1. Ears

2. right ear, viewed from front

8. semicircular canals
Rotation of the head results in the inertial flow of fluid through the semicircular canals, displacing the gelatinous cupula and bending the hair cells

16. Organ of Corti

22. hair cells 3,500 inner hair cells per cochlea.
11,000 outer hair cells in three rows.
Mechanical losses (damping) in a completely passive cochlea would result in broad and insensitive tuning.
The inner hair cells are sensory, outer cells are motors.
Active vibrations by the outer hair cells overcome the damping, and allow a sensitive and selective tuning.
Most vertebrates apparently amplify sounds by actively wiggling the outer cells’ stereocilia from side to side.
Mammals have evolved an electrostatic motor protein prestin from a chloride channel, which can alter the length of the outer hair cells at audio frequencies.

24. Elgoyhen et al (2009) The nicotinic receptor of cochlear hair cells: a possible pharmacotherapeutic target? Biochem. Pharmacol. 78, 712–719.
Amplification by the outer hair cells is controlled by the medial olivocochlear system (MOC).
Acetyl choline released by MOC neurons reduces amplification
Control is applied in a frequency selective manner, so that attention can be focused on the most significant sounds.
Think of the cochlea as a 3,500 channel graphic equalizer, skillfully adjusted to tune out unwanted noise…

25. transduction mechanism
Endocochlear potential (+80mv) adds to the hair cell membrane potential and drives ions through the gating pore – gives large amplifier gain
Internal calcium tracks the input waveform
Glutamate release from ribbon synapse preserves phase and amplitude
This information is used within the brain to determine the position and nature of the sound source.
Bats can do this at 70kHz – how?

26. Mammano et al (2007) Ca2+ Signaling in the Inner Ear
Mammano et al (2007) Ca2+ Signaling in the Inner Ear. Physiology 22,
high potassium, low calcium endolymph
the stria vascularis and endolymph compartment are bounded by a tight junction network
purinergic receptor expression in hair cells

27. Mammano et al (2007) Physiology 22, 131-144
Afferent ribbon synapse. Internal calcium controls the rate of glutamate release.
Efferent cholinergic synapse controls the amplitude of the prestin-driven oscillations.

28. Mammano et al (2007) Physiology 22, 131-144
photolytic release of caged calcium

29. hearing adaptation
Multiple frequency selective adaptation systems with very different time scales.
Fastest is based on calcium binding within the gating pore – required for amplifier stability?
Medium speed uses the adaptation motors to pull the tip links until the pore is about to open
Reflex relaxation of the tiny tensioning muscles within the middle ear in response to loud noises
Cover your ears to prevent mechanical damage to hair cells

30. Volrath et al (2007) Annu. Rev. Neurosci. 30, 339-365

31. Volrath et al (2007) Annu. Rev. Neurosci. 30, 339-365
Stereocilia bundles are held together by a variety of protein links, which vary slightly between species, but only the delicate tip links are essential for transduction.

32. Drosophila antenna: Gordon Beakes © University of Newcastle upon Tyne

33. Bechstedt & Howard (2008) Current Biology 18, R869-R870

34. horses for courses (again)
mammals
Endolymph compartment
Microvilli based sensors
Mechanically gated pores
Gating springs
Active amplification
Nanometer sensitivity
Actin-based skeleton
Myosin adaptation motor
Ribbon synapses can follow sound waves
Excellent frequency discrimination
Drosophila
Receptor lymph
Cilia based sensors
Mechanically gated pores
Gating springs
Active amplification
Nanometer sensitivity
Tubulin-based skeleton
Dynein adaptation motor?

35. end of the slide show
Now switch to the website and read some of the papers listed at