Auditory System Flashcards
Hair cells
. Polarized epithelial cells
. Apical end specialized for reception and translation of mechanical energy into receptor currents
. Basal end specialized for transmission of info to the CNS via synaptic contacts w/ primary afferent neuron (housed into spinal ganglion)
Cochlea
. Part fo bony labyrinth
. Bony tube that winds spirally 23/4 turns around a modiolus (central pillar of bone)
Cochlear duct
. Part of membranous labyrinth
. Enter cochlea and is attached at edge of the 2 sides of the cochlea
. Divides cochlea into 2 long spiral chambers (Scala vestibuli above duct and scala tympani below duct)
. Scala vestibule and tympani communicate w/ each other at apex of modiolus via helictrema (small opening) and are both filled w/ perilymph
. Duct is filled w/ endolymph
Roof of cochlear duct
Vestibular (Reissner’s) membrane
Lateral wall of cochlear duct
. Spiral ligament
. Thickening of endosteum that lines cochlea
. Upper part of ligament is stria vascularis (vascularized epithelium that produces endolymph)
.
Floor of cochlear duct
. Basilar membrane that stretched from spiral ligament to osseous spiral lamina
Osseous spiral lamina
. Bony plate that winds around modiolus like threads of a screw
. Composed of 2 bony plates w/ n. Fibers in between
. Periosteum of upper surface forms an elevation (spiral limbus)
Tectorial membrane
. Thin, jelly-like membrane that arises from spiral limbus and overlies hair cells of organ of Corti
Organ of corti lies on ____
Basilar membrane
Organ of corti
. Complex epithelial structure consisting of neuroepithelial hair cells and supporting cells
. Hair cells arranged in rows: inner form single row, outer hairs for 3 rows
. Free surfaces of hair cells covered w/ sterocilia but no kinocilium
. Kinocilium embedded in tectorial membrane
. Base of each hair cell is contacted by afferent and efferent n. Endings
Spiral ganglion
. Located w/in modiolus of cochlea
. Contains 30,000 bipolar neurons
. 90% innervate hair cells w/ 1 fiber innervating 1 inner hair cell
. Each hair cell receives 10 fibers
. Inner cells are tuned to a specific tone
. 10% innervate outer hair cells w/ 1 fiber innervating many outer hair cells
. Outer hair cells receive olivocochlear synapses and function in fine-tuning aural input
Cochlear n. And nuclei
. Central process of spiral ganglion cells form cochlear n.
. Primary afferent fibers making up cochlear n. Bifurcate prior to termination in cochlear nuclei
. 1 collateral terminated in ventral cochlear nucleus
. Other collateral bifurcated once more to terminate in ventral cochlear nucleus and dorsal cochlear nucleus
. Yield 3 branches from each primary afferent fiber
Mechanical transduction through ear
. Airborne sound waves strike tympanic membrane which vibrates in response
. Vibrations transmitted across tympanic cavity via 3 bony ossicles
. Foot plate of stapes moves to and from oval window, transmitting vibration to fluid-filled inner ear
. Pressure from compression of perilymph is released via round window
How pressure waves get from perilymph to endolymph
. Pressure waves in perilymph result from oscillations of foot plate of stapes
. Pressure waves enter scala vestibuli and via helicotrema are transmitted to scala tympani
. Pressure waves transmitted through endolymph of cochlear duct makes basilar membrane vibrate
Characteristics of basilar membrane
. Narrow and taut near base of cochlea Vibrates preferentially to sounds of high pitch
. Membrane is wide and floppy neat apex of cochlea and vibrates preferentially to low pitch sounds
. Continuous resonance spectrum exists between 2 sides of membrane
Interactions between basilar membrane and hair cells
. Oscillations of basilar membrane alter the conformation between hair cell stereocilia and tectorial membrane
. Deformation of hair cells causes alteration of the pattern oof discharge in contacting afferent fibers
. APs generated in activated spiral ganglion cells and are propagated along central processes of bipolar neurons (cochlear n.) as it exits modiolus passing through internal acoustic meatus w/ vestibular component of CN VIII and enters brainstem at pontocerebellar angle
Dorsal and ventral cochlear nuclei
. Located dorsal and lat. to inf. Cerebellar peduncle
. Nuclei organized tonotopically
Acoustic striae
. Fibers in 3 striae cross midline to enter contralat.
. Axons from dorsal cochlear nucleus form dorsal acoustic stria and ascend to form lat. lemniscus
. Axons from post. Portion ventral cochlear nucleus form intermediate acoustic stria, main branch projects to lat. lemniscus and collaterals project bilat. To sup. Olivary complexes
. Axons from ant. Portion of ventral cochlear nucleus form trapezoid body (ventral acoustic stria) and is largest of 3 striae, collaterals project bilat. To sup. Olivary complex
Superior olivary complex in auditory system
. In caudal pons
. Consists of med. and lat. sup. Olivary nucleus and nucleus of trapezoid body
. 1st point in auditory pathway that input from both sides converges
. Cells sensitive to differences in time of arrival of auditory stimuli from both ears
. Function to localize sound in space but auditory cortex still necessary to interpret input
. Send axons bilaterally to lat. lemnisci
Lateral lemniscus
. Receives secondary fibers from contralat. Cochlear nuclei via acoustic striae, and tertiary fibers from ipsilat and contralat. Sup. Olivary complex
. In lat. pons, fibers either enter nucleus of lat. lemniscus, enter RF to become part of reticular activating system (RAS), or enter inf. Colliculus
Nucleus of lateral lemniscus
. Communicate w/ one another via a commissure
. Send axons to ipsilat. Inf. Colliculus
Inf. Colliculus in auditory system
. Located in caudal midbrain
. Receives afferents from ipsilat lat. lemniscus, bilateral sup. Olivary nuclei, and contralat. Cochlear nuclei
. Sends fibers to opposite inf. Colliculus via commissure of inf. Colliculus and to ipsilat. Med. geniculate nucleus of thalamus via brachium of inf. Colliculus
Medial geniculate nucleus (MGN)
. Located in caudal and ventral thalamus
. Axons project to ipsilat. Primary auditory cortex via sublenticular part of internal capsule
Primary auditory cortex
. Located deep w/in lat. fissure
. Also called transverse temporal gyrus of Heschl (area 41,42)
. Cells show tonotopic organization
Auditory assoc. cortex
. Post. 2/3 of the sup. Temporal gyrus (part of area 42, area 22)
. In dominant hemisphere
. Largely synonymous w/ Wernicke’s area
Descending auditory pathways
. Olivocochlear bundle arises from nuclei close to sup. Olivary complex
. Axons leave brainstem in vestibular division of CN VIII, crossing into cochlear division in internal acoustic meatus
. Axons terminate on hair cells in organ of Corti or presynpatically on afferent fibers
. Pre and postsynaptic inhibition is used to inhibit all hair cells except those in the region of max. Basilar displacement
. Called auditory sharpening: results in dec. background noise
Conduction deafness
. Vibrations do not reach oval window
. Hear better via bone conduction than normal route of air conduction
. Causes: otosclerosis (fixation of foot plate of stapes from abnormal growth of annular ligament), otitis media, excess cerumen (ear wax) in external auditory canal
Sensoryneural deafness
. Due to disease of cochlea, cochlear division of CN VIII, or cochlear nuclei in medulla
. Causes: prolonged exposure to loud noise, med side effects, acoustic neuroma (Schwann cells grow too much in space of pontocerebellar angle impeding n. Transmission), BM, rubella, syphilis, Ménière’s disease
Anacusis
Absent hearing
Hypacusis
. Reduction in hearing
Presbycusis
. Hearing loss assoc. w/ aging
. Most common cause of hearing loss
. Gradual bilateral loss w/ high-frequency tones lost first
Ipsilateral hearing loss
. Caused by damage to cochlear division of CN VIII or to cochlear nuclei
Lesions of primary auditory cortex
. Affect ability to localize sounds in space
. Due to commissures connecting both sides of cortices, unilateral lesion is difficult to recognize clinically
Air conduction screening
. Hearing by air conduction requires intact outer, middle, and inner ear structures
. Damage to any of these could result in hearing loss via air conduction
Bone conduction screening
. Hearing via bone conduction only requires intact inner ear structures
. Sensorineural deficit indicated when hearing loss is measured by bone conduction
Unilateral conduction deafness
. Patient does not hear sound when tuning fork is next to ear
Unilateral sensorineural deafness
. Patient hears the sounds somewhat better when tuning fork is next to ear than on mastoid process
Weber test
. Normal: sound come inside patient’s head
. Unilateral conduction: sounds louder in abnormal ear
. Unilateral sensorineural: sounds louder in normal ear
Cochlear implant
. Partial resorption of hearing from cartilage or complete hair cell loss
. Consists of microphone to detect sound, electronic processor that transform sound waves into code of electrical stimuli
. Array of stimulating electrodes in cochlea inserted through round window into scala tympani where it lies close to peripheral axons of primary auditory neurons
. Electrodes positioned to stimulated n. Fibers
Success of cochlear implant depends on ____
. Number of surviving primary auditory neurons
. Orientation of electrodes w/ respect to neurons
. Coding scheme of processor used
