Credit: C. Liberman/MEEI
On the left, the schematic shows an inner hair cell, with its hair bundle at one end and with three of its roughly 20 synapses with cochlear nerve terminals at the other end. The cell's nucleus is shown in red, and each synapse is marked by a smaller red dot. Immunostain recognizes a molecule present in the nucleus and also in a key synaptic component within the hair cell, called a "ribbon".
The images shown on the right were taken with a confocal microscope. The upper image shows eight inner hair cells from a normal ear (each cell has one large red nucleus) and about 150 small red dots (arrowheads), each of which shows a synapse between a hair cell and a cochlear nerve fiber. The bottom image shows another ear a few days after exposure to a noise that caused only a transient threshold elevation. The confocal image shows no loss of hair cells, but a striking loss of synapses, which will never recover.