Ventricles of the Brain
The human brain is so vital and delicate that it is fully encased in a bony vault in order to protect it from damage. To add even more protection, the brain is wrapped in three meningeal layers – dura mater, arachnoid mater and pia mater. However, even with all those layers, there is still space surrounding the brain that makes it vulnerable to injury. This space is therefore occupied by a clear fluid that suspends the brain within the cranial vault. The fluid (cerebrospinal fluid) is produced in the ventricular system of the brain. There are four such hollow spaces in the brain that house cerebrospinal fluid (CSF): two lateral ventricles, a third ventricle and a fourth ventricle. This article will look at the structure of this system and how it helps the brain.
Each ventricle is home to a choroid plexus. The vascular part of the pia mater, which is called the tela choroidea, folds into the cavity of the ventricle and is further covered by ependymal. It contains choroid epithelium, which is simply cuboidal or low columnar epithelium. The extensive folding of the membrane gives the structure an expansive surface area. The capillaries of the plexus are fenestrated with specific permeability.
The choroid plexuses in each ventricle are responsible for the synthesis of cerebrospinal fluid (CSF). The fluid consists of water and other plasma components, amino acids, and glucose that nourish brain tissue. In addition to providing nutrients for the brain to complete its metabolic activity, CSF travels through the ventricles and eventually surrounds the entire brain in the subarachnoid space (between the arachnoid mater and the pia mater). It therefore acts as a shock absorbent in instances of mild or severe head injury. The choroid plexus of the lateral ventricles produces the most CSF, followed by that of the third ventricle then that of the fourth ventricle.
There are two C-shaped cavities called the lateral ventricles; one in each cerebral hemisphere. These ventricles have three horns projecting into the lobes for which they are named. The central part of the lateral ventricle is located in the region of the parietal lobe. It is roved by the corpus callosum and floored by the dorsal thalamus and the tail of the caudate nucleus inferolaterally. The floor of the central part also has the thalamostriate vein, and the stria terminalis (fibers from the amygdaloid body) and the fornix inferomedially. In between the fornix and the thalamus is a groove known as the choroid fissure. Not only do the choroid plexuses of the lateral ventricles live here, but this region, which is also complete with ependymal and pia mater from each lateral ventricle, forms the medial boundary of the ventricles.
An anterior projection from the level of the interventricular foramen of Monro extends into the frontal lobe. It’s known as the frontal horn and is also roved by the corpus callosum. The frontal horns of each lateral ventricle are separated medially from each other by the septum pellucidum (bridge between corpus callosum superiorly and fornix inferiorly) on the medial side. Anteriorly, the genu of the corpus callosum borders the space. Its floor contains the head of the caudate nucleus.
The temporal horn is the most inferior aspect of the cavity. It extends into the temporal lobe and houses its own choroid plexus. Additionally, it contains parts of the limbic system. The tail of the caudate nucleus is adjacent to the temporal horn. The anterior part of its floor contains the pes hippocampi (anterior end of the hippocampus that resembles a lion’s paw). The middle part of the floor contains the dentate gyrus, the fimbriae of the hippocampus, the hippocampus and collateral eminence (proximal part of collateral trigone) from medial to lateral.
The occipital horn extends variably as a finger-like projection from the posterior aspect of the concavity of the ventricle. Its floor contains the calcar avis (related to the calcarine fissure) and the collateral trigone. This part of the lateral ventricle is encircled by white matter.
The third ventricle is located in the diencephalic part of the brain. It is a narrow slit that is bordered laterally by the medial nuclei of each thalamus, the hypothalamus and interrupted anteriorly by the interthalamic adhesion. The roof of the cavity is formed anteriorly by the fornix and posteriorly by the splenium of the corpus callosum. Anteriorly, the space is limited by the lamina terminalis and the anterior commissure. Inferiorly, it continues into the infundibular and supraoptic recesses of the hypothalamus and the tuber cinereum. Posterosuperiorly the cavity extends into the pineal recess with the Habenular commissure making an impression in the region.
Its lateral wall, on either side, is indented by the hypothalamic sulcus running from the foramen of Monro to the opening of the cerebral aqueduct of Sylvius. It should also be noted that the foramen of Monro provides a passage way for the choroid plexus of the lateral ventricles to enter the third ventricle. The plexus then resides in a groove inferior to the fornix and splenium of the corpus callosum. Posteroinferiorly, the posterior commissure makes a slight extension above the opening of the aqueduct of Sylvius.
The fourth ventricle is the most inferior of the four ventricles. It is situated in the brainstem where the ventricular surface of the rhombencephalon constitutes its floor (rhomboid fossa): inferior to the midbrain, posterior to the pons, anterior to the cerebellum and superior to the medulla oblongata. The nuclei of several cranial nerves make important impressions on the floor of the fourth ventricle. Protuberances of equal size, known as the medial eminence, are observed on either side of the floor extending craniocaudally. The left and right medial eminences are separated by a dorsal median sulcus. At the inferior part of the medial eminence, the fibers of each facial nerve produce a larger bulge known as the facial colliculus. Lateral to the medial eminence and facial colliculus (on either side) is the sulcus limitans; it continues caudally to the termination of the region. The locus coeruleus (pigmented area that responds to stress) is anterolateral to the medial eminence. Further inferior to the medial eminence are the hypoglossal trigone, the vagal trigone and the obex (in that craniocaudal order). A bundle of fibers, called the striae medullares, cross the floor horizontally at its midpoint towards the foramen of Luschka.
The roof of the fourth ventricle is formed by the superior and inferior medullary vela of the cerebellum. Laterally, the inferior cerebellar peduncles limit the space. On either side are apertures (foramina of Luschka) that open into the quadrigeminal cisterns. Similarly, in the inferior roof of the fourth ventricle is another aperture known as the foramen of Magendie that opens into the cerebellomedullary cistern.
The subarachnoid space is described as a cistern at points where spaces exist between it and the underlying pia mater. At different points around the brain, the cisterns are described with respect to adjacent anatomical landmarks. Notable cisterns include the suprasellar or chiasmatic cistern, the interpeduncular cistern, prepontine cistern, and the cistern of the corpus callosum.
Flow of Cerebrospinal Fluid (CSF)
Once CSF is produced in the lateral ventricle, it fills the cavity then leaves to enter the third ventricle by way of the interventricular foramen of Monro. In addition to the CSF from the lateral ventricle, the CSF produced in the third ventricle then exits the space through the cerebral aqueduct of Sylvius to enter the fourth ventricle. Very little CSF is produced in the fourth ventricle; however, it – along with that coming from the above ventricles – exit the fourth ventricle to either enter the central canal of the spinal cord or by the foramina of Luschka and foramen of Magendie to enter the cisterns. CSF surrounds the brain, and then leaves by way of arachnoid granulations to enter the superior sagittal sinus and subsequently join systemic circulation.
It is extremely important that the production of CSF is balanced with its removal from the cranial vault. Congenital abnormalities relating to the development of the interventricular pathways – namely the aqueduct of Sylvius – can result in an obstruction of CSF flow. This condition consequently leads to an accumulation of CSF in the ventricles, called non-communicating hydrocephalus. It should be noted that tumors or traumatic lesions that obstruct the interventricular pathway can also lead to non-communicating hydrocephalus. In other instances, where there is obstruction in the cisterns or dural sinuses, CSF accumulation is known as communicating hydrocephalus.
If this process takes place in an individual prior to the fusion to the fontanelles, then the patient can present with an encephalomegaly (enlarged head). However, if the fontanelles have fused, then it is most likely that herniation of the adjacent tissue would occur. Other pathological presentations would be dependent on the nuclei and nerves that are compressed by the excess CSF.