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16 Auditory Pathway Learning Objectives

• When you have learned the material in this lecture, you should be able to: • Draw and label the auditory pathway from the sense organ to the Primary

auditory cortex. • Describe the acoustic reflex in terms of the sensory sand motor pathways

that underlie the reflex. • Distinguish between central and peripheral sensory hearing loss.

Transduction of Sound - Review Transduction of sound can be considered in three steps: Sound waves in the fluid of the bony labyrinth moves the basilar membrane up and down. As the basilar membrane vibrates, it sets up convection currents in the endolymph which cause the cilia of the hair cells to bend back and forth.

Bending of the cilia opens and closes mechanically-gated ion channels in the hair cell membrane, changing the membrane voltage. This, in turn, opens voltage-gated Ca++ channels, allowing Ca++ influx leading to an increase in the rate of neurotransmitter release, thereby increasing the frequency of neurotransmitter release.

The increase in neurotransmitter release increases the signal carried to the central nervous system by the spiral ganglion neurons. This change is interpreted as sound. Figure 16-01. Steps in sound transduction

K+

Ca++

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Figure 16-02. Central structures involved in the auditory pathway.

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Figure 16-03 Central Auditory pathway

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The Auditory Cortex. The primary auditory cortex is located in the transverse temporal gyri, also known as Heschel’s gyri, located on the superior aspect of the temporal lobe. The auditory cortex is tonotopically organized. High pitch sounds are perceived in the medial parts of the gyri and low pitch sounds are perceived at the lateral aspect of the gyri. The cortex encodes three fundamental aspects of sound: its intensity, its location and its pitch. Sounds that are related to speech are distinguished from other kinds of sounds in the Auditory association cortex, also know as Wernicke’s area. Wernicke’s area is located in the posterior on third of the superior temporal gyrus,

Figure 16-4. Location of the superior temporal gyrus and its tonotopic organization.

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Auditory Reflex Reflex pathways, by definition, include a sensory limb (input), central integration and a motor limb (output). The auditory reflex is a protective mechanism that limits the amount of energy entering the cochlea when the sound entering the ear is approximately 85 ± 15 dB or higher. The sensory signals travel to the cochlear nuclei via the auditory component of the eighth cranial nerve (vestibulocochlear nerve). From the cochlear nucleus signals travel to the superior olivary complex bilaterally, and from there to the lower motor neurons in the facial nucleus that innervate the stapedius muscle. Motor signals travel via the branchial motor division of cranial nerve seven (Facial nerve) to the stapedius muscle. The tensor tympani, innervated by the motor division of the trigeminal nerve, is also activated, but its effect is not as important since it does not have the mechanical advantage that the stapedius muscle. Has.

Figure 16-5 Auditory reflex pathway. People with Bell’s Palsy, (usually due to a viral inflammation of the facial nerve), can complain of ‘sound that is too loud’ on the affected side, because of their inability to activate the stapedial muscle. In fact, the presence or absence of an acoustic reflex can help localize where the seventh nerve is damaged. If the acoustic reflex is present, then the damage is distal to the middle ear.