Fire alarms. Thunder claps. Explosions. These jarring noises make us jump out of our seats and cause our hearts to race. Our ability to respond immediately to such startling stimuli—and yet still appreciate the soft sound of a gentle breeze or a baby breathing—reflects an ingenious design of the human ear.
The cochlea is the central part of the human ear that permits the detection of sounds. Stereocilia, hair-like structures within the cochlea, respond to the sound-induced movements of fluid in the ear. The bulk (90-95%) of the sensory neurons in the cochlea connect to just one particular hair-like structure, transmitting signals to the brain in response to sound input. These are labeled type I neurons and are well-understood by researchers. On the other hand, the function of the less abundant type II neuron has puzzled scientists. Recent research sheds light on the utility of type II neurons.
In contrast to type I neurons, type II neurons contact many individual stereocilia and the cells that support them. Though they exhibit greater networking, type II neurons have much smaller axons—the nerve fiber that transmits signals. The small axons make it difficult to detect any signals transmitted through them. Consequently, these neurons receive less input despite the greater degree of networking. Nevertheless, a team of neuroscientists were able to detect signals from type II neurons and determine the cause of the excitation. Their results highlight an important function of the human ear.
Type I neurons respond to very low levels of sound stimulation and transmit progressively greater signals as the sound level increases. Once the sound level reaches a threshold, the type I neuron output saturates. In contrast, the small and infrequent response of type II neurons indicates that they must receive a large sound input in order to be stimulated at all. Additionally, the presence of adenosine triphosphate, a nucleotide known release during tissue damage, stimulates type II neurons. Taken together, these properties indicate that type II neurons respond only to very loud, damaging sounds and likely serve to signal the presence of danger.
Without type I neurons, we would not be able to sense and distinguish the vast range of sounds we encounter. Without type II neurons we would not detect the danger that certain sounds portend. The mammalian ear seems optimally designed to allow humans to understand and appreciate the sounds that enrich life while ensuring quick responses to danger—all by analyzing the diversity of noises around us.