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. The cerebrum (prosencephalon or forebrain) comprises the telencephalon (cerebral hemispheres) and the diencephalon. 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.
This article will describe the anatomy of the structures visible in the midsagittal section of the brain.
Commissures: Corpus callosum, anterior commissure, posterior commissure, habenular commissure
Lobes: Frontal, temporal, parietal, occipital, insular, limbic
Gyri: Cingulate gyrus (limbic lobe); medial frontal gyrus (frontal lobe); paracentral lobule (frontal and parietal lobes); precuneus (parietal lobe); cuneus (occipital lobe)
Sulci: Sulcus of corpus calosum, cingulate sulcus, marginal sulcus, paracentral sulcus, central sulcus, parietooccipital sulcus, calcarine sulcus
|Cerebellum||Cerebellar cortex (grey matter)
Arbor-vitae - tree of life (white matter)
The telencephalon corresponds to the cerebral hemispheres and includes the cerebral cortex and several subcortical structures. The cerebral cortex is composed of gyri and sulci and divided into six lobes including the frontal, temporal, parietal, occipital, insular and limbic lobes. The divisions between lobes are primarily identified by fissures. The midsagittal aspect of the cerebrum also reveals structures spanning the longitudinal cerebral fissure, such as the corpus callosum. The corpus callosum is a prominent structure on this midsagittal surface composed of commissural fibers that permit the direct communication between the structures of the two hemispheres. The septum pellucidum is a thin membrane connected to the inferior surface of the corpus callosum and located at the midline of the brain.
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 (this posterosuperior extension of the cingulate sulcus is called marginal sulcus). The area between the cingulate sulcus and the corpus callosum is the gyrus cinguli (or cingulate gyrus), which is a part of the so called limbic lobe. 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 (or 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 anterior border of the paracentral lobule is the paracentral sulcus, which separates this lobule from the medial frontal gyrus. The paracentral sulcus is a very short sulcus which is continuous with the cingulate sulcus. The medial frontal gyrus and anterior part of the paracentral lobule are part of the frontal lobe.
Between the parietooccipital 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 form the medial surface of the parietal lobe.
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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, which is a part of the occipital lobe. This triangular area is defined anterosuperiorly 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 sulcus, and posteriorly by the superomedial border of the cerebral cortex. The calcarine sulcus extends forward beyond its junction with the parietooccipital 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 sulcus) and the splenium of the corpus callosum.
The brain can be divided into Brodmann areas according to its cytoarchitecture and function. The medial frontal gyrus contains Brodmann area 6 (premotor and supplementary motor cortex) and area 8 (part of the motor association cortex). The paracentral lobule surrounds the medial end of the central sulcus and, because of that, it contains the Broadman area 4 (the primary motor cortex), areas 3, 1, and 2 (the primary somatosensory cortex) and area 5 (the somatosensory association cortex). The cuneus contains Brodmann areas associated with the process of visual information (areas 17, 18 and 19). These areas are not exclusively seen in the medial view of the brain and extend into the lateral aspect of the cerebral hemispheres.
The diencephalon is a phylogenetically older part of the brain lying between the telencephalon and the brainstem. It is composed of the thalamus, the hypothalamus, the subthalamus and the epithalamus.
The thalamus is a large mass of grey matter with symmetrical halves located on each side of the third ventricle. The thalamus has several functions that include relaying of sensory and motor information to the cerebral cortex. The interthalamic adhesion expands between the two halves of the thalamus.
The hypothalamus is the part of the diencephalon that lies below the thalamus. It comprises several subdivisions and nuclei that are responsible for controlling important physiological functions such as hormone regulation. It works closely with the pituitary gland to perform this function. Just anterior inferior to the hypothalamus is the optic chiasm, where nerves from each eye cross over allowing binocular vision.
The main component of the epithalamus is the pineal gland or epiphysis, which produces a hormone called melatonin that regulates circadian rhythms.
The subthalamus lies inferior to the posterior part of the thalamus, just posterior and lateral to the hypothalamus. The largest division of the subthalamus is the subthalamic nucleus. This nucleus plays a fundamental role in the circuitry of the basal ganglia (i.e. movement regulation).
The cerebellum, or commonly called the 'small brain', lies just posterior to the brainstem. It has a superficial layer of grey matter, the cerebellar cortex. It consists of two hemispheres separated by the medially positioned vermis. 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, and the inner white matter of the cerebellum gets a branched, tree-like appearance, to which the term arbor-vitae (tree of life) is ascribed. The cerebellum is involved in the coordination of voluntary movements, and in the maintenance of muscle tone, balance and posture.
The brainstem is the “bridge” between the brain and the spinal cord. It is made up of the midbrain, pons and the medulla oblongata. It is continuous above with the diencephalon, below with the spinal cord and posteriorly with the cerebellum. The brainstem contains cranial nerve nuclei and important neural pathways, being responsible for regulating fundamental body functions such as breathing, heart rate and blood pressure.
The sagittal view of the midbrain reveals its two portions: tectum and tegmentum. The tectum is the region of the midbrain posterior to the cerebral aqueduct of Sylvius. It is composed of the two posterior bulges called the superior and inferior colliculi. They are involved in processing of visual and auditory stimuli respectively. The tegmentum is the larger portion of the midbrain situated anterior to the tectum, between the cerebral aqueduct and the substantia nigra. The tegmentum contains two areas named after specific colours: the red nucleus (involved in the coordination of movements); the periaqueductal grey matter (involved in pain processing). Note that the tegmentum in the broader sense extends through the whole brainstem.
The pons is divided into two main regions: the basilar part is located anteriorly and contains descending tracts made of myelinated fibers (white matter), nuclei (gray matter) and transverse pontine fibers; the tegmentum is located posteriorly, forms the floor of the fourth ventricle and contains cranial nerve nuclei and associated tracts.
The medulla oblongata is composed of three parts from anterior to posterior: the basal area that contains the pyramidal decussation of the corticospinal tract, the tegmentum where we can find the inferior olivary nuclei and nuclei of cranial nerves, and the tectum which composed of the inferior medullary velum (the posteroinferior part of the fourth ventricle).
The fourth ventricle lies dorsal to the pons and medulla oblongata and ventral to the cerebellum. This ventricle communicates with the third ventricle through the cerebral aqueduct. The fourth ventricle continues inferiorly with the central canal of the medulla which becomes the central canal of the spinal cord.
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