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2.6. CSF and the Ventricles
Overview
The ventricles are fluid-filled spaces within the brain. There are four ventricles in total. Beginning in the cerebrum and moving inferiorly, the ventricles are called:
- The two lateral ventricles (one on the left and one on the right)
- The third ventricle
- The fourth ventricle
What is CSF?
Cerebrospinal fluid, or CSF, is a clear liquid produced by the choroid plexuses that lie within the four ventricles. It flows throughout the ventricular system and subarachnoid space surrounding the brain and spinal cord. Each choroid plexus actively secretes CSF into the ventricular lumens, although the choroid plexuses in the lateral ventricles produce the most CSF. Ependymal cells (see Section 2.5. Glial Cells) are crucial to the functioning of the choroid plexuses – they actively secrete Na+ ions and use their cilia to waft CSF through the ventricular system.
CSF is formed from the filtrates of blood that passes through the choroid plexuses (see Figure 2.6.1.). The composition of CSF is as follows:
There are very few cells present in CSF; in fact, the presence of white blood cells can indicate a serious infection of the meninges. The process of forming CSF is described as 'active' – this is because the secretion of Na+ ions from the ependymal cells requires energy (ATP). As expected, water from the blood naturally follows the Na+ ion movement by osmosis. So, what is the function of CSF? The brain is essentially suspended in CSF and the meningeal layers. As such, these structures act as shock absorbers to the brain when the head moves. They dissipate any kinetic energy from the ‘floating’ brain and thereby prevent damage caused by the brain (and dura) forcefully hitting the skull. CSF is also thought to play a protective role by circulating nutrients filtered from the blood and removing toxins from the CNS.
CSF is formed from the filtrates of blood that passes through the choroid plexuses (see Figure 2.6.1.). The composition of CSF is as follows:
- Water, which makes up 99% of CSF
- Ions, including Na+, K+, Ca2+, Cl- and Mg2+ ions. There is more Na+, Cl- and Mg2+ in CSF than in blood, but less Ca2+ and K+ than in blood;
- Molecules, including glucose and proteins (again, normally present in lower concentrations in CSF compared to blood).
There are very few cells present in CSF; in fact, the presence of white blood cells can indicate a serious infection of the meninges. The process of forming CSF is described as 'active' – this is because the secretion of Na+ ions from the ependymal cells requires energy (ATP). As expected, water from the blood naturally follows the Na+ ion movement by osmosis. So, what is the function of CSF? The brain is essentially suspended in CSF and the meningeal layers. As such, these structures act as shock absorbers to the brain when the head moves. They dissipate any kinetic energy from the ‘floating’ brain and thereby prevent damage caused by the brain (and dura) forcefully hitting the skull. CSF is also thought to play a protective role by circulating nutrients filtered from the blood and removing toxins from the CNS.
Flow of CSF through the Ventricular System
The lateral ventricles are essentially 3D C-shaped spaces that lie within the cerebrum (see Figure 2.6.2.). The lateral ventricles are lined with ependymal that produce and secrete cerebrospinal fluid (CSF) into the centre of the ventricles. The CSF then flows through the interventricular foramina* (again, there is a left foramen and a right foramen). These foramina carry the CSF into the third ventricle, which is sandwiched between the left and right parts of the thalamus. (*By the way, foramina are basically just holes through which the fluid flows!)
CSF then flows downwards from the third ventricle, through a thin tube called the cerebral aqueduct, which leads into the fourth ventricle. The fourth ventricle is situated between the pons and cerebellum, and it’s described as being shaped like a fairly flat tent. The CSF then leaves the fourth ventricle by one of three openings (also called apertures): the two lateral apertures (also called ‘foramina of Luschka’) and the median aperture (‘foramen of Magendie’). It will eventually leave this canal and move into the subarachnoid space surrounding the spinal cord and brain. The production of new CSF in the lateral ventricles is balanced by the absorption of older CSF into the dural venous sinuses at the arachnoid villi (see Figure 2.4.2.).
CSF then flows downwards from the third ventricle, through a thin tube called the cerebral aqueduct, which leads into the fourth ventricle. The fourth ventricle is situated between the pons and cerebellum, and it’s described as being shaped like a fairly flat tent. The CSF then leaves the fourth ventricle by one of three openings (also called apertures): the two lateral apertures (also called ‘foramina of Luschka’) and the median aperture (‘foramen of Magendie’). It will eventually leave this canal and move into the subarachnoid space surrounding the spinal cord and brain. The production of new CSF in the lateral ventricles is balanced by the absorption of older CSF into the dural venous sinuses at the arachnoid villi (see Figure 2.4.2.).
Clinical Top Tip:
Hydrocephalus
Hydrocephalus is essentially excess CSF surrounding the brain. The most common reason is a blockage of CSF flow somewhere in the ventricular system. CSF builds up and the ventricles dilate. Hydrocephalus can be present at birth or be acquired during later life, e.g. after a head injury. The pressure increases in the skull and can cause headaches, vomiting and blurry vision.
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