The Inner Ear SPA 4302 Summer 2004

Sensory Endorgans encased within very dense bone INNER EAR Sensory Endorgans encased within very dense bone Two Halves: Vestibular--transduces motion and pull of gravity Cochlear--transduces sound energy (Both use Hair Cells)

Subdivision into spaces containing endolymph (blue), and spaces containing perilymph (red)

Cochlea is Divided into 3 “Scala” Scala Vestibuli Reissner’s Membrane Scala Media Basilar Membrane Scala Tympani Helicotrema - the opening between 2 outer Scala

Fluids filling the Inner Ear Perilymph- in S. Vestibuli and S. Tympani High Sodium / Low Potassium concentrations Low Voltage (0 to +5 mV) Endolymph- in S. Media High Potassium / Low Sodium concentrations High Positive Voltage (85 mV)

Cross-Section of the Cochlea Third Turn Second Turn First Turn

A Cross Section Shows the 3 Scala

Within S. Media is the Organ of Corti

I = Inner Hair Cells P = Pillar Cells O = Outer Hair Cells D = Deiter’s Cells

The Stereocilia on IHCs and OHCs OHCs (at top) V or W shaped ranks IHC (at bottom) straight line ranks

Stereocilia bent toward tallest row Potassium flows into cell Calcium flows into cell Voltage shifts to a less negative value More neurotransmitter is released

Cochlear Afferent (Sensory) Neurons Type I (95%) synapse w/ IHC 1 IHC to 20 neurons well myelinated Type II (5%) Synapse with OHCs Many OHCs to 1 neuron unmyelinated

Cochlear Functions Transduction- Converting acoustical-mechanical energy into electro-chemical energy. Frequency Analysis-Breaking sound up into its component frequencies

Development of the Inner Ear Beginning in week 3, Auditory placode develops, Invaginates to form auditory pit, Which then closes off, leaving an auditory capsule.

Capsule divides into saccular, utricular divisions Saccular: cochlea (begins forming in week 6) Utricular: semicirc. canals, endolymphatic sac & duct Organ of Corti forms from week 10 Inner ear fully formed by 25 weeks

Hearing Loss and Disorders of the Inner Ear Vast majority of SNHL results from inner ear disorder. Cochlear pts often hear, but cannot distinguish what they hear very clearly: dysacusis. Arises from frequency and other distortions associated with changes in inner ear function.

Inner Ear Disorders: Prenatal Causes

Anatomical Anomalies Often seen as Bony malformations Examples: Mondini (incomplete cochlea) Enlarged Duct (shown here)

Age Effects

Noise Damage Temporary Threshold Shift (TTS) Permanent Threshold Shift (PTS) Duration, Timing and Intensity influence Typical “Noise Notch” often seen between 3 and 6 kHz first. Notch widens and deepens over time, with hearing loss spreading to adjacent frequencies, and increasing in degree.

Ototoxicity Substances poisonous to the ear Medicines Aminoglycocide antibiotics Chemotherapy drugs for CA Aspirin Industrial Chemicals (tolulene, etc) May affect balance Degree of hearing loss varies Synergy with noise or other ototoxic agents