Neuroanatomy for Speech-Language Pathology and Audiology 2nd Ed. Matthew H Rouse

Chapter 10. The Neurology of Hearing and Balance


It is time to apply neuroscience to communication and communication disorders. This chapter will discuss the neurology of both hearing and balance as well as survey select disorders that affect these systems.


In this chapter, we will . . .

 Survey the anatomy and physiology of the peripheral hearing system

 Explore the anatomy and physiology of the central auditory system

 Examine disorders of the auditory system

 Survey the anatomy and physiology of the peripheral vestibular system

 Explore the anatomy and physiology of the central vestibular system

 Examine disorders of the vestibular system


1. The learner will list and briefly describe the peripheral auditory system.

2. The learner will list and briefly describe the central auditory system.

3. The learner will list and describe the function of the main central auditory structures.

4. The learner will define the following auditory disorders: sensorineural hearing loss, auditory processing disorder, pure word deafness, and other disorders that may affect auditory comprehension.

5. The learner will briefly describe the peripheral and central vestibular systems.

6. The learner will define the following vestibular disorders: vestibular schwannoma and labyrinthitis.



 The Neurology of Hearing

 The Peripheral Auditory System

 The Central Auditory System

 Select Disorders of the Auditory System

 The Neurology of Balance

 The Peripheral Vestibular System

 The Central Vestibular System

 Select Vestibular Disorders


 Summary of Learning Objectives

 Key Terms

 Draw It to Know It

 Questions for Deeper Reflection

 Case Study

 Suggested Projects



Hearing, also known as audition, is a process whereby acoustic or sound energy waves are changed into neural impulses. This is why the ear is known as a transducer or energy changer. Anatomically, the ear can be divided into the peripheral auditory system and the central auditory system (FIGURE 10-1). The peripheral auditory system includes the outer ear, middle ear, inner ear, and cranial nerve VIII, while the central auditory system involves several structures in the brainstem, thalamus, and cerebral cortex. The purpose of this chapter is to survey these two systems, perform an in-depth exploration of the central auditory system, and examine select neurological disorders of hearing.

 The Neurology of Hearing

The Peripheral Auditory System

The outer ear includes the pinna (or auricle) and the external auditory meatus. In the outer ear, hearing involves the pinna locating, collecting, and funneling acoustic energy (i.e., sound) to the middle ear via the external auditory canal or meatus (FIGURE 10-2). In the middle ear, acoustic energy is changed, or transduced, into mechanical energy as the tympanic membrane begins to vibrate due to sound waves hitting it. This mechanical energy is then transmitted through the ossicular chain, which is composed of three small bones—the malleus, incus, and stapes (FIGURE 10-3). This causes the footplate of the stapes to rock in and out of the vestibule’s oval window. At this point, the inner ear has been reached and several other energy changes will occur. The cochlea is filled with two fluids, perilymph and endolymph. Perilymph occupies the scala vestibuli and scala tympani, and endolymph is found in the scala media. As the footplate of the stapes rocks in and out of the cochlea, mechanical energy from the middle ear is changed into hydraulic energy through the creation of waves in these cochlear fluids. Hydraulic energy is the power of moving fluids. As these waves travel through the cochlea, hair cells in the organ of Corti are displaced, causing two more energy changes at these hair cells. First, there is a hydraulic to mechanical energy change at the hair cell cilia as they bend. Second, there is a mechanical to electrochemical energy change at the synapse of the hair cell to the afferent auditory neuron at the base of the hair cell.

FIGURE 10-1 The anatomy of the ear.

FIGURE 10-2 The process of hearing in the peripheral auditory system. Movements of fluid in the inner ear stimulate the hairs of the auditory hair cells, which result in stimulation of auditory nerve endings.

FIGURE 10-3 The size of the ossicles in relation to a penny (a penny is 20 mm [about % inch] in diameter).

© Assessment Technologies, Institute, LLC. All rights reserved.

Resting hair cells in the organ of Corti are in a highly polarized state, just like neurons. The environment outside an individual hair cell is positively charged, whereas the environment in the hair cell is negatively charged. The movement of the stapes causes perilymph displacement that leads to disruption of the basilar membrane. This disruption causes stereocilia at the very end of the hair cells to bend, leading to potassium channels opening at the ends of the stereocilia. Potassium ions (K+) enter the hair cell from the endolymph through the cilia and change the hair cell’s inner environment from negative to positive (FIGURE 10-4). This is the process of depolarization. The hair cell will repolarize later through active transport facilitated by the sodium-potassium pump. Hair cell depolarization causes calcium channels to open on the bottom sides of the hair cells, allowing calcium (Ca++) to enter the hair cell. This chemical activity within the hair cell triggers the opening of vesicles at the bottom of the hair cells to fuse with the cell membrane; these vesicles release the neurotransmitter glutamate, which travels across the synaptic cleft and excites the auditory nerve branch’s dendrites. At this point, neuron function takes over and an electrochemical impulse is sent from the organ of Corti to the central auditory system via cranial nerve VIII. Approximately 30,000 cochlear nerve fibers pass from the cochlea through the internal auditory canal (or meatus) to cranial nerve VIII (Martin & Clark, 2000). In addition to these fibers, vestibular nerve fibers course through the internal auditory canal to the brainstem. Cranial nerve VII fibers and the internal auditory artery also pass through the internal auditory canal.

FIGURE 10-4 A. A cross-section of the cochlea showing the scalas and organ of Corti in the scala media. B. The organ of Corti and the inner and outer hair cells. C. An individual hair cell undergoing depolarization. A pressure wave bends the hair cell's cilia, causing potassium channels to open and allow potassium to enter the hair cell. As a result, the hair cell depolarizes. Depolarization causes calcium channels at the bottom of the hair cell to open and allow calcium to enter the hair cell.

This activity triggers the synaptic vesicles to fuse with the cell membrane and release glutamate across the synaptic cleft to the afferent neuron.

© Sakurra/Shutterstock.

As mentioned previously, hydraulic energy is changed into electrochemical energy in the organ of Corti. These energy changes occur at specific places along the organ of Corti, which runs the length of the cochlea. Where the energy change occurs is related to the frequency of the sound. Different sensitivities to different frequencies occur all along the basilar membrane. The cochlear base is more sensitive to high-frequency sounds, and the cochlear apex is more sensitive to low-frequency sounds. The basilar membrane’s sensitivity to different sound frequencies is known as tonotopic organization (FIGURE 10-5). Nerve fibers leave their corresponding hair cells along the basilar membrane; each is responsible for communicating different frequencies and intensities to the brain, which we will see is tonotopically arranged also. In conclusion, the cochlea is a finely tuned instrument, able to decipher different sound intensities and frequencies.

FIGURE 10-5 The tonotopic organization of the cochlea.

FIGURE 10-6 Inferior view of the brain showing where cranial nerve VIII inputs into the brainstem.

Modified from: © illustrator/Shutterstock.

The Central Auditory System

Brainstem Organization

Cochlear Nucleus

Cranial nerve VIII carries the neural impulse approximately 17 to 18 mm (about 0.7 in) from the internal auditory meatus to the brainstem (Martin & Clark, 2000). It inputs near where the pons and cerebellum meet to form the cerebellopontine angle (FIGURE10-6). It is here that the vestibular and cochlear portions of cranial nerve VIII diverge and go their separate ways. All cochlear fibers synapse at the cochlear nucleus (CN). A nucleus is a collection of neuron cell bodies that function to relay and integrate neural signals as well as play an important role in the reflex arc.

The CN resembles a fish tail, as can be seen in FIGURE 10-7, and it has a few important divisions. The posterior CN (also called the dorsal CN) is an area made up of what are called pyramidal cells, so named because the cells look like little pyramids. Their function is a bit of a mystery because destroying them in animals does not seem to affect their hearing. The anterior CN (also called the ventral CN) is made up of three different nuclei. First, the most anterior nucleus is made up of spherical bushy cells. Second, the middle nucleus is made up of globular bushy cells, which are sensitive to very specific frequencies. The last nucleus is made up of octopus cells, so named because of their long, tentacle-like dendrites. These cells are sensitive to a wider range of frequencies. The rest of the CN is made up of various multipolar neurons, sensitive to the beginning, ending, or change of a sound in terms of frequency and/or intensity.

As one can see, the anterior CN preserves the tonotopic arrangement of the basilar membrane. It is represented not just once but several times throughout the nervous system, which is key to our ability to hear complex sounds. As we will see shortly, this tonotopic organization is also preserved in the primary auditory cortex of the cerebral cortex.

Superior Olivary Complex

Vast arrays of neural connections are made throughout the central auditory nervous system projecting through the brainstem, diencephalon, and cerebrum, as can be seen in FIGURE 10-8. From the CN, fibers course to the trapezoid body where most (about two- thirds) of nerve fibers decussate from the CN to the superior olivary complex, giving binaural representation in the central auditory nervous system. The superior olivary complex (SOC) in the pons gets the name olivary because its shape resembles that of an olive. Overall, this complex is important in integrating auditory information together. It has two major components, the medial superior olivary complex (MSOC) and the lateral superior olivary complex (LSOC). The MSOC (the center, or pit, of the olive) specializes in low-frequency hearing, specifically integrating low-frequency hearing from the right and left ears to create binaural hearing. Binaural hearing is hearing with two ears that provides us with several benefits—principally, our ability to determine the location of a sound. Because of the decussation of nerve fibers at the trapezoid body, each superior olive in the complex receives information from both ears. The SOCs on both sides of the brainstem analyze the slight differences in time and intensity of sound coming from each ear to locate the sound. Our ears are quite tuned for localizing sound and can do it precisely. The MSOC is the first place in the central auditory system to do this type of integration of our two ears. The LSOC (the fleshy part of the olive) specializes in higher frequency hearing. Both the medial SOC and the lateral SOC play a role in sound localization.

FIGURE 10-7 The cochlear nucleus.

In addition, the medial and lateral SOCs are responsible for the stapedius reflex, which is when the stapedius muscle contracts during loud sounds to stiffen the ossicular chain, thus preventing damage to the cochlea. When a sound is too loud, it does not need to travel all the way to the cerebral cortex. The SOC determines if it is too loud and, if so, sends a signal down the facial nerve (cranial nerve VII) to the stapedius muscle in both ears to contract. This action pulls the stapes out of the oval window, decreasing the intensity of the vibrations and protecting the delicate hair cells in the cochlea. This reflex lasts 45 seconds, so it is protection only for transient loud sounds. Its presence is used routinely by audiologists as a valid diagnostic test to evaluate the outer ear to the SOC.

Lateral Lemniscus and Inferior Colliculus

There are two other important brainstem regions. The lateral lemniscus (Latin for “ribbon”) is a tract of six pathways (or axons) that travel from the CN and SOC to the inferior colliculus in the midbrain. Two pathways travel directly from the CN to the inferior colliculus, and the remaining four travel to the inferior colliculus via the SOC. There are also nuclei scattered throughout the lateral lemniscus. The inferior colliculus (Latin for “lower mound” or “lower hill”) is the auditory center of the midbrain. There are two inferior colliculi, one of the right and one of the left. They maintain the tonotopic organization we first discussed in the cochlea. They also regulate our acoustic startle reflex, which is when we suddenly move in response to an unanticipated sound. People with hyperacusis (hypersensitivity to sound) are easily startled (Davis, Gendelman, Tischler, & Gendelman, 1982).

FIGURE 10-8 The connections of the central auditory pathway.

Diencephalon Organization

The thalamus’s auditory center is the medial geniculate body (Latin for “knee” or “bent abruptly”) as compared to the lateral geniculate body, which is the thalamic visual center. Overall, the medial geniculate body acts as a relay station, directing auditory tracts from the inferior colliculus to the auditory cortex of the cerebrum’s temporal lobe. It has three parts: the ventral, medial, and dorsal divisions. Most auditory tracts input into the ventral division, which maintains the tonotopic organization previously discussed.

Specifically, the ventral division receives the following auditory information: sound source, sound location, sound onset and offset, frequency, intensity, and binaural information. Axons leave the ventral division and pass into the internal capsule, ascending to the primary auditory cortex.

The dorsal division is thought to play a role in establishing and maintaining our attention to a sound source (i.e., auditory attention). Similar to the reticular formation, the medial division may play a role in our arousal system (i.e., telling the brain to pay attention to something). These two divisions also exit the thalamus, pass into the internal capsule, and input into the cerebral cortex. All these connections are summarized in FIGURE 10-9.

Cerebral Cortex Organization

The Primary Auditory Cortex

The central auditory pathway ends at the primary auditory cortex (Brodmann areas [BAs] 41 and 42), which sits on the superior part of the temporal lobe on the superior temporal gyrus and what is known as the gyri of Heschl (also called the transverse temporal gyri) (FIGURE 10-10). As can be seen by the name gyri (as opposed to gyrus) of Heschl, there are typically multiple gyri (one to three) that make up this area. The number of gyri also differs between the left and right temporal lobe (e.g., the left might have two, but the right might have three). This fact illustrates the variability between human brains. There is no known functional reason for this difference.

The tonotopic organization that began in the cochlea and that is maintained through the rest of the central auditory system is preserved in the primary auditory cortex. In other words, there are neurons in this area that are sound frequency specific. Neurons at one end respond best to low frequencies, and neurons on the other end react best to higher frequencies. Functionally, BAs 41 and 42 locate, perceive (i.e., recognize it is there), and discriminate (i.e., identify auditory segments in terms of frequency and intensity) sound; damage to these areas can result in a loss of awareness of sound, but with preserved auditory reflexes due to anatomical integrity at the level of the SOC.

FIGURE 10-9 A simplified diagram of the central auditory pathways.

FIGURE 10-10 Brodmann map showing important cortical auditory areas 41,42, and 22.

Wernicke's Area

The auditory information processed in the primary auditory cortex is then sent to BA 22 in the dominant cerebral hemisphere (Figure 10-10). This area consists of the posterior two-thirds of the superior temporal gyrus and a structure called the planum temporale (Latin for “temporal plain”). BA 22 is known as Wernicke area (named after the German neurologist Karl Wernicke’s) or the auditory association cortex. The planum temporale is in the shape of a flat triangle and is larger in the dominant cerebral hemisphere, which in most people is the left hemisphere. For those who are not left hemisphere dominant for language, this structure will be of equal size in both hemispheres or larger in the right hemisphere than in the left hemisphere.

The function of Wernicke’s area is not well understood, but those with damage to this area have what is called Wernicke aphasia. One of the hallmark characteristics of this aphasia type is severely impaired auditory comprehension. Because of this observation, neuroscientists believe Wernicke’s area in the dominant hemisphere is important in attaching meaning to others’ speech. Specifically, they believe that it processes dominant word meanings of ambiguous words (e.g., if I say “bank,” you associate the word “teller”).

The area corresponding to Wernicke’s area in the nondominant hemisphere may play a role in nondominant word meanings of ambiguous words (e.g., if I say “bank,” you associate “river”) (Harpaz, Levkovitz, & Lavidor, 2009).

Broca's Area

Wernicke’s area (BA 22) connects to Broca’s area (BAs 44, 45) via the arcuate fasciculus (Latin for “curved bundle”), an axonal tract that leaves BA 22 and inputs into BAs 44 and 45 and the prefrontal cortex as well as the angular gyrus (BA 39) and supramarginal gyrus (BA 40). Because those with Broca aphasia have difficulty understanding complex grammatical constructions, Broca’s area appears to be recruited into the task of understanding these constructions.

Select Disorders of the Auditory System

Sensorineural Hearing Loss

sensorineural hearing loss is a hearing loss caused by problems with either the inner ear or the vestibulocochlear nerve (cranial nerve VIII). When the loss is located in the inner ear, it is usually due to damage in the hair cells located in the organ of Corti. This damage can be induced by loud noise (noise-induced hearing loss) or by other conditions, such as Meniere disease, which is caused by a buildup of endolymph in the inner ear. Most cases of sensorineural hearing loss are due to inner ear issues involving the hair cells (BOX 10-1).

BOX 10-1 Sensorineural Hearing Loss and Cochlear Implants

Cochlear implants (FIGURE 10-11) are a treatment option for people with severe to profound cochlear hearing losses. When inner ear hair cells are damaged, the sound processing flow is broken in the inner ear and does not reach the brain. Cochlear implants are designed to fill in for this broken inner ear link. There are four parts to a cochlear implant, three of which are external and one that is internal. The one internal part is the intracochlear electrode array, which is tonotopically arranged. It is surgically implanted in the cochlea and takes advantage of the inner ear's tonotopic organization by stimulating the nerve directly. The inner ear's hair cells are essentially bypassed by this electrode, which directly stimulates the auditory branch of the vestibulocochlear nerve. The nerve then takes the auditory signal to the brain through the central auditory pathway, where it is processed. This signal is not perceived as in normal hearing, so it requires time and training to learn to interpret the signals. The external parts of the cochlear implant are the microphone, the processor, and the transmitter. The microphone picks up auditory signals from the environment and sends this signal to the processor, which turns it into an electrical signal. The processor then turns over this electrical signal to the transmitter (or coil), which sends it to the internal electrode array. This last external piece is held in place by a magnet implant just under the skin near the mastoid bone (FIGURE 10-12).

Controller with

FIGURE 10-11 The external and internal components of a cochlear implant. © Assessment Technologies, Institute, LLC. All rights reserved.

FIGURE 10-12 A 7-year-old boy who has been fitted with a cochlear implant. © James King-Holmes/ScienceSource.

Auditory Processing Disorder

Auditory processing is “what we do with what we hear” (Katz, 1992). An auditory processing disorder (APD) can be thought of as dyslexia of the ears in that there is difficulty processing and interpreting auditory symbols similar to how people with dyslexia have difficulty interpreting written symbols. The peripheral auditory system is intact in those who suffer from APD, but their brain struggles to process auditory information. The condition can be congenital or acquired through brain injuries, strokes, or other mechanisms.

In terms of specific difficulties, the American Speech-Language-Hearing Association (2005) reports that APD can include problems with the following:

 Sound localization and lateralization

 Auditory discrimination

 Auditory pattern recognition

 Temporal aspects of audition, including temporal integration, temporal discrimination (e.g., temporal gap detection), temporal ordering, and temporal masking

 Auditory performance in competing acoustic signals (including dichotic listening)

 Auditory performance with degraded acoustic signals

These problems are not believed to be caused by general attention problems, language disorders, or cognitive impairments, but APD may have associations with these conditions. Hearing and intelligence are intact in children with APD.

The National Institute on Deafness and Other Communication Disorders (2004) has listed warning signs of APD, which include the following:

 Trouble paying attention to and remembering information presented orally

 Problems carrying out multistep directions

 Poor listening skills

 Increased time to process information

 Low academic performance

 Behavior problems

 Language difficulty (e.g., they confuse syllable sequences and have problems developing vocabulary and understanding language)

 Difficulty with reading, comprehension, spelling, and vocabulary

Auditory training is the main method of treating APD; however, auditory training methods lack evidence to support their efficacy. There is some evidence that treatment targeting phonological disorders improves not only phonology but also auditory processing (Leite, Wertzner, & Matas, 2010).

Pure Word Deafness

Pure word deafness or auditory verbal agnosia is caused by bilateral damage to the superior temporal lobes, resulting in the inability to distinguish phonemes and, thus, comprehend speech. Patients do retain the ability to hear and comprehend nonspeech sounds, like a doorbell ringing or a bird singing, as well as the abilities to speak, read, and write. Pure word deafness is one of the first symptoms of Landau- Kleffner syndrome (LKS), a rare epileptic syndrome that occurs in children between 3 and 7 years of age (Metz-Lutz, 2009). It can also be a symptom of Alzheimer disease (Kim et al., 2011).

Deficits in Auditory Comprehension

Though not technically a disorder of the central auditory system, aphasia is a multimodality language disorder that can affect mapping meaning to sounds. Most patients with aphasia suffer from at least some high-level difficulties in auditory comprehension. When thinking about the classical aphasias, three stand out in terms of significant deficits in auditory comprehension: global, Wernicke, and transcortical sensory aphasia. What these three types of aphasia have in common is damage to the temporal lobe where the primary auditory cortex and Wernicke’s area are located.

The name global aphasia sounds bad, and the name is fitting because this is the most severe form of aphasia. Severe deficits are found in all the modalities of language, including verbal formulation, auditory comprehension, reading, and writing. Verbally, patients are nonfluent and are either mute or produce just a few sounds or words. Repeating words is often very difficult. In terms of auditory comprehension, patients struggle to follow even one-step commands and to complete simple tasks like pointing to a picture in a field of two pictures after being given the name of one picture. Reading is equally compromised, with patients unable to match a single written picture to a picture in a field of two pictures. Often patients with global aphasia cannot write their name or other identifying information.

Wernicke aphasia causes severe deficits in auditory comprehension; however, unlike with global aphasia, these patients have fluent verbal output (100-200 words per minute), though it is filled with paraphasias (sound or word substitutions). They are described as having word salad in their verbal output, meaning it seems someone placed their words in a bowl, stirred them up, and then what was dumped out was their verbal output. Repeating is severely impaired, as is writing and reading.

Patients with transcortical sensory aphasia (TSA) are fluent like patients with Wernicke aphasia but have paraphasias in their speech. They also demonstrate echolalia, which is repeating what other people say. Unlike people with Wernicke aphasia, patients with TSA can repeat words said to them. Auditory comprehension is poor, as is reading and writing.

► The Neurology of Balance

Human beings are sensory creatures. We have five senses through which we experience the world: vision, hearing, smell, taste, and touch. Right now, if you are reading the print version of this text, the most prominent sense you are using is vision. Touch is also used as you touch and turn the pages, as is perhaps smell if the book still has that “new book” smell. However, there is a sixth sense working in the background of which you are probably not aware. It keeps you sitting upright as you read this text and coordinates your head and eye movements. The only time you may have been aware of the stealthy system behind this sense is when you had an inner ear infection and experienced vertigo or when you were on a boat in choppy waters and felt seasick. The sense is your sense of balance, and the main system behind it is called the vestibular system.

The vestibular system contributes to spatial orientation by constantly sending information about your head’s position in space to your brain. When there is a change in head position, information concerning the direction and speed is relayed to the brain. This system does not work alone to maintain spatial orientation but rather works with your sense of vision and proprioception. (Proprioception is your body’s eyes for itself.) This redundancy illustrates how important this sense is to living in and navigating our environment. The vestibular system will be examined in this chapter.

The Peripheral Vestibular System

The vestibular system is located in the same place as the cochlea—the inner ear. In fact, these two structures are connected to each other (FIGURE 10-13). Just as the cochlea contains the main organ of the hearing system, so the semicircular canals and the vestibule’s utricle and saccule contain the main organs of the vestibular system.

Three semicircular canals are interconnected with each other. All three branch from the labyrinth and are filled with endolymph. Because we live in a threedimensional world, each canal senses head movement in a particular dimension. The canals sit at right angles to each other to accomplish this (FIGURE 10-14). To imagine this arrangement, look over to a corner of the

FIGURE 10-13 The semicircular canals.

room you are sitting in. Look at where the floor and two walls meet. The floor is the horizontal semicircular canal, which is sometimes referred to as the lateral canal; it senses head movement in the transverse or horizontal plane. For example, you use this canal when crossing the street, looking left and then right to make sure your path is free of cars. Continuing the analogy, one of the walls is the anterior semicircular canal, also known as the superior canal, which senses movement in the coronal plane. For example, if you touch your ear to your shoulder, you are activating this canal. The final wall is the posterior semicircular canal. It senses movement in the sagittal plane. This canal is activated when you nod your head yes. You can see that all planes of head movement are covered by these canals: horizontal, coronal, and sagittal. Movement can be a combination of multiple planes (e.g., left upward movement); when this is the case, multiple canals are involved. How this movement is specifically sensed is our next topic.

Each semicircular canal has an enlargement called an ampulla, and each ampulla contains a sensory epithelium called a crista (FIGURE 10-15). Each of these cristae contains a ridge of epithelium, the ampullary crest, which contains sensory hair cells similar to the organ of Corti in the cochlea. Each hair cell has stereocilia that protrude into a gelatinous structure called the cupula. These stereocilia are sensitive to rotary movements of the head (i.e., head turning left and right in relation to the body) in the horizontal plane.

FIGURE 10-14 A. The arrangement of the semicircular canals. B. The semicircular canals and their planes. Notice the enlargement of each semicircular canal as it attaches at the vestibule. This enlargement is the ampulla.

When these movements occur, waves are produced in the endolymph that cause the stereocilia to move, triggering depolarization of the hair cell.

In the vestibule, there are two important structures, the utricle and the saccule. Each of these structures contains a sensory epithelium, called the macula, that is similar to the crista. One macula is called the macula utriculus (located in the utricle), and the other is the macula sacculus (located in the saccule). In both the utricle and saccule, the macula is made up of support cells layered with hair cells with stereocilia embedded in a gelatinous otolithic membrane with otoconia (calcium carbonite crystals) imbedded in it.

In the utricle, the macula is on the “floor” and the hair cells point up. These hair cells respond to linear acceleration on the horizontal plane (forward/backward) and right and left head tilt. The otolith membrane with the otoconia “leans” with the motion, causing depolarization of the hair cells. In contrast, in the saccule the macula is situated on the “back” and the hair cells point out. These hair cells respond to linear acceleration on the vertical plane, or moving up and down (as in an elevator). The depolarization of the hair cells with Ca++ and K+ entering the cell occurs as in the cochlea, but now it happens with movement.

Like in the cochlea, the hair cells are transducers of hydraulic energy into nerve impulses. At the end of the hair cells are two kinds of cilia, stereocilia and kinocilia. There are approximately 100 stereocilia on each hair cell, but only 1 kinocilium per hair cell. When the stereocilia bend toward the kinocilium, potassium and calcium channels open, allowing Kand Ca++ to flow from the endolymph into the hair cells. This action depolarizes the hair cell (i.e., neural firing), exciting it to release the neurotransmitter glutamate at its base into the synaptic cleft. The neural signal is then transferred to the Scarpa ganglion of the vestibular branch of cranial nerve VIII, which transmits the signal to the brain. If the stereocilia sway away from the kinocilium, then the signal produced is inhibitory in nature, meaning action potentials are not sent to the brain. This is quite useful information to the brain. When the sensory structures from one inner ear are sending action potentials to the brain because the stereocilia are moving toward the kinocilium, the other inner ear’s structures are not if the stereocilia are moving away from the kinocilium (such as when turning the head to the right). This dynamic tells the brain which way the head is turning.

The Central Vestibular System

Brainstem Organization and Connections

Vestibular Nucleus

The vestibular portion of cranial nerve VIII leaves the semicircular canals and the utricle and saccule serving the left and right vestibules, joins the cochlear portion, and travels through the internal auditory canal before inputting into the brainstem at the cerebellopontine angle. Fibers from it input into the vestibular nucleus (VN), a structure consisting of four nuclei on each side of the brainstem—the superior, inferior, medial, and lateral (Deiter) nuclei. Axons from these nuclei make connections with various nervous system structures that form the five functional systems that will be considered next.

FIGURE 10-15 A. A semicircular canal's ampulla and an enlarged picture of its crista. B. The vestibule and its utricle and saccule with an enlarged picture of the saccule's macula.

Universal Images Group North America LLC / Alamy Stock Photo.

Cerebellar Connections and Balance

Vestibular fibers project to the cerebellum and back from the cerebellum to the VN. The cerebellum coordinates motor movement, so these connections facilitate the coordinated movements necessary to preserve the body’s balance.

Brainstem Nuclei Connections and Eye Movements

Fibers project from the VN to form the medial longitudinal fasciculus that inputs into various nuclei in the brainstem that control eye movements (FIGURE 10-16). These nuclei include the abducens nucleus in the pons, the trochlear nucleus in the midbrain, and the oculomotor nucleus in the midbrain. From these nuclei, cranial nerves III, IV, and VI project to the extrinsic eye muscles that move the eyeballs. These connections allow you to keep your eyes fixed on a target while moving your head (try it now). We do this without thinking about it, so this ability is called the vestibulo-ocular reflex.

Brainstem Connections and Nausea

The VN make connections to the reticular formation (RF). The RF coordinates visceral/autonomic functions, which may result in the feelings of nausea related to motion sickness and possible vomiting.

Spinal Cord Connections and Head/Neck Movements

Axons of the medial descend as the medial vestibulospinal tracts (FIGURE 10-17). These tracts input in the lower motor neuron portions of the cervical spinal cord. This results in our ability to rotate our head in one direction and our body in the other direction. This is a reflex called the vestibulocollic reflex.

Spinal Cord Connections and Arm/Leg Adjustments

Axons from the lateral VN descend to form the lateral vestibulospinal tract (Figure 10-17). This ipsilateral tract terminates in the thoracic and lumbar lower motor neurons of the spinal cord. The lower motor neuron’s axons then pass to the extensor muscles of the arms and legs, resulting in the vestibulospinal reflex. You can test this reflex by bending over to pick something up. When you do, you will usually unconsciously extend one or more of your limbs to keep yourself balanced.

FIGURE 10-16 An example of vestibular neural organization using the horizontal semicircular canal as an example.

1. The turning motion of the head.

2. The effect of the head's motion on the horizontal canals and their fluid.

3. The left and right vestibular nuclei.

4. Important cranial nerve nuclei and their connections to eye muscles.

5. The effect on particular eye muscles and the turning of the eyes in the opposite direction of the head movement.

The vestibular system works in the background, hardly being noticed. This is because there are few connections between this system and the cerebral cortex. An indirect pathway does exist between the VN and the thalamus. The thalamus then projects to a small part of the postcentral gyrus (sensory cortex) and the middle temporal gyrus. It would appear that the cortex activates our conscious awareness of our vestibular system when there is something wrong with it (e.g., vertigo), such as in the case of the following select examples of vestibular disorders.

Select Vestibular Disorders

Vestibular Schwannoma

Schwann cells are the myelin producers of the peripheral nervous system. A schwannoma is a slow-growing, unilateral, benign tumor that occurs in the peripheral nervous system, most commonly on cranial nerves. When a schwannoma occurs on cranial nerve VIII, it typically arises on one of the two vestibular branches of the nerve and therefore is properly called a vestibular schwannoma. The first symptoms patients typically report are hearing loss in one ear and tinnitus (i.e., a sensation of a sound, typically, a high-pitched ringing) in the same ear. Other symptoms include headache, balance issues, vertigo, and nausea. Schwannomas are diagnosed through neuroimaging techniques, like magnetic resonance imaging. Once diagnosed, treatment is usually surgery to remove the tumor (Victor & Ropper, 2001). All cranial nerve VIII schwannomas begin in the internal auditory canal; and as they grow, they expand out this canal; into the base of the skull. Preservation of hearing in surgical cases is dependent on time of detection and location in the internal auditory canal. Schwannomas that are completely within the internal auditory canal (“intracanalicular”) have a better chance of hearing preservation postoperatively. With larger tumors expanding into the skull base, most likely hearing will be lost postoperatively, and as well, there is an attendant risk of damage to the facial nerve (cranial nerve VII) because it is in this same region. Compression of cranial nerve VII can cause facial paresis or paralysis. Schwannomas are generally unilateral (95% of the time) but can be bilateral and develop at a younger age in people with a genetic problem called neurofibromatosis-2.


Labyrinthitis is an infection of the inner ear that affects the vestibular nerve. It is usually caused by a virus but can also be bacterial in origin. Onset is usually rapid, with patients reporting vertigo, nausea, and vomiting. People suffering from this condition report that they need to remain in bed to lessen the symptoms. Interestingly enough, patients do not usually report tinnitus or hearing loss, illustrating that the condition is affecting only the vestibular portions of the inner ear. Virally caused labyrinthitis usually will go away on its own in a matter of a few days to a week without treatment. Bacterial labyrinthitis may require antibiotics (Victor & Ropper, 2001).

FIGURE 10-17 Dorsal view of the brainstem illustrating the central vestibular pathway.

► Conclusion

In this chapter, the route of the central auditory pathway has been traced from the cochlea to the CN through various brainstem structures and finally to the cerebral cortex via the thalamus. The route of the central vestibular pathway was also traced. An important observation was made in this chapter about hearing: Tonotopic organization, which begins in the cochlea, is maintained through the whole pathway and even in

the cerebral tissues responsible for processing sound and attaching meaning to it.

Why should a speech-language pathologist (SLP) or audiologist know this information? The SLP will be interested to know how damage to the system at the various neuroanatomical levels impairs a person’s ability to understand speech and language. An audiologist is obviously interested in the same thing but also in how tests, like auditory brainstem response, examine this pathway and assess where damage is in the system (BOX 10-2). Knowing this information leads to appropriate treatment for the patient, especially babies and others incapable of behavioral audi-ological measures.

BOX 10-2 Auditory Brainstem Response

Auditory brainstem response (ABR) is a test that evaluates the integrity of cranial nerve VIII and the central auditory pathway. It is also sometimes known as an auditory evoked potential. ABR is used on people who cannot respond to typical hearing examinations. For example, pure tone hearing tests require the listener to raise a hand on the side on which he or she hears a short beep. Some people, such as babies or people with disorders of consciousness, cannot respond to these types of tests, so ABR is an appropriate substitute. To perform the test, electrodes are placed on the subject's skin and then hooked to a computer (FIGURE 10-18). Sounds are then played through earphones, and the

FIGURE 10-18 A child undergoing an auditory brainstem response screening.

Courtesy of Natus Neurology Incorporated (Grass Brand), 3150 Pleasant View Road, Middleton, WI.

FIGURE 10-19 Auditory brainstem response (ABR). ABR involves waves I through V.

Courtesy of Natus Neurology Incorporated (Grass Brand), 3150 Pleasant View Road, Middleton, WI.

equipment records the evoked potentials generated (FIGURE 10-19). ABR and otoacoustic emissions (OAE), a test that tests up to the outer hair cells in the cochlea, are the instruments of choice for screening the hearing of newborn infants. If the baby fails the screening, further audiological examination is warranted. Screening infants is part of the Early Hearing Detection and Intervention (EHDI) program in which every newborn infant receives a hearing screening using ABR and/or OAE. All 50 states have EHDI programs in place (American Speech Language Hearing Association, 2018).


The following were the main learning objectives of this chapter. The information that should have been learned is below each learning objective.

1. The learner will list and briefly describe the peripheral auditory system.

 Outer ear: Involves the location, collection, and funneling of acoustic energy to the middle ear via the external auditory meatus.

 Middle ear: Involves acoustic energy being transduced into mechanical energy as the tympanic membrane begins to vibrate due to sound waves hitting it. This mechanical energy is then transmitted through the ossicular chain, which is composed of three small bones—the malleus, incus, and stapes. This causes the footplate of the stapes to rock in and out of the oval window of the cochlea.

 Inner ear: The rocking stapes causes waves in the fluid-filled cochlea (hydraulic energy); as these waves travel through the cochlea, hair cells in the organ of Corti are disrupted, causing final energy changes—hydraulic energy to mechanical energy and then mechanical to electrochemical energy.

 Cranial nerve VIII: Nerve fibers from the cochlear branch of cranial nerve VIII, the vestibulocochlear nerve, pick up the neural impulses and transmit them to the brain.

2. The learner will list and briefly describe the central auditory system of hearing.

 The brainstem: It begins the central auditory system by receiving and routing auditory information to the cerebral hemispheres.

 The cerebral hemispheres: Auditory areas of the brain decode and identify the electrical impulses.

3. The learner will list and describe the function of the main central auditory structures.

 Cranial nerve VIII: Connects the inner ear to the central nervous system, transferring cochlear signals to the brainstem.

 Cochlear nucleus (CN): several groups of nuclei that receive auditory signals from cranial nerve VIII.

 Superior olivary complex (SOC): Integrates auditory information and specializes in low-frequency hearing, specifically integrating low-frequency hearing from the right and left ears into what is called binaural hearing.

 Lateral lemniscus: a tract of six pathways/ axons that travels from the CN and SOC to the inferior colliculus in the midbrain.

 Inferior colliculus: the auditory center of the midbrain that regulates our acoustic startle reflex, which is when we suddenly move in response to an unanticipated sound.

 Medial geniculate body: the auditory center of the thalamus that acts as a relay station, directing auditory tracts from the inferior colliculus to the auditory cortex of the cerebrum’s temporal lobe.

 Primary auditory cortex: Locates, perceives (i.e., recognizes it is there), and discriminates (i.e., identifies auditory segments in terms of frequency and intensity) sound information.

 Wernicke’s area: area in the dominant hemisphere important in attaching meaning to others’ speech.

4. The learner will define the following auditory disorders: sensorineural hearing loss, auditory processing disorder, pure word deafness, and other disorders that may affect auditory comprehension.

 Sensorineural hearing loss: a hearing loss caused by problems with the inner ear.

 Auditory processing disorder: Can be thought of as dyslexia of the ears in that there is difficulty processing and interpreting auditory symbols.

 Pure word deafness: an inability to distinguish phonemes and, thus, comprehend speech; nonspeech sound interpretation is preserved.

 Other disorders that affect auditory comprehension. Aphasia is a multimodality language disorder. Some forms of aphasia involve impaired auditory comprehension.

5. The learner will briefly describe the peripheral and central vestibular systems.

 Semicircular canals: three canals in the inner ear that sense motion in three dimensions

 Utricle and saccule: structures in the vestibule of the inner ears that sense linear acceleration in the horizontal and vertical planes

 Vestibular nucleus: a group of nuclei that contain axons that make connections with various nervous system structures that form the five functional systems: balance, eye movements, nausea, head/neck movements, and arm/leg adjustments

6. The learner will define the following vestibular disorders: vestibular schwannoma and labyrinthitis.

• Vestibular schwannoma: a slow-growing, benign peripheral nervous system tumor of cranial nerve VIII

• Labyrinthitis: an infection of the inner ear that can affect the vestibular branch of cranial nerve VIII



Auditory association cortex Auditory brainstem response (ABR)

Auditory processing disorder (APD)

Binaural hearing

Central auditory system Cochlear nucleus (CN) Global aphasia Inferior colliculus

Inner ear

Lateral lemniscus

Lateral superior olivary complex (LSOC)

Lateral vestibulospinal tract

Medial superior olivary

complex (MSOC)

Medial vestibulospinal tracts

Meniere disease

Middle ear

Noise-induced hearing loss

Outer ear

Peripheral auditory system Planum temporale Sensorineural hearing loss

Superior olivary complex (SOC)

Tonotopic organization

Transcortical sensory aphasia (TSA)


Vestibular nucleus (VN) Vestibulocollic reflex Vestibulo-ocular reflex Vestibulospinal reflex Wernicke aphasia


1. Draw a cross-section of the cochlea (see Figure 10-4) and label the following: scala vestibuli, scala media, scala tympani, organ of Corti, inner hair cells, and outer hair cells.

2. Draw and label the central auditory pathway (see Figure 10-9).


1. In essay form, describe the process of hearing from when a sound is heard through when the brain processes the signal.

2. In essay form, describe how the vestibular system works.

3. List and define the central auditory disorders discussed in this chapter.


Julie is a 46-year-old college professor who has experienced tinnitus and a slow loss of her hearing in her left ear over the past year and a half. Her neurologist ordered an MRI, which revealed a mass on the vestibular branch of her left cranial nerve VIII. Julie’s neurologist counseled her that it would be best to monitor the growth of the tumor, but that a day might come when she might need to have it surgically removed.

1. What condition to you think Julie has? Why do you think this?

2. What could happen to Julie is the mass continues to grow?


1. Borrow a portable audiometer from your clinic and test the hearing of five people. Did you detect any hearing losses? Is so, what kind?

2. Choose one of the disorders discussed in this chapter and write a two- to three-page paper with the following sections: cause, signs and symptoms, diagnosis, and treatment.

3. Read Chapter 3 of Oliver Sack’s The Man Who Mistook His Wife for a Hat. Write one or two pages about how this clinical tale relates to the vestibular system.


American Speech-Language-Hearing Association (ASHA). (2005).

ASHA practice policy. Retrieved from

American Speech-Language-Hearing Association. (2018).

Early hearing detection and intervention (EHDI). https:// -and-Intervention/

Davis, M., Gendelman, D. S., Tischler, M. D., & Gendelman, P. M. (1982). A primary acoustic startle circuit: Lesion and stimulation studies. Journal of Neuroscience, 2(6), 791-805.

Harpaz, Y., Levkovitz, Y., & Lavidor, M. (2009). Lexical ambiguity resolution in Wernicke’s area and its right homologue. Cortex, 45(9), 1097-1103.

Katz, J. (1992). Classification of auditory processing disorders. In J. Katz, N. A. Stecker, & D. Henderson (Eds.), Central auditory processing: A transdisciplinary view (pp. 81-92). St. Louis, MO: Mosby Year Book.

Kim, S. H., Suh, M. K., Seo, S. W, Chin, J., Han, S. H., & Na, D. L. (2011). Pure word deafness in a patient with early-onset

Alzheimer’s disease: An unusual presentation. Journal of Clinical Neurology, 7(4), 227-230.

Leite, R. A., Wertzner, H. F., & Matas, C. G. (2010). Long latency auditory evoked potentials in children with phonological disorder. Pro-Fono Revista de Atualizagao Cientfica, 22(4), 561-566.

Martin, F. N., & Clark J. G. (2000). Introduction to audiology (7th ed). Needham Heights, MA: Allyn & Bacon.

Metz-Lutz, M. N. (2009). The assessment of auditory function in CSWS: Lessons from long-term outcome. Epilepsia, 50(s7), 73-76.

National Institute on Deafness and Other Communication Disorders. (2004). Auditory processing disorders in children (NIH Pub. No. 01-4949). Retrieved from http://www.ldonline .org/article/8056

Victor, M., & Ropper, A. H. (2001). Principles of neurology. New York, NY: McGraw-Hill Medical.

If you find an error or have any questions, please email us at Thank you!