Definition
A ventricle is an internal cavity of the brain. Within the normal human brain, there is a connecting system of ventricles, commonly referred to as the ventricular system, which is filled with cerebrospinal fluid (CSF). The ventricular system within the brain develops from the cavity of the neural tube in the embryo.
Description
The ventricular system is composed of two lateral ventricles and two midline ventricles, referred to as the third and fourth ventricles. The chambers are connected to allow the flow of cerebrospinal fluid via two interventricular foramen (referred to as the foramen of Monro) and the cerebral aqueduct (referred to as the aqueduct of Sylvius).
The chambers of the ventricular system are lined or covered with ependymal cells and are continuous with the central canal enclosed within the spinal cord. Ependymal cells also line the central canal of the spinal cord.
Basic anatomy
The lateral ventricles
The lateral ventricles are separated by the septum pellucidum and do not communicate directly (i.e., do not allow the flow of cerebrospinal fluid) with each other. Cerebrospinal fluid within the individual lateral ventricles must flow to the third ventricle via the interventricular foramen associated with each lateral ventricle.
Lateral ventricles themselves are descriptively divided into a body with anterior, posterior, and inferior horns.
The third ventricle
The third ventricle is a narrow cavity or cleft located between the two thalami. The third ventricle also contains two saclike recesses called the anterior supraoptic recess and the infundibular recess. The massa intermedia, the neural tissue that connects both halves of the thalamus in some brains, runs through the third ventricle. Posteriorly, the third ventricle communicates with the fourth ventricle via the cerebral aqueduct, a narrow channel that allows the flow of cerebrospinal fluid from the third to the fourth ventricle. There is no choroids plexus within the cerebral aqueduct.
The fourth ventricle
The fourth ventricle is a wide and flattened space located just anterior to the cerebellum and posterior to the upper, or superior, half of the medulla oblongata and the pons. The fourth ventricle also has two lateral saclike pouches that are called the lateral recesses. The fourth ventricle is continuous with the upper (superior) terminal end of the central canal of the spinal cord. The fourth ventricle also connects with the subarachnoid space via three small foramina: the two foramina of Luschka (one in each of the lateral recesses) and the foramen of Magendie.
The subarachnoid space continues as the space between the arachnoid matter and the pia mater (meningal tissues that surround the brain and spinal cord) and is filled with CSF. The subarachnoid space also surrounds cranial and spinal nerves.
CSF flow and blockage of the ventricular system
The normal flow of cerebrospinal fluid—produced from brain surface tissue and the choroids plexuses within the ventricles—is from the two lateral ventricles through their respective interventricular foramina into the third ventricle. Then the CSF flows from the third ventricle through the cerebral aqueduct into the fourth ventricle and from there it can flow into the subarachnoid space where it is reabsorbed into the bloodstream.
Swellings or structures within the ventricular system may be due to congenital defect, trauma, or tumor.
If there is a blockage of the ventricular system the flow of CSF is interrupted. If, for example, there is a blockage within the cerebral aqueduct, the normal flow of fluid formed in the lateral ventricles and the third ventricle is interrupted, and the lateral ventricles and third ventricle begin to swell with cerebrospinal fluid. The swelling or enlargement is termed hydrocephalus. Hydrocephalus can also result from the formation of CSF (as can occur with a tumor in one of the choroid plexuses) that exceeds the amount that can flow through the ventricular system, or from a downstream-diminished capacity to absorb cerebrospinal fluid.
A tumor in one of the interventricular foramen connecting a lateral ventricle to the third ventricle obstructs the flow of cerebrospinal fluid from the same side lateral ventricle and results in an asymmetrical swelling of the blocked lateral ventricle.
Blockage of the flow of CSF through the foramen connecting the fourth ventricle to the subarachnoid space usually produces asymmetrical swelling or dilation of the entire ventricular system. The entire ventricular system can also swell in cases of meningitis in which the flow of cerebrospinal fluid over the outer surface of the brain is obstructed.
Resources
BOOKS
Bear, M., et al. Neuroscience: Exploring the Brain. Baltimore: Williams & Wilkins, 1996.
Goetz, C. G., et al. Textbook of Clinical Neurology. Philadelphia: W.B. Saunders Company, 1999.
WEBSITES
"Development of the Ventricular System." Temple University Department of Neuroanatomy. May 10, 2004 (May 27, 2004). http://courses.temple.edu/neuroanatomy/lab/embryo/ventlate.htm.
Paul Arthur
