Audition, Neural Basis

Peripheral Processing

Hearing in terrestrial vertebrates begins as sound waves strike the head and body. Sound waves can be thought to be composed of two parameters: frequency (pitch) and intensity (loudness). The outer ear, or pinna, plays a key role in shaping the acoustic stimulus as it enters the ear canal and travels to the tympanic membrane (eardrum). The unique shape of the pinna causes different frequencies of the sound to be reflected and resonated, causing some frequencies to be amplified and others to be dampened. This can result in a quite different sound that reaches the tympanic membrane compared to what was in the air. These cues are important for localizing sounds, particularly in elevation, and are the reason that sounds seem to originate from inside the head when using earphones, where the amplification and dampening does not occur.

The vibrations of the tympanic membrane are amplified by the three ossicles (small bones) of the inner ear, which then converts these vibrations into a traveling wave within the cochlea, much like a pebble dropped into a pond creates waves toward the shore. The cochlea is a spiral structure within the temporal bone that contains the mechanical and neural elements that allow transduction of the sound energy into neural signals. This is done by the traveling wave moving the basilar membrane, on which sit the sensory receptors, the hair cells. The basilar membrane is cleverly designed such that the motion is greatest for low frequency sounds at one end, and the location of the peak motion moves from one end to the other as the frequency increases. Thus, there are specific hair cells that will be activated the most for each particular frequency that the person or animal is sensitive to. There are two classes of hair cells: the outer hair cells and the inner hair cells. The inner hair cells provide the sensory information to the brain, whereas outer hair cells influence how the basilar membrane moves and do not provide any [Page 86]sensory information. The inner hair cells release the neurotransmitter glutamate onto an afferent neuron called a spiral ganglion cell (SGC), so named because the cell body of this neuron is located in the spiral ganglion. The SGCs are the first to generate action potentials, and there are approximately 20 SGCs for each sensory hair cell. The axons of these afferents combine to form the auditory-vestibular, or eighth, nerve, which then enters the skull and makes contact with the cochlear nucleus of the brain stem.

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