THE SPINAL CORD

The anatomy

Astonishingly as it may seem the spinal cord is only 45cms in length and about 12mm or half an inch thick.

The spinal cord and its exiting nerves are encased by three separate layers of membranes also called meninges From the outside inwards these are: the dura mater, the arachnoid mater and the pia mater.

The pia mater is closely applied to the spinal cord and carries the blood vessels.

Between it and the arachnoid mater there is a space called the sub-arachnoid space, containing the cerebrospinal fluid.

The Arachnoid membrane is a web-like lining, covering the spinal cord and the nerves arising from it.

The arachnoid itself is covered by another much thicker and stronger layer called the dura, meaning 'tough' in Latin.

The nerves they cover, run down to the bottom of the spine, in the shape of a horse's tail, hence known as 'Cauda Equina'.

The arachnoid membrane is a fine, transparent and fragile layer.

Interestingly, it has no blood supply of its own. It does not have a circulatory system, to bring nutrients or a clearance system to deal with unwanted metabolic end-products. However it lies very close to both the pia mater and the dura mater which is richly supplied with blood vessels.

The physiology
What about its functions?

The best know function is that of a liquid buffer. By enclosing the cerebrospinal fluid, which is continuous with the cerebrospinal fluid in the brain, it allows pressure on the brain and the spinal cord to be kept equal at all times and at all parts.

It also acts as a cushion, against impact, local trauma and sudden movement.

There are other significant but less well known functions.

By directly bathing the nerve roots it allows for diffusion of nutrients to the nerves and for waste-products of nerve activity to be removed.

It allows for lymphatic drainage from larger blood vessels going along nerves.

More recent evidence shows that in fact up to 50% of the nutrition of a nerve root may be derived from this cerebrospinal fluid.

In addition the cerebrospinal fluid could act as a transport media for a number of neurochemicals acting at a distance from their release source.

Finally, despite its lack of a blood supply, and thus lack of white blood cells to fight infections and foreign bodies, new studies have shown that this fragile membrane does have the ability to indicate to the Immune system (the body's defence forces) when to initiate an attack on foreign bodies lying on it.

Therefore it is little wonder that when this highly functional but fragile membrane is affected that so much can go wrong.

The pathophysiology
What happens if these functions are affected?

If there is a block to the normal flow of the cerebrospinal fluid as a result of narrowing in the subarachnoid space, and this block is in the lower part of the spine, also known as the caudal cul-de-sac, there will be increased pressure here, causing pain and weakness.

If due to the block the cerebrospinal fluid can not move about and replenish itself of nutrients and get rid of waste-products, there may be dysfunction of the nerve roots, also known as radiculo-ischemia.

This may cause pins and needles, numbness, weakness and or pain.

Again the release of certain neurochemicals from distant areas may not get to the nerve roots if the block is complete.

Because the cerebrospinal fluid pressure helps maintain the arterial pressure of vessels supplying the nerves and allows drainage of lymph from the lymphatics along the veins, this function may be affected, leading to venous congestion and decreased arterial (fresh) blood supply to the nerve root, thus compromising it.