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on the hair cells will be most intense. B, The effect of frequency. Lower frequencies produce a maximal effect at the apex of the basilar membrane, where it is the widest and least stiff. Pure tones affect a single location, complex tones affect multiple loci. (Modified from von Bekesy G. Experiments in Hearing. New York: McGraw-Hill, 1960.)

can be separately distinguished. In the midrange of hearing (around 1,000 Hz), the human auditory system can sense a difference in frequency of as little as 3 Hz. The tonotopic organization of the basilar membrane has facilitated the invention of prosthetic devices whose aim is to provide some replacement of auditory function to people suffering from deafness that arises from severe malfunction of the middle or inner ear (see Clinical Focus Box 4.1).

Central Auditory Pathways. Nerve fibers from the cochlea enter the spiral ganglion of the organ of Corti, from there, fibers are sent to the dorsal and ventral cochlear nuclei. The complex pathway that finally ends at the auditory cortex in the superior portion of the temporal lobe of the brain involves several sets of synapses and considerable crossing over and intermediate processing. As with the eye, there is a spatial correlation between cells in the sensory organ and specific locations in the primary auditory cortex. In this case, the representation is called a tonotopic map, with different pitches being represented by different locations, even though the firing rates of the cells no longer correspond to the frequency of sound originally presented to the inner ear.

The Function of the Vestibular Apparatus. The ear also has important nonauditory sensory functions. The sensory receptors that allow us to maintain our equilibrium and balance are located in the vestibular apparatus, which consists (on each side of the head) of three semicircular canals and two otolithic organs, the utricle and the saccule (Fig. 4.23). These structures are located in the bony labyrinth of the temporal bone and are sometimes called the membranous labyrinth. As with hearing, the basic sensing elements are hair cells.

The semicircular canals, hoop-like tubular membranous structures, sense rotary acceleration and motion. Their interior is continuous with the scala media and is filled with endolymph,- on the outside, they are bathed by perilymph. The three canals on each side lie in three mutually perpendicular planes. With the head tipped forward by about 30 degrees, the horizontal (lateral) canal lies in the horizontal plane. At right angles to this are the planes of the anterior

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