Ears

right ear, viewed from front

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

Organ of Corti

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.

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…

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?

Mammano et al (2007) Ca2+ Signaling in the Inner Ear Mammano et al (2007) Ca2+ Signaling in the Inner Ear. Physiology 22, 131-144 high potassium, low calcium endolymph the stria vascularis and endolymph compartment are bounded by a tight junction network purinergic receptor expression in hair cells

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.

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

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

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

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.

Drosophila antenna: Gordon Beakes © University of Newcastle upon Tyne

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

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?

end of the slide show Now switch to the website and read some of the papers listed at http://www.bmb.leeds.ac.uk/illingworth/bioc3800/index.htm#lect5