Presentation on theme: "Chapters 9,10 Auditory and Vestibular Systems"— Presentation transcript:
1Chapters 9,10 Auditory and Vestibular Systems Chris RordenUniversity of South CarolinaNorman J. Arnold School of Public HealthDepartment of Communication Sciences and Disorders
2AuditionThe ear converts sound energy into patterns of neural firing: transductionOuter ear: collect and amplify sound, aid in localizationMiddle ear: impedance matchingCochlea: frequency and intensity analysisAuditory pathway: complex signal processing
4Anatomy and functionPinna: the projecting part of the ear lying outside of the head (also called auricle, or just ear lobe)Reflection of sound in pinna provides spectral cues about elevation of a sound source
5Outer Ear – Auditory Canal External auditory meatusProvides communication between middle and inner ears by conducting sound to the ear drumS-shaped 2.5cm long 7mm wideLining of the lateral 1/3rd of canal has cilia and glandsCerumen (ear wax): protects ear canal from drying out and prevents intrusion of insects
6Outer Ear - Ear drum Tympanic membrane Separates outer and middle ear CompliantThin, three-layered sheetEpithelium of EAM: outer layerMiddle layer: fibrous (strong) tissueInner layer of middle ear: mucous membrane
7Outer Ear - Ear drum Slightly concave to EAM, cone-shaped Most depressed and thinnest point is called the umboEnd of the attachment of malleus‘Cone of light’ from umbo to periphery reflects light when viewed with otoscopeSlightly oval, taller than wideOtoscope: if you pull the pinna up and back the tympanic membrane is visible
9Role of outer earTo augment the sound shadowEar canal protects delicate parts of middle and inner ear from impact.To heighten our sensitivity to soundsEar canal boosts sounds 15 to 16 dB between 1.5 and 8 kHz (in the area of speech)This is due to resonance of ear canalJust like vocal tract this tube amplifies and dampens certain frequencies based on its length and composition
10Localization and shadowing Intensity differences: louder if nearer, less shadedInter-aural timing differencesFrequencies influenced by location relative to pinna.
11Middle Ear – Eustachian Tube Establishes communication between middle ear and nasopharynx~ 35 to 38 mm long, typically closedBiological functions:To permit middle ear pressure to equalize with external air pressureOn the air plane, change in atmospheric pressure but not pressure in middle earYawning or swallowing opens pharyngeal orifice of tube to equalize pressuresTo permit drainage of normal and diseased middle ear secretions into the nasopharynx
12Middle Ear - Ossicles 3 of the smallest bones Malleus (hammer)Incus (anvil)Stapes (stirrup)Ossicular chain: Transmits acoustic energy from tympanic membrane to inner earActs as lever: large weak motion of TM causes small forceful movement of stapes.Takes force from gas (air) and matches impedance to liquid (inner ear).Muscles allow movement to be attenuated: Prevents the inner ear from being overwhelmed by excessively strong vibrations
14Middle Ear – Ossicles - Malleus Malleus (hammer) 9 mm longManubrium (handle): attaches to tympanic membrane; pulls the drum mediallyCaput (head): jointed (quite inflexibly) to Incus
15Middle Ear – Ossicles - Incus The ossicles give the eardrum mechanical advantage via lever action and a reduction in the area of force distributionPressure = Force/Area; so less area = more pressurethe resulting vibrations would be much smaller if the sound waves were transmitted directly from the outer ear to the oval window.The movements of the ossicles is controlled muscles attached to them (the tensor tympani and the stapedius).These muscles can dampen the vibration of the ossicles, in order to protect the inner ear from excessively loud noise and that they give better frequency resolution at higher frequencies by reducing the transmission of low frequencies
16Middle Ear – Ossicles - Stapes Head (caput) jointed to incusAnterior and posterior crura (legs)Footplate: joins oval window of inner ear (opening in temporal bone) via annular ligament
22Travelling waveAlways starts at the base of the cochlea and moves toward the apexIts amplitude changes as it traverses the length of the cochleaThe position along the basilar membrane at which its amplitude is highest depends on the frequency of the stimulus
23High frequencies have peak influence near base and stapes Traveling waveHigh frequencies have peak influence near base and stapesLow frequencies travel further, have peak near apexA short movie:Green line shows 'envelope' of travelling wave: at this frequency most oscillation occurs 28mm from stapes.
24Cochlear structureCross-section shows the coiling of the cochlear duct The red arrow is from the oval window, the blue arrow points to the round window.Scala media – filled w Endolymphscala vestibuli filled w Perilymphscala tympani filled w Perilymphspiral ganglionnerve fibres
26Inner Ear – Organ of Corti Both types of hair cells protrude into endolymph of scala media,
27Inner Ear – OHC & IHC Inner Hair Cells Outer Hair Cells Non-motile Vibrates when triggered – acts as preamplifier.Hair cells are mechanically gated ion channels: deflection of hairs depolarizes the cell, resulting in a receptor potential – causing calcium ions to enter, which in turn stimulates the release of neuroreceptors.
28Neural connectionsInner hair cells: many nerve fibers for each cell (many-to-one innervation) 3500Outer hair cells: each nerve fiber connected to many hair cells (one- to-many innervation)
29Function of the cochlea First stage of auditory processing1. Spectral analysisExtracts frequency and amplitude information from sound waves2. Temporal analysisBasic temporal characteristics of sounds
30The ear codes frequency in two ways: Position of neural responses along basilar membrane changes with frequency - tonotopic organization or the place codingTiming of neural responses follows the time waveform of sound – phase-locking
31Place codingPlace coding: Auditory frequency coded by location of stimulation.BaseHigh FreqApexLow Freq.
32The rate of neural firing matches the sound's frequency. Phase lockingThe rate of neural firing matches the sound's frequency.Problem: some auditory frequencies much faster than neurons can fireEach neuron can only fire around 200 times per sec.Solution: volley principle: large numbers of neurons that are phased locked can code high frequencies.
33Afferent and efferent innervation Afferent: signals from sense organ to brainAuditory signalsEfferent: signals from brain to sense organInhibits auditory signalsBoth cochlear and ossiclesImproves signal-to-noise ratio by suppressing noise
34Primary auditory cortex Medial geniculateBody (thalamus)Inferior colliculus(in midbrain)Auditory radiationCochlear and Superior olivaryComplex in the Medulla
35Central Auditory Mechanism Auditory input projects to the cortex bilaterally, with stronger contralateral connections.The superior olive and the inferior colliculus send efferent fibers back to attenuate motion of the middle ear bones (dampen loud sounds)
36Cochlear NucleusEvidence of signal processing (monaural)Superior Olivary Complex (SOC)Binaural processingLocalization of sound sourceLow frequency sounds: arrival time comparedHigh frequency sounds: intensity level compared
37Inferior Colliculus (IC) Auditory PathwayLateral LemniscusFiber tract within CNSFrom SOC to ICInferior Colliculus (IC)Bilateral innervationFrequency, intensity and temporal processingMedial Geniculate Body (MGB)Tonotopic mappingComplex responses to contralateral signals
38Cerebral cortexSignal comes primarily from contralateral ear via ipsilateral MGBHeschl’s gyrusTonotopic mapping in columnsEach column has one characteristic frequencyNeurons in column responsive to different stimulus parameters, like frequency and intensity
39Anatomy and functionMany sound features are encoded before the signal reaches the cortex- Cochlear nucleus segregates sound information- Signals from each ear converge on the superior olivary complex - important for sound localization- Inferior colliculus is sensitive to location, absolute intensity, rates of intensity change, frequency - important for pattern categorization- Descending cortical influences modify the input from the medial geniculate nucleus - important as an adaptive ‘filter’cortexmedial geniculatebodyinferior colliculuscochlear nucleuscomplexcochleasuperior olivary complex
40Clinical Notes Conductive Sensorineural Central Cerumen in canal, Otitis Media of Middle Ear (ear infection)SensorineuralMeniere’s disease (abnormality in the fluids of the inner ear = vertigo), Presbycusis (age related hearing loss)CentralPathology in cortexBilateral auditory cortex lesions result in:Profound loss of auditory discriminative skillsImpaired speech perceptionHearing loss