|Brain: Cerebral ventricles|
The ventricular system accounts for the production and circulation of cerebrospinal fluid.
Rotating 3D rendering of the four ventricles and connections. From top to bottom:
Blue - Lateral ventricles
Cyan - Intraventricular foramina (Monro)
Yellow - Third ventricle
Red - Cerebral aqueduct (Sylvius)
Purple - fourth ventricle
Green - continuous with the central canal
(Apertures to subarachnoid space are not visible)
The ventricular system is a set of four structures, the ventricles, containing cerebrospinal fluid (CSF) in the brain. It is continuous with the central canal of spinal cord. The ventricle lining consists of an epithelium-like membrane called ependyma. The ventricles are interconnected, allowing the flow of cerebrospinal fluid. CSF is produced by the ependymal cells in the choroid plexus, a network of these cells within each of the ventricles.
The system comprises four ventricles:
There are several foramina, openings acting as channels, that connect the ventricles. The interventricular foramina (also called the foramina of Monro) connect the lateral ventricles to the third ventricle through which the cerebrospinal fluid can flow.
|interventricular foramina (Monro)||lateral ventricles||third ventricle|
|cerebral aqueduct (Sylvius)||third ventricle||fourth ventricle|
|median aperture (Magendie)||fourth ventricle||subarachnoid space via the cisterna magna|
|right and left lateral aperture (Luschka)||fourth ventricle||subarachnoid space via the cistern of great cerebral vein|
The cavities of the hollow human brain are called ventricles. There are four ventricles in the human brain: two lateral ventricles in the cerebrum; the third ventricle in the diencephalon of the forebrain; and the fourth ventricle in the medulla of the hind brain.
As the part of the primitive neural tube that will develop into the brainstem, the neural canal expands dorsally and laterally, creating the fourth ventricle, whereas the neural canal that does not expand and remains the same at the level of the midbrain superior to the fourth ventricle forms the cerebral aqueduct. The fourth ventricle narrows at the obex (in the caudal medulla), to become the central canal of the spinal cord.
In more detail, around the third week of development, the embryo is a three-layered disc. The embryo is covered on the dorsal surface by a layer of cells called endoderm. In the middle of the dorsal surface of the embryo is a linear structure called the notochord. As the endoderm proliferates, the notochord is dragged into the middle of the developing embryo. The notochord becomes a canal within the embryo known as the neural canal.
As the brain develops, by the fourth week of embryological development several swellings have formed within the embryo around the canal, near where the head will develop. These swellings represent different components of the central nervous system, and are three in number: the prosencephalon, mesencephalon and rhombencephalon. These in turn divide into five sections. As these sections develop around the neural canal, the inner neural canal becomes known as primitive ventricles. These form the ventricular system of the brain:
- The prosencephalon divides into the telencephalon, which forms the cortex of the developed brain, and the diencephalon. The ventricles contained within the telencephalon become the lateral ventricles, and the ventricles within the diencephalon become the third ventricle.
- The rhomencephalon divides into a metencephalon and mesencephalon. The ventricles contained within the rhomencephalon become the fourth ventricle, and the ventricles contained within the mesencephalon become the aqueduct of Sylvius.
Flow of cerebrospinal fluid
The ventricles are filled with cerebrospinal fluid (CSF) which bathes and cushions the brain and spinal cord within their bony confines. CSF is produced by modified ependymal cells of the choroid plexus found in all components of the ventricular system except for the cerebral aqueduct and the posterior and anterior horns of the lateral ventricles. CSF flows from the lateral ventricles via the foramina of Monro into the third ventricle, and then the fourth ventricle via the cerebral aqueduct in the brainstem. From there it can pass into the central canal of the spinal cord or into the cisterns of the subarachnoid space via three small foramina: the central foramen of Magendie and the two lateral foramina of Luschka.
The fluid then flows around the superior sagittal sinus to be reabsorbed via the arachnoid villi (or granulation villi) into the venous sinuses, after which it passes through the jugular vein and major venous system. CSF within the spinal cord can flow all the way down to the lumbar cistern at the end of the cord around the cauda equina where lumbar punctures are performed.
The cerebral aqueduct between the third and fourth ventricles is very small, as are the foramina, which means that they can be easily blocked, causing high pressure in the lateral ventricles. This is a common cause of hydrocephalus (known colloquially as "water on the brain"), which is an extremely serious condition due to both the damage caused by the pressure as well as nature of whatever caused the block (e.g. a tumour or inflammatory swelling).
Protection of the brain
The brain and spinal cord are covered by the meninges, the three protective membranes of the tough dura mater, the arachnoid mater and the pia mater. The cerebrospinal fluid (CSF) within the skull and spine provides further protection and also buoyancy, and is found between the pia mater and the arachnoid mater.
The CSF that is produced in the ventricular system is also necessary for chemical stability, and the provision of nutrients needed by the brain. The CSF helps to protect the brain from jolts and knocks to the head and also provides buoyancy and support to the brain against gravity. (Since the brain and CSF are similar in density, the brain floats in neutral buoyancy, suspended in the CSF.) This allows the brain to grow in size and weight without resting on the floor of the cranium, which would destroy nervous tissue.
Diseases of the ventricular system include abnormal enlargement (hydrocephalus) and inflammation of the membranes or ventricles, (meningitis or ventriculitis) caused by infection or introduction of blood following trauma or hemorrhage.
The scientific study of CT scans of the ventricles in the late 1970s revolutionized the study of mental disorder. Researchers found that individuals with schizophrenia had (in terms of group averages) enlarged ventricles compared to healthy subjects. This became the first "evidence" that schizophrenia was biological in origin and led to a reinvigoration of the study of such conditions via modern scientific techniques. Whether the enlargement of the ventricles is a cause or a result of schizophrenia has not yet been ascertained, however. Still, this finding was not revolutionary at the time, as enlarged ventricles are found in various other types of organic dementia. In fact, ventricle volumes have been found to be "mainly explained by environmental factors" and to be extremely diverse between individuals, such that the percentage difference in group averages in schizophrenia studies (+16%) has been described as "not a very profound difference in the context of normal variation" (ranging from 25% to 350% of the mean average). Nowadays, magnetic resonance imaging (MRI) has superseded the use of CT in research in the role of detecting ventricular abnormalities in psychiatric illness.
This article uses anatomical terminology; for an overview, see anatomical terminology.
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- ventricular system and CSF (concise description, University of Washington)
- CSF at answers.com (brief description, excellent diagram of CSF flow)