AUDITORY I: PERIPHERAL MECHANISMS

I. Introduction and Organization of the Lecture

1. Conductive Hearing Loss
• Occurs in 100 million Americans a year.
• Almost totally reversible; spontaneously or by treatment.
• Represents a blockage of sound conduction to the cochlea.
• An outer or middle ear problem.
2. Sensorineural Hearing Loss
• Occurs currently in 22-24 million Americans.
• Is graded from mild to severe to profound to deaf.
• Virtually non-reversible.
• Most commonly occurs as damage or loss to hair cell receptors, damage or loss to the fibers of the auditory nerve, or more rarely as damage to the auditory regions of the CNS.
3. This lecture will attempt to provide an introduction to the operations of the peripheral ear.

1. Pinna and Outer Ear
• Role of pinna in collecting and funneling sound.
• Role of external auditory meatus.
2. Middle-Ear Structures
• Tympanic membrane as the collector of sound.
• Ossicles: malleus, incus, and stapes.
• Middle ear muscles: tensor tympani and stapedius.
• Impedance matching function.
• Eustachian tube as a pressure equalizer.
3. Cochlear Organization
• The labyrinth is located in the temporal bones.
• The labyrinth consists of two parts: vestibular and cochlear.
• Vestibular apparatus used for balance.
• Cochlear portion: a snail shaped structure, 33-mm in length with 3 chambers (upper, middle, and lower).
• Scalae vestibuli and scala tympani are upper and perilymphatic chambers; called the bony labyrinth chambers.
• Scalae media is the middle or endolymphatic chamber; also called the membranous labyrinth.
• Perilymph has an ionic concentration where Na K; perilymph is a filtrate of CSF.
• Endolymph has an ionic concentration where K Na; endolymph is secreted by stria vascularis; stria vascularis secrets charged K; the ionic gradient between scala vestibuli and scala media is +80mV (referenced against scala media.
• The upper and lower chambers are inter-connected at the top (apex) of the cochlea.
• The stapes footplate inserts into the oval window at the cochlear base; the oval window is at the lower extent of scala vestibuli.
• The basal extent of scala tympani is the round window which opens onto the air space of the middle ear.
• The Organ of Corti is found in the middle chamber.
• The Organ of Corti has two receptor cell types: outer and inner hair cells.
• There are 3750 inner hair cells and 11250 outer hair cells.
• These cells are organized into a single row of inner and three rows of outer hair cells extending the length of the cochlea.
• Each inner hair cell is innervated by 8-12 afferent nerve fibers; 40,000 of these fibers are bundled into the auditory option of the 8^th cranial nerve.
• Each outer hair cell is innervated by one afferent and one or two efferent nerve fibers; each efferent fiber synapses on 5 - 10 outer hair cells.
• The Organ of Corti rests on the basilar membrane; this membrane contains radial and longitudinal fibers; the thickness and width of the membrane vary from one end of the cochlea to the other producing a gradient of stiffness from base to apex (the base is where the stapes footplate inserts into the cochlea, the apex is at the top of the snail shell).
• Each hair cell has a tuft of sensory hairs at the apical end of the cell; this tuft is called the sensory hair bundle or stereocilia; the hairs are organized into three rows of descending height.
• Lying above the hair cells is a structure called the tectorial membrane; this is a gelatinous structure; the tallest row of outer hair cell hairs are embedded in this; the tallest row of inner hair cell hairs are not in contact with the hair bundle.

1. Inner-Ear Mechanics
• Pressure variations at the tympanic membrane are converted to mechanical movements in the ossicles; movements of the stapes footplate sets up fluid pressure waves in the cochlea.
• This pressure wave progresses from the upper (scala vestibuli) through the middle (scala media) to the lower (scala tympani) cochlear chamber.
• As the pressure wave moves through scala media, a tiny portion of the energy is absorbed by the basilar membrane and it is set in vibration.
• Movement of the basilar membrane is translated into motion of the sensory hairs.
• Hair bundle movement leads to hair cell depolarization, neurotransmitter release, and nerve excitation.
• The gradient of basilar membrane stiffness can be viewed as a variable impedant barrier between the upper and lower cochlear chambers.
• This impedant barrier is frequency dependent; at each cochlear location only certain frequencies will have the pressure wave flow from the upper to lower chamber.
• The impedant gradient causes a frequency to place translation along the cochlear length; every frequency causes a maximal basilar membrane vibration at a slightly different cochlear location.
• Higher frequencies are coded in the cochlear base while low frequencies are found toward the apex.
• This cochlear property performs a spectral analysis; the multiple frequencies found in complex sound are broken down along the basilar membrane and represented as more or less discrete locations of mechanical activity.
• Of course, hair cells at these locations are activated when the corresponding cochlear location is set in motion.
2. Hair Cell Transduction
• Mechano-sensitive transduction channels are found in the shafts and tips of the stereocilia.
• The hair cells are organized so that the motion applied to the hair bundle results in movements in the plane of the staircase; the hairs are deflected toward or away from the tallest hairs in the bundle.
• This motion causes a stretch or relaxation of the tip links and opening or closing of the transduction channel.
• The transduction current is carried by the potassium ion in scala media; this ion is highly charged and is driven into the cell when the channel is opened.
• The receptor potential is a saturating, non-linear, voltage analogue of hair bundle motion.
• Movements of the hair bundle in the direction of the tallest hairs results in membrane depolarization; movement in the opposite direction causes the cell to hyperpolarize.
• This process follows the stimulus on a cycle-per-cycle basis at low frequencies; at high frequencies charge builds on the plasma membrane and the cell exhibits a net depolarization.
• Stimulus intensity is proportional to hair bundle displacement; displacement is proportional to the degree of membrane depolarization; depolarization is proportional to neurotransmitter release, and this release is proportional to the number of discharges in the 8th nerve fiber.
3. Auditory Nerve Properties
• Type I afferent auditory fibers (90%) are myelinated and communicate with the inner hair cells; Type II are unmyelinated and connect to the outer hair cells.
• The cell bodies of the auditory nerve form the spiral ganglion.
• All afferent auditory nerves are spontaneously active.
• All auditory nerve fibers exhibit a "primary like" PST histogram.
• Each auditory nerve fiber exhibits frequency selective properties; ti shows a preferred response at a selected range of frequencies.
• Frequency selectivity is determined from the tuning curve.
• The tuning curve defines the receptive field of the fiber.
• Tuning curves have tip and tail regions; the most sensitive part of the tip is the CF or characteristic frequency.
• A the CF, the discharge rate of the neuron goes from threshold to saturation only over a 40 dB range; how then is our ability to code loudness over a 100 dB range coded?
• The tuning curve of a fiber is traceable to the hair cell of origin and the frequency coding at the cochlear location.
• The organization of pitch then is largely a "labeled line" phenomenon.
• The loudness or intensity of a sound is coded in the discharge rate within the auditory nerve fiber.