March 17, 2011
We have to go deeper

Why it’s only the Central Nervous System! Hello there! ‘Ow’s yer father?! I know, it’s totally basic stuff that pales in comparison to some of the other stuff I harp on about but it’s so elegant! The CNS = The Brain + The Spinal Cord.

Within the CNS is a fluid filled cavity which is known as the ventricular system. The Ventricles start in the brain, and extend into the spine. The ventricles contain circulating cerebrospinal fluid (CSF). The walls of the ventricles are lined with ependymal glial cells. In the lateral and the 4th ventricles, the ependyma form the choroid plexus, which is the main site of CSF production. The CSF starts circulation in the lateral ventricles within their respective hemispheres of the brain and circulates through the Foramen of Monro into the 3rd Ventricle. From here the CSF circulates through the Cerebral Aqueduct, the narrowest region of the ventricular system and the region most likely to become blocked. The CSF flows through the peculiarly shaped 4th ventricle into the central canal of the spine, or through the foramens of luschka and magendie to enter the subarachnoid space.

The Ventricular System

What is the subarachnoid space? Well to know that, I’ll introduce you to the meninges. The meninges are the outer coverings of the CNS. These surround the entire CNS. The outermost layer is the Dura Mater. The middle layer is the Arachnoid Mater. The thinnest layer which is in close contact with the very surface of the brain itself, is the Pia Mater. The Subarachnoid Layer is the region between the Arachnoid and the Pia. CSF enters this space via the aforementioned foramina and circulates to surround the CNS. It acts as mechanical cushion - a very effective one at that. The CSF also serves to remove harmful metabolites from the neurons, provide a balanced pH for the brain to be bathed in and is an exchange medium between the blood and the brain. The CSF contains little to no protein, contrary to blood plasma. It also contains absolutely no cells. Presence of cells (determined by extracting the CSF via a lumbar puncture) is a bad thing and indicates infection of the CNS, probably some kind of meningitis inducing infection.

Diagram of the Meninges

The Dura Mater contains nerves which transmit pain. The nerves arise from cranial nerve V, aka the Trigeminal Nerve. This is where mosst headaches arise from. Stretching of Dura stimulates the nerve fibres causing the characterstic headache. Meningitis is inflammation of the meninges. The inflammation causes swelling of the meninges (aka oedema, due to the immune response to foreign pathogens present in the CSF or meninges), and this leads to pain - which is why headaches are but one symptom of meningitis.

The  Trigeminal Nerve and its peripheral branches

The back grows faster than the spinal cord so by adulthood, your spinal cord is just over halfway down your back (as depicted on the first CNS diagram, by the yellow spine). The rest of your spine contains diffuse nerves and cerebrospinal fluid. The bottom of your back is a big bag of CSF, and is known as the Lumbar Cistern. Cisterns are deep regions of Sub Arachnoid Space. The Lumbar Cistern is a very deep region of Subarachnoid space, and a site of drainage for CSF, where it is reabsorped into circulation. The depth and absence of spinal cord (which you usually don’t want to damage with a needle) makes it ideal to extract the stuff for clinical analysis. This is known as a Spinal Tap or a Lumbar Puncture, as the needle goes in between the vertebrae of L3 and L5. The CSF can then be analysed to check for signs of infection, disease or bleeding. If a lumbar puncture is not viable, then CSF may be extracted from the Cisterna Magna (another very deep cistern) which is a cistern just below the cerebellum. This is very risky, as the decrease in pressure may cause herniation of the brain through the back of the skull which can be fatal.

Spinal Tap/Lumbar Puncture

I mentioned earlier that the Cerebral Aqueduct (aka the Aqueduct of Sylvius) is narrow and is more liable to occlusion. Occlusion of the cerebral aqueduct causes build up of CSF in the brain as it can’t circulate through the brainstem into the spine. This causes hydrocephalus. This can be congenital or acquired. If it occurs in the womb, it is usually as a result of some kind of mishap in development which blocks it. The CSF builds up in the babies’ skull and as the skeleton is still ‘soft’, the intracranial pressure of CSF buildup causes expansion of the brain and the skull, leading to very large headed babies being born. Acquired hydrocephalus occurs in a developed skull, which means that the skull cannot expand so the brain is compressed, leading to compression of the meninges and intense pain. Left untreated, this can irreversibly damage the brain and even cause death. This can be treated by inserting a catheter into the ventricles which then drains the fluid artificially. Acquired hydrocephalus can occur from tumours compressing the cerebral aqueduct. In particular, pineal gland tumours are liable to cause this due to their position.

An MRI of a large pineal tumor causing occlusion of the cerebral aqueduct.

So there you go. The depths of the nervous system contain more than just neurons.

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