Lecture 04 - Ears 1 Flashcards
Subdivisions of the ear
3 parts
- external (pinna, ext. auditory meatus) - funnels sound
- middle (drum, air-filled cavity behind drum)
- 3 bones transfer drum vibrations to oval window and fluid behind oval window
- auditory tube only opening of middle ear to outside world
- inner (fluid filled sacs inside bony sac behind oval window)
- holds sensory hair cells for hearing and balance
- vestibular and cochlear nerves –> CN8
hair cells
innervated by CN 8
Development
Pouch 1 - middle ear space and eustacian tube
Cleft 1 - ext. acoustic meatus
***membrane btw pouch and cleft 1 - ear drum
Branchial arch 1 - malleus and incus
Branchial arch 2 - stapes
otic placode invaginates to form otic vesicle, which gives rise to inner ear structures, including hair cell receptors and ganglion cells that form CN8
Ext. Ear innervation
Sensory innervation (touch, pressure, 2pt, pain temp) is complex
- from branches of mandibular division (V3, ariculotemporal)
- from branch of cervical plexus (greater auricular nerve)
- from small branches of facial (CN7) and vagus (CN10)
cranial nerve branches of 5, 7, 10 terminate in trigeminal nuclear complex of brainstem
- touch/pressure - chief nucleus of 5
- pain/temp - spinal trigeminal nucleus of 5
External auditory meatus
S-shaped from top down
-layered, slightly cone-shaped ear drum at end with malleus attached to backside
Ear drum
Healthy drum is smooth, pink and translucent
- innervation of drum is complex (outer surface is V3, 7, 10, inner surface is 9)
- Auditory tube is path for infection nasopharynx
- eardrum reddens (otitis media)
- can get fluid build up
- too much pressure = drum rupture
Sound
air molecules/particles in constant random motion, bumping into each other and exerting small pressure variations
- pressure wave transmitted through air with each particle only moving a small distance
- pressure wave pushes on ear drum
- sinusoidal wave of pressure moves ear drum in and out
Features of pure tones
Single frequency
- period = 1/f (time it takes to complete one cycle of wave
- distance between successive peaks is wavelength
- sound transmitted at velocity dependent on density and elasticity of substance
Characteristics of sound
Amplitude = loudness (decibels) frequency = pitch complexity = Timbre
Sensitivity
Humans can hear over a huge range of intensities
- hearing sensitivity not the same at all frequencies –> best in speech frequency range
- in range, 16Hz to 16kHz heard (conversation = middle)
- aging affects high frequency hearing (presbicusis)
Middle ear
- narrow, air-filled, mucous membrane-lined chamber in temporal bone between ext. acoustic meatus and inner ear
- eustachian tube connects outside world to middle ear to nasopharynx
- Branch of CN9 (tympanic) innervates lining of middle ear and carries parasympathetics for the otic ganglion
- facial CN7 runs in back wall of middle ear and gives off chordae tympani that runs through middle ear space carrying taste and parasymp fibers
- 3 bones malleus, incus and stapes = hammer, anvil and stirrup, to transfer vibrations of drum to fluid of inner ear
- two muscles: tensor tympani - malleus (CN5), stapedius muscle - stapes (CN7)
***major function to AMPLIFY airborne sound wave so it can be transferred to fluid of inner ear to activate hair cells (overcome impedance mismatch)
Inner ear
cochlea (hearing) and vestibular apparati (balance)
- cochlea = membranous duct sitting inside bony duct (coiled like a seashell - 2.5 turns)
- hair cells for hearing sit inside membranous labyrinth
Membranous labyrinth
separates bony labyrinth into 3 spaces:
- scala vestibuli -> and tympani filled with perilymph
- scala tympani
- scala media - has hair sells –> filled with endolymph (high K, low Ca)
formed by reissner’s membrane (separate from vestibuli)
stria vascularis pasted to bony wall and secretes endolymph
basilar membrane separates from scala tympani and auditory hairs cell sit here
tectorial membrane sits over hair cells
Cochlea
Sound wave transmitted to fluid via oval window –> scala vestibuli –> scala tympani via helicotrema –> round window bulge into middle ear –> pressure differential across membranous labyrinth (move up and down)
***only part of the length of the basilar membrane vibrates to a given frequency
Basilar membrane
width and stiffness varies from one end (base) to the other (apex)
- change resonant properties
- stiff narrow base vibrates at high freq
- apex frequency of vibration systematically becomes lower
