Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings  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 RegionPartDescription and function External earAuricle/pinnaSurrounds and protects the ear canal and funnels vibrations toward the auditory canal and eardrum External acoustic canal * contains ceruminous glands that 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 RegionPartDescription and function Middle ear/tympanic cavity Pharyngotympanic tube / Eustachian tube Connects between the middle ear and the nasopharynx. Has a role in equalization of pressure inside and outside the ear drum Auditory ossicles3 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

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings The Cochlea  The cochlea is divided into three chambers:  Scala vestibuli  Scala media  Scala tympani Reissner’s membrane Scala media Scala vestibuli Scala tympani Outer hair cell Inner hair cell Basilar membrane Auditory nerve Tectorial membrane

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