Fourth ventricle

Lateral walls

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.

It has two major extensions, known as the 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 (foramina of Luschka).

Roof

The roof of the fourth ventricle has presents a 'tent-like' apex at the intersection of it's superior and inferior parts. This apex, also known as the fastigium, extends into the white core of the cerebellum.

The superior 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.

Cavity

The cavity or fossa of the fourth ventricle communicates with the third ventricle superiorly as a continuation of the cerebral aqueduct. The inferior portion of the cavity is known as the obex (Latin for barrier), and extends into 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.

Floor

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 posterior median sulcus, and into a superior and inferior triangle by the striae medullares.

• The upper triangular part is formed by the posterior surface of the pons. 
• The lower triangular part is made up of the upper part of the posterior surface of the medulla and an intermediate part at the junction of the medulla and pons.
• 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.

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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 superiormost 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 pontine tegmentum . Lower down the sulcus limitans is a depression referred to as the superior fovea. At the level of this depression, the medial 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.

For more details about the brain ventricles and flow of CSF, take a look below:

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 functions:

• 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.

Solidify your knowledge about the ventricles of the brain with the quiz below!