No one knows exactly why dogs or humans decline in their mental abilities when they age. One theory suggests that as the genetic material (DNA) reproduces itself in each new cell, the successive transcriptions become less accurate, sort of like making copies of copies of copies on a photocopier, where each one gets progressively grainier and harder to read. Damage to the DNA can also come about due to natural radiation from cosmic rays and from more terrestrial sources, such as breathing in air pollutants or fumes from certain solvents. Other theories of aging blame simple wear and tear, suggesting that various physical and neural systems break down from frequent use and may break down even faster if they are put under stress.
Regardless of the source of aging effects, the brain and nervous system of dogs (and people) change markedly as they age. Old dogs have smaller, lighter brains than young dogs. The change is quite significant and the older brain might be up to 25 percent lighter. It is important to note that this change is not necessarily due to brain cells dying off. Actually, we mostly lose parts of the nerve cells, the branches (dendrites and axon filaments) that connect with other nerve cells. These connections to other cells start to break down with age. If we think of the brain as a complexly wired computer, it would be as if various circuits in the central processor simply stopped functioning because connections were broken. For the most part, it is the loss of these connections that reduces the size and the weight of the brain.
With age, there are also chemical changes occurring in the brain that affect behaviour, memory, and learning. In dogs and humans, the mitochondria, little strand-like structures in the nucleus of cells that are responsible for converting nutrients into energy, begin to release “free radicals,” chemicals that oxidize compounds essential for normal cell function. The loss of these compounds places the cell at risk. As the tissues degenerate, protein deposits called amyloids accumulate in the brain. High levels of amyloids, especially when associated with clusters of dead and dying nerve cells, are taken as part of the evidence that an individual is suffering from Alzheimer’s disease. Physical evidence, found only in autopsies, reveals similar degenerative brain lesions in aging dogs and aging humans. Studies conducted at the University of Toronto by a team of researchers including psychologist, Norton Milgram, have shown that dogs with high levels of amyloids in their brains have poorer memories and difficulty learning new material, especially if it involves more complex thinking and problem solving. This equivalent to Alzheimer’s disease in dogs is called Canine Cognitive Dysfunction Syndrome.
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