The Auditory System: An Overview

Auditory System Overview

• Sensory systems in animals are compact and efficient, with the auditory system standing out.
• Starts with capturing energy from atomic movements by the eardrum.
• Uses a small frequency analyzer (cochlea) with few receptors and sensory neurons.
• Analyzes a variety of sounds, but specialized parts mean damage causes specific problems.

Nature of Sound

• Pushing against a medium like air or water increases pressure locally.
• This creates compression waves that move through the medium.
• Pulling back creates rarefaction waves.
• Alternating compresses and rarefies, based on oscillation frequency and amplitude.
• The auditory system: captures wave energy, converts it to electrical signals, interprets the signals.

Range of Audible Frequencies

• Sound is frequencies that cause electrical changes in a species' auditory system.
• Human range: 20 to 20,000 Hz (Hertz, or cycles per second).
• Other animals: audible spectra displaced to lower (elephants, 5 Hz to 12 kHz) or higher frequencies (bats, 2 kHz to 110 kHz).
• Sensitivity varies; humans are most sensitive in the lower mid-frequency range, around 3 kHz, good for speech.

Decibel Scale

• Humans can hear over a remarkable range of sound pressure levels.
• Threshold: minimal meaningful hearing ability.

Outer, Middle, and Inner Ear

• Fish hear easily because sound moves from water to their auditory receptor cells.
• Land vertebrates: transfer airborne sound vibrations to fluids around inner ear receptor cells.
• Challenge: Water density makes it hard to move, 99.9% of sound is reflected at an air-water interface.
• Solution: air-filled outer and middle ears.
• Outer ear funnels vibrations, emphasizing frequencies.
• Middle ear transfers energy to the fluid-filled inner ear.

Outer Ear

• Direct air connection, has the auricle (pinna) and external auditory canal (ear canal).
• Ends at the tympanic membrane (eardrum), the boundary between outer and middle ears.
• Auricle: a cartilaginous plate with elevations and depressions.
• Important part: the concha ("sea shell"), which funnels sound into the ear canal.
• Concha and ear canal resonate at certain frequencies, amplifying sound pressure in a mid-frequency range, peaking at about 3 kHz, by up to 20 dB by the time it reaches the tympanic membrane.
• Corrugations helps localize sound, different amounts of high frequencies are absorbed based on direction, animals learn patterns to decide sound source direction.

Middle Ear

• Air-filled cavity in the temporal bone, transforms the airborne sound to deal with air-water interface.
• Tympanic membrane: lateral wall.
• Periosteal surface of temporal bone: medial wall.
• Air connection to the pharynx (eustachian, or auditory, tube) maintains pressure.
• Ossicles bridge the cavity: malleus ("hammer"), incus ("anvil"), and stapes ("stirrup").
• Malleus attaches to the tympanic membrane and incus.
• Incus attaches to stapes.
• Stapes footplate occupies a hole in temporal bone (oval window).
• Fluids of the inner ear are on the other side of the oval window.
• Stapes acts like a piston, inward/outward movement results in pushes/pulls on the footplate.
• Reciprocal movements of membrane accommodate pushes and pulls to in ensure fluidity.
• Malleus and incus: lever system, force increase at stapes footplate with mechanical advantage.
• Tympanic membrane area is nearly 20 times that of the oval window, middle ear system converts large-amplitude, low-force vibrations at the tympanic membrane into vibrations of smaller amplitude but sufficient force to move inner ear fluids.
• Energy is nearly perfectly transferred with only 3 dB loss at about 1 kHz.

Middle Ear Muscles

• Tiny muscles dampen vibrations into inner ear fluids, attached to two of the middle ear ossicles.
• Tensor tympani (trigeminal nerve) extends from a canal above the auditory tube to the malleus handle.
• Contraction pulls on the tympanic membrane, diminishing its ability to vibrate.
• Stapedius (facial nerve) occupies a cavity in temporal bone, tendon attaches to malleus.
• Stapedius pulls the stapes sideways, diminishing the footplate's movement.
• Stapedius contracts in response to a loud sound, tensor tympani contracts to painful sounds, they decrease transmission of low-frequency sounds, enhancing high-frequency environmental sounds during normal activities.

Middle Ear Dysfunction

• Pathologic conditions can impede transfer of energy through the middle ear.
• Ex: infections (otitis media), trauma, or bony growths (otosclerosis).
• Conditions cause inner ear to absorbs 0.1% of the incident sound with 30 dB loss.
• Hearing loss may be up to 60 dB because airborne sound hits both oval and round windows, it tries to compress incompressible fluid.
• Vibrations delivered directly help assess damage.
• Air and bone conduction are assessed with an audiometer for a pure-tone audiogram.
• Brief single-frequency tones cover part of the audible spectrum (250 Hz to 8 kHz).
• Tones are delivered through headphones or a vibrating probe, intensity gradually increased until audible.
• Audiometer accounts for sensitivity irregularities and greater efficiency of air conduction, threshold is plotted as 0 dB at all frequencies with a normal ear.
• Hearing loss causes outer ear obstruction and middle ear damage.

Inner Ear

• Tortuous system of fluid-filled channels called the labyrinth contains receptor cells of auditory and vestibular systems.
• Two systems of channels: bony labyrinth (cavity in temporal bone) and the membranous labyrinth (suspended within).
• Semicircular canals and cochlea of bony labyrinth house semicircular ducts and cochlear duct, vestibule houses utricle and saccule.
• Cochlea is for hearing, remaining components are in Chapter 11.
• Bony and membranous labyrinths appear spatially complex, each is continuous.
• Spaces contain different fluids, diffusion barriers in the walls of the membranous labyrinth separate one from the other.
• Bony labyrinth is filled with perilymph, has a composition typical of extracellular fluids and is continuous with the CSF around the base of the brain.
• Membranous labyrinth is filled with endolymph, has an ionic composition more like that of cytoplasm which produces fluid thought to involve labyrinthine pathology.

Hair Cells

• Subdivisions of the membranous labyrinth includes discrete hair cells, modified epithelial cells that are characteristic receptors.
• "Hairs:" actin-filled microvilli, called stereocilia, arranged by height.
• Each contains a kinocilium, a true cilium near the tallest stereocilia (degenerates in adult cells).
• Tallest stereocilia attach to overlying gelatinous mass; movement causes stereocilia deflection.
• Actin core makes stereocilia rigid, secured movement as a unit.
• Filamentous proteins called tip links join each stereocilium to its neighbor, attachment opens mechano-sensitive cation channel.
• Hair bundle deflection in the direction of the tallest stereocilia increases tension on tip links and opens channels, allowing K+ influx that depolarizes hair cells.
• Tension Deflection away from tall stereocilia causes hyperpolarization, deflection parallel has little effect.
• Receptor potentials are converted to increased and decreased glutamate release, resulting in changes in eighth nerve fibers.

Cochlea

• Pea-sized structure that curves through 2½ turns of a tapering spiral that surrounds the bony core (the modiolus).
• Base and apex orientation the spiral with osseous lamina extending as a shelf with subdivisions.
• Scala vestibuli and scala tympani perilymph-filled compartments opening to round window which are connected by an aperture.
• Scala vestibuli to vestibule, Scala tympani ends at the round window membrane.
• Cochlear duct (scala media) extends from osseous spiral lamina to cochlea wall, completes separation of Scala vestibuli from Scala tympani.
• Cochlear duct is triangular, made of Reissner's membrane and the basilar membrane separating fluids.
• Organ of Corti containing cochlear hair cells spirals around with basilar membrane.
• Oscillations of stapes footplate cause waves of compression and rarefaction that move through cochlear perilymph, reaching round window membrane.
• Low frequencies move through the helicotrema, audible frequencies cut across the cochlear duct, deflecting the basilar membrane.

Basilar Membrane

• Reissner's membrane: solely a diffusion barrier between scala vestibuli and scala media.
• Thicker and collagenous basilar membrane has mechanical properties smoothly from base to apex, allowing function as miniature frequency analyzer.
• Perilymph oscillations causes traveling waves of deformation to maximum. Deformation is dependent on frequency.
• The audible spectrum is mapped out, from low to high frequencies, beginning of a tonotopic mapping of sound frequencies.

Organ of Corti

• Cochlear hair cells work with supporting cells, spiral around on basilar membrane composing the organ of Corti.
• Inner hair cells are flask-shaped, numbering about 3500, form a single strip.
• Outer hair cells are numerous (12,000), wide suspended above basilar membrane by supporting cells.
• Outer hair cells are inserted with vibration causing deflection of stereociliary bundles with different pivot points, unlike inner hair cells.
• Inner hair cells extend across cleft but are not attached to tectorial membrane and as such, are stimulated by endolymph squirts.
• Inner hair cells provide information stimuli which conveys CNS by VIII and outer power amplifier hinted by innervation.
• Spinal ganglion houses auditory afferent for cells bodies.

Auditory Connections in the Central Nervous System

• Auditory parts of the CNS figure out the identity and location of sounds.
• Deciphering the identity of a sound is based on analyzing frequencies used by frequency information.
• Different parts of the basilar membrane are tuned to different frequencies.
• Nerve fibers form the basis of a place code for frequency and multiple fibers have particular parts.

Subcortical Auditory Connections

• Auditory cortex sends signals back to cochlear.
• Cochlear nerve fibers terminate in the ipsilateral cochlear nuclei.
• Cochlear nuclei project bilaterally to brainstem nuclei, important ones include the superior olivary nucleus and the inferior colliculus.
• Projections are bilateral and indicate that hearing loss strictly confines it.
• Some neurons primarily convey duration of sounds it projects to both inferior.