Cell Death in the Inner Ear

Abstract

The inner ear transduces sound energy and head motion into neural impulses. These signals are detected by six sensory patches in the fluid-filled spaces of the inner ear. The snail-shaped cochlea detects sound, whereas the vestibular system serves balance and gravity-detection functions. All six sensory patches in the inner ear use mechanosensory hair cells to transduce fluid motion signals into neurotransmitter release. These sensory cells are sensitive to death from noise trauma, aging, and certain therapeutic drugs. Hair cells in nonmammalian vertebrates are regenerated after they die, resulting in functional recovery of hearing and balance. In contrast, mammalian sensory hair cells are not regenerated, and their loss results in permanent hearing and/or balance disorders. Cochlear hair cells make synaptic connections with spiral ganglion neurons. Spiral ganglion neurons are bipolar cells with dendrites that synapse with the basal surfaces of hair cells and axons that comprise the eighth cranial nerve. Hair cells provide trophic support to spiral ganglion neurons. Therefore, death of hair cells is often followed by spiral ganglion neuron degeneration. Hearing loss is the most common sensory impairment in humans and is the sixth most common chronic health problem in the United States. This chapter addresses apoptotic death of sensory hair cells in response to ototoxic drugs and the subsequent death of spiral ganglion neurons (SGNs).