lies anterior to the paracentral lobule, which is wound around the end of the central sulcus. The paracentral lobule is separated from the anterior part by a very short sulcus which is continuous with the cingulate sulcus. Between the parieto-occipital sulcus and paracentral lobule is a quadrilateral area called the precuneus. Anteroinferiorly, the precuneus is separated from the posterior part of the gyrus cinguli by the suprasplenial (or subparietal) sulcus. The precuneus and the posterior part of the paracentral lobule forms the medial surface of the parietal lobe. Further posterior to the paracentral lobule and gyrus cinguli, are two major sulci that cut off a triangular area. This area is called the cuneus. This triangular area is defined antero-superiorly by the parieto-occipital sulcus (which crosses the superomedial border to appear as a very short sulcus on the superolateral surface), inferiorly by the calcarine fissure, and posteriorly by the superomedial border of the cerebral cortex. The calcarine fissure extends forward beyond its junction with the parieto-occipital sulcus and ends a little below the splenium of the corpus callosum (posterior part of the corpus callosum) to define a small area called the isthmus which lies between it (calcarine fissure) and the splenium of the corpus callosum.
Important functional Areas of the Cerebrum
Cortices of the cerebral cortex have long been divisible by their specific functions. These areas can be defined in terms of the gyri and sulci described above. However, upon investigation of the microscopic structure of the cerebral cortex there is considerable variation from region to region and that definition of these functional areas is not confined to the boundaries of gyri and sulci, but often cross them. Most of these authors have also worked out “maps” of the cerebral cortex indicating areas of differing structure. The best known scheme is that by Brodmann, who represented different areas of the cortex by numbers. On the midsagittal view of the cerebrum, the numbers and areas most commonly referred to are represented in the following diagram, and described as follows:
Motor Area (Primary Motor Cortex): This is the area corresponding to area 4 of Brodmann and possibly to the part of area 6 which lies in the precentral gyrus. The motor area is located in the precentral gyrus on the superolateral surface and in the anterior part of the paracentral lobule on the medial surface. This area of the cortex is responsible for initiation of voluntary movement. Within the motor area, subpopulations of neurons are responsible for activating different areas of the body. Each neuronal population responsible for activating a physical area neighbours the population that activates the physical area adjacent, this is called topographical organisation, and in this way the brain forms a representation of the body. While topographic organisation depicted in the “Homunculus” shows the association between the physical body areas and their representations, it also depicts the size of the excitability of each representation, and the relative control the neuronal populations have over each physical location. The topographical motor representation of the body begins with the toes and feet deep in the paracentral lobule, and progresses into the legs and genitalia superiorly, before progressing laterally and inferiorly with the rest of the body. As indicated by the homunculus, the lower limbs have a relatively small neuronal motor input.
Pre-motor Area (Motor Association Cortex): This is the area corresponding to areas 6, 8, 44 & 45 of Brodmann. It is located just anterior to the motor area, occupying the posterior parts of the superior, middle and inferior frontal gyri. The part of the premotor area located in the superior and middle frontal gyri corresponds to area 6 and 8 of Brodmann.
Sensory Area (Primary Somatosensory Cortex): This functional area is located in the postcentral gyrus. It corresponds to area 1, 2 and 3 of Brodmann. It extends into the medial surface, from the lateral surface, where it lies in the posterior part of the paracentral lobule. As with the motor area, the concept of Homunculus, and associated topographical organisation - called somatotopic organization - applies to the sensory area. The somatotopic representation denoted by the homunculus displays a similar organisation to the motor cortex, with the legs, and genitalia being visible from the midsagittal aspect, and being responsible for sensations of those regions. The area of the cortex that receives sensations from a particular part of the body is not proportional to the size of that part, but rather to the complexity of sensations received from it (e.g., the hands have high cortical investment, whereas the feet, with a similar mass, have very little cortical investment). Immediately posterior to the somatosensory area is the Sensory Association Cortex. This cortex is responsible for processing of multisensory information.
Visual Areas: The areas concerned with vision are located in the occipital lobe, mainly on the medial surface, both above and below the calcarine sulcus (area 17). Area 17 extends into the cuneus, and into the lingual gyrus. Posteriorly, it may extend onto the superolateral surface where it is limited anteriorly by the lunate sulcus. Area 17 is continuous, both above and below with area 18 and beyond this area with area 19. Areas 18 and 19 are described as psychovisual areas (or visual association areas) and are responsible for the interpretation of visual impulses reaching area 17. The pathways leading between the eyes and visual areas can also be observed from a midsagittal aspect, especially at the optic chiasm.
Occipital eye field: Fibres from the visual areas reach the frontal eye field which is concerned with eye movements. The visual areas are therefore regarded as partly motor in function. This view is substantiated by the fact that movements of the eyeballs and head can be produced by stimulation of areas 17 and 18 which constitutes an occipital eye field. Efferents from the visual areas also reach the superior colliculus, the pretectal region, and the nuclei of cranial nerves supplying muscles that move the eyeballs.
Therefore, as a way of summary, the visual areas can be categorized as follows:
- visual area – area 17 – sensory
- occipital eye field – area 17 & 18 – motor
- psychovisual area – area 18 & 19 – sensory
Prefrontal Areas (Prefrontal Cortex): The part of the frontal lobe excluding the motor, premotor and motor association cortex is referred to as the prefrontal area. It includes the parts of the frontal gyri anterior to the motor association area, most of the anterior parts of the orbital gyri, most of the medial frontal gyrus, and the anterior part of the gyrus cinguli. This area is concerned with normal expression of emotions, the ability to predict consequences of actions, and complex thoughts. The medial part of the prefrontal area is associated with auditory and visual functions.
Olfactory Cortex: This region of the cerebral cortex plays a role in olfaction (the sense of smell). It receives direct fibres from the olfactory bulb. The olfactory cortex is composed of the prepiriform cortex (which includes the lateral olfactory gyrus and the gyrus ambiens), gyrus semilunaris, anterior olfactory nucleus, as well as the entorhinal area.