Scientists at Johns Hopkins have discovered the function of a group of nerve fibers that originates in the brain stem and fires off signals even before newborn rats can hear. The signals may be a process for wiring the ears and brain for hearing, much like laying down circuits on a computer chip.
The study, published in the June 30 issue of Science, moves researchers a step closer to understanding the development of human and mammalian hearing.
Rats are born deaf and do not hear for the first two weeks of life. It is during this time, according to the Hopkins researchers, that the brain appears to provide a feedback mechanism onto the sensory organ that may turn the system on and off. Scientists have found a similar process in the eyes and brains of newborn cats, who are born blind.
Specifically, the investigators report the identification of a working "inhibitory synapse," a nerve cell communications pathway, between neurons in the brain and hair cells in a part of the inner ear known as the cochlea. They also developed a method to directly record this synaptic activity, a chemical signal released from a neuron that activates an electric impulse in the hair cell.
"Even before an animal can make sense out of sound, we have long surmised that as with the visual system's connection between eye and brain, there is some spontaneous activity that allows for the development of more hard-wired connections between the brain and the hair cells," said Elisabeth Glowatzki, Ph.D., lead author of the article. "This has to occur because it's the hair cells of the cochlea (the spiral tube in the inner ear containing nerve cells) that respond to sound, but the brain that feeds signals back through the pathway to regulate the sensory information," she added.
In the past, scientists found no evidence that these synapses have a particular purpose before the animals begin hearing. The Hopkins work, however, suggests that they play a role in the development as a feedback system will organize hearing about two weeks after birth.
"What's unusual and striking is that that is happening before the animal hears. That says that this isn't only there as a way of filtering the world in real time, it must also play some complex role in setting up the relationship between the sense organ and the brain," says Paul Fuchs, Ph.D., professor of otolaryngology and co-author of the work.
Working with the cochlea surgically removed from newborn rats, Glowatzki placed electrodes directly on the hair cells to record electrical activity and found that the neurons from the brain release the neurotransmitter acetylcholine, which inhibits or turns off the hair cell's signal to the brain.
"The purpose of this inhibition may be that it imposes some kind of rhythmicity or pattern onto the developing auditory pathway," she said.
The Hopkins researchers plan to use this information to continue to map out the activities of the auditory system.
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