The fourth ventricle is one of the interconnected fluid-filled cavities within the human brain. There are four of these cavities in the brain, three of which are located within the cerebrum and called the lateral ventricles (first and second ventricles), and the third ventricle. These cavities and their content constitute the ventricular system.
The fourth ventricle lies dorsal to the pons and medulla (of the brainstem) and ventral to the cerebellum. It extends from the cerebral aqueduct (aqueduct of Sylvius) to the obex, thus communicating freely with the central canal of the spinal cord inferiorly and the third ventricle superiorly. Its surface is lined by an epithelial layer called the ependyma, and is filled with cerebrospinal fluid (CSF).
The fourth ventricle has an anterior floor with a characteristic diamond shape, named the rhomboid fossa, and a posterior tent-shaped roof. It also has a cavity and lateral walls. CSF produced and/or flowing into the fourth ventricle can exit to the subarachnoid space through lateral apertures and a single median aperture within the roof of the fourth ventricle.
The lateral walls of the fourth ventricle are formed by the cerebellar peduncles. The superior part of these walls is formed by the superior cerebellar peduncle. The inferior part is formed by the inferior cerebellar peduncle and by the gracile and cuneate tubercles of the brainstem.
The roof of the fourth ventricle has an apex at the intersection of the upper and lower parts of the roof. Thus, the roof of the fourth ventricle is tent-shaped and is divisible into an upper and a lower part. The apex extends into the white core of the cerebellum.
The upper part of the roof is formed by the superior cerebellar peduncles and the superior medullary velum (thin sheet of white matter). The inferior part of the roof is made of non-nervous tissue, the inferior medullary velum. However, like other parts of the ventricle, it is lined by a membrane consisting of ependyma and a double fold of pia mater which constitutes the tela choroidea of the fourth ventricle. Laterally on each side of the midline, this membrane extends and joins the inferior cerebellar peduncles. The lower part of the membrane has a large aperture, the foramen of Magendie. This is the median aperture of the fourth ventricle, through which the entire ventricular system communicates.
The cavity or fossa of the fourth ventricle communicates with the third ventricle superiorly as a continuation of the cerebral aqueduct. Inferiorly it extends as the central canal of the brainstem, which in turn runs through the vertebral column. The cavity also communicates with the subarachnoid space through the three apertures mentioned above.
It has several extensions, including two lateral dorsal recesses which lie above the inferior medullary velum, and a median dorsal recess extending into the white core of the cerebellum and lying superior to the nodule of the cerebellum. The largest extensions are the two lateral recesses, one on either side of the midline. These recesses extend laterally between the inferior cerebellar peduncle and the peduncle of the flocculus of the cerebellum, to open into the subarachnoid space as the lateral apertures of the fourth ventricle (foramina of Luschka).
The floor of the fourth ventricle is also referred to as the rhomboid fossa because of its shape. It is divisible into a right and left half by the median sulcus and a superior and inferior triangle by the striae medullares.
- The upper triangular part is formed by the posterior surface of the pons.
- The upper part of the posterior surface of the medulla and an intermediate part at the junction of the medulla and pons make up the lower triangular part.
- The intermediate part is prolonged laterally over the inferior cerebellar peduncle as the floor of the lateral recess. Its surface is marked by the presence of delicate bundles of transversely running fibres that constitute the striae medullares.
- The lowest part of the floor of the fourth ventricle is referred to as the calamus scriptorius. Each inferolateral margin of the floor is marked by a narrow white ridge called taenia. The right and left taeniae meet at the inferior apex of the floor to form a small fold called the obex.
For ease of description of the floor of the fourth ventricle, the median sulcus is used as a major feature. On either side of this sulcus lies a longitudinal elevation called the medial eminence. This eminence is limited laterally by the sulcus. The area is called the vestibular area and houses the vestibular nuclei. Thus the vestibular area lies partly in the pons and partly in the medulla.
The uppermost part of the sulcus limitans overlies an area called the locus coeruleus, deep to which there is a nucleus called the nucleus coeruleus extending into the tegmentum of the midbrain. Lower down the sulcus limitans is a depression referred to as the superior fovea. At the level of this depression, the median eminence shows a swelling called the facial colliculus.
Within the medullary part of the floor, the sulcus limitans is marked by a depression, the inferior fovea. Inferior to this inferior fovea is an oblique sulcus running towards the midline and dividing the medial eminence into two triangles called the hypoglossal and vagal triangles (or the hypoglossal and vagal trigones). The hypoglossal triangle lies medial and the vagal, lateral. These triangles house the hypoglossal and vagal nuclei respectively. The vagal triangle defines an area with the gracile tubercle called the area postrema.
Tela Choroidea And Choroid Plexuses
The tela choroidea and choroid plexus of the fourth ventricle are other notable features of the fourth ventricle. The tela choroidea is made up of two layers of pia mater. The choroid plexus is a highly vascular structure, and is similar in structure to those of the lateral and third ventricles. It lies within the folds of pia mater forming the tela choroidea.
These structures are responsible for the production of the cerebrospinal fluid (CSF), which has several roles:
- fills the subarachnoid space and ventricles
- plays a protective role to the brain
- supplies nutrients to the brain
- removes waste products from the brain, which are absorbed by the arachnoid villi.
Hydrocephalus is one of the conditions that can result from blockage of the median and lateral apertures. In Arnold Chiari malformation (Type II Chiari malformation), the medulla and the tonsils of the cerebellum come to lie in the vertebral canal by descending through the foramen magnum. The median and lateral apertures are blocked by this condition leading to obstruction of CSF flow. This causes a type of hydrocephalus called internal hydrocephalus. Chiari II can also present with syringomyelia due to the development of CSF-filled cyst or syrinx.
Medulloblastoma is the most common malignant brain tumour in children, which arises in the cerebellum and can therefore impinge on the roof of the fourth ventricle. The area postrema of the caudal region of the fourth ventricle is also of clinical significance because of its role in the control of vomiting.