Researchers have known for some time that 95 percent of the cells that shuttle sound to the brain are large, lively neurons found deep within the ear. These big boys have accounted for most of what scientists know about how hearing works. That said, however, scientist have also speculated that there could be a second set of rare, tiny cells that carry signals from the from the inner ear to the brain and play a part in how sound is processed.
A team of Johns Hopkins researchers report in the Oct. 22 issue of Nature that rat experiments have measured and recorded electrical activity of the type II neurons in the cochlea, snail-shell-like structure within the ear, for possibly the first time. Their research seems to confirm what had previously been posited that the cells do carry signals from the ear to the brain, and the sounds they likely respond to would need to be loud, such as sirens or alarms that might be even be described as painful or traumatic.
According to the researchers, they've also discovered that these cells perform by responding to glutamate, a neurotransmitter found throughout the nervous system that stimulate the cochlear to transmit acoustic information to the brain, released from sensory hair cells of the inner ear.
"No one thought recording them was even possible," says Paul A. Fuchs, Ph.D., the John E. Bordley Professor of Otolaryngology-Head and Neck Surgery and co-director of the Center for Sensory Biology in the Johns Hopkins University School of Medicine, and a co-author of the report. "We knew the type II neurons were there and now at last we know something about what they do and how they do it."
Working with week-old rats, neuroscience graduate student Catherine Weisz removed live, soft tissue from the fragile cochlea and, guided by a powerful microscope, touched electrodes to the tiny type II nerve endings beneath the sensory hair cells. Different types of stimuli were used to activate sensory hair cells, allowing Weisz to record and analyze the resulting signals in type II fibers.
Results showed that, unlike type I neurons which are electrically activated by the quietest sounds we hear, and which saturate as sounds get louder, each type II neuron would need to be hit hard by a very loud sound to produce excitation, Fuchs says.
Fuchs and his team postulate that the two systems may serve different functional roles. "There's a distinct difference between analyzing sound to extract meaning - Is that a cat meowing, a baby crying or a man singing? - versus the startle reflex triggered by a thunderclap or other sudden loud sound." Type II afferents may play a role in such reflexive withdrawals from potential trauma."
Source: Paul Fuchs, Ph.D., Johns Hopkins University School of Medicine
Writer: Walaika Haskins