Hearing - Sound • Hearing is the reception of air sound waves that are converted to fluid waves, which ultimately stimulate mechanosensitive hair cells that send impulses to the brain for interpretation

Sound waves are created when an object moves: • Air molecules that are displaced by object movement are pushed forward into the adjacent area, adding to air molecules already in that space • Creates an area of high pressure due to compression of molecules together • When an object returns to its original position, the area it leaves has low pressure - rarefaction

Sound is described by two physical properties: Frequency Number of waves that pass a given point in a given time • Shorter wavelength = higher frequency of sound. Amplitude Height of the crests • Higher the crest, louder the sound • Measured in decibels (dB)

Tympanic membrane = Eardrum • Forms the boundary between the outer ear (auricle and external acoustic meatus) and middle ear • Thin connective tissue, membrane covered by skin externally and mucosa internally • Soundwaves make it vibrate

Ossicles = three of the smallest bones in the body • Transfer vibration of eardrum to oval window, which forms the boundary between the middle and inner ear • Tympanic membrane is about 20X larger than the oval window, so vibration is amplified about 20X

Hearing – Transmission of sound to the internal ear: Cochlea • A small spiral, conical, bony chamber, size of a split pea • Extends from vestibule • Contains cochlear duct, which houses the spiral organ and ends at cochlear apex (helicotrema) Cavity of cochlea is divided into three chambers: • Scala vestibule: contains perilymph • Scala media (cochlear duct): contains endolymph • Scala tympani: contains perilymph

Spiral organ contains cochlear hair cells arranged in one row of inner hair cells and three rows of outer hair cells • Sandwiched between tectorial and basilar membranes • The cochlear nerve runs from spiral organ to the brain

Transmission of sound to the internal ear: 1. Sound waves vibrate the tympanic membrane. 2. Auditory ossicles vibrate, Pressure is amplified. 3. Pressure waves created by the stapes pushing on the oval window move through fluid in the scala vestibuli. 4. a. Sounds with frequencies below the hearing range travel through the helicotrema and do not excite hair cells. 4.b. Sounds in the hearing range go through the cochlear duct, vibrating the basilar membrane and deflecting hairs on inner hair cells.

Resonance: movement of different areas of basilar membrane in response to a particular frequency • Basilar membrane changes along its length: • Fibers near oval window are short and stiff • Resonate with high-frequency waves • Fibers near cochlear apex are longer, floppier • Resonate with lower-frequency waves So basilar membrane mechanically processes sound even before signals reach receptors

Movement of basilar membrane deflects hairs of inner hair cells • Cochlear hair cells have microvilli that contain many stereocilia (hairs) that bend at their base • Longest hair cells are connected to shortest hair cells via tip links • Tip links, when pulled on, open ion channels they are connected to • Stereocilia project into K+-rich endolymph

Movement of basilar membrane deflects hairs of inner hair cells • Perception of pitch: impulses from hair cells in different positions along basilar membrane are interpreted by the brain as specific pitches (sound wave frequency) • Detection of loudness is determined by the brain as an increase in the number of action potentials (frequency) that result when hair cells experience larger deflections • Localization of sound depends on relative intensity and relative timing of sound waves reaching both ears

Outer hair cells • Nerve fibers coiled around these cells are efferent neurons that convey messages from brain to the ear • Outer hair cells can contract and stretch, which changes the stiffness of basilar membrane • Increases the responsiveness of inner hair cells by amplifying the motion of basilar membrane • Protect inner hair cells from loud noises by decreasing motion of basilar membrane

Equilibrium: Equilibrium (balance) is maintained in response to various movements of our head that relies on input from the inner ear, eyes, and stretch receptors (muscles and tendons) • Vestibular apparatus: equilibrium receptors in semicircular canals and vestibule • Vestibular receptors monitor static equilibrium • Semicircular canal receptors monitor dynamic equilibrium.

Maculae: sensory receptor organs that monitor static equilibrium • One located in the saccule wall and one in the utricle wall • Monitor the position of head in space • Play a key role in controlling posture

Maculae Each is a flat epithelium patch containing hair cells with supporting cells • Hair cells have stereocilia and kinocilium that is located next to the tallest stereocilia Stereocilia are embedded in otolith membrane • Jelly-like mass studded with otoliths (Calcite stones) that give the membrane weight

Maculae Utricle maculae are horizontal with vertical hairs • Respond to change along a horizontal plane, such as tilting head • Forward/backward movements stimulate utricle Saccule maculae are vertical with horizontal hairs • Respond to change along a vertical plane • Up/down movements stimulate saccule

Macula • Hair cells release neurotransmitters continuously • Acceleration/deceleration causes a change in amount of neurotransmitter released and leads to a change in action potential frequency to brain. • The density of the otolith membrane causes it to lag behind movement of hair cells when our head changes positions • However the base of stereocilia moves at the same rate as head, causing hair to bend • Ion channels open, and depolarization occurs

Receptor for rotational acceleration is crista ampullaris (crista) • Found at the base of each semicircular canal • Cristae are excited by rotational movements • Semicircular canals are located in all three planes of space, so cristae can pick up on all rotational movements of head