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Chapter 12 The Auditory System: Deafness Input from each ear reaches the auditory cortex of both hemispheres Fig. 16-6c Primary auditory cortex (AI) = area 41 Fig. 16-6b (pinna) naso- Middle ear (tympanic cavity): 3 ossicles 2 muscles tensor tympani (CNV) – dampens low tones stapedius (CN VII) – decreases sound intensity 3 naso- sound vibrations external acoustic meatus tympanic membrane malleus incus stapes oval window (left ear) Otitis media: Opening of auditory (Eustachian) tube Nasopharynx mastoid air cells Internal ear: in the temporal bone has vestibular parts and an auditory part fluid-filled spaces (2 ½ coils) When the stapes moves the oval window inward, the round window moves outward (and vice versa) Perilymph waves in scalae vestibuli and tympani movement of fluid in cochlear duct movement of basilar membrane stimulation of auditory receptors scala vestibuli & scala tympani cochlear duct (parts of bony labyrinth) (part of membranous labyrinth) contain perilymph contains endolymph Spiral organ (of Corti): neuroepithelial receptor cells (hair cells) and supporting cells (3-5) s Each inner hair cell synapses with 1 spiral ganglion cell (up to 20 spiral ganglion cells may synapse with the same inner hair cell) Each spiral ganglion cell synapses with ~10 outer hair cells (16,000 hair cells in each cochlea) stereocilia (bipolar neurons) cochlear nerve The hearing process: airborne pressure waves mechanical energy (vibration of tympanic membrane and ear ossicles) sinusoidal pressure waves in scala vestibuli and scala tympani upward movement of basilar membrane depolarization of hair cells propagation of action potentials Decibel (dB): measure of intensity (loudness) the more intense the stimulus, the more the afferent neurons increase the frequency of their action potentials Fig. 12-3 (frequency = tone) tonotopic localization of the basilar membrane 4 neurons in the auditory pathway Fig. 12-2 tonotopic localization of the primary auditory cortex Tectospinal tracts axons of spiral ganglion cells synapse in the cochlear nuclei Fig. 12-4 spiral ganglion cochlear nuclei lateral recess of 4th ventricle acoustic tubercle cochlear nuclei D V inferior cerebellar peduncle axons from the cochlear nuclei form 3 acoustic striae and ascend into the tegmentum of the pons and decussate (contains nuclei) nuclei of trapezoid body: • receive input from both ipsilateral and contralateral cochlear nuclei • project output to both lateral lemnisci superior olivary nuclei: ditto all auditory fibers ascending in the lateral lemniscus synapse in the inferior colliculus of the midbrain lateral lemniscus (receives lemniscal fibers, send axons to both ipsi and contra lateral lemnisci) brachium of inferior colliculus Fig. 3-4 fibers from the inferior colliculi ascend bilaterally to the medial geniculate nuclei (MG) fibers pass thru the post. limb of the internal capsule to the primary auditory cortex medial geniculate nucleus (thalamic auditory center) Unilateral lesion of: the spiral organ of Corti, spiral ganglion, cochlear nerve, or cochlear nuclei deafness on ipsilateral side Unilateral lesion of: either the auditory cortex or ascending paths distal to (above) the cochlear nuclei difficulty localizing the direction and distance of sounds reaching the contralateral ear (1st order neurons) (2nd order neurons) 2 types of deafness: Nerve deafness (perception deafness) damaged receptor cells of spiral organ or cochlear n. hearing losses in both air conduction and bone conduction (extent of deafness depends on amount of damage) Conduction deafness interference of passage of sound waves in the external or middle ear is never complete (total) because of transmission of sound waves through the skull still occurs cerumen Audiogram hearing aids ear horn cochlear implant system Tuning fork tests: Weber lateralizes to good ear in nerve deafness lateralizes to bad ear with conduction deafness Rinne air conduction should exceed bone conduction Acoustic neuroma: Schwann cell tumor of the vestibular nerve in the internal acoustic meatus Dysequilibrium Progressive deafness Facial weakness (the tumor can enter the posterior cranial fossa near the cerebellar angle ipsilateral limb ataxia, ipsilateral loss of corneal reflex, and ipsilateral loss of somato-sensation of the face) Selective hearing does exist: to modulate the ascending auditory activity Auditory cortex medial geniculate nucleus Auditory cortex inferior colliculus Inferior colliculus, nuclei of lateral lemniscus, and superior olivary nuclei cochlear nuclei Superior olivary nuclei outer hair cells of spiral organ and their afferent terminals allows for selective attention to certain sounds Auriculotherapy points: The dog with the largest ears in the world as recognised by the new 2004 Guiness Book of World Records, Mr Jeffries the Bassett Hound, is seen in this handout photo made available Wednesday, Sept. 24, 2003. Jeffries, whose ears measure 29.2 cm (11.5 in) lives with his owner, Phil Jeffries, in West Sussex, England. Mr Jeffries' full name is Knightsfollie Ladiesman and he is the grandson of Biggles, the face of Hush Puppies shoes. His ears are insured for 30,000 pounds ($47,800). (AP Photo/Guinness World Records) Longest human ear hairs Chapter 12 know the components of the external, middle, and internal ear know the muscles in the middle ear and their innervations and functions know the path of auditory vibrations know the spaces within the cochlea and the membranes which separate them know the locations of the endolymph and perilymph know the components of the organ of Corti know the tonotopic localization of the basilar membrane know the tonotopic localization of the auditory cortex know the location of the neurons involved in the auditory pathway know the pathways of the axons in the auditory pathway know the structures responsible for the bilateralism of the auditory pathway know the hearing deficits associated with lesions of the components of the auditory pathway know the structures affected by acoustic neurinomas and the associated signs and symptoms know the two types of deafness and the locations of the lesions responsible for them know the purpose of efferent input into the auditory pathway