The auditory system is built to allow us to hear. It is remarkable for its sensitivity. It is especially important in humans because it provides the sensory input necessary for speech recognition.
ANATOMY AND FUNCTION
The cochlea, within the inner ear, is the specialized organ that registers and transduces sound waves. It lies within the cochlear duct, a portion of the membranous labyrinth within the temporal bone of the skull base (Fig 16–1; see also Chapter 11). Sound waves converge through the pinna and outer ear canal to strike the tympanic membrane (Figs 16–1 and 16–2). The vibrations of this membrane are transmitted by way of three ossicles (malleus, incus,and stapes) in the middle ear to the oval window, where the sound waves are transmitted to the cochlear duct.
FIGURE 16–1 The human ear. The cochlea has been turned slightly. Middle ear muscles have been omitted to make the relationship clear.
FIGURE 16–2 Schematic view of the ear. As sound waves hit the tympanic membrane, the position of the ossicles (which move as shown in blue and black) changes.
Two small muscles can affect the strength of the auditory signal: the tensor tympani, which attaches to the eardrum, and the stapedius muscle, which attaches to the stapes. These muscles may dampen the signal; they also help prevent damage to the ear from very loud noises.
The inner ear contains the organ of Corti within the cochlear duct (Fig 16–3). As a result of movement of the stapes and tympanic membrane, a traveling wave is set up in the perilymph within the scala vestibuli of the cochlea. The traveling waves propagate along the cochlea; high-frequency sound stimuli elicit waves that reach their maximum near the base of the cochlea (ie, near the oval window). Low-frequency sounds elicit waves that reach their peak, in contrast, near the apex of the cochlea (ie, close to the round window). Thus, sounds of different frequencies tend to excite hair cells in different parts of the cochlea, which is tonotopically organized.
FIGURE 16–3 Cross section through one turn of the cochlea.
The human cochlea contains more than 15,000 hair cells. These specialized receptor cells transduce mechanical (auditory) stimuli into electrical signals.
The traveling waves within the perilymph stimulate the organ of Corti through the vibrations of the tectorial membrane against the kinocilia of the hair cells (Figs 16–3 and 16–4). The mechanical distortions of the kinocilium of each hair cell are transformed into depolarizations, which open calcium channels within the hair cells. These channels are clustered close to synaptic zones. Influx of calcium, after opening of these channels, evokes release of neurotransmitter, which elicits a depolarization in peripheral branches of neurons of the cochlear ganglion. As a result, action potentials are produced that are transmitted to the brain along axons that run within the cochlear nerve.
FIGURE 16–4 Structure of hair cell. (Reproduced, with permission, from Hudspeth AJ: The hair cells of the inner ear. They are exquisitely sensitive transducers that in human beings mediate the senses of hearing and balance. A tiny force applied to the top of the cell produces an electrical signal at the bottom, Sci Am 248(1):54–64, 1983.)
AUDITORY PATHWAYS
The axons that carry auditory information centrally within the cochlear nerve originate from bipolar nerve cells in the spiral (or cochlear) ganglion, which innervate the cochlear organ of Corti. Central branches of these neurons course in the cochlear portion of nerve VIII (which also carries vestibular fibers). These auditory axons terminate in the ventral and dorsal cochlear nuclei in the brain stem where they synapse. Neurons in these nuclei send both crossed and uncrossed axons rostrally (Fig 16–5; see also Chapter 7). Thus, second-order fibers ascend from the cochlear nuclei on both sides; the crossing fibers pass through the trapezoid body, and some of them synapse in the superior olivary nuclei. The ascending fibers course in the lateral lemnisci within the brain stem, which travel rostrally toward the inferior colliculus and then project to the medial geniculate body. Because some ascending axons cross and others do not cross at each of these sites, the inferior colliculi and medial geniculate bodies each receive impulses derived from both ears (Fig 16–6). From the medial geniculate body (the thalamic auditory relay), third-order fibers project to the primary auditory cortex in the upper and middle parts of the superior temporal gyri (area 41; see Figs 10–11 and 16–6).
FIGURE 16–5 The vestibulocochlear nerve.
FIGURE 16–6 Diagram of main auditory pathways superimposed on a dorsal view of the brain stem.
Auditory signals are thus carried from the inner ear to the brain by a polysynaptic pathway, unique in that it consists of both uncrossed and crossed components, including the following structures:
Cochlear hair cells → Bipolar cells of cochlear ganglion → Cochlear (VIII) nerve → Cochlear nuclei → Decussation of some fibers in trapezoid body → Superior olivary nuclei → Lateral lemnisci → Inferior colliculi → Medial geniculate bodies → Primary auditory cortex.
Reflex connections pass to eye muscle nuclei and other motor nuclei of the cranial and spinal nerves via the tectobulbar and tectospinal tracts. These connections are activated by strong, sudden sounds; the result is reflex turning of the eyes and head toward the site of the sound. In the lower pons, the superior olivary nuclei receive input from both ascending pathways. Efferent fibers from these nuclei course along the cochlear nerve back to the organ of Corti. The function of this olivocochlear bundle is to modulate the sensitivity of the cochlear organ.
Tonotopia (precise localization of high-frequency to low-frequency sound-wave transmission) exists along the entire pathway from cochlea to auditory cortex.
CLINICAL CORRELATIONS
Tinnitus
Ringing, buzzing, hissing, roaring, or “paper-crushing” noises in the ear are frequently an early sign of peripheral cochlear disease (eg, hydrops or edema of the cochlea).
Deafness
Deafness in one ear can be caused by an impairment in the conduction of sound through the external ear canal and ossicles to the endolymph and tectorial membrane; this is called conduction deafness. Nerve (sensorineural)deafness can be caused by interruption of cochlear nerve fibers from the hair cells to the brain stem nuclei. Tests used to distinguish between nerve and conduction deafness are shown in Table 16–1. Nerve deafness is often located in the inner ear or in the cochlear nerve in the internal auditory meatus; conduction deafness is the result of middle or external ear disease. Progressive ossification of the ligaments between the ossicles, otosclerosis, is a common cause of hearing loss in adults.
TABLE 16–1 Common Tests with a Tuning Fork to Distinguish between Nerve and Conduction Hearing Loss.
A peripheral lesion in the eighth nerve with loss of hearing, such as a cerebellopontine angle tumor, usually involves both the cochlear and vestibular nerves (Fig 16–7). Central lesions can involve either system independently. Because the auditory pathway above the cochlear nuclei represents parts of the sound input to both ears, a unilateral lesion in the lateral lemniscus, medial geniculate body, or auditory cortex does not result in marked loss of hearing on the ipsilateral side.
FIGURE 16–7 Magnetic resonance image of a horizontal section through the head at the level of the lower pons and internal auditory meatus. A left acoustic nerve schwannoma with its high intensity is shown in the left cerebellopontine angle (arrow).
Hearing loss becomes a significant handicap when there is difficulty in communicating by speech. Beginning impairment has been defined as an average hearing-level loss of 16 dB at frequencies of 500, 1,000, and 2,000 Hz. Sounds of these frequencies cannot be heard when their strength is 16 dB or less (a loud whisper). A person is usually considered to be deaf when the hearing-level loss for these three frequencies is at or above 82 dB (the noise level of heavy traffic). Early hearing loss often appears initially at a high frequency (4000 Hz) in both children with conduction impairment and adults with presbycusis (lessening of hearing in old age).
CASE 22
A 64-year-old woman was evaluated for progressive hearing loss, facial weakness, and increasing headaches, all on the right side. Her hearing loss had been present for at least 5 years, and 2 years before admission, she noted the gradual development of unsteadiness in walking. During recent months, she began to experience weakness and progressive numbness of the right side of the face as well as double vision.
Neurologic examination showed beginning bilateral papilledema, decreased pain and touch sensation in the right half of the face, moderate right peripheral facial weakness, and absence of both the right corneal reflex and blinking with the right eye. Tests of air and bone conduction showed that hearing was markedly decreased on the right side. Caloric labyrinthine stimulation was normal on the left; there was no response on the right. On gaze to the right, there was mild weakness of abduction of the right eye (weakness of the abducens). Examination of the motor system, reflexes, and sensations yielded normal results, with the exception of three findings: a broad-based gait, bilateral Babinski signs, and the inability to walk with feet tandem.
What is the differential diagnosis? What is the most likely diagnosis?
Cases are discussed further in Chapter 25.
REFERENCES
Allum JM, Allum-Mecklenburg DJ, Harris FP, Probst R (editors): Natural and Artificial Control of Hearing and Balance. Elsevier, 1993.
Hudspeth AJ: How hearing happens. Neuron 1997;19:947.
Luxon LM: Disorders of hearing. Pages 434–450 in: Diseases of the Nervous System: Clinical Neurobiology. 2nd ed. Asbury AK, McKhann GM, McDonald WI (editors). WB Saunders, 1992.
Mencher, DT: Audiology and Auditory dysfunction: Allyn & Bacon, 1996.
Musiek, FE, Baran JA: Auditory System: Anatomy, Physiology and Clinical Correlates, Pearson Higher Education, 2006.
BOX 16–1 Essentials for the Clinical Neuroanatomist
After reading and digesting this chapter, you should know and understand:
• Anatomy and function of the auditory system
• Cochlea and peripheral mechanisms of hearing
• Tonotopic organization
• Central pathways (cochlear nuclei, trapezoid body, superior olivary nucleus, inferior colliculus, primary auditory cortex)
• Clinical correlations: tinnitus and hearing loss