by Anthony J. Brown, MD
Last Updated: 2007-10-31 14:03:00 -0400 (Reuters Health)
NEW YORK (Reuters Health) – New research indicates that non-nerve cells play a critical role in stimulating auditory nerve firing in the absence of sound, in preparation for the development of hearing.
"It was known that this ‘spontaneous’ activity helps auditory nerves make proper connections with other nerve cells in the brain, which enables the accurate encoding of sound; however, the trigger that initiates this activity was not known," senior author Dr. Dwight E. Bergles told Reuters Health. "We discovered that non-nerve cells in the developing inner ear stimulate electrical activity in nerves that carry sound information from the ear to the brain."
Dr. Bergles, from the Johns Hopkins School of Medicine in Baltimore, said the discovery that non-nerve cells were involved in the initial stimulatory activity was particularly surprising since it had been thought that these cells were merely bystanders.
The findings, which appear in the November 1st issue of Nature, are based on a series of experiments conducted on developing rat cochlea.
The authors found that supporting cells in the organ of Corti spontaneously release ATP, which leads to the release of glutamate by nearby inner hair cells. This, in turn, triggers action potentials in primary auditory neurons. After hearing onset, this ATP-dependent signaling quickly ceases so that accurate encoding of sound can occur.
Prior to the current study, "it was assumed that the ‘spontaneous’ activity observed in developing auditory nerves was entirely due to the intrinsic activity of hair cells. Our findings indicate that the ATP is a major extrinsic influence on these sensory cells," Dr. Bergles said.
As for clinical implications, he said that the current findings may suggest an underlying mechanism for tinnitus. Namely, if support cells begin to release ATP (possibly after some injury), perception of "ringing in the ears" might occur in the absence of external stimuli.