Innervation Anteriorly auriculotemporal nerve V3 posterior superior CN 7

1. 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 • Innervation • Anteriorly auriculotemporal nerve V3 • posterior superior CN 7 • posterior inferior and floor CN 9 (Jacobsen) +10 (Arnold) • clockwise

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

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

4. 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 25-30 dB gained • Transformer ratio

5. 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

6. Stapedial reflex is bilateral • Protects cochlea esp. <2000Hz from sounds > 90dB • Delay of 10 ms

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

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

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

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

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

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

13. 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 • 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

14. OAE • Sound detected in EAC emanating from cochlea • Spontaneous OAE • Transient evoked OAE • Frequency matched deficit • 20-30 dB loss will lose OAE • Used in newborn hearing

15. 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

16. 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

17. 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

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