Scientists believe they may have solved the mystery of why some people stop breathing fatally in their sleep.
They say a cumulative loss of cells in the area of the brain that controls breathing is to blame - triggering a condition called central sleep apnoea.
However, they believe many such deaths in elderly people are misdiagnosed as heart failure.
The study, by the University of California, Los Angeles, is published in Nature Neuroscience.
Central sleep apnoea: Triggered by problems with the brain's breathing centre Obstructive sleep apnoea: Breathing stops when the airway collapses
They had also identified a small group of cells within this area as being responsible for issuing the commands, reports BBC News.
According to Daily Mail, researchers focused on a brainstem region called the preBotzinger complex (preBotC) which contains specialised neurons that trigger breathing. Rats were injected with a chemical designed to target and kill more than half the preBotC neurons.
The results were dramatic. Breathing stopped completely when the rats entered REM sleep - the mentally active phase of sleep characterised by dreaming - forcing the animals to wake up. Over time, the breathing lapses increased in severity and spread into non-REM, deeper sleep. Eventually they occurred when the rats were awake as well.
The US scientists believe the findings, reported in the online edition of the journal Nature Neuroscience, are relevant to humans. Mammalian brains are all organised in a similar fashion. Rats have about 600 of the specialised preBotC cells, and humans are thought to have a few thousand. The cells are lost as part of the ageing process, and not renewed.
Dr Jack Feldman, who led the team from the University of California at Los Angeles, said: "Our research suggests that the preBotzinger complex contains a fixed number of neurons that we lose as we age. Essentially, we sped up these cells' ageing process in the rats over several days instead of a lifetime."
Long before the rats had difficulty breathing when awake, they developed breathing problems during sleep. Dr Feldman suspects the same thing happens as people grow older. He added: "We speculate that our brains can compensate for up to a 60% loss of preBotC cells, but the cumulative deficit of these brain cells eventually disrupts our breathing during sleep. There's no biological reason for the body to maintain these cells beyond the average lifespan, and so they do not replenish as we age."
The scientists suspect central sleep apnea also strikes people suffering the late stages of neurodegenerative disorders, such as Parkinson's disease, Lou Gehrig's disease and multiple system atrophy, all serious conditions that lead to movement problems. "People with these diseases breathe normally when they are awake, but many of them have breathing difficulties during sleep," said Wiktor Janczewski, assistant researcher in neurobiology. "When central sleep apnea strikes, they are already very ill and their sleep-disordered breathing may go unnoticed. "As the patients grow sicker, their nighttime threshold for wakefulness rises," he added. "Eventually, their bodies reach a point when they are unable to rouse themselves from sleep when they stop breathing, and they die from lack of oxygen."
The UCLA team will repeat their research with elderly rats in order to learn why central sleep apnea first strikes during REM sleep. The group also plans to analyze the brains of people who die from neurodegenerative diseases to determine whether these patients show damage in their preBцtzinger complexes.
The National Heart, Lung and Blood Institute funded the research, informs Medical News.
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