9. The Auditory System
Revised July 21, 2010
The learning objectives of this chapter are to:
- Describe the organization of the auditory system. In particular, be able to describe the
- Categorize hearing loss as conductive, sensorineural, or central according to the location of the causative lesion.
- Describe where in the auditory system a unilateral lesion may cause ipsilateral hearing loss.
- Explain hemisphere dominance and why inability to understand speech is only one type of aphasia.
a. Ear and auditory receptors
b. Spiral ganglion and cochlear nerve
c. Central auditory pathway
d. Wernicke’s area
Lesions of the auditory system cause
- Tinnitus ("ringing" or "tinkling") (Pronounced TINN-ih-tuhs)
- Hearing loss (deafness)
Ringing in the ear: perception of noise in the absence of external sound.
Examples are whistling like a teakettle and roaring like listening to a seashell.
I. EAR AND AUDITORY RECEPTORS
The auditory parts of the ear are the external ear, the middle ear, and the auditory portion of the inner ear, the cochlea, part of the bony labyrinth (#7348). (The vestibular parts of the bony labyrinth, the vestibule and the semicircular canals, are described in Chapter 8: The Vestibular System).
A. External ear and middle ear
Sound waves travel down the external auditory canal and cause the tympanic membrane to vibrate (#7371). Attached to the tympanic membrane is the malleus, the first of three ossicles in the middle ear (#7401): the malleus (“hammer”), incus (“anvil”) and stapes (“stirrup”). The stapes is attached to the oval window of the vestibule (#7423), which is continuous with the base of the cochlea. Consequently, as the tympanic membrane vibrates so does the membrane of the oval window (#7408).
Conductive hearing loss is hearing loss due to a lesion of the external or middle ear.
The structure of the cochlea (”snail shell”) can be appreciated by unwinding it as in #7430. A section through a turn of the cochlea shows three chambers (#7433). Two of these chambers, the scala vestibuli and the scala tympani (#7431), are filled with perilymph. They are continuous at the apex of the cochlea at a point called the helicotrema (#7435). The cochlear duct (scala media) (#7430) is a spiraled tube within the cochlea that contains endolymph and the organ of Corti, in which the auditory receptors are located.
Vibration of the membrane of the oval window produces waves in the perilymph of the bony labyrinth (#7424, #7974) that in turn create traveling waves along the basilar membrane. The organ of Corti (#6351) rests on the basilar membrane (#7432, #7927). The auditory receptors are hair cells (#7437, #6353, #3290) that are excited by the deflection of their stereocilia as the local part of the basilar membrane is displaced.
The cochlea is tonotopically organized along its length with high tones represented at the base and low tones at the apex. Tonotopic representation continues in the cochlear nerve and central auditory pathway to the auditory cortex.
II. SPIRAL GANGLION AND COCHLEAR NERVE
At the base of each hair cell are terminals of the peripheral processes of the primary sensory neurons of the auditory system (#7438). The cell bodies of these bipolar neurons are in the spiral ganglion (#6342). The spiral ganglion is within the modiolus (“hub,” pronounced moh-DYE-uh-luhs), the bony central pillar of the cochlea. The central axonal processes of these primary sensory neurons form the cochlear (auditory) nerve (#6341, #7439, #7975), a division of the vestibulocochlear nerve (VIII). The cochlear nerve leaves the inner 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 vestibulocochlear 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. The cochlear nerve attaches to the brainstem at the rostralmost medulla (#5602), at the margin of the lateral recess, where its axons end in the ipsilateral dorsal cochlear nucleus (#4480) and ventral cochlear nucleus (#9752. Note also the dorsal cochlear nucleus, and the lateral recess dorsal to the dorsal cochlear nucleus).
Sensorineural hearing loss is hearing loss due to a lesion of the cochlea (sensory), the cochlear nerve (neural), or both.
III. CENTRAL AUDITORY PATHWAY
The cochlear nuclei of each side give rise to bilateral pathways to the primary auditory cortex.
A. Brain stem auditory pathway
The main nuclei of the brain stem auditory pathway are located at each of the three levels of the brain stem: the medulla, the pons and the midbrain.
Medulla: cochlear nuclei. At the rostralmost medulla, the dorsal and ventral cochlear nuclei (fig 9a) look like saddle bags thrown over the inferior cerebellar peduncle. They contain the secondary sensory neurons of the auditory system. They give rise to the secondary sensory axons, which form the dorsal, intermediate and ventral acoustic striae. In humans, the secondary sensory axons project bilaterally to two main targets: the superior olivary complex and, via the lateral lemniscus (#6365, #6368), the inferior colliculus.
Pons: superior olivary complex. The superior olivary complex (fig 9b, labeled Superior Olivary Nucleus) is in the pons. It processes sound localization. In addition, efferent olivocochlear axons to the organ of Corti enhance auditory selective attention and the processing of speech in the presence of noise. The superior olivary complex projects via the lateral lemniscus mainly to the inferior colliculus.
Midbrain: inferior colliculus. The lateral lemniscus terminates mainly in the central, or main, nucleus of the inferior colliculus (fig 9c), in the midbrain. 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.
B. Thalamus: medial geniculate nucleus
The medial geniculate nucleus (fig 9e) forms a surface elevation, the medial geniculate body. It is the relay nucleus of the thalamus for the auditory system. Axons of the medial geniculate nucleus project through the most caudal part of the posterior limb of the internal capsule (#6383) to end mainly in the primary auditory cortex. These thalamocortical axons constitute the auditory radiations (#6383).
C. Auditory cortex
Primary auditory cortex. The transverse temporal gyrus (Heschl’s gyrus) is on the superior surface of the superior temporal gyrus, so that it is hidden in the Sylvian (lateral) fissure (#8437, #5374). It is commonly two gyri (#5374), or bifid laterally. Only a small part of it extends onto the lateral surface of the temporal lobe (#4352). The primary auditory cortex (Brodmann’s area 41) is on the medial two-thirds of Heschl’s gyrus, corresponding approximately to the medial (anterior) Heschl’s gyrus when there are two gyri. It represents perceived pitch, tonotopically organized with low tones laterally.
Secondary auditory cortex (Brodmann’s area 42) forms a lateral crescent around the primary auditory cortex. Further auditory areas are on the anterior two-thirds of the superior temporal gyrus (anterior two-thirds of area 22) (#4340, superior temporal gyrus). These areas (42 and anterior two-thirds of 22) represent the phonological representation of words at a presemantic (pre-meaning) level.
Central hearing loss is hearing loss due to a lesion of the central auditory pathway and is rare.
- Bilateral lesions are generally necessary to produce central hearing loss
- Patients with lesions of the central auditory pathway are usually not deaf in the usual sense. Pure tone audiometry is usually normal, and more sophisticated tests such as tests of speech reception threshold or word recognition are required for evaluation.
- Lesions of the CNS (except for the cochlear nuclei) do not result in unilateral hearing loss.
IV. RECEPTIVE SPEECH REGION: WERNICKE'S AREA
Wernicke’s area. Wernicke’s area processes the understanding of speech. It is adjacent to the auditory cortex, mainly in the posterior third of the superior temporal gyrus, of the dominant, generally left, hemisphere (#4213. Ignore the included parietal area.).
[The superior surface of the temporal lobe posterior to Heschl’s gyrus is the planum temporale, which is usually larger on the left. The planum temporale behind the auditory cortex of the dominant hemisphere is also part of Wernicke’s area (posterior third of area 22).]
Wernicke’s aphasia. Aphasia means a language deficit caused by brain damage. This may include defective production or comprehension of spoken or written vocabulary or grammar (sentence structure). Aphasia occurs with lesions of the dominant hemisphere, which, by definition, is the cerebral hemisphere that is dominant for language.
Lesions of Wernicke’s area result in Wernicke’s (fluent, sensory, receptive) aphasia. These patients are unable to understand speech or repeat words. Yet, they speak fluently, although with many incorrect words.
Branches of what cerebral artery vascularize auditory cortex and Wernicke's area?
Sound Transduction from Cranial Nerves in Health and Disease, Linda Wilson-Pauwels 2002
Vestibulocochlear nerve (VIII) with the cochlear nuclei on the side of the inferior cerebellar peduncle colored black. From John Sundsten, University of Washington.
Brain Stem Auditory Pathway from John Sundsten, University of Washington.
Auditory Pathway from: VIII to MGN, 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. Charleston, South Carolina.
Planum temporale from John Sundsten, University of Washington.
Auditory Radiations from John Sundsten, University of Washington.
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