The third ventricle is a narrow funnel-shaped cavity of the brain. It is located in the midline, comprising the central part of the ventricular system of the brain. As such, the third ventricle directly communicates with other ventricles:
- It communicates with each lateral ventricle via the foramen of Monro,
- It communicates with the fourth ventricle via the aqueduct of Sylvius.
The third ventricle can be described as a cuboid structure that has a roof, floor and four walls (anterior, posterior, and two lateral). Similar to the other brain ventricles, the main function of the third ventricle is to produce, secrete and convey cerebrospinal fluid.
This article will discuss the anatomy and function of the third ventricle.
|Structure||Four walls (anterior, posterior, two lateral), roof and floor|
|Function||Production and transportation of the cerebrospinal fluid|
|Tela choroidea and choroid plexus||Located on the roof of the ventricle
Cavity and recesses
Choroid plexus and tela choroidea
- Clinical relevance
The floor of the third ventricle extends from the optic chiasm anteriorly to the aqueduct of Sylvius posteriorly. The anterior portion of the floor is formed by the hypothalamus, while the posterior portion is formed by the midbrain (mesencephalon). Going from the anterior to posterior, the structures that comprise the floor of the third ventricle are the:
- Optic chiasm
- Infundibulum of hypothalamus
- Tuber cinereum
- Mammillary bodies
- Posterior perforated substance
- The anterior part of the tegmentum of the midbrain
When the floor of the third ventricle is viewed from above, there is a noticeable prominence in the anteriormost part of the ventricle, formed by the optic chiasm. Note that the optic chiasm is located at the junction of the floor and anterior wall of the ventricle. Posterior to the optic chiasm, there is a prominence formed by the mamillary bodies. Between these two prominences, the cavity of the third ventricle extends into the pituitary stalk, forming the infundibular recess. The posterior half of the floor (posterior to the mammillary bodies) lies above the posterior perforated substance and tegmentum and it is generally smooth and unremarkable.
The roof of the third ventricle extends from the foramen of Monro anteriorly to the suprapineal recess posteriorly. The roof lies immediately below the body of fornix.
The roof has for separate layers which include:
- Neural layer
- Tela choroidea (two layers)
- Vascular layer
The most superficial layer is the neural layer, formed by the body and the crura of the fornix. Below this layer, there are two thin membranous layers of tela choroidea. The tela choroidea is semi-transparent double-layered structure derived from the pia mater and we’ll speak about its function further in the text.
In between the layers of the tela choroidea, there is a vascular layer that mainly consists of medial posterior choroidal arteries and their branches.
The third ventricle has four walls in total; anterior, posterior, and two lateral walls.
The anterior wall extends from the foramina of Monro superiorly to the optic chiasm inferiorly. From superior to inferior, the structures that participate in the formation of the anterior wall are as follows:
- Foramina of Monro
- Columns of fornix
- Anterior commissure
- Lamina terminalis
- Optic recess
- Optic chiasm
Note that the foramen of Monro is located at the junction between the anterior wall and the roof of the third ventricle. When the brain is viewed from the frontal aspect, only the inferior two-thirds of the anterior wall are visible. The superiormost portion is covered by the rostrum of the corpus callosum.
The posterior wall extends from the suprapineal recess superiorly, to the aqueduct of Sylvius inferiorly. When viewed from the anterior perspective, the posterior wall is formed by five structures which are (from superior to inferior):
- Suprapineal recess
- Habenular commissure
- Pineal body and its recess
- Posterior commissure
- Aqueduct of Sylvius
The only structure visible from the posterior aspect of the brain is the pineal gland. Inferior to the posterior commissure the ventricle is continuous with the cerebral aqueduct of the midbrain (of Sylvius).
The lateral walls of the third ventricle are formed by the medial aspects of the thalamus and hypothalamus, which are separated by the hypothalamic sulcus. When viewed from the medial perspective, the outlines of lateral walls can be described as the silhouette of the bird's head with open beaks. The majority of the lateral wall (the head of the bird) is formed by the medial aspect of the thalamus. The superior beak represents the optic recess, while the inferior beak is formed by the infundibular recess.
Cavity and recesses
The cavity of the third ventricle extends into four recesses:
- The supra-optic recess is located superior to the optic chiasm and inferior to the lamina terminalis.
- The infundibular recess is found on the floor of the third ventricle, between the optic chiasm and the mamillary bodies. It extends inferiorly, into the pituitary stalk.
- The pineal recess extends posteriorly. More specifically, it projects into the pineal body, between the cranial and caudal lamina of the pineal gland.
- The suprapineal recess is located between the superior portion of the pineal gland and the inferior aspect of the tela choroidea in the roof of the ventricle. It extends posteriorly, behind the posterior wall of the third ventricle.
Choroid plexus and tela choroidea
Tela choroidea is the vascular pia matter which is closely connected to the ependymal lining of the ventricles. In the third ventricle, the tela choroidea is, in fact, embedded in the multi-layered roof of the ventricle. The tela choroidea gives rise to the choroid plexus.
The choroid plexus is a collection of capillaries that produce and secrete the CSF into the ventricular system of the brain. The epithelium of the choroid plexus constitutes the blood–CSF barrier.
The blood for the choroid plexus is mainly supplied by the anterior choroidal branch of the internal carotid artery and choroidal branches of the posterior cerebral artery. The venous blood is drained by a single choroidal vein. When viewed from the sagittal section, the choroid plexuses of the lateral and third ventricles are continuous.
The function of the third ventricle is to produce and secrete the CSF, as well as to contribute to the continuous circulation of the CSF through the ventricular system. This way, the ventricle participates in cushioning and protecting the brain from injury, as well as in the transport of nutrients and waste in and out of the neural tissue. If the CSF flow through the ventricles is obstructed for any reason (e.g. a tumor that blocks the foramina through which the ventricles communicate), the CSF starts to accumulate and results in hydrocephalus.
Do you want to know more about the ventricles of the brain? Check out our study unit:
Ventriculography and hydrocephalus
Ventriculography is a tracing method used to study the ventricular system in living subjects. In this procedure, radiographs are taken after injecting a radio-opaque dye into the ventricular system. Parts of the ventricles can also be seen using computed tomography (CT) scans and magnetic resonance imaging (MRI). Hydrocephalus is one of the well known clinical conditions associated with the ventricular system malformations and can be observed using ventriculography.
Hydrocephalus is a condition in which there is an abnormal accumulation of CSF in the central nervous system (CNS) due to a disturbance of CSF flow or absorption. This leads to an increase in the volume occupied by CSF in the CNS, causing increased intracranial pressure. It can also cause progressive enlargement of the head if it occurs in childhood, potentially causing convulsion, tunnel vision, slowing of mental capacity, cognitive deterioration, headaches, neck pain suggesting tonsillar herniation, vomiting, blurred vision, double vision, difficulty in walking secondary to spasticity, drowsiness and other forms of mental disabilities, amongst other symptoms.
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