Sometimes, the biggest of mysteries are found in the smallest of places. While we can largely explain how the ear allows us to perceive sounds, we have never been able to pinpoint the function of a certain set of cells in the inner ear, the type II neurons. Now, scientists are starting to find answers.
In the inner ear, the snail-shaped cochlea converts incoming sound waves into signals that the brain can understand. The cochlea contains cells with hairs that bend in response to the sound waves that vibrate nearby membranes.
Type I neurons make up 90 to 95 percent of the cochlear nerve and usually come in contact with the inner hair cells, eventually shuttling signals to the brain. The type II neurons have been more mysterious. For a long time, little was known about them, except that they were typically associated with the outer hair cells.
A team of Hopkins scientists, perhaps for the first time ever, has been able to document the electrical activity of the type II neurons. Through their observations and measurements, they have concluded that like the type I neurons, these type II cells also transmit signals from the ear to the brain and are excited by the transmitter glutamate. In addition, type II neurons can also be excited by ATP.
The researchers experimented by removing live tissue from the cochlea of week-old rats and observing the response of type II neurons to different stimuli under a powerful microscope. Unlike type I neurons, which can be activated by the quietest of sounds, type II neurons were shown to require extremely loud and even painful sounds to become electrically excited.
"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," Paul Fuchs, who co-authored the report, said in a press release. Fuchs is co-director of the Center for Sensory Biology in the Hopkins School of Medicine. "Type II afferents may play a role in such reflexive withdrawals from potential trauma."
Another interesting difference is that while all neurons in the cochlea have dendrites or little projections that come in contact with sensory hair cells, type II neurons have projections that contact dozens of hair cells over a great distance, while type I neurons have a dendrite that touches a sensory cell in just one spot.
"Somewhat counter-intuitively, the type II cell that contacts many hair cells receives surprisingly little synaptic input," Fuchs said. "In fact, all of its many contacts put together yield less input than that provided by the one single hair cell touching a type I neuron."
Whereas at one time it did not seem feasible to examine the minuscule type II neurons of the inner ear, we are now beginning to discover their unique qualities and just how big of a role they may really play.