Chapter 16: The Auditory System

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.

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.

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.

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.

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).

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.