Auditory Nerve Pathways

The cochlear branches of the vestibulocochlear nerves enter the auditory nerve pathways that extend into the medulla oblongata and proceed through the midbrain to the thalamus. From there they pass into the auditory cortices of the temporal lobes of the cerebrum, where they are interpreted. On the way, some of these fibers cross over, so that impulses arising from each ear are interpreted on both sides of the brain. Consequently, damage to a temporal lobe on one side of the brain is not necessarily accompanied by complete hearing loss in the ear on that side (fig. 12.17).

Table 12.4 summarizes the pathway of vibrations through the parts of the middle and inner ears. Clinical Application 12.4 examines types of hearing loss.

Units called decibels (dB) measure sound intensity. The decibel scale begins at 0 dB, which is the intensity of the sound that is least perceptible by a normal human ear. The decibel scale is logarithmic, so a sound of 10 dB is 10 times as intense as the least perceptible sound; a sound of 20 dB is 100 times as intense; and a sound of 30 dB is 1,000 times as intense.

On the decibel scale, a whisper has an intensity of about 40 dB, normal conversation measures 60-70 dB, and heavy traffic produces about 80 dB. A sound of 120 dB, such as that commonly produced by the amplified sound at a rock concert, produces discomfort, and a sound of 140 dB, such as that emitted by a jet plane at takeoff, causes pain. Frequent or prolonged exposure to sounds with intensities above 90 dB can cause permanent hearing loss.

Describe the external, middle, and inner ears.

Explain how sound waves are transmitted through the parts of the ear.

Describe the tympanic reflex.

Distinguish between the osseous and membranous labyrinths.

Explain the function of the organ of Corti.

Incus

Malleus

Cochlear duct (endolymph)

Vestibular membrane

Tympanic membrane

Incus

Malleus

Cochlear duct (endolymph)

Vestibular membrane

Tympanic membrane

Tympanic cavity

Round window

Equilibrium Pathway Ear

Figure 12.16

Receptors in regions of the cochlear duct sense different frequencies of vibration, expressed in cycles per second (cps).

Tympanic cavity

Round window

Figure 12.16

Receptors in regions of the cochlear duct sense different frequencies of vibration, expressed in cycles per second (cps).

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