Chapters 9,10 Auditory and Vestibular Systems Chris Rorden University of South Carolina Norman J. Arnold School of Public Health Department of Communication Sciences and Disorders

Audition The ear converts sound energy into patterns of neural firing: transduction Outer ear: collect and amplify sound, aid in localization Middle ear: impedance matching Cochlea: frequency and intensity analysis Auditory pathway: complex signal processing

Ear structures Peripheral Central Outer ear Middle ear Inner ear Auditory nerve Central Brainstem Midbrain Cerebral

Anatomy and function Pinna: 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

Outer Ear – Auditory Canal External auditory meatus Provides communication between middle and inner ears by conducting sound to the ear drum S-shaped tube @ 2.5cm long and @ 7mm wide Lining of the lateral 1/3rd of canal has cilia and glands Cerumen (ear wax): protects ear canal from drying out and prevents intrusion of insects

Outer Ear - Ear drum Tympanic membrane Separates outer and middle ear Compliant Thin, three-layered sheet Epithelium of EAM: outer layer Middle layer: fibrous (strong) tissue Inner layer of middle ear: mucous membrane

Outer Ear - Ear drum Slightly concave to EAM, cone-shaped Most depressed and thinnest point is called the umbo End of the attachment of malleus ‘Cone of light’ from umbo to periphery reflects light when viewed with otoscope Slightly oval, taller than wide Otoscope: if you pull the pinna up and back the tympanic membrane is visible

Middle Ear – Tympanic Membrane

Role of outer ear To augment the sound shadow Ear canal protects delicate parts of middle and inner ear from impact. To heighten our sensitivity to sounds Ear 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 canal Just like vocal tract this tube amplifies and dampens certain frequencies based on its length and composition

Localization and shadowing Intensity differences: louder if nearer, less shaded Inter-aural timing differences Frequencies influenced by location relative to pinna.

Middle Ear – Eustachian Tube Establishes communication between middle ear and nasopharynx ~ 35 to 38 mm long, typically closed Biological functions: To permit middle ear pressure to equalize with external air pressure On the air plane, change in atmospheric pressure but not pressure in middle ear Yawning or swallowing opens pharyngeal orifice of tube to equalize pressures To permit drainage of normal and diseased middle ear secretions into the nasopharynx

Middle Ear - Ossicles 3 of the smallest bones Malleus (hammer) Incus (anvil) Stapes (stirrup) Ossicular chain: Transmits acoustic energy from tympanic membrane to inner ear Acts 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

Middle Ear – Ossicles

Middle Ear – Ossicles - Malleus Malleus (hammer) 9 mm long Manubrium (handle): attaches to tympanic membrane; pulls the drum medially Caput (head): jointed (quite inflexibly) to Incus

Middle Ear – Ossicles - Incus The ossicles give the eardrum mechanical advantage via lever action and a reduction in the area of force distribution Pressure = Force/Area; so less area = more pressure the 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

Middle Ear – Ossicles - Stapes Head (caput) jointed to incus Anterior and posterior crura (legs) Footplate: joins oval window of inner ear (opening in temporal bone) via annular ligament

Cochlea and neighbors

Inner Ear - Cochlear

Osseous cochlea Oval window Round window Connects scala vestibuli and middle ear Round window Connects scala tympani and middle ear

Cochleus from 5mo fetus: Cochlear Structures Cochleus from 5mo fetus: Oval window (blue arrow) Round window (yellow arrow)

Tonotopic Base High Freq Apex Low Freq.

Travelling wave Always starts at the base of the cochlea and moves toward the apex Its amplitude changes as it traverses the length of the cochlea The position along the basilar membrane at which its amplitude is highest depends on the frequency of the stimulus

High frequencies have peak influence near base and stapes Traveling wave High frequencies have peak influence near base and stapes Low frequencies travel further, have peak near apex A short movie: www.neurophys.wisc.edu/~ychen/auditory/animation/animationmain.html Green line shows 'envelope' of travelling wave: at this frequency most oscillation occurs 28mm from stapes.

Cochlear structure Cross-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 Endolymph scala vestibuli filled w Perilymph scala tympani filled w Perilymph spiral ganglion nerve fibres www.iurc.montp.inserm.fr/cric/audition/english/cochlea/fcochlea.htm

Inner Ear - Labyrinth Endolymph K+ ~100 mV Reissner’s Membrane Perilymph Na+ ~20mV Reissner’s Membrane Basilar Membrane

Inner Ear – Organ of Corti Both types of hair cells protrude into endolymph of scala media,

Inner 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.

Neural connections Inner hair cells: many nerve fibers for each cell (many-to-one innervation) 3500 Outer hair cells: each nerve fiber connected to many hair cells (one- to-many innervation). 12000

Function of the cochlea First stage of auditory processing 1. Spectral analysis Extracts frequency and amplitude information from sound waves 2. Temporal analysis Basic temporal characteristics of sounds

The ear codes frequency in two ways: Position of neural responses along basilar membrane changes with frequency - tonotopic organization or the place coding Timing of neural responses follows the time waveform of sound – phase-locking

Place coding Place coding: Auditory frequency coded by location of stimulation. Base High Freq Apex Low Freq.

The rate of neural firing matches the sound's frequency. Phase locking The rate of neural firing matches the sound's frequency. Problem: some auditory frequencies much faster than neurons can fire Each neuron can only fire around 200 times per sec. Solution: volley principle: large numbers of neurons that are phased locked can code high frequencies.

Afferent and efferent innervation Afferent: signals from sense organ to brain Auditory signals Efferent: signals from brain to sense organ Inhibits auditory signals Both cochlear and ossicles Improves signal-to-noise ratio by suppressing noise

Primary auditory cortex Medial geniculate Body (thalamus) Inferior colliculus (in midbrain) Auditory radiation Cochlear and Superior olivary Complex in the Medulla

Central 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)

Cochlear Nucleus Evidence of signal processing (monaural) Superior Olivary Complex (SOC) Binaural processing Localization of sound source Low frequency sounds: arrival time compared High frequency sounds: intensity level compared

Inferior Colliculus (IC) Auditory Pathway Lateral Lemniscus Fiber tract within CNS From SOC to IC Inferior Colliculus (IC) Bilateral innervation Frequency, intensity and temporal processing Medial Geniculate Body (MGB) Tonotopic mapping Complex responses to contralateral signals

Cerebral cortex Signal comes primarily from contralateral ear via ipsilateral MGB Heschl’s gyrus Tonotopic mapping in columns Each column has one characteristic frequency Neurons in column responsive to different stimulus parameters, like frequency and intensity

Anatomy and function Many 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’ cortex medial geniculate body inferior colliculus cochlear nucleus complex cochlea superior olivary complex

Clinical Notes Conductive Sensorineural Central Cerumen in canal, Otitis Media of Middle Ear (ear infection) Sensorineural Meniere’s disease (abnormality in the fluids of the inner ear = vertigo), Presbycusis (age related hearing loss) Central Pathology in cortex Bilateral auditory cortex lesions result in: Profound loss of auditory discriminative skills Impaired speech perception Hearing loss