How ageing affects the nervous system
Face facts! If you are over 20, you probably have two miles less wiring in your brain than this time yesterday. If you had a few drinks last night it may be less still. Luckily, you were born with around 100 billion nerve cells (neurons) so at first you can afford these losses.
The nervous system compensates by sprouting new connections from surviving neurons. Surviving motor neurons in our spinal cord, for example, more than double in size so that when some are lost during ageing we can still control our muscles.
Our brains function differently as we start to use different pathways. Our ability to learn might reduce, but we already know more than we used to.
However, somewhere around late middle age there is a small, but detectable decrease in cognitive ability, and if we are unlucky enough to develop a neurodegenerative disorder such as Alzheimer’s disease or motor neuron disease, we find that normal ageing has reduced our ability to compensate just when we need it most.
This is one reason why normal ageing is the biggest single risk factor for most neurodegenerative disorders.
What causes these changes?In most parts of our nervous system we only ever have the neurons we were born with. Each one that is lost is not replaced. More importantly, however, there are far greater losses in the wires that carry electrical impulses between neurons (axons), and their insulating sheaths (myelin) and in the brain and spinal cord these axons cannot regrow.
The result is rather like still having your phone but gradually shortening your contacts list.
The ability to reshape those connections, known as plasticity, also declines, so we find it harder to learn. “One lesson for every year of your age” is the rule of thumb for how many lessons are needed to pass a driving test. Our senses also decline as axons connecting our eyes, ears and skin to the brain reduce in number.
The fastest firing axons are especially vulnerable, which is one reason why our reaction times slow and we stop hearing highly pitched sounds.
What are scientists trying to do about it?Scientists have found clear evidence that dietary restriction in simple species such as flies and nematode worms extends lifespan and healthspan. This includes the health of the nervous system and resistance to age-related neurodegenerative disorders.
Initial studies now indicate that this is true in laboratory mice too. There is much research into the underlying mechanism, aiming to find other ways to achieve the same outcome since the degree of dietary restriction needed is considerable.
Other studies have shown that the birth of new neurons, which is restricted to some specific areas of our brains, is increased by exercise and this enhances cognitive ability.
Premature ageing models in laboratory mice help to further understand the underlying mechanisms and non-invasive imaging in humans is revealing changes in our brains and how we use them. The sequence of events in age-related neurodegenerative disorders is slowly being pieced together.