Chapter 7D - Auditory System
There are three components of the ear subserving hearing function: outer ear, middle ear, inner ear. The outer ear consists of the pinna, which collects sound waves, and the external auditory meatus, ending at the tympanic membrane. Internal to this tympanic membrane is the middle ear containing three bones, the malleus, incus and stapes. Pressure in the middle ear is equilibrated with the external environment via the auditory (Eustachian) tube. The ossicular chain ends at the oval window on the medial wall of the middle ear. The footplate of the stapes and its ligaments seal the opening into the inner ear that contains the cochlea and the vestibular apparatus (the semicircular canals and organs of the vestibule, discussed in the next section). The chief functions of the outer and middle ear are to transmit sound waves from air into fluid of the inner ear via the tympanic membrane and ossicular chain. Since it takes almost 13 times more energy to vibrate fluid than it does air, sound waves from a large amount of air have to be focused on a small area, a process called impedance matching. This is why the tympanic membrane is about 13 times larger than the footplate of the stapes. Anything that impedes vibration of the tympanic membrane or ossicular chain would produce "conductive deafness" in which a vibrating tuning fork placed on the head would actually seem to be louder in the deaf ear (vibrations of the skull bypass the ossicles).
There are two muscles that can modulate the movement of the bones of the inner ear. The tensor tympani muscle, innervated by the trigeminal nerve, is attached to the handle of the malleus. The stapedius muscle, innervated by the facial nerve, is attached to the stapes. Contraction of these muscles attenuates the movement of the bones, and therefore diminishes sound before it reaches the inner ear.
The frequency of vibration in the air indicates the pitch, while the amplitude of the vibrations equates to the loudness of the sound. Human hearing is in the range of 20 to 20000 Hz. This upper limit of frequency exceeds the maximum frequency at which a nerve can conduct action potentials. Therefore, these frequencies of sound cannot be translated by the auditory system directly into frequency of action potentials.
The inner ear consists of a boney cavity (the bony labyrinth), which is filled with perilymph and a watertight membrane-lined sac (membranous labyrinth) containing endolymph inside (the membranous labyrinth is, much like a fluid-filled balloon inflated in a bottle filled by a different fluid) (figure 17). The receptive cells for hearing are contained within the cochlea. This cochlea, which is coiled for 2.5 turns, contains three fluid filled compartments. The oval window, noted above, displaces perilymph in the vestibule of the inner ear. The vestibule is continuous with the scala vestibuli. At the tip of the cochlea (the helicotrema), the perilymph of the scala vestibuli communicates with the perilymph of the scala tympani, which ends blindly at the round window. The round window appears on the medial wall of the middle ear and is closed by a membrane. Displacement of the oval window by movement of the stapes produces a pressure wave that is transmitted along the scala vestibuli and tympani, ultimately vibrating the membrane closing the round window. The perilymph that fills the scala vestibuli and tympani has a composition very similar to that of extracellular fluid and may represent an ultrafiltrate of plasma.
Separating the scala vestibuli from the scala tympani is a duct that is part of the membranous labyrinth called the scala media or cochlear duct. The cochlear duct is triangular, with the vestibular (Reisner's) membrane separating it from the scala vestibuli and the basilar membrane separating it from the scala tympani. The third side of the triangle is comprised of the spiral ligament that contains the blood vessels of the stria vascularis, which ultimately, produce endolymph. The cochlear duct is a blind sac, filled with endolymph, which differs in composition from the perilymph in that the potassium concentration is much higher. This endolymph more closely resembles intracellular fluid. Due to this ion gradient, there is a potential difference of +80 mv between the endolymph and perilymph.
Hair cells are the receptive cells of the inner ear. They are stretched along the cochlear duct in two rows (the inner and outer hair cells) positioned on a thickened ridge of the basilar membrane. Upon each segment of the basilar membrane are three outer hair cells and one inner hair cell. In humans, outer hair cells are more numerous (approx. 12,000) than inner hair cells (approx 3500). The "hairs" on these cells project toward the inside of the cochlear duct and are embedded in a covering membrane (the tectorial membrane). Collectively, these structures are termed the organ of Corti and this is the location of sound transduction. The "hairs" consist of a series of cilia and one longer steriocilia (called the kinocilium), which is located off to one side of the hair cell (figure 18B). At the apex of the cilia there are protein filaments that attach to adjacent cilia. These filaments are associated with ion channels that open when tension is placed on the filament. Deformation of the cilia towards the kinocilium places tension on the filaments. This depolarizes the hair cell and increases release of neurotransmitter from the base of the hair cells. The hair cells have a resting membrane potential of approximately -65 mv, compared to the endolymph. When the hair cell is depolarized, calcium enters the cell and neurotransmitter (probably glutamate) is released onto peripheral terminals of afferent fibers in the vestibulocochlear nerve.
Despite the fact that there are more outer hair cells, the inner hair cells have much denser innervation and the auditory system receives more afferent input from the inner than outer hair cells. Another difference between inner and outer hair cells is in their efferent innervation. Both receive projections from the brain stem, mostly the superior olivary nucleus (olivocochlear bundle). When activated, these fibers release neurotransmitter, probably acetylcholine, which hyperpolarizes the inner and outer hair cells. Such inhibition is more effective for the outer hair cells since efferents end directly on the cell body (as opposed to ending on the afferent axons of the inner hair cells). This olivocochlear efferent innervation can "fine tune" auditory perceptions.
Different frequencies of vibration in the perilymph will produce particular patterns of maximal vibration in the basilar membrane due to varying length, and stiffness of the membrane at different parts of the cochlea. For example, low frequencies produce a maximal displacement of the apical end of the cochlea whereas high frequencies produce a maximal displacement of the base. This mapping of frequency of vibration (i.e., pitch) onto location in the cochlea is the reason that the brain is able to detect different frequencies of sound. These frequencies are transmitted to the central nervous system via the central process of the neurons of the spiral ganglia. These processes comprise the vestibulocochlear nerve.
The hallmark of the central auditory pathways is divergence. Such divergence commences immediately upon the afferent fiber entering the brainstem, with a bifurcation to terminate in the dorsal and ventral cochlear nucleus. The cochlear nuclei have a "tonotopic" organization with cells of progressively higher frequency characteristics being arrayed in a progression along the axis of the nuclei. Neurons of the ventral cochlear nucleus send fibers to the medial and lateral superior olivary nuclei on the ipsilateral and contralateral sides of the brain. Divergence continues with neurons of the olivary nuclei projecting to both the ipsilateral and contralateral lateral lemniscus. The lateral lemniscus projects to neurons of the inferior colliculus of the midbrain, which relay the signal to the medial geniculate nucleus of the thalamus, this nucleus, in turn, relays the signal to the primary auditory cortex of the superior temporal lobe (transverse gyri of Heshel). At all levels of the system, a tonotopic representation is maintained. The auditory association area surrounds the primary auditory cortex and is necessary for recognition of the significance of sounds. In the dominant hemisphere, this is particularly important for recognition of language (Wernicke's area) and includes much of the angular and supramarginal gyrus.
This divergence of auditory information from the cochlea is evidenced by the finding that many neurons along the "auditory pathway" receive information from both ears. There is a difference in arrival time from each ear (the "binaural beat") that the brain uses to lateralize sounds. Also, progressively more complicated stimuli are necessary in order to activate neurons at progressively higher centers of the auditory pathway. Thus, many neurons in the brainstem do not respond to a pure tone, but only to complicated frequency patterns.
The divergence within the auditory system also makes it exceedingly improbable that any single lesion involving the brainstem or cerebral cortex will cause deafness in both ears or even total deafness in one ear.