The brain is essentially a post-mitotic organ. That is science-speak that can be loosely translated to mean each nerve cell (also called a neuron) is as old as the owner of the brain. Many of the other organs of the body produce individual cells on a continuous basis. For example, the lining cells of the intestinal tract are recycled about every 3 to 5 days. Because of this, 'aging' has a very different meaning to a cell that lives for a week than it does for cells that must survive for 80 or 90 years. Neurons live a long time and are exposed to numerous dents and dings every day. They must also contend with insults that don't individually affect cell function in any meaningful fashion, but when such 'sub-threshold' changes accumulate over time, they collectively conspire to produce alterations that very dramatically impair neuronal characteristics. One example of this type of insult is called oxidation. That is what happens when a nail rusts. The same process can affect living cells and tissues by chemically altering vital structural attributes subsequently producing detrimental functional implications. When nerve cell membranes are oxidized, they become stiffer. There are proteins imbedded within the membranes that act as 'receptors' for molecules that enable nerve cells to communicate. These molecules are called neurotransmitters and go by the familiar names serotonin (important in mood disorders), dopamine (the 'feel-good' transmitter), and norepinephrine (for focus and attention). There are many others, more than 50 all together, that serve a variety of uses. For them to do their job, they must attach to a receptor molecule that lives in the membrane. When this pair (neurotransmitter-receptor) is formed, it changes shape. A mental picture of this process might be a key in a lock. After being inserted, the key is turned which opens the lock. Some locks can become 'sticky' or even 'frozen.' When this occurs, the key may be inserted without unlocking the lock. A similar process can develop in neuronal membranes. When they are oxidized, they get stiff and receptors respond less actively when bound to a neurotransmitter. Since this process is the basis for nerve cell communication, when it is impaired brain function is compromised. This is one mechanism that is associated with aging and contributes to memory alterations, for example. Short-lived cells are not around long enough for the buildup of multiple oxidative changes to occur, which takes longer than the 5 days these cells live. 

Neurons must also contend with more catastrophic insults such as strokes, traumatic events, and hemorrhages. These tend to produce damage in a local, isolated region of the brain. They tend to kill cells at the center, leave cells at a distance alone, and produce an intermediate zone of cells that are alive but not functioning optimally.

If part of another organ, such as the liver, is lost, the remaining cells take over and cover for their lost compatriots because each cell in these organs does essentially the same thing. The brain is quite different in this regard. No two neurons are identical and different regions of the brain are wired differently and provide different functions. Given these constraints, how is it possible to 'recover' lost function, and how does the brain cope with chronic disease such as that which accumulates as we age? In other words, what is the brain's backup plan?  

Generally speaking, there are several processes involved. The sick nerve cells can be made healthier. Other functioning nerve cells can learn to compensate for their neighbors who have died, and new nerve cells can be formed. To understand how the brain addresses such problems, it is helpful to consider a simpler example. For this I will use a severed nerve in the hand whose job it is to provide sensation to a specific area of skin. When such a peripheral nerve is cut, several things happen. You lose sensation to a portion of the skin. The body detects this and initiates a response. New nerve cells start to grow out from the spinal cord and the surrounding nerves in the hand start to respond. The nerves supplying touch sensation to contiguous areas of skin realize the presence of the damaged nerve nearby and start to grow and expand into the 'denervated' numb spot. Various nerve growth factors and other chemicals and electrical signals orchestrate this complex process.

The brain reacts in a similar fashion. Neighboring neurons sense the nearby injury and send out branches that integrate into the network of nerves that were injured. Because of the high degree of connectivity between one brain cell and thousands of others, many widely separated regions of the brain are put on notice. They start to reconfigure, or rewire, their connections in ways designed to return functions that were lost or decreased. This process can be visualized on special brain scans called fMRI scans (for functional Magnetic Resonance Imaging). When we perform certain tasks, whether it is moving a finger or adding 3+5, it requires brain activity in specific regions of the brain. This activity is identified by the fMRI scan and is identified as an area of the brain that 'lights up.' Certain tasks cause a portion of the left frontal lobe to light up. When this portion of the brain gets sick or is injured, it lights up less vigorously. However, in time, when the same task is undertaken, something interesting is seen on the fMRI scan. Nearby areas 'turn on' as do brain regions at a distance from the damaged area. Sometimes they show up as mirror image regions on the other side of the brain, in this case the right frontal lobe. Other times they show up as bright spots on the same side of the brain but in the left parietal or left occipital lobe region. These represent signs that the brain is compensating for the loss of function at another area. Again, a host of growth factors, proteins and electrical signals coordinate the process.

What is important to know is that we have a degree of control over what is happening. By training our brains we can facilitate this entire process. In part, this helps explain why different types of therapy work to help stroke patients. Physical activity of various sorts, unrelated to the area involved, acts to enhance the recovery response. You would think the mechanism would be merely augmented blood flow to the brain, yet there are many other pathways that are switched on in the brain by physical exercise of all sorts. They make nerve cells more resistant to future insults, speed up nerve communication, and increase nerve to nerve contact points. Proper sleep is restorative to healthy brains and has additional benefits for sick brains. Stress causes neurons to shrivel up; something that is counterproductive to a healing brain. Stress reduction facilitates brain health and healing. Dietary approaches that lower insulin levels and are anti-inflammatory reinforce each of the above mechanisms and healing pathways. So, the brain can do a lot to heal itself, but the power to turbocharge the response is in your hands!