Hair Cells Lecture (2023)

Hair cells  Key concepts Describe how ion concentrations affect hair cell functioning Describe the mechanisms underlying hair cell functioning in the auditory and vestibular system Describe the differences between how outer and inner hair cells function Describe the r...

Hair cells  Key concepts Describe how ion concentrations affect hair cell functioning Describe the mechanisms underlying hair cell functioning in the auditory and vestibular system Describe the differences between how outer and inner hair cells function Describe the role of the inner and outer hair cells  Big picture Outer Middle Inner ear Basilar Hair cells ear ear membra Ascendin ne g Air Mechanic Fluid Mechanic Neural auditory pressure al moveme al signal pathway changes moveme nt moveme nt nt Descendi ng auditory pathway  Hair cells Vestibular system Auditory system Responsible for balance Responsible for hearing Movement of the head Movement of the ear causes fluid to move in drum causes fluid to move the semicircular canals inside the cochlea Both systems rely on hair cells  Physics of the semicircular canals If you’re in a car and it rapidly accelerates, you are pushed backwards As it keeps going, you are gradually pushed back less When it stops suddenly you fly forward  Physics of the semicircular canals The semicircular canals work in a similar way They are filled with a fluid called endolymph As the head moves, the canals also move, but the endolymph lags acceleration, catches up, and then keeps moving briefly after the head stops  Endolymph Perilymph High K+ concentration High Na+ concentration Low Na+ concentration Low K+ concentration Typical for rest of nervous system Hair cell stereocilia are surrounded by endolymph Hair cell body is surrounded by perilymph  Hair cells in the vestibular system Inside the Utricle and Saccule (attached to semicircular canals) are hair cells These have mechanosensitive receptors Utricle  Gated receptors Ligand-gated receptors – open based on a ligand binding to them Voltage-gated receptors – open based on depolarization Mechanosensitive receptors – open based on mechanical force Ear Skin Muscles  Hair cells Attached to the top of the hair cells is a gelatinous sheet called an otolithic membrane Within and on top of the otolithic membrane is otoconia  A complex chain of events When the head moves the canals move This causes the otoconia to move That motion is transferred to the otolithic membrane That deflects the hair cells Smaller big picture (how hair cells work) Stereocili Pull on “Trap K+ a bend tip link door” enters opens cell Depolarizati on Voltage- gated Ca2+ channels open Neurotransmitt er released  Hair cells in the vestibular system Deflecting the hair cells causes channels to open because of the tip link labspace.open.ac.uk  Vestibular and auditory system The hair cells in the vestibular system and the auditory system have the same underlying mechanisms What leads to the cells responding differs in the two systems  Stimulating the hair cells in the cochlea Fluid movement causes the basilar membrane to move The hair cells are attached to the basilar membrane This movement causes the hair cells to bend when moving against the tectorial membrane  Transduction Deflecting the hair cells causes channels to open because of the tip link R Pujol labspace.open.ac.uk  Endolymph Perilymph High K+ concentration High Na+ concentration Low Na+ concentration Low K+ concentration Typical for rest of nervous system Hair cell stereocilia are surrounded by endolymph Hair cell body is surrounded by perilymph  Flow of K+ K+ flows into the stereocilia This depolarizes the cell and opens voltage-gated channels There are voltage-gated K+ channels as well as Ca2+ - gated channels that open Ca2+ -gated channels cause release of neurotransmitter K+ flows out through the cell body  Reticular Lamina Reticular lamina is formed of tight junctions between hair cells and supporting cells This keeps endolymph and perilymph separated  Outer hair cells Outer hair cells respond to movement of the basilar membrane using the same mechanism as inner hair cells What happens next is quite different Outer hair cells change shape when they are depolarized  Behind the Science Joseph Santos-Sacchi Studies hair cell motility More information available on canvas  They really do move! http://amara.org/en/videos/tmb3aj4hoRR3/in fo/dancing-hair-cell/  Outer hair cells The structure of the outer hair cell bodies is made of a protein called Prestin Prestin changes shape in a voltage-dependent manner As a result Hair cell depolarization -> prestin contracts -> hair cell contracts Hair cell repolarization -> prestin expands -> hair cell elongates This voltage-sensitive shape change feeds mechanical energy back into the basilar membrane  Outer hair cells This OHC-mediated amplification is referred to as the active mechanism or the cochlear amplifier It causes both an increase in amplification and an increase in frequency selectivity Florentine et al., 1980 http:// www.cochlea.eu/  Key concepts Describe how ion concentrations affect hair cell functioning Describe the mechanisms underlying hair cell functioning in the auditory and vestibular system Describe the differences between how outer and inner hair cells function Describe the role of the inner and outer hair cells