Auditory Sensation (Hearing)

Auditory Sensation (Hearing) 1 Objectives ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ ⚫ Define decibel (intensity) and Hz (frequency) Describe the ossicular system and explain its function Follow up sound transmission up to the cochlea Outline the structure of cochlea, and the organ of Corti Describe the mechanism of sound transduction Follow up the auditory pathway to the cerebral cortex Describe auditory abnormalities (types of deafness) 2 3 Frequency of sound wave: Audible sound wave pure tones 20 – 20,000 Hz Speed of sound is 335 m/sec in Air 4 5 6 Decibel: a measure of sound intensity ⚫ ⚫ Decibel (dB) = 10 log I/IR ⚫ I = intensity of sound, IR= reference intensity ⚫ Acoustic intensity is proportional to the square of sound pressure level ⚫ Sound pressure is more conveniently measured than sound intensity Sound pressure level (SPL) unit is decibel ⚫ SPL (dB) = 20 log P/PR ▪ P= the sound pressure in N/m2 (N=Newton, m = meter) ▪ PR= reference pressure (either 0.0002 dynes/cm2, the absolute threshold for human hearing and equal 20 micropascal, or 1 dyne/ cm2 ) 7 8 The Tympanic Membrane and the Ossicular System ➢ Tympanic membrane functions to transmit vibrations in the air to the cochlea ➢ Amplifies the signal because the area of the tympanic membrane is 17 times larger than the oval window (55 sq. mm Vs. 3.2 sq. mm) ➢ Tympanic membrane connected to the ossicles ➢ malleus ➢ incus ➢ Stapes ➢ Ossicular system works as a lever system and amplifies the sound 1.3 time ➢ Total amplification is 22 times (17x1.3) called Impedance Matching (match the resistance of sound wave movement in fluid vs. the resistance in air) 9 Components of the Auditory System 10 11 12 Attenuation of Sound by Muscle Contraction ➢ Two muscles attach to the ossicles ➢ Stapedius (supplied by facial Nn VII) ➢ tensor tympani (supplied by Trigeminal Nn V) ➢ A loud noise initiates reflex contraction after 40 - 80 milliseconds ➢ Attenuates vibration going to cochlea (Attenuation reflex) ➢ Serves to protect cochlea and damps (mask) low frequency sounds in loud environment .i.e., your own voice 13 Cochlea ➢system of three coiled tubes separated by membranes into the scala tympani, scala media, scala vestibuli ➢sound waves cause back and forth movement of the tympanic membrane which moves the stapes back and forth ➢this causes displacement of fluid in the cochlea and induces vibration in the basilar membrane 14 Components of the Auditory System 15 Structural Components of the Cochlea 16 Structure of the Human Cochlea 17 18 19 BasilarMembrane ➢ contains about 30,000 fibers which project from the bony center of the cochlea, the modiolus ➢ fibers are stiff reed-like structures fixed to the modiolus and embedded in the loose basilar membrane ➢ because they are stiff and free at one end they can vibrate like a musical reed ➢ the length of the fibers increase and the diameter of the fibers decrease from base to the helicotrema, overall stiffness decreases 100 times, high frequency resonance occurs near base, low near apex 20 Organ of Corti ➢ receptor organ that generates nerve impulses ➢ lies on the surface of the basilar membrane, contains rows of cells with stereocilia called hair cells ➢ the tectorial membrane lies above the stereocilia of the hair cells ➢ movement of the basilar membrane causes the stereocilia of the hair cells to shear back and forth against the tectorial membrane 21 22 23 The Organ of Corti 24 movement of the basilar membrane causes the stereocilia of the hair cells to shear back and forth against the tectorial membrane. 25 Malleus Incus Stapes vibrating Helicotrema in oval window Cochlea Sound waves Perilymph 3 7 4 5 1 2 6 9 External auditory canal 8 Scala tympani Scala vestibuli Basilar membrane 8 Spiral organ (organ of Corti) Tectorial membrane Vestibular membrane Cochlear duct (contains endolymph) Tympanic membrane Secondary tympanic membrane vibrating in round window Middle ear Auditory tube 26 Nerve Impulse Origination ➢ The stereocilia, when bent in one direction cause the hair cells to depolarize, and when bent in the opposite direction hyperpolarize. ➢ this is what begins the neural transduction of the auditory signal ➢ Auditory signals are transmitted by the inner hair cells. ➢ 3-4 times as many outer hair cells than inner hair cells around 12000 outer hair cells and 3500 inner hair cells ➢ outer hair cells may control the sensitivity of the inner hair cells for different sound pitches 27 28 29 Determination of Sound Frequency and Amplitude (sound intensity) ➢ Place principle determines the frequency of sound perceived. ➢ Different frequencies of sound will cause the basilar membrane to oscillate at different positions (basilar membrane is tonotopically organized) ➢ Position along the basilar membrane where hair cells are being stimulated determines the pitch of the sound being perceived. ➢ Phase-locked (volley) principle at lower frequencies of sound where firing rate determines the phase of sound wave (i.e frequency of sound wave) ➢ Amplitude is determined by how much the basilar membrane is displaced ( by frequency of impulses from the nerve fiber and the No. of nerve fibers stimulated) 30 The “Place Principle” 31 Phase-locked (volley) principle 32 33 Threshold of hair cells 34 Decibel Unit of Sound ➢unit of sound ➢expressed in terms of the logarithm of their intensity ➢a 10 fold increase in energy is 1 bel ➢0.1 bel is a decibel ➢1 decibel is an increase in sound energy of 1.26 times 35 Central Auditory Pathway 36 37 38 from medial geniculate to auditory cortex from inferior colliculus to medial geniculate from superior olivary nucleus to inferior colliculus via the lateral lemniscus some fibers pass to the ipsilateral superior olivary nucleus. 2nd order neurons project through trapezoid body to the contralateral superior olivary nucleus. fibers enter dorsal and ventral cochlear nuclei of the upper part of the medulla. 39 Auditory Cortex and Association Areas - arranged by tonotopic maps - high frequency sounds at one end of map - low frequency sounds at other end - discrimination of sound patterns 40 Determining the Direction of Sound ➢superior olivary nucleus divided into lateral and medial nuclei ➢lateral nuclei detects direction by the difference in sound intensities between the two ears ➢medial nuclei detects direction by the time lag between acoustic signals entering the ears 41 Deafness ➢nerve deafness ➢impairment of the cochlea or the auditory nerve ➢conduction deafness ➢impairment of tympanic membrane or ossicles 42 Audiometery 43 Weber test Rinne test 44 How do doctors conduct Rinne and Weber tests? Rinne and Weber tests both use 512-Hz tuning forks to test how you respond to sounds and vibrations near your ears. Rinne test 1. The doctor strikes a tuning fork and places it on the mastoid bone behind one ear. 2. When you can no longer hear the sound, you signal to the doctor. 3. Then, the doctor moves the tuning fork next to your ear canal. 4. When you can no longer hear that sound, you once again signal the doctor. 5. The doctor records the length of time you hear each sound. Weber test 1. The doctor strikes a tuning fork and places it on the middle of your head. 2. You note where the sound is best heard: the left ear, the right ear, or both equally. 45 What are the results of Rinne and Weber tests? Rinne and Weber tests are noninvasive and cause no pain, and there are no risks associated with them. The information they provide determines the type of hearing loss you may have, especially when the results of both tests are used together. Rinne Test results • Normal hearing will show an air conduction time that is twice as long as the bone conduction time. In other words, you will hear the sound next to your ear twice as long as you will hear the sound behind your ear. • If you have conductive hearing loss, the bone conduction is heard longer than the air conduction sound. • If you have sensorineural hearing loss, air conduction is heard longer than bone conduction, but may not be twice as long. Weber Test results • Normal hearing will produce equal sound in both ears. • Conductive loss will cause the sound to be heard best in the abnormal ear. • Sensorineural loss will cause the sound to be heard best in the normal ear. 46 The Internal Ear (Site of Equilibrium Receptors) 47