PERIPHERAL PATHWAYS FOR SOUND RECEPTION

The sound transduction process involves complex mechanical transduction of sound waves through the external ear and the external acoustic meatus and across the tympanic membrane; there it is leveraged as a mechanical force by the bones of the middle ear (ossicles) via the oval window to produce a fluid wave in the cochlear duct. This fluid wave causes differential movement of the basilar membrane, stimulating hairs on the apical portion of hair cells to release neurotransmitters that stimulate primary sensory axons of neurons of the cochlear (spiral) ganglion. The basilar membrane in the cochlea shows maximal displacement spatially according to the frequency of impinging tones, with low frequencies maximally stimulating the apex (helicotrema) and high frequencies maximally stimulating the base. The eustachian (pharyngotympanic) tube permits pressure equilibrium between the middle ear and the outside world.

CLINICAL POINT

Hearing loss may be partial or total and can involve virtually any range of detectable frequencies. The most devastating for human communication is a loss in the frequencies of speech (300 to 3000 Hz) of 40 or more decibels. In general, hearing loss can be subdivided into two categories: sensorineural and conductive. Sensorineural hearing loss involves damage to the hair cells, the auditory nerve, or central auditory pathways. Because of the neural damage, both air conduction and bone conduction are diminished. Conductive hearing loss involves damage to the outer or middle ear. Air conduction is impaired because the sound is not properly transduced into the inner ear, but bone conduction is normal. These two types of hearing loss can be tested for at the bedside by using a tuning fork of 512 Hz. The Weber test involves placing the vibrating tuning fork on the center of the forehead. Normally, the patient hears the fork equally in both ears. With sensorineural loss, the sound is heard best in the unaffected ear; with conductive loss, the sound is heard best in the affected ear. The Rinne test involves holding the vibrating tuning fork against the mastoid bone. When the fork is no longer heard, it is immediately placed just outside the external auditory meatus. Normally, air conduction is more effective than bone conduction, and the fork will again be heard when moved adjacent to the external auditory meatus (air conducting sound better than bone). If conductive hearing loss is present, once bone conduction is no longer heard, air conduction also will not be heard (bone conducting sound better than air). If sensorineural hearing loss is present, air conduction may be greater than bone conduction, although both may be diminished

BONY AND MEMBRANOUS LABYRINTHS

The relationship between the cochlea and the vestibular apparatus (utricle, saccule, semicircular canals, and ducts) and the bony labyrinth that surrounds them is illustrated. The ossicles (malleus, incus, stapes) leverage the movement of the tympanic membrane to produce movement of the oval window. Movement of the oval window causes a fluid wave to move through the scala vestibuli and the scala tympani of the cochlea and ricochet onto the round window. The three semicircular canals are located at 90-degree angles to each other, representing tilted X, Y, and Z axes.

CLINICAL POINT

The semicircular canals (ducts) contain the ampullae that have hair cells that respond to angular acceleration. The utricle contains the otolith organ in the macula that responds to linear acceleration and detects gravitation. The saccule responds best to vibratory stimuli. The cochlea contains the hair cells that respond to fluid movements in the scalae vestibuli and tympani, brought about by the leveraging of the ossicles against the oval window; this movement affects hair cells in the cochlear duct.

The activity of the utricle can sometimes become distorted when debris moves away from the hairs and induces activation of the hair cells in the ampulla of the posterior semicircular canal. This produces vertigo and nystagmus that are associated with a specific position of the head (benign postural or positional vertigo). This disorder is the most common cause of vertigo seen in neurological practice. These attacks commonly occur when the patient is lying down, moving to a particular position, or tilting the head back; they may recur either briefly or for a longer period of days or weeks. Attacks can be induced by an examiner through the Hallpike maneuver (tilting the patient's head back and then 30 degrees to the side), resulting in a brief attack of vertigo and nystagmus. No pharmacological treatment is available. Attempts to reposition the debris by deliberate Hallpike-like head movements have met with some success.

VIII NERVE INNERVATION OF HAIR CELLS OF THE ORGAN OF CORTI

Primary sensory axons of the spiral (cochlear) ganglion innervate inner and outer hair cells of the organ of Corti, located on the basilar membrane. The axons are activated by release of neurotransmitters from the hair cells, which occurs when the hairs on the apical surface are moved by shearing forces resulting from movement of the basilar membrane (fluid wave through the scalae vestibuli and tympani) in relation to the more rigidly fixed tectorial membrane. This represents the complex transduction process of the conversion of external sound waves to action potentials in spiral ganglion axons. The ionic potentials (in mV) are indicated for the scala tympani and vestibuli (perilymph) and the cochlear duct (endolymph). These potential differences contribute to the excitability of the hair cells.

CLINICAL POINT

Hair cells in the organ of Corti respond to fluid movements in the scalae vestibuli and tympani that induce shearing motion of the tectorial membrane relative to the basilar membrane. Each region of the spiraled cochlea contains hair cells that respond optimally to movement of the basilar membrane; low frequencies stimulate hair cell movement in the apex (helicotrema), and high frequencies stimulate hair cell movement in the basilar coils of the cochlea. The hair cells can be damaged by many pathological processes, such as viral infections (e.g., mumps); drugs (e.g., quinine); antibiotics; exposure to sustained loud noise; and age-related deterioration caused by free-radical damage. Exposure to loud noises above 85 decibels can selectively damage hair cells, especially those in the basilar coils of the cochlea that transduce high-frequency sounds. High-pitched machinery noise (jet engines), gunfire without ear protection, exposure to loud music at concerts or by earphones, and loud ambient noise in construction or industrial sites can induce temporary damage that can become permanent with repeated exposure. Environmental protection regulations now require ear protection in personnel working at such sites.

COCHLEAR RECEPTORS

Fluid movement through scala vestibuli, around the helicotrema, and back through the scala tympani differentially moves the basilar membrane on which the organ of Corti and its hair cells reside. Movement of hairs on the apical portion of the hair cells by shearing forces of the tectorial membrane results in their depolarization and the release of neurotransmitters. This release stimulates action potentials in the primary afferent axons of spiral ganglion cells. Efferent axons from the olivocochlear bundle, controlled by descending central auditory pathways, can modulate the excitability of hair cells and the sensory transduction process.

AFFERENT AUDITORY PATHWAYS

Central axon projections of the spiral ganglion neurons terminate in dorsal and ventral cochlear nuclei in several tonotopic maps (receptor origination shown in the cochlea in colors). These cochlear nuclei project into the lateral lemniscus via acoustic striae; many of these projections remain ipsilateral. The lateral lemniscus terminates in the nucleus of the inferior colliculus, which in turn projects via the brachium of the inferior colliculus to the medial geniculate body (nucleus) of the thalamus. The thalamus sends tonotopical projections to the primary auditory cortex on the transverse gyrus of Heschl. Several accessory auditory brain stem nuclei (the superior olivary nucleus for lateral sound localization, the nuclei of the trapezoid body [not shown], and the lateral lemniscus) send both crossed and uncrossed projections through the lateral lemniscus. Sound is represented throughout the afferent auditory pathways bilaterally; thus, a unilateral lesion in the lateral lemniscus, the auditory thalamus, the auditory radiations, or the auditory cortex does not produce contralateral deafness. With such a lesion, there is a diminution in hearing and auditory neglect contralateral to the lesion with bilateral simultaneous stimulation.

CLINICAL POINT

The cochlear nerve contains axons that innervate the hair cells of the organ of Corti in the spirals of the cochlea. Primary cochlear axons enter the lateral portion of the caudal pons, terminating in the dorsal and ventral cochlear nuclei with several tonotopically representative maps of the auditory frequency world. The auditory nerve can be damaged by infections, tumors (e.g., acoustic Schwannoma), and traumas, particularly those associated with the petrous portion of the temporal bone. Irritation of auditory nerve fibers can produce tinnitus, a sense of ringing in the ears (or buzzing, humming, clicking, or other sounds). When the nerve is actually destroyed, the tinnitus stops and hearing loss ensues. Auditory nerve damage has symptoms that are present on the ipsilateral side with respect to the damage. In the brain stem, the acoustic striae project axons to a host of nuclei in bilateral fashion, including the superior olivary nuclei, the nuclei of the trapezoid body, the nuclei of the lateral lemnisci, and the inferior colliculi. The inferior colliculi, a mandatory synaptic processing site for central auditory processing, receive information from both ears. These projections proceed to the medial geniculate nucleus and then via the auditory radiations to the auditory cortex (transverse gyrus of Heschl). Damage in the interior of the brain stem or, more likely, in the temporal lobe, generally caused by a vascular infarct, tumor or abscess, or trauma, may result in diminished hearing but not unilateral deafness.

CENTRIFUGAL (EFFERENT) AUDITORY PATHWAYS

Descending pathways travel from the auditory cortex, the medial geniculate body of the thalamus, the inferior colliculus, and accessory auditory nuclei of the brain stem to terminate in caudal structures in the pathway, such as the cochlear nuclei and the superior olivary nucleus. These centrifugal connections permit descending control of incoming auditory information. The olivocochlear bundle, from the superior olivary nuclei, projects back to the hair cells in the organ of Corti and modulates the transduction process between the hair cells and the primary afferent axons. The motor nuclei of V and VII send LMN axonal projections to the tensor tympani and stapedius muscles, respectively, for reflex dampening of the ossicles in the presence of sustained loud noise.