Sound is a funny thing. Sometimes, even when you think the room is way too loud, you can still make yourself heard. A team based at the University of Buffalo and Johns Hopkins University School of Medicine might have discovered the reason. It seems your ears are pretty smart. In non-human experiments, it has been revealed that the process by which sound is transferred from the ear to the brain can change in direct response to the level of background noise.
The study, which was published this week in the scientific journal Proceedings of the National Academy of Sciences, describes how cells in the auditory nerve change both their behavior and structure when they are they are exposed to loud volumes for long periods. It is thought that this flexibility is an evolutionary advantage which allows the ear to hear the important stuff — like communications from others — even when environmental noise levels are raised. Accordingly, when sound levels return to normal, so do the functions of the ear.
"The brain is amazingly adaptable," says Matthew Xu-Friedman, associate professor of biological sciences at UB, "the way it receives information can change to accommodate for different conditions, and this is what we see in our research." Xu-Friedman's team conducted their tests on mice in order to study the effects of living in a noisy environment for a week. The team was keen to stress that the mice were only exposed to "nondamaging noise."
When sound waves enter the ear, they cause the eardrum to vibrate. These vibrations pass through three bones of the middle ear and cause waves in the fluid of the inner ear. In turn, the fluid bends thousands of delicate hair-like cells, the movement of which triggers nerve impulses. These signals are carried to the brain by the auditory nerve where they are interpreted as sound. The key to the new study is the way "sound" is passed along the auditory nerve by neurotransmitters — chemicals which form a vital chain of communication to the brain, linking one synapse to the next.
The problem is that each cell only has a limited supply of the chemical. When the ear is bombarded with a wide range of frequencies or noise at high volume — such as at a rock concert or in an industrial work environment — the supply can run out and the ear loses its hotline to the brain. Xu-Friedman believes that the adaptations that mice exhibited in the new study are geared toward addressing this complication.
It was discovered that the mice used in Xu-Friedman's experiments had a cunning solution to their new environment. When exposed to noise similar to that made by a lawn mower or hair dryer, the animals' auditory nerve cells became more economical with the chemical transmitter, using less of it in response to loud sounds when compared to animals in quieter habitats. They preserved the neurotransmitter in order to transmit any new sound information. "The changes could help the animals deal with loud conditions and not go deaf," Xu-Friedman said. "Instead of draining your limited supply, you save some of it so you can continue processing new stimuli."
As well as altering their behavior, the animals' auditory nerve cells also changed their structure by enlarging their synaptic endings. The synapse is the region of the cells where neurotransmitters are stored, and the increase in size, says Xu-Friedman, implies that the cells were "upping their inventories of the chemicals."
As was expected, when the mice were placed back into a quiet environment, their ear structure and behavior returned to normal. The team says the auditory nerve cells of the mice began, once again, releasing neurotransmitters at a level comparable to those of mice who had always led a quiet life. The team says that their results show the brain's adaptability to changes in environment.
"What we see is that the cells in the auditory nerve adjust. They change themselves so they can respond to a different, heightened level of activity," says Xu-Friedman. "If the brain needs to process information under many different conditions, it's helpful if there's a set of rules to follow, ways to behave when activity is high and when activity is low. That appears to be happening with regard to these cells in the auditory nerve."
No matter how good your ears might be at keeping a little neurotransmitter in reserve, always protect your hearing in noisy environments. NoisyPlanet has some pretty sound advice with regard to avoiding noise-induced hearing loss. Try to keep things turned down for you and your loved ones and, if you're going to be near something noisy, use some protection in the form of plugs or ear defenders. You hear me?