Ear: Hearing and Equilibrium Nestor T. Hilvano, M.D., M.P.H.

Learning Objectives 1.Describe the structures and functions of the external, middle, and internal ear. 2.Describe the receptors for vestibular sense (equilibrium) and explain how they function. 3.Trace the vestibular pathway. 4.Describe the cochlear duct and the receptor for hearing. 5.Discuss the reception and transduction of sound energy or hearing process. 6.Trace the auditory pathway. 7.Explain common clinical conditions associated with vestibular and auditory problems.

Anatomy of Ear External Ear: - collect/detect sound waves ____- fleshy and cartilagenous ____ passageway ____ thin, semitransparent sheet Middle Ear: - transmit sound to inner ear. ____- communicates the middle ear with nasopharynx. ____- 3 small bones, conduct vibrations of sounds; tensor tympani (VCN) – stiffens t.m.; stapedius (VIICN reduces movement of stapes at oval window a. external auditory canal b. pinna (auricle) c. tympanic membrane d. auditory (eustachian) tube e. ossicles

Anatomy of Ear Inner Ear – receptors for equilibrium and hearing Bony labyrinth surrounds membranous labyrinth Bony labyrinth subdivided into: a) Vestibule – encloses saccule and utricle b) Semicircular canals – contain semicircular ducts c) Cochlea – contains cochlear duct Round window – separates perilymph from air spaces of middle ear Oval window – connected to base of stapes

Equilibrium Maculae - receptors which perceived the sense of gravity and acceleration;when head is tilted, weight of membrane bends the stereocilia Cristae ampullaris- receptor for sense of rotation; orientation causes ducts to be stimulated

Vestibular Projection Pathways Vestibular receptor to sensory neuron and axons form the vestibular nerve Nerve synapse to vestibular nuclei within pons & medulla Information sent to targets: ____- unconscious control of head and posture ____- to nuclei of CN III, IV, VI ____- innervate antigravity muscles (peripheral m. tone) ____- conscious sense of head position and movement a.vestibulo-cerebral tracts b. vestibulospinal tracts c.Vestibulo-ocular tracts d. vestibulo-cerebellar tracts * Nystagmus – due to damage of BS or inner ear

Cochlear Duct – Organ of Hearing Scala media (cochlear duct) separated from scala vestibuli by vestibular membrane, and from scala tympani by basilar membrane. Spiral organ of corti contains receptor cells called ___. a. bipolar cells b. hair cells c. dendritic cells d. sustentacular Frequency (pitch) of sound - determined by which part of cochlear duct is stimulated. Intensity (loudness) - determined by number of hair cells stimulated. Physiology of hearing- trace the passage of sound from the external ear to the internal ear

Auditory Pathway Hair cells of organ of corti axons form cochlear (VIII) nerve synapse in cochlear nuclei ascends to the inferior colliculi of the midbrain –process fluctuations in pitch during speech –produce startle response; head turning to loud sound auditory information synapse in the Medial Geniculate Bodies of the thalamus and projected to primary auditory cortex (Heschl gyri) of temporal lobe.

Nature of Sound Pitch – frequency vibrates specific parts of ear - hearing range is 20 (low pitch) - 20,000 Hz (cycles/sec) -speech is where hearing is most sensitive Loudness – amplitude; intensity of sound energy (decibels) Audiometry test – differentiate high and low frequency sound.

Clinical correlation Aging – tympanic membrane less flexible, ossicles stiffens, and round window ossify. Types of Deafness: a. Conduction deafness – Rhine’s test BC better than AC; Weber’s test sound appear louder in abnormal ear) b. Sensorineural (Neural) deafness – Rhine’s test both are decrease, but AC remain better than BC; Weber’s test sound appear louder in normal ear) Vertigo – damage to vestibular receptor Nystagmus, gait problems – seen in lesion of vestibular nuclei Identify type of deafness in: ____ atrophy of cochlear nerve; ____ otitis media ____ impacted cerumen; ____ hereditary deafness

Homework (Self-Review) 1.Give the function/s of the following: a) cerumen, b) auricle, c) macula, d) crista ampullaris, e) basilar membrane, f) tectorial membrane, g) spiral ganglion, h) organ of corti, i) tympanic membrane. 2.What is auditory (eustachian) tube? Discuss the occurrence of ear pain when it is blocked. 3.Identify the subdivisions of bony labyrinth and membranous labyrinth. 4.Trace the auditory pathway from the receptor to brain. 5.Type of deafness in: a) otitis media, torn tympanic membrane, and impacted cerumen, and b) atrophy of cochlear nerve and damage of organ of corti. 6.What fluid is contained in scala media? In scala vestibuli and scala tympani? 7.Study anatomy illustration of ear and it’s parts (refer to text reference).