Innervation Anteriorly auriculotemporal nerve V3 posterior superior CN 7 posterior inferior and floor CN 9 (Jacobsen) +10 (Arnold) clockwise EAC -2.5 cm long S-shaped curved anteriorly -lateral 1/3 cerumen cartilaginous -resonance at 3500 Hz, gain of 15 dB for adults *resonance is 8000Hz for baby until 2.5 years of age

Head Shadow effect Head blocks sound Head blocks sound Shorter wavelengths > 2000Hz can’t bend and will have interaural INTENSITY difference Shorter wavelengths > 2000Hz can’t bend and will have interaural INTENSITY difference Longer wavelengths can bend around head and will have interaural TIME difference Longer wavelengths can bend around head and will have interaural TIME difference

Middle ear must compensate for loss of energy from air to fluid transition (impedence) Impedence match

Area of TM to area of footplate 17 : 1 Handle of malleus and long process of incus 1.3 : 1 Shape of TM allows difference in reception of oval and round window 22 : 1 advantage dB gained Transformer ratio Transformer ratio

Cochleariform process houses tensor tympani which attaches to handle of malleus (CN V) and points to facial Stapedieus emanates from the pyrimidal process CN 7 Both are smallest striated mm of body

Stapedial reflex is bilateral Protects cochlea esp. 90dB Protects cochlea esp. 90dB Delay of 10 ms Delay of 10 ms

COCHLEA 2.5 turns Helicotrema connects vestibuli and tympani at apex 3 compartments Scala vestibuli, media, tympani Scala vestibuli, media, tympani Endolymph Endolymph Intracellular fluid low Na; high K Perilymph Perilymph Vestibuli and tympani (extracellular) high Na; low K

Footplate attached to vestibule of labyrinth Contiguous with scala vestibuli Contiguous with scala vestibuli Walls of scala media Reissner’s membrane Reissner’s membrane Basilar membrane – organ of Corti Basilar membrane – organ of Corti Lateral wall – stria vascularis Lateral wall – stria vascularis Na-K ATPase Cochlear implant into scala tympani

Organ of Corti Outer hair cells (3) Inner hair cells Supporting cells Tectorial membrane

Inner Hair Cells Type 1 neurons Type 1 neurons many spiral ganglion cells to 1 inner hair cell Efferents project lateraly Outer Hair Cells More numerous More numerous Innervated by type 2 afferent neurons Innervated by type 2 afferent neurons One spiral ganglion cell branches to many outer Efferents project medially

Sound vibrates basilar membrane Stiffer at base than apex Stiffer at base than apex Tonotopically constructed with high freq maximal displacement at base Tonotopically constructed with high freq maximal displacement at base Cochlear amplification Cochlear amplification Outer hair cells enhance frequency pick up

Hair cells have stereocilia Directly contact tectorial membrane in outer Directly contact tectorial membrane in outer Deflection causes K+ influx depolarization Deflection causes K+ influx depolarization

Auditory Nerve CN 8 Auditory nerve function measured by tuning curves of type I cells (inner) Sound is presented an frequency and intensity adjusted until change in firing rate Sound is presented an frequency and intensity adjusted until change in firing rate Nadir is where nerve is best at that frequency Nadir is where nerve is best at that frequency Characteristic frequency SNHL loses tips Presbycusis caused by dysfunction of stria vascularis Normal two-tone suppression Normal two-tone suppression

OAE Sound detected in EAC emanating from cochlea Spontaneous OAE Spontaneous OAE Transient evoked OAE Transient evoked OAE Frequency matched deficit dB loss will lose OAE Used in newborn hearing

Auditory Central Nervous System Cell bodies in spiral ganglion with afferent to hair cells, axons sent to cochlear nucleus Mostly contralateral innervation to superior olivary complex, small ipsilateral contribution Stimulation of the contralateral ear is usually stimulatory and ipsilateral is inhibitory

Medial portion of superior olivary complex is where crossed efferents to outer cell originates Lateral superior olivary complex is where efferents to uncrossed inner originates

Next synapse is at inferior colliculus (crossing) Medial geniculate body (crossing) Slyvian fissure of the auditory cortex in temporal lobe (no crossing) There is tonotopic orginaztion There is tonotopic orginaztion

Auditory Brainstem Response 7 waves measured after given stimulus I and II: 8 th nerve III : Cochlear nucleus IV: superior olive V : inferior colliculus Auditory steady-state Response Continuous tones used with Continuous tones used with