Brain neurons blamed for dizzy spells.

Brain neurons blamed for dizzy spells

In a bygone era, fainting or dizzy spells signified a response to great passion or trauma. Modern science has since taken the romance out of these episodes, ascribing many of them to the mundane activity of standing up too quickly.

When a person stands up, a complex system called the baroreflex kicks into action: Nerve endings in key blood vessels detect the resulting drop in pressure and send a message to brain neurons, which respond by boosting blood pressure. Scientists know that the baroreflex often begins to fail with age, leading to dizziness, fainting spells and even stroke. But the exact nature of this failure has remained elusive.

George Hajduczok, a physiologist at the University of Iowa in Iowa City, now reports evidence suggesting that the blame lies with malfunctioning neurons. He and his colleagues studied 11 elderly purebred beagles and five young beagles. They increased blood pressure within the dogs' carotid arteries, which contain pressure-sensing nerve endings. Young dogs quickly sensed that change and lowered the activity of the sympathetic nervous system -- which boosts blood pressure -- by 70 percent, whereas elderly beagles damped such activity by only 20 percent, the researchers found.

A separate experiment showed that elderly dogs can lower sympathetic nervous system activity as much as young dogs, but only for a few seconds -- a finding that suggests the problem lies with the brain neurons rather than the nerve endings.

Such studies may help explain why some elderly people experience dangerous swings in blood pressure. A faulty baroreflex, says Hajduczok, can cause fainting when pressure dips too low, or a stroke when pressure soars too high.