How Is The Brain Protected From Dangerous Organisms?

The brain we all know is a wonder of nature and still holds many secrets yet to be discovered. It can also be vulnerable to attack from pathogenic organisms and disease. However, one of the brain's most enigmatic features is how it can protect itself against these harmful invasions. The main barrier against dangerous organisms and disease is the blood-brain barrier.

According to Science Daily:

"The blood-brain barrier -- the filter that governs what can and cannot come into contact with the mammalian brain -- is a marvel of nature. It effectively separates circulating blood from the fluid that bathes the brain, and it keeps out bacteria, viruses and other agents that could damage it."

However, there is much about this 'marvel of nature' that is still not fully understood. Nevertheless, we can get a good grasp of how the brain protects itself from the knowledge that has been discovered.

Brain protection starts with a number of layers. Beginning from the outside in, we'll look at these defensive shields and the job they do.

The brain is encased in the skin layer of the scalp and the skull that not only guards the brain itself from trauma and organisms, but both create a shield for other protective devices around the brain.

The Meninges

There are three of these protective layers. The meninges are better known through the medical condition - meningitis - which is an infection of these layers. The three meninges are called the:

1. Dura mater - this is a thick layer that prevents the brain from being thrown around within the skull. This layer is found all around the brain.
2. Arachnoid mater - sits between the dura and pia. As well as protection this area helps to move cerebrospinal fluid back into the blood stream.
3. Pia Mater - this is the inner most layer. This is the only membrane that actually touches the brain itself.

Cerebrospinal fluid (CSF)

Moving within and around the brain is cerebrospinal fluid (CSF). This is a clear liquid that is produced in areas of the brain called ventricles. The main functions of this fluid are:

1. It acts as a cushion for delicate brain structures
2. Brings nutrients to the brain
3. Removes waste matter away from the brain

CSF is often used to diagnose medical conditions especially of the nervous system, such as multiple sclerosis.

However the main protective structure of brain that prevents dangerous organisms from entering is the blood-brain barrier (BBB).

The blood-brain barrier (BBB)

This is a semi-permeable membrane that protects the brain by allowing some substances to get through, such as nutrients, but not others - for example bacteria.

Within the body there are endothelial cells that have spaces in between, lining the tiny blood capillaries that allow the exchange of many chemicals. Within the brain, the endothelial cells that form the BBB are packed very tightly together making it much more difficult for substances to pass through. Many of the materials needed by the brain for health, have special carriers that get them through the barrier.

In addition, glial cells or astrocytes surround the blood vessels in the brain forming a lining. It is thought that these cells are both responsible for constructing the blood-brain barrier and for the transportation of ions out of the brain into the blood stream.

The blood brain barrier's main functions are:

• Protecting the brain from harmful organisms or substances that may try to enter.
• Hormones and neurotransmitters that circulate around the body could potentially harm the brain, so the BBB keeps them out.
• The blood-brain barrier is also responsible for maintaining the correct and a healthy environment for the brain to function properly.

There are conditions that can harm the blood brain barrier and cause it to malfunction:

1. Exposure - if the brain is exposed to things such as radiation and microwaves this can begin to break down the blood-brain barrier so allowing other harmful substances and organisms to enter.
2. Birth - newly born babies can be at risk from infection in particular because the blood brain barrier is not yet fully formed.
3. Infection - if the barrier is exposed to infectious agents they can begin to weaken it.
4. Hypertension (high blood pressure) - this condition causes weakness in the barrier causing it to open up.
5. Hyperosmolity: a high concentration of certain substances in the blood will open up the barrier. Osmolity is the steady state of water and salt content in our body. Hyperosmolity is the abnormal increase in osmolity of a body fluid.
6. Medical conditions - there are certain medical conditions or injuries that weaken and allow the blood-brain barrier to open up, such as - trauma, ischaemia, inflammation and pressure.

Apart from the situations previously mentioned that can weaken the BBB, there are additional circumstances where this normally formidable barrier is breached. For example in medical conditions such as bacterial meningitis - an inflammation of the meninges covering the brain - the blood-brain barrier can become disrupted allowing substances to pass through that wouldn't normally. In addition some viruses and bacteria can sometimes infiltrate the brain by being transported in infected immune cells.

Other conditions that can cause damage to the BBB are:

• Encephalitis
• Strokes
• Tumours
• Multiple sclerosis

The difficulty doctors and pharmacists have is finding drugs that will be accepted by the BBB so that they can fight diseases and infections that affect the brain. Although there are a few that have been developed that can do this. The problem with drugs is their size. Some medicines that have been produced such as for epilepsy have small enough molecules that can pass through the barrier. However, many drugs have very large molecules that just can't get in. Interestingly viruses also have very large molecules and bacteria are even bigger. Therefore, the protection put in place for the brain, can also prevent treatment being given.

However, the infections by bacteria and viruses, although we've heard of most of them, are still rare and will continue to be so, as long as we have the formidable shield of the blood-brain barrier. In addition, medical science is advancing constantly, so perhaps it's just a matter of time before all the secrets of the brain's protective devices are revealed.