Interpreting the cadences of human speech requires that information be transmitted from the ear to the brain with exquisitely precise timing. Neurons in general are no slackers at transmitting signals, but specialized neurons in the ear known as inner hair cells perform at an even higher level. Two papers published this week offer insights into how these cells respond so speedily.
Neurons store neurotransmitters, molecules that serve as the bits and bytes of communication, within small membrane-bound compartments called synaptic vesicles. Upon stimulation, the vesicles fuse with the outer membrane of the neuron and release neurotransmitters that excite the next neuron in line. During signaling, neurons need to constantly replenish their supply of vesicles. Most neurons make new vesicles using bits of recycled cell membrane, a relatively slow process.
In a study published online in Nature today, physiologist Claudius Griesinger of University College London and colleagues show that hair cells rev up this process by making vesicles from scratch and storing them in the cytoplasm, instead of culling them from the membrane. The preformed vesicles are then shipped to the presynaptic ribbon, a structure that organizes vesicles near the site of their release. This difference helps maintain a seemingly inexhaustible supply of vesicles while sustaining a vesicle release rate 100 times higher than that of a conventional neuron, say the researchers. Another paper in Nature, by Tobias Moser of the University of Goettingen, Germany, and colleagues, suggests that ribbons enable the release of multiple vesicles in parallel during the initial burst of signaling by a stimulated hair cell.
“Both of these studies further emphasize the remarkable properties of synaptic ribbons of hair cells," says Paul Fuchs, a neuroscientist at Johns Hopkins School of Medicine in Baltimore, Maryland. "These tiny vesicular vending machines greatly out-perform other chemical synapses in their release capacity.” Moser notes that this work also has potential implications for sensorineural hearing loss. This disorder, in which people can often hear sounds but cannot decipher speech, is sometimes caused by problems with hair cells or ribbons.