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Hearing – allows us to detect and interpret sound waves Equilibrium – inform us of the position of the head in space The receptors for both senses are mechanoreceptors called hair cells. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings The Ear: Hearing and Balance The three parts of the ear are the inner, outer, and middle ear The outer and middle ear are involved with hearing The inner ear functions in both hearing and equilibrium Receptors for hearing and balance: Respond to separate stimuli Are activated independently Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Region Part Description and function External ear Auricle/pinna Surrounds and protects the ear canal and funnels vibrations toward the auditory canal and eardrum External acoustic * contains ceruminous glands that canal secret cerumen – waxy material that protects the ear * contains small hairs that protect the ear and increase sensitivity Tympanic membrane (ear drum) * Converts sound waves into mechanical movements * separate external and middle ear Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Region Part Description and function Middle Pharyngotympanic Connects between the middle ear and the ear/tympanic tube / Eustachian nasopharynx. Has a role in equalization cavity of pressure inside and outside the ear tube drum Auditory ossicles 3 bones: mallus – attached to the eardrum incus – middle bone stapes – attached to the oval window When the eardrum moves it moves the ossicles which in turn move the oval window Tensor tympani and stapedius muscles Protect the eardrum and ossicles from violent movement under very noisy conditions. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Inner ear The inner ear is housed within a bony labyrinth. Passageways within the bone are lined with membranous labyrinth. Between bone and membrane is a fluid called perilymph Endolymph fills the chamber within the membranous labyrinth. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Bony labirinth http://thalamus.wustl.edu/course/audvest.html Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings The Cochlea The cochlea is divided into three chambers: Scala vestibuli Reissner’s membrane Scala media Scala vestibuli Scala tympani Scala media Scala tympani Tectorial membrane Auditory nerve Outer hair cell Basilar membrane Inner hair cell Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings The Cochlea The scala tympani terminates at the round window The scalas tympani and vestibuli: Are filled with perilymph Are continuous with each other via the helicotrema The scala media is filled with endolymph Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Sound and Mechanisms of Hearing Sound vibrations beat against the eardrum The eardrum pushes against the ossicles, which presses fluid in the inner ear against the oval and round windows This movement sets up shearing forces that pull on hair cells Moving hair cells stimulates the cochlear nerve that sends impulses to the brain Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Transmission of Sound to the Inner Ear The route of sound to the inner ear follows this pathway: Outer ear – pinna, auditory canal, eardrum Middle ear – malleus, incus, and stapes to the oval window Inner ear – scalas vestibuli and tympani to the cochlear duct Stimulation of the organ of Corti Generation of impulses in the cochlear nerve Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings The Organ of Corti Is composed of supporting cells and outer and inner hair cells Afferent fibers of the cochlear nerve attach to the base of hair cells The stereocilia (hairs): project into the endolymph Touch the tectorial membrane Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Excitation of Hair Cells in the Organ of Corti Bending cilia: Opens mechanically gated ion channels Causes a graded potential and the release of a neurotransmitter (probably glutamate) The neurotransmitter causes cochlear fibers to transmit impulses to the brain, where sound is perceived Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Mechanisms of Equilibrium and Orientation Vestibular apparatus – equilibrium receptors in the semicircular canals and vestibule Maintains our orientation and balance in space The position of the body with respect to gravity (static equilibrium) – the vestibule The motion of the body (dynamic equilibrium) – the semicircular canals Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings