19th February 2024

Cerebrum

Clinical anatomy

The cerebral cortex consists of 2 hemispheres along with the diencephalon and basal ganglia centrally. The longitudinal cerebral fissure divides the cerebral hemisphere. The diencephalon and basal ganglia are located deep within the cerebral cortex. The diencephalon consists of the thalamus and hypothalamus. These structures sit superior to the tentorium cerebelli. 

Each cerebral hemisphere can be divided into four separate lobes: Frontal, Parietal, Temporal and Occipital. The anterior-most portion of the cerebral hemisphere is the frontal lobe and is divided from the parietal lobe posteriorly by the central sulcus. The lateral sulcus divides the temporal lobe inferiorly and the parietal lobe superiorly. The occipital lobe is found posteriorly, with the parieto-occipital sulcus separating the two lobes. 

The function of the lobes is as follows:

• Frontal lobe – Personality, higher executive functions, primitive reflexes, motor function (precentral gyrus), olfactory sensation.
• Parietal lobe – Language; sensation (Post-central gyrus); visuospatial awareness, visual pathway; ability for calculation, writing and learned movements. 
• Temporal lobe – Visual pathways, auditory processing, memory formation. 
• Occipital lobe – Visual processing

Image Source: Wikimedia Commons (2016). Cerebrum lobes, Licensed under a creative commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:Cerebrum_lobes.svg (Accessed: 9 January 2024).

Each lobe of the cerebral hemisphere will control the contralateral side of the body. 

The basal ganglia is a subcortical structure that consists of several nuclei, including the caudate nucleus, lentiform nucleus, subthalamic nucleus and substantia nigra. While its function is complex, it is primarily a feedback circuit to help modulate and refine motor functions. 

Image Source: Wikimedia Commons (2018). The structures of the basal ganglia, Licensed under a creative commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:The_structures_of_the_basal_ganglia.png (Accessed: 9 January 2024).

The thalamus and hypothalamus have separate functions. The thalamus serves as a sensory processing centre as it receives input from the sensory organs and relays this information to relevant areas of the cerebral cortex. The hypothalamus controls many essential bodily functions, including hormonal release, thermoregulation, sleep regulation, appetite control and memory formation. 

The pituitary gland is situated underneath the cerebrum and is suspended by the pituitary stalk. It sits within the sella turcica and is superiorly covered by the diaphragma sellae. The optic chiasm also runs superior to the pituitary gland. It is divided into two parts, the anterior pituitary and posterior pituitary, but both regions’ primary function is hormonal secretion.

Vasculature

The arterial supply to the brain mainly stems from the internal carotid artery and vertebral artery. The internal carotid has no extracranial branches and enters the skull via the carotid canal in the petrous part of the temporal bone. The carotid canals also contain the venous plexus and carotid sympathetic plexus. The internal carotid courses through the cavernous sinus in close proximity to CN III, IV, and VI within the carotid groove on the sphenoid body. It terminates as the anterior and middle cerebral arteries and forms the anterior circulation of the brain. 

The posterior circulation of the brain originates from the vertebral artery, the first branch of the first part of the subclavian artery. Each vertebral artery will ascend through the transverse foramina of the cervical vertebrae and enter the cranium through the foramen magnum. The intracranial vertebral arteries unite to form the basilar artery. The basilar artery terminates at the Circle of Willis as it divides into the posterior cerebral artery. 

Communicating arteries exist to allow anastomosis within the cerebral circulation. The anterior communicating artery connects the anterior cerebral arteries, while the posterior communicating artery connects the internal carotid artery to the posterior cerebral artery. The anterior, posterior and middle cerebral arteries, along with the anterior and posterior communicating arteries, complete the Circle of Willis.

Image Source: Wikimedia Commons (2013). 2123 Arteries of the Brain, Licensed under a creative commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:2123_Arteries_of_the_Brain.jpg (Accessed: 9 June 2022).

The venous drainage of the brain begins through the cerebral veins, which carry deoxygenated blood into the dural venous sinuses. The draining cerebral veins pierce through the arachnoid and dura mater as bridging veins to empty their contents into the dural sinus. Arachnoid granulations/villi are outpouchings of the arachnoid mater that enter the dural sinus to allow drainage of CSF into the venous system. 

There are 11 dural sinuses, with the main sinuses being the superior sagittal sinus, inferior sagittal sinus, straight sinus, transverse sinus, sigmoid sinus and cavernous sinus. The confluence of sinuses is the location where the transverse, superior sagittal, straight and occipital sinuses meet. Blood is drained from the confluence of sinuses through the transverse sinus and sigmoid sinus into the internal jugular vein. 

The cavernous sinus contains several venous plexus within the body of the sphenoid. It is important as many intracranial structures pass through it, including the internal carotid artery, CNIII, CNIV, and CNVI, along with the ophthalmic and maxillary division of CNV.

Image Source: Wikimedia Commons (2016). 1315 Brain Sinuses, Licensed under a creative commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:1315_Brain_Sinuses.jpg (Accessed: 9 January 2024).

Meninges

The cranial meninges are the protective lining surrounding the brain and spinal cord. This also serves as the lining to the subarachnoid space and acts as a supportive structure for the cranial vasculature. There are three distinct layers to the cranial meninges: dura mater, arachnoid mater and pia mater.

The periosteal dura mater adheres to the calvarium’s internal surface and cranial sutures. In contrast, the meningeal dura mater lines the brain and forms a continuous layer along the foramen magnum into the spinal cord. Apart from the areas where the dural sinuses exist, both layers of the dura mater are fused. The meningeal dura mater also creates infolding that forms partitions within the brain, including:

• Falx cerebri – Lies in the longitudinal cerebral fissure
• Tentorium cerebelli – Lies between the occipital lobe and cerebellum
• Falx cerebelli – Lies inferior to the tentorium cerebelli and partially separated the cerebellar hemispheres
• Diaphragm sella – Suspended between the clinoid process and forms a roof over the hypophysial fossa. 

The arachnoid mater and pia mater both form the lining of the leptomeninx. The space between the arachnoid mater and the pia mater is the subarachnoid space. The pia mater is a highly vascular structure adherent to the brain’s surface. 

Ventricular system and cerebrospinal fluid (CSF) production

The ventricular system in the brain consists of 4 main ventricles: 2 lateral ventricles, the third ventricle and the fourth ventricle. The two lateral ventricles connect to the third ventricle via the interventricular foramen, while the cerebral aqueduct within the midbrain connects the third and fourth ventricles. Two lateral apertures and a median aperture allow CSF to drain from the fourth ventricle into the subarachnoid space around the brain and spinal cord. 

CSF production begins in the choroid plexus within the ventricular system. It will circulate through the various ventricles and is excreted around the subarachnoid space. Reabsorption of the CSF occurs at the dural sinuses, where arachnoid granulations project into the superior sagittal sinus. The CSF’s primary function is to cushion the brain and blood vessels from the cranial vault. 

Image Source: Wikimedia Commons (2013). Blausen 0896 Ventricles Brain, Licensed under a creative commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:Blausen_0896_Ventricles_Brain.png (Accessed: 9 January 2024).