9. The Auditory System

The objectives of this chapter are to:

  1. Examine the structure of the peripheral auditory apparatus.
  2. Study the organization of the auditory pathway.

The structure of the auditory system--as it is frequently pointed out--appears to be diffuse. Although the auditory system has a well-ordered structure, the structure is not as obvious as in the somatic sensory and visual systems. Several nuclear groups are intercalated between the organ of Corti in the periphery and the primary auditory cortex.

Sound waves travel down the auditory canal and cause the tympanic membrane to vibrate (#7371). Attached to the tympanic membrane is the malleus, one of the three ossicles in the middle ear (#7401). The stapes, the third ossicle in the chain, is attached to the oval window of the vestibule (#7423), which is continuous with the cochlea. Consequently, as the tympanic membrane vibrates so does the membrane of the oval window (#7408). This produces waves in the perilymph of the bony labyrinth (#7424, #7974) that in turn displaces the endolymph in the cochlear duct (membranous labyrinth). The structure of the cochlea can be appreciated by unwinding it as in #7430. The bony labyrinth of the cochlea consists of two chambers, the scala vestibuli and the scala tympani (#7431), both filled with perilymph. They are continuous at the apex of the cochlea, called the helicotrema (#7435). The cochlear duct (#7430) is a spiraled tube within the cochlea that contains endolymph and the organ of Corti, in which the auditory receptors are located. A section through a turn of the cochlea shows these three chambers (#7433).

The organ of Corti (#6351) rests on the basilar membrane (#7432, #7927). The receptors are hair cells (#7437, #6353, #3290) that are excited by the deflection of their cilia. At the base of each hair cell is an ending of a primary sensory neuron of the auditory system (#7438), whose cell body is located in the spiral ganglion (#6342). The central axonal processes of these primary sensory neurons form the cochlear nerve, a division of cranial nerve VIII (#6341, #7439, #7975). The cochlear nerve leaves the internal ear and enters the posterior cranial fossa. It enters this fossa through what foramen (#7298)? What other cranial nerve passes through this foramen (#7297)?

The attachment of the cochlear nerve to the brain stem is seen in #5304. This general area, at the junction of the medulla, pons, and cerebellum, is known as the cerebellopontine angle. Tumors of the Schwann cells in cranial nerve VIII are called acoustic neuromas (#10866, #7966). They press on the cochlear and vestibular nerves, first irritating the axons and then causing them to degenerate. As the tumor increases in size, it will affect adjacent cranial nerves. What nerves will be affected (#5575)?

The axons in the cochlear nerve (#5602) terminate in the dorsal and ventral cochlear nuclei (fig 9a), which look like saddle bags thrown over the inferior cerebellar peduncle. The dorsal (#4480) and ventral (#9752) cochlear nuclei represent the secondary sensory neurons of the auditory system. The secondary sensory axons leave these nuclei and travel through the tegmentum either to terminate in an auditory relay nucleus, e.g. the superior olivary nucleus (fig 9b), or to join the lateral lemniscus (#6365, #6368). The superior olivary nucleus receives input from the cochlear nuclei on both sides of the brain stem. This binaural input to each superior olivary nucleus is undoubtedly important for localizing the source of a sound.

The trapezoid body (#6360) is formed by axons from the ventral cochlear nucleus and the superior olivary nucleus that cross the midline and enter the lateral lemniscus. They intersect the medial lemniscus (#6175), but the relationship between the two is purely physical.

The lateral lemniscus terminates in the central, or main, nucleus of the inferior colliculus (fig 9c). Axons from the inferior colliculus form the brachium of the inferior colliculus (fig 9d) which can be seen on the surface of the brain stem. In cross section (#6374), the brachium is a band on the surface of the midbrain. The brachium of the inferior colliculus ends in the medial geniculate nucleus (#11716, #8291) of the thalamus. This is the specific sensory nucleus of the thalamus for the auditory system. The medial geniculate nucleus (fig 9e) sends axons to the primary auditory cortex. These thalamocortical axons constitute the auditory radiations (#6383). (Note: the occipital pole is up).

The primary auditory cortex is located on the transverse gyri of Heschl in the lateral fissure (#8437, #5374). Only a small part of it extends onto the lateral surface of the temporal lobe (#4352). Primary auditory cortex is organized in a tonotopic manner, as are the medial geniculate nuclei, inferior colliculus, dorsal cochlear nucleus, and the cochlea. Unilateral destruction of the primary auditory cortex usually impairs the ability to localize sound in space, especially when the sound is coming from a source on the side opposite the affected hemisphere.

Primary auditory cortex is adjacent to the posterior third of the superior temporal gyrus, which in the dominant hemisphere is called Wernicke's area (#4213). Lesions of this cortical area in the dominant hemisphere produce receptive aphasia. Although able to speak, these patients are unable to comprehend speech. Lesions involving the adjacent angular gyrus (#4218) in the dominant hemisphere cause difficulties with visual and auditory associations that are central to such things as reading and performing calculations. Branches of what cerebral artery vascularize the primary auditory cortex, Wernicke's area, and the angular gyrus?

Click for the Syllabus Quiz.
Click for the Pathway Quiz
Click for the Movie From: "Parallel Pathways, A Tour Through the CNS". Edward Allen Neilson, W. Curtis Wise, Henry F. Martin. Department of Physiology, The Medical University of South Carolina. Charlston, South Carolina.

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