The brain undergoes some physical changes with advancing age. It's likely that these physical changes account for at least some of the noticeable cognitive changes. But discovering the exact links has so far proved elusive. Nonetheless, given the evidence accumulated thus far, some theories have been proposed, and more research is being conducted. To date, only a few studies have attempted to make a direct correlation between the physiologic changes of the brain and the cognitive and other effects.

The brain is composed of numerous structures, each composed of nerve cells (also called neurons) and supporting cells called glia. Nerve cells transmit electrical and chemical signals, and this transmission of signals between neurons underlies cognition. Three basic types of glia also exist in the brian:

Oligodendroglia cells wrap projections, termed axons, from neurons with myelin. The myelin thus forms a sheath around the axon facilitating the transmission of electrical signals from one neuron to another.

Astroglia cells are found throughout the brain and play many different "housekeeping" roles, providing nourishment and protection for the neurons.

Microglial cells ar are important in the brain's injury response as they scavenge cellular debris and help to clean up damage.

In addition to the work of these cells, brain functioning depends on a complex interaction between the brain regions. The cortex, which is the outer coating where higher mental activities occur, is divided into several lobes, which are further subdivided into regions. Physical changes that take place in the brain affect some areas while leaving others intact.

Brain shrinkage
Lost connections
Plasticity
Theoretical links

Brain shrinkage
Throughout adulthood, there is a gradual reduction in the weight and volume of the brain. This decline is about 2% per decade. Contrary to previously held beliefs, the decline does not accelerate after the age of 50, but continues at about the same pace from early adulthood on. The accumulative effects of this are generally not noticed until older age.

It has long been thought that the reason for brain shrinkage is the loss of neurons. Some past studies estimated that adults lose as many as 100,000 neurons a day. However, improved testing techniques have revealed that the actual loss of neurons is far less significant than previously thought. While some brain cells are lost, the reduction in brain volume is more a function of the neurons themselves shrinking in size, making them less effective messengers.

While the brain does shrink in size, it does not do so uniformly. Certain structures are more prone to shrinkage. For example, the hippocampus and the frontal lobes, two structures involved in memory, often become smaller. This is partly due to a loss of neurons and partly due to the atrophy of some neurons. Many other brain structures suffer no loss in size.

In a study published in The Lancet in 2001, researchers from the Institute of Neurology at the University College of London reported results from a small study that used a technique called serial magnetic resonance imaging (serial MRI) to look at certain brain regions in a group of four people with a genetic mutation that predisposed them to Alzheimers disease. At the beginning of the study, none of these people had Alzheimers' symptoms, but all developed the disease during the next five-eight years. Researchers determined that in the pre-symptomatic patients, compared to a group of 20 age-matched, disease-free participants, there was significant atrophy in the medial temporal lobes, as well as losses in two other brain regions. With additional testing, it is hoped that this kind of technique could be used to diagnose early stage or pre-clinical Alzheimers and to monitor the effects of various therapeutic interventions for Alzheimers.

Lost connections
The complexity of the brain is due in part to the intricate system of interconnections between neurons in the different parts of the brain. Neurons communicate with one another via specialized chemicals called neurotransmitters, of which there are several. Changes in this network of communication may account for some of the cognitive changes seen with age. Some of the connections may be lost, and new connections may not be made as readily. In addition, levels of two neurotransmitters (acetylcholine and dopamine) are thought to decline with age.

Plasticity
On the bright side, the brain has a great capacity for adaptation, modification, and repair. The term plasticity refers to the ability of the brain to modify its structure and function. This capability continues throughout life. For one thing, there is a certain amount of redundancy in the brain. If one network of neurons is damaged or dies, another network can take over the function. The death of neurons can also be compensated for by surrounding neurons sprouting new connections to take the place of the lost ones.

The brain is a dynamic, not a static, system. The neurons respond to mental stimulation and environmental factors. And there is the capacity to respond to age-related changes. It appears likely that cognitive changes are noticed at a point when the compensatory mechanisms of the brain are unable to overcome physical changes taking place. There may be strategies, either with medications, mental exercises, or something else, to enhance the brain's natural capacity for plasticity and thus forestall cognitive declines associated with aging.

Theoretical links
The brains of older adults show a patchwork pattern of declines and preservation. Given the complexity of the changes, it is unlikely that only one factor is responsible for the changes in mental activity that accompany aging. It's more likely a combination of factors. However, some theories have been proposed for some of the changes seen.

The slowing of mental processing may be caused by the deterioration of neurons, whether they are lost, shrink, or lose connections. This depletion of fully functioning neurons makes it necessary to recruit additional networks of neurons to manage mental tasks that would otherwise be simple or automatic. Thus, the process is slowed down.

A portion of the frontal lobe, called the prefrontal cortex, is involved in monitoring and controlling thoughts and actions. The atrophy that occurs in this brain region may account for the word finding difficulties many older adults experience. It may also account for forgetting where the car keys were put or general absentmindedness.

The shrinkage of both the frontal lobe and the hippocampus are thought to be responsible for memory difficulties.

Many of the neurons involved in motor function utilize the neurotransmitter dopamine. And dopamine is one of the neurotransmitters that declines with age. Therefore, it's been suggested that the effects of aging on dopamine account for some of the decline in motor function. Specifically how this would happen has not been discovered.

Several researchers have suggested that high blood pressure in the brain may be responsible for some of the detrimental effects.