Hearing: the function of the outer, the middle and inner ear. Hearing tests. The auditory pathways

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1 Hearing: the function of the outer, the middle and inner ear. Hearing tests. The auditory pathways Dr. Gabriella Kékesi 74. Hearing: the function of the outer, the middle and inner ear. Hearing tests. The auditory pathways Define the following categories: pure (basic) ton, sound (musical tone), noise, frequency, loudness and intensity of the sound, propagation of the sound, sound pressure level (db). Describe the function of the outer, middle, and inner ear structures in the mechano-electrical transduction process of sound energy into nerve impulses. Describe the acustic impedance matching. Describe the differences between bone and air conduction Describe the nerves and muscles in the middle ear and explain their role in withdrawal reflexes Define the difference between conductive, sensory and neural loss of hearing. Explain the frequency analysis performed by the cochlea on the basis of its physical properties (Békésy theory, tonotopy). Identify the neuronal elements of the organ of Corti. Explain the function of inner and outer hair cells. Explain how deformations of the basilar membrane are converted into action potentials in auditory nerve fibers. Describe the auditory pathway. Describe the role of frequency code and population code in hearing and explain the binaural hearing. Normal values: frequency range of human hearing: Hz, sound pressure level of human hearing: db, reference sound pressure level: 20 µpa, threshold of human hearing: 0 db, frequency range of human speech: Hz, reference frequency of the phon scale: 1000 Hz 1

2 Sound: longitudinal waves that propagates through compressible media (cannot travel through vacuum) Sound is the oscillation of pressure (vibration), a series of compressions (where molecules are dense) and rarefactions (where molecules are sparse) pressure difference Sound characteristics Sound characteristics: Frequency: measure of how many vibrations occur in one second, and directly corresponds to the pitch of a sound (Hz) - The higher the frequency the higher the pitch Amplitude: the higher the amplitude the higher the volume Wave length Sound pressure Intensity Velocity Acoustic impedance indicates how much sound pressure is generated by the vibration of molecules of a particular acoustic medium at a given frequency = resistance of the medium against the sound propagation 2

3 A pure tone is a tone with a sinusoidal waveform A complex tone is any musical tone that is periodic and can be described as a sum of simple tones with harmonically related frequencies of a single frequency. A harmonic series is the sequence of all multiples of a base frequency. Human hearing Sensitivity range: Hz (0-120 db) Speach range: Hz Hearing threshold: Hz (0 db) Infrasounds: below 20 Hz (elephant, owl) Ultrasounds: upper than Hz (bat, delfin) 3

4 Sound Pressure Level (SPL) Intensitiy of the sound: reference sound pressure of 20 micropascals (μpa), which is considered the threshold of human hearing at 2000 Hz Alexander Graham BELL 0 db Expression unit: db (decibel) SPL [db] =20 log P/20µPa The phon is a unit of loudness level for pure tones. Its purpose is to compensate for the effect of frequency on the perceived loudness of tones At 1000 Hz: db=phon Ear: is the organ that detects sound. It not only receives sound, but also aids in balance and body position. The ear is part of the auditory system. Portions: Outer ear: auricle (pinna) and ear canal, surface of ear drum (tympanic membrane) Middle ear: Couple the sound from the opening of the ear canal to the cochlea. Increases the soud pressure and transmits sound waves. Inner ear: organ of hearing (cochlea) and vestibular apparatus (labyrinth) 4

5 Outer ear Auricula (pinna) Flesh covered cartilage appendage Ear canal Partially cartilage, partially lies on the bone of the skull Ear wax (cerumen) is produced by glands. Hairs Functions as a resonator Tympanic membrane (ear drum) responsibilities: plays an important role in the conduction of sound. Help to get sound (imposes filtering) Help localize the direction of the sound source Serving as a resonator: it increases the pressure of the incoming acoustic signal by some db Serving as a transducer, the membrane converts acoustic pressure waves into mechanical motion. otoscopy 5

6 Middle ear air filled cavity cavum thympani Ear drum Eustachian-tube (tuba auditiva) Connects from the chamber of the middle ear to the back of the nasopharynx ventilation Oval and round window Oval window connects to the stapes; round window is closed by the secondary eardrum Ear bones - ossicles malleus hummer incus anvil Stapes - stirrup Muscles in the middle ear: Tensor tympanic muscle: attached to the malleus and keeps the tympanic membrane tensed allowes sound vibrations on the tympanic membrane to be transmitted to the ossicles. Pulls the malleus inward. Stapedius muscle: pulls the stapes outward (from ovale window), protects against overly loud vibrations 6

7 Tympanic (acoustic) reflex - attenuation reflex physical response to overly loud noises protect the cochlea from damage The movement of the ossicles may be stiffened by two muscles, the stapedius and tensor tympani, which are under the control of the facial nerve and trigeminal nerve, respectively. These muscles contract in response to loud sounds, thereby reducing the transmission of sound to the inner ear. bilateral reflex protect against low frequency sounds do not protects against single sounds with high intensity (fulmination, rifle shot) Tympanometry A tone of 226 Hz is generated by the tympanometer into the ear canal, where the sound strikes the tympanic membrane, causing vibration of the middle ear, which in turn results in the conscious perception of hearing. Some of this sound is reflected back and picked up by the instrument. Most middle ear problems result in stiffening of the middle ear, which causes more of the sound to be reflected back. 7

8 Ear bones ossicles malleus, incus, stapes: they are suspended by ligaments in such a way that the combined malleus and incus act as a single lever. responsibilities: ossicles works to efficiently couple the sound from the opening of the ear canal to the cochlea. There are several simple mechanisms that combine to increase the sound pressure - Impedance matching 1. hydraulic principle : Sound energy strikes the tympanic membrane and is concentrated to the smaller footplate. 2. lever principle : increase in the force applied to the stapes footplate compared with that applied to the malleus 3. round window protection channels the sound pressure to one end of the cochlea, and protects the other end from being struck by sound waves abnomralities: conductive hearing loss Conduction of sound from the tympanic membrane to the cochlea Ossicles conduct sound from the tympanic membrane through the middle ear to the cochlea Handle of malleus is attatched to the eardrum Incus moves with malleus (bounded with ligaments) Incus also articulates with the stem of the stapes Footplate (base) of the stapes lies against membraneous labyrinth of the cochlea in the openinig of the oval window Every time the tympanic membrane moves inward the stapes push forward the oval window (cochlear fluid); and to pull backward on the fluid every time the malleus moves outward Impedance matching by the ossicular system Increase the force of movement about 1.3 times Surface area differences of the tympanic membrane and stapes (17 times) 8

9 Inner ear Consists bony labyrinth in the temporal bone of the skull with a system of passages comprising two main functional parts: organ of hearing (cochlea) vestibular apparatus (labyrinth): vestibule of the ear and semicircular canals Innervation: VIII. cranial nerve Membraneous labyrinth runs inside of the bony labyrinth (between the perilymph fluid) Frequence analyzing Mechano-electrical transduction Air conduction: outer ear middle ear ossicles inner ear Bone conduction: vibration of skull bones inner ear (without middle ear) Cochlea System of coiled tubes Stapes foramen ovale portions: Fluid filled hollows Scala vestibuli, perilympha Reissner s membrane Scala media: Corti-organ, endolympha, n. cochlearis Basilar membrane Scala tympani, perilympha role: frequence encoding Faceplate of stapes 9

10 Organ of Corti Contains electromechanically sensitive hair cells. They are the receptors and organ that generate nerve impulses in response to soud vibration. Structure: two specialized types of epthelial cells Three raws of outer/external hair cells: Attach to tectorial membrane Stereocilia are contracted as the result of depolarization, thus pull the membrana tectoria closer to the inner hair cells (amplifier) Sensitive to sounds with high intensity PRESTIN Certain medicines may destroy them (pl. streptomycin, ASA) Single row of inner/internal hair cells: Do not attache to tectorial membrane Tectorial membrane The bases of the hair cells synapse with a network of cochlea nerve endings Generates nerve impulses in response to vibration of the basilar membrane. The auditory sense organ. Martin Schwander et al. J Cell Biol 2010;190: Schwander et al. 10

11 Tonotopy: is the spatial arrangement of where sounds of different frequency are processed in the brain. Tones close to each other in terms of frequency are represented in topologically neighbouring regions in the brain. Different regions of the basilar membrane in the organ of Corti, vibrate at different sinusoidal frequencies due to variations in thickness and width along the length of the membrane. Nerves that transmit information from different regions of the basilar membrane therefore encode frequency tonotopically: base: sounds of high pitch helicotrema: sounds of low pitch mechanisms: Traveling wave along the basilar membrane Cochlear amplifier Best frequency phase locking Georg von Békésy (Békésy György) hungarian biophycist In 1961, he was awarded the Nobel Prize in Physiology or Medicine for his research on the function of the cochlea in the mammalian hearing organ. Research Békésy developed a method for dissecting the inner ear of human cadavers while leaving the cochlea partly intact. By using strobe photography and silver flakes as a marker, he was able to observe that the basilar membrane moves like a surface wave when stimulated by sound. Because of the structure of the cochlea and the basilar membrane, different frequencies of sound cause the maximum amplitudes of the waves to occur at different places on the basilar membrane along the coil of the cochlea. He concluded that his observations showed how different sound wave frequencies are locally dispersed before exciting different nerve fibers that lead from the cochlea to the brain. He theorized that the placement of each sensory cell (hair cell) along the coil of the cochlea corresponds to a specific frequency of sound (the so-called tonotopy). Békésy later developed a mechanical model of the cochlea, which confirmed the concept of frequency dispersion by the basilar membrane in the mammalian cochlea. But this model could not provide any information as to a possible function of this frequency dispersion in the process of hearing

12 A. J. Hudspeth, Integrating the active process of hair cells with cochlear function. Nature Reviews Neuroscience 15, (2014) doi: /nrn

13 Inner ear hair cells. Coloured scanning electron micrograph (SEM) of sensory hair cells from the cochlea of the inner ear. The hairs are surrounded by a fluid (endolymph). As sound enters the ear it causes waves to form in the endolymph, which in turn cause these hairs to move. The movement is converted into an electrical signal, which is passed to the brain. Each crescent-shaped arrangement of hairs lies on the top of a single cell. Nervus cochlearis Efferentation Action potential 1 n. cochlearis afferent fiber 1 inner hair cell 1 inner hair cell receives more inputs Trm release ggl. spirale Analysis of intensity in the cochlea Frequence code: the intensity of the sound is proportional to the frequency of the action potential Population code: the sound intensity is proportional to the deflection of the m. basilaris adjacent afferent fibers are also stimulated with higher sound intensity Phase locking: Characteristic frequence The maximum of the frequency of the action potential correlates with the maximum of the soud pressure lateral olivocochlear bound decreases the transmitter release from the inner hair cells - adjusts the sensitivity of the cochlear afferent fibers medial olivocochlear bound inhibits the amlifier role of the outer hair cells on the contralateral cochlea 13

14 Phase locking: an observed phenomenon (in support of the volley principle) where neurons fire in synchrony with the phase of a stimulus. No individual neuron could fire at each peak, but a bunch of phase-locked neurons working together can produce a burst of activity at each peak, and so the firing frequency of a collection of neurons can indeed mimic the frequency of the stimulus. Central auditory processing Br 41, 42 14

15 Examination of hearing loss Conductive or sensorineural Weber test. It can detect unilateral (one-sided) conductive hearing loss (middle ear hearing loss) and unilateral sensorineural hearing loss (inner ear hearing loss). a vibrating tuning fork (256Hz) is placed on top of the head equi-distant from the patient's ears. The patient is asked to report in which ear the sound is heard louder. A normal Weber test has a patient reporting the sound heard equally in both sides. In an affected patient, if the defective ear hears the Weber tuning fork louder, the finding indicates a conductive hearing loss in the defective ear. In an affected patient, if the normal ear hears the tuning fork sound better, there is sensorineural hearing loss on the other ear (defective ear). Air vs bone conduction Rinne test - presence of conductive hearing loss. The Rinne test is performed by placing a vibrating tuning fork (512 Hz) against the patient's mastoid bone and asking the patient to tell you when the sound is no longer heard. Quickly position the still vibrating tuning fork 1-2 cm from the auditory canal, and again ask the patient to tell you if they are able to hear the tuning fork. A normal or positive Rinne test is when the sound heard outside of the ear (air conduction) is louder than bone conduction. In conductive hearing loss, bone conduction is better than air, a negative Rinne. 15

16 Audiometer Audiogram: to determine the nature of hearing disabilities Figure Audiogram of the old-age type of nerve deafness. Figure Audiogram of air conduction deafness resulting from middle ear sclerosis. 16

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