The Ear

The Ear: Hearing and Balance Structure of the Ear -divided into external ear, middle ear, and internal ear -the external ear and middle ear structures are involved with hearing only and are simple -the internal ear functions in both equilibrium and hearing and is complex External Ear -consists of the auricle and the external acoustic meatus -the auricle (pinna) is the shell-shaped projection surrounding the opening of the external acoustic meatus -composed of elastic cartilage with thin skin -its outer rim is known as the helix -the lobule (earlobe) is dangling and lacks supporting cartilage -functions to direct sound waves into the external acoustic meatus -the external acoustic meatus (auditory canal) is a short, curved tube that extends from the auricle to the eardrum -the canal is lined with skin bearing hairs, sebaceous glands, and ceruminous glands that secrete cerumen which traps foreign particles and repels insects -sound waves entering the external acoustic meatus eventually hits the tympanic membrane (eardrum) which acts as the boundary between the outer and middle ears -eardrum is a thin, translucent, connective tissue membrane covered by skin on its external face and by a mucosa internally -shaped like a flattened cone; its apex protrudes medially into the middle ear -sound waves make the eardrum vibrate, which then transfers the sound energy to the tiny bones of the middle ear and makes them vibrate Middle Ear -known as the tympanic cavity -is a small, air-filled, mucosa-line cavity -surrounded by the eardrum laterally and medially by a bony wall with two openings -the superior oval (vestibular) window -the inferior round (cochlear) window -the tympanic cavity reaches upward as the epitympanic recess which functions as the roof of the middle ear cavity -the mastoid antrum is a canal in the posterior wall of the tympanic cavity and allows it to communicate with mastoid air cells in the mastoid process -the anterior wall contains the opening of the pharygotympanic (auditory) tube -this tube runs obliquely downward to link the middle ear cavity with the nasopharynx; the mucosa of the middle ear is continuous with the lining of the pharynx (throat) -the tube is normally flattened and closed; swallowing and yawning opens it briefly to equalize pressure in the middle ear cavity with the external air pressure -the eardrum vibrates freely only if the pressure on both of its surfaces is the same; otherwise, sounds are distorted -pressure equalization can be felt as the ear-popping sensation otitis media: middle ear inflammation; usually result of a sore throat -the tympanic cavity is spanned by the 3 smallest bones in the body known as the auditory ossicles -bones are named for their shape 1) malleus (hammer) 2) incus (anvil) 3) stapes (stirrup) -ligaments suspend the ossicles and mini synovial joints link them together into a chain -ossicles transmit the vibratory motion of the eardrum to the oval window, which sets the fluids of the internal ear into motion, eventually exciting the hearing receptors -2 skeletal muscles are associated with the ossicles -the tensor tympani arises from the wall of the pharyngotympanic tube and inserts on the malleus -the stapedius runs from the posterior wall of the middle ear cavity to the stapes -when the ears hear loud sounds, these muscles contract reflexively to prevent damage to hearing receptors -the tensor tympani tenses the eardrum by pulling it medially -the stapedius checks vibration of the whole ossicle chain and limits the movement of the stapes in the oval window -dampening muscles are an example of CNS control over sensory input - protects the auditory receptor cells -innervated by cranial nerves Internal Ear -known as the labyrinth because of its complicated shape -has 2 major divisions: 1) Osseous (Bony) Labyrinth -made up of 3 regions -vestibule, cochlea, and the semicircular canals -filled with perilymph (like cerbrospinal fluid) 2) Membranous Labyrinth -a continuous series of membranous sacs and ducts contained within the bony labyrinth and followings its contours -suspended in perilymph + -interior contains endolymph which is similar to K -rich extracellular fluid -perilymph and endolymph conduct the sound vibrations involved in hearing and respond to the mechanical forces occurring during changes in body position and acceleration Vestibule -central egg-shaped cavity in the bony labyrinth -posterior to the cochlea, anterior to the semicircular canals, and flanks the middle ear medially -suspended in its perilymph -2 membranous labyrinth sacs are united by a small duct -the 2 membranous sacs are known as the saccule and utricle -the smaller saccule is continuous with the membranous labyrinth extending anteriorly into the cochlea -the utricle is continuous with the semicircular ducts extending into the semicircular canals posteriorly -the saccule and utricle house equilibrium receptor regions called maculae that respond to the pull of gravity and report changes of head position Semicircular Canals -lie posterior and lateral to the vestibule -project from the posterior aspect of the vestibule; each oriented in 1 of the 3 planes in space -made up of an anterior, posterior, and lateral semicircular canal in each internal ear -anterior and posterior canals are oriented at right angles to each other in the vertical plane -lateral canal lies horizontally -the semicircular duct communicates with the utricle anteriorly -each duct has an enlarged swelling at one end called the ampulla that hosues an equilibrium receptor region called the crista ampullaris -these receptors respond to angular (rotational movements) of the head Cochlea -spiral, conical, bony chamber -extends from the anterior part of the vestibule and coils around a bony pillar called the modiolus -the cochlear duct runs through the centre and ends blindly at the cochlear apex -the cochlear duct houses the spiral organ (of Corti) which acts as the receptor organ for hearing -the cochlear duct and the osseous spiral lamina divides the cavity of the bony cochlea into 3 separate chambers or scalae 1) Scala Vestibuli -lies superior to the cochlear duct -continuous with the vestibule and abuts the oval window -part of the bony labyrinth and filled with perilymph 2) Scala Media -makes up the actual cochlear duct -is part of the membranous labyrinth -filled with endolymph 3) Scala Tympani -ends at the round window -inferior to the cochlear duct -part of the bony labyrinth and filled with perilymph -the perilymph-containing chambers are continuous with each other at the cochlear apex known as the helicotrema -the roof of the cochlear duct is known as the vestibular membrane and separates the scala media from the scala vestibuli -stria vascularis makes up the duct's external wall and is composed of vascularized mucosa that secretes endolymph -the floor of the cochlear duct is composed of bony spiral lamina and the flexible / fibrous basilar membrane which supports the spiral organ (of Corti) -basilar membrane is involved in sound reception -the cochlear nerve (division of the vestibulocochlear nerve VIII) runs from the spiral organ through the modiolus to the brain Physiology of Hearing -sounds set up vibrations in air that beat against the eardrum that pushes a chain of tiny bones that press fluid in the internal ear against membranes that set up shearing forces that pull on tiny hair cells that stimulate nearby neurons that give rise to impulses that travel to the brain which interprets them Sound -sound is a pressure disturbance that involves areas of high and low pressure, produced by a vibrating object and propagated by the molecules of the medium -sound waves involve a series of compression (high-pressure) areas and rarefaction (low- pressure) areas -the energy of the wave declines with time and distance -illustrated by a sine wave -frequency is defined as the number of waves that pass a given point in a given time -the frequency range of human hearing is from 20 to 20 000 Hz (waves per second) -we are most sensitive to frequencies between 1500 and 4000 Hz -the distance between 2 consecutive crests or troughs is called a wavelength -shorter wavelength = higher frequency of sound = higher pitch -longer wavelength = lower frequency of sound = lower pitch -we perceive different sound frequencies as differences in pitch -the amplitude (height of sine wave crests) represent the sound's intensity -high amplitude = loud -low amplitude = soft -loudness refers to the subjective interpretation of sound intensity -measure in decibels (dB) Transmission of Sound to the Internal Ear -hearing occurs when the auditory area of the temporal lobe cortex is stimulated -sound waves must be propagated through air, membranes, bones, and fluids to reach and stimulate receptor cells in the spiral organ of Corti -the path involves: 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 1) airborne sound entering the external acoustic meatus strikes the tympanic membrane and causes it to vibrate at the same frequency -greater intensity = greater vibratory motion = greater displacement of membrane 2) the motion of the tympanic membrane is amplified and transferred to the oval window by the ossicle lever system, which transfers the same total force hitting the eardrum to the oval window -the tymapnic membrane is larger than the oval window, so the pressure exerted on the oval window is greater than that on the tympanic membrane -this increased pressure sets the cochlear fluid into wave motion Resonance of the Basilar Membrane -as the stapes rocks back and forth against the oval window, the perilymph is set into a back-and-forth motion as well -this creates a pressure wave that travels through the perilymph from the basal end toward the helicotrema -low frequency sounds / inaudible sounds (below 20 Hz) creates pressure waves