Tympanic Membrane and Ossicular System

• This chapter explains how the ear collects sound waves
• The ear differentiates their frequencies
• Auditory information is sent to the central nervous system for interpretation

Tympanic Membrane and Ossicular System

• The tympanic membrane, also known as the eardrum, works with the ossicles to conduct sound
• Sound travels from the tympanic membrane through the middle ear to the cochlea
• The malleus connects to the tympanic membrane
• The malleus is linked to the incus via small ligaments
• Movement of the malleus results in movement of the incus
• The opposite end of the incus connects to the stapes
• The stapes' faceplate rests against the membranous labyrinth of the cochlea, specifically at the oval window opening
• The malleus' handle tip is attached to the center of the tympanic membrane
• The tensor tympani muscle constantly pulls on this attachment point, keeping the tympanic membrane taut
• This tension enables sound vibrations on any part of the tympanic membrane to be transmitted to the ossicles
• The malleus and incus act as a single lever, balanced approximately at the tympanic membrane's border
• The articulation between the incus and stapes allows the stapes to push forward on the oval window
• The stapes then pulling backward on the fluid each time the malleus moves outward

Impedance Matching

• The stapes faceplate's movement amplitude is only three-fourths of the malleus handle's amplitude
• The ossicular lever decreases movement distance but amplifies movement force by about 1.3 times
• The tympanic membrane's surface area is about 55 square millimeters, while the stapes' area is about 3.2 square millimeters
• This 17-fold difference, combined with the lever system's 1.3 ratio, results in about 22 times more force being exerted on the cochlea's fluid
• The force is relative to the force exerted by sound waves against the tympanic membrane
• Impedance matching occurs between sound waves in the air and sound vibrations in the cochlea's fluid
• The matching is about 50% to 75% effective for sound frequencies between 300 and 3000 cycles/sec
• This ensures that most of the incoming sound waves' energy is utilized
• Without the ossicular system and tympanic membrane, sound waves can still reach the cochlea through the middle ear air at the oval window
• The sensitivity for hearing is reduced by 15 to 20 decibels, equivalent to decreasing from a medium to a barely perceptible voice level

Sound Attenuation

• Loud sounds trigger a reflex causing contraction of the stapedius muscle and, to a lesser extent, the tensor tympani muscle after a latent period of 40 to 80 milliseconds
• The tensor tympani muscle pulls the malleus's handle inward, while the stapedius muscle pulls the stapes outward
• These opposing forces increase the ossicular system's rigidity, reducing the conduction of low-frequency sounds (mainly below 1000 cycles/sec)
• The attenuation reflex can reduce the intensity of lower-frequency sound transmission by 30 to 40 decibels
• The function: protect the cochlea from damage and mask low-frequency sounds in loud environments
• Masking removes background noise, aiding concentration on sounds above 1000 cycles/sec, where most voice communication information exists
• The tensor tympani and stapedius muscles decrease sensitivity to a person's own speech
• Collateral nerve signals activate this effect simultaneously with the brain activating the voice mechanism

Bone Transmission

• Vibrations of the entire skull can cause fluid vibrations in the cochlea because the inner ear sits inside the bony labyrinth
• A tuning fork or electronic vibrator on any bony part of the skull, especially the mastoid process, can cause the person to hear the sound
• Ordinary loud airborne sound isn't sufficient to cause hearing via bone conduction without an amplifying device

Cochlea Anatomy

• The cochlea consists of three coiled tubes: the scala vestibuli, scala media, and scala tympani
• Reissner's membrane (or vestibular membrane) separates the scala vestibuli and scala media
• The basilar membrane separates the scala tympani and scala media
• The organ of Corti contains electromechanically sensitive hair cells
• These hair cells serve as the receptive end organs, generating nerve impulses in response to sound vibrations

Cochlea Function

• Reissner's membrane is thin and easily moved, so it does not obstruct sound vibrations from the scala vestibuli into the scala media
• The scala vestibuli and scala media are considered a single chamber for fluid conduction of sound
• Reissner's membrane maintains a special fluid in the scala media, which is required for the function of hair cells
• Sound vibrations enter the scala vestibuli from the stapes' faceplate at the oval window
• The faceplate connects to the window's edges by a loose annular ligament, enabling inward and outward movement with sound vibrations
• Inward movement causes fluid to move forward, and outward movement causes the fluid to move backward

Basilar Membrane

• The basilar membrane separates the scala media from the scala tympani and contains 20,000 to 30,000 basilar fibers
• Fibers project from the modiolus (bony center) toward the outer wall
• These fibers are stiff, elastic, reed-like structures fixed at their basal ends in the modiolus but not at their distal ends
• The fibers can vibrate freely like the reeds of a harmonica
• Fiber lengths increase from 0.04 millimeter near the oval and round windows to 0.5 millimeter at the helicotrema
• Fiber diameters decrease from the oval window to the helicotrema, so their stiffness decreases more than 100-fold
• Stiff, short fibers vibrate best at high frequencies
• Long, limber fibers vibrate best at low frequencies
• High-frequency resonance occurs near the base, where sound waves enter through the oval window
• Low-frequency resonance occurs near the helicotrema because of less stiff fibers and increased loading with extra masses of fluid

Transmission of Sound Waves

• When the foot of the stapes moves inward against the oval window, the round window must bulge outward
• A sound wave entering at the oval window causes the basilar membrane at the base to bend
• Elastic tension in the basilar fibers initiates a fluid wave that travels along the membrane toward the helicotrema
• Each wave is relatively weak at first but strengthens upon reaching the membrane section with a natural resonant frequency
• Vibration causes energy dissipation, and the wave dies at this point
• High-frequency waves travel a short distance to their resonant point
• Medium-frequency waves travel about halfway
• Low-frequency waves travel the entire distance

Basilar Membrane Vibration

• Traveling waves move fast along the initial basilar membrane portion but slows down
• High elasticity near the oval window and a decreasing coefficient further along the membrane causes the differing wave speeds
• Rapid initial transmission allows the high-frequency sounds to travel far enough to spread out and separate
• Frequency cannot be discriminated if high-frequency waves bunch together
• Basilar membrane vibration amplitude patterns are generated by sound waves
• Maximum amplitude for 8000 cycles/sec sound (high-frequency) happens closer to the cochlea's base
• Maximum amplitude for frequencies less than 200 cycles/sec (low-frequencies) happens at the tip of the basilar membrane near the helicotrema

Function of the Organ of Corti

• The organ of Corti lies on the basilar membrane and generates nerve impulses in response to its vibration
• Two types of hair cells are sensory receptors
• The single row of internal (inner) hair cells numbers about 3500
• They have diameters of about 12 micrometers
• The three or four rows of external (outer) hair cells number about 12,000
• They have diameters of only about 8 micrometers
• The hair cells are connected via synapses to a network of cochlear nerve endings
• Between 90% and 95% terminate on inner hair cells, showing their significance for sound detection
• Nerve fibers that are stimulated by the hair cells connect to the spiral ganglion of Corti, in the cochlea's modiolus
• Spiral ganglion neuronal cells transmit around 30,000 axons into the cochlear nerve
• They then enter the central nervous system at the upper medulla level

Hair Cell Excitation

• Minute hairs project upward from the hair cells
• Those hairs touch or get embedded in the coating above the stereocilia in the scala media called the tectorial membrane
• Bending hairs in one direction depolarizes them, and bending in the opposite direction hyperpolarizes them
• The basilar membrane's vibration excites the hair endings
• The hair cell's outer ends are fixed tightly in the reticular lamina that moves as a cohesive unit
• Upward movement rocks the reticular lamina upward and inward toward the modiolus
• Hairs shear back and forth against the tectorial membrane when the basilar membrane vibrates
• As a result, the hair cells are excited

Auditory Signals

• Auditory signals are transmitted mainly by inner hair cells, even if more exist of the other type
• If the outer cells are damaged but inner cells are still functional, vast hearing loss can still occur
• Control of the inner hair cells' sensitivity at different pitches is proposed via outer hair cells
• Retrograde nerve fibers support this concept
• They pass from the brain stem to the vicinity of the outer hair cells
• Outer hair cell shortening and changes in stiffness can occur
• A retrograde nervous mechanism for controlling the ear's pitch sensitivity, activated through the outer hair cells is suggested

Hair Cell Receptor Potentials

• The stereocilia protruding from the hair cells have rigid protein
• Each hair cell has around 100 hairs that become progressively longer on the side away from the modiolus
• They attach to adjacent longer hairs, so the tips are tugged outward when the cilia are bent
• This transduction opens 200 to 300 cation-conducting channels
• This allows rapid movement of positive potassium ions into the stereocilia, which causes depolarization of the hair cell membrane
• Calcium channels open via depolarization and causes its influx
• Calcium channels exit through calcium-sensitive potassium channels
• The scala vestibuli cause depolarization, and in the opposite direction results in hyperpolarization
• It releases the neurotransmitter glutamate which stimulates the cochlear nerve endings at the hairs' synapses during depolarization

Endocochlear Potential

• Different concentrations exist in the endolymph and perilymph
• The perilymphatic scala vestibuli and scala tympani communicate
• The stria vascularis secretes endolymph
• Endolymph contains a high concentration of K+ but a low concentration of Na+
• An electrical potential of about +80 millivolts exists between endolymph and perilymph, with positivity inside the scala media and negativity outside
• This happens from continuous secretion of positive potassium ions into the scala media by the stria vascularis
• The electrical potential at the tips of the stereocilia sensitizes the cell

Determination of Sound Frequency

• Low-frequency sounds activate the basilar membrane near the apex of the cochlea
• High-frequency sounds activate the basilar membrane near the base
• Intermediate-frequency sounds activate at intermediate distances
• There is spatial organization of the nerve fibers
• Specific brain neurons activate depending on frequency
• The nervous system determines the stimulation's place, called frequency principle

Frequency

• It's hard to understand differentiation down to 20 cycles/sec if only low (distal end in the basilar membrane at the helicotrema) is stimulated
• The discrimination happens mainly by the volley principle with impulses and their frequencies
• Lower frequencies are from 20 to roughly 2000 cycles per second
• Can cause nerve impulses synchronized at the same frequencies and thus can be distinguished
• Destruction of apical half doesn't completely eliminate sensitivity

Determination of Loudness

• The auditory system determines loudness by amplitude and stimulating more endings by the outer hair cells
• Amplitude of vibration and the hair cells' excitement of nerve endings increase with loudness
• The range of different sounds results in spatial summation of stimulating impulses
• It only happens until a vibration reaches high intensity
• Those apprise the cells and the nervous system to note that the sound is loud

Loudness Changes

• Changes from intensity are interpreted proportionally
• Changes from a sensation are proportional to intensity
• The ear interprets differences of intensity greatly compressed

Decibel Unit

• Expressed usually in logarithms from sensitivities
• 10-fold increase = 1 bel
• .1 bel = 1 decibel
• 1 decibel roughly an increase of 1.26 times

Sound Thresholds

• The thresholds depend on the pressure and sound
• Can hear 3000 cycles with relatively low intensity
• Can detect 100 cycles only if its intensity is 10000 times
• Frequencies range from hearing from 20-20000 cycles for the young but it depends on loudness
• In old age range is reduced to 50-8000 cycles

Auditory Pathways

• Pathway is spiral ganglion --> cochlear nuclei and synapse
• Fibers go to opposite part of the superior olivary nucleus and same side
• Upload through lateral lemniscus
• Inferior colliculus and all synapse --> the way passes to the medial geniculate nucleus --> auditory radiation and cortex
• Signals go to both brains with contralateral transmission
• Crossing over between pathways occurs and transmitted

Nerves and Sounds

• Activate nervous system responding to loud sounds and activate cerebellum
• Sound frequencies terminate spatially along with cortical, colliculi, and nuclei patterns

Firing Rates

• Entering auditory nerves firing rates are proportional to loudness
• Impulses get synchronized up to a few thousand cycles
• Firing not synchronized higher up and shows signals dissect information rather than go unchanged
• The sound directs to the cortex to direct neurons

Function of the Cortex

• In Figure 53-10 auditory signals and the cortex that lies
• Excited by geniculate body and association areas that process
• maps have been described by audition
• Frequencies from auditory association and perception are high to low
• Auditory neurons respond to noise vs frequency specifically

Patterns

• Patterns allow identification
• Remove hearing --> reduce sounds or pitch patterns
• The cortex is extremely important for sequential patterns of sounds so it can be important for communication
• Lesions in the temporal gyrus affect word comprehension
• Sound location via 2 means: time lag and intensity
• The mechanisms are for frequencies high and above with directions determined by ear
• Direct towards sound results in sound reaching the same --> brain and ears the same but if not same side it enters first

Nerve Signals

• Neural analyses occurs at the superior and goes to the auditory
• Detect the directions by intensity and ears
• Medial superior has mechanisms to detect lag with signals from intensity and frequency with other signals

Centrifugal Signals

• Signals from the body and lower centers that are retrograde
• Inhibitory signals from the inferior has certain qualities to reject
• Divided into 2 types - 1 for nerve deafness and from conducting
• Destruction results in bone for the cochlea

Audiometer and Conducts

• Conducts sound to the skull and bone
• Depressed transmission is shown
• Air conduction can be surgically removed and replaced