Auditory Physiology Richard M. Costanzo, Ph.D. OBJECTIVES After studying the material of this lecture, the student should be able to: 1. Describe the morphology and function of the following structures: a. outer ear b. middle ear c. inner ear 2. Define the frequency range over which human auditory threshold is at its lowest level. 3. Explain what is meant by the term acoustical impedance matching. Give two examples of how this is accomplished in the human auditory system. 4. Discuss the physical characteristics of the basilar membrane and how they function to encode the frequency components of sound. 5. Discuss what is meant by the tonotopic organization of the auditory system. 6. Describe two clinical tests used to evaluate auditory function. I. PROPERTIES OF SOUND A. Sound Waves Sound waves are produced by an oscillating body or mass. Oscillations result in compression of air molecules (increased pressure) followed by rarefaction (decrease in pressure). These fluctuations are propagated over long distances and are referred to as sound waves. Figure 1: Periodic Pressure Fluctuations of a Sound Wave Two parameters of a sound wave are: 1. Sound pressure (amplitude of periodic fluctuations) 2. Pitch (frequency of the pressure fluctuations)
Sound waves of a single frequency are called tones. B. Decibels The intensity of sound is expressed as a relative measure called the decibel. X decibel = 20 log P/P o P is the sound pressure being measured and P o is the sound pressure it is being referred to (i.e., the initial pressure or the reference). If a sound pressure P is 10 times that of its initial value P o, it is said to have increased 20 decibels. In audition the reference pressure P o most often used is.0002 dynes/cm 2 ("triple- O-two") --this value is equivalent to the average human detection for a 1000 Hz tone. When this reference is used to calculate sound intensities, the resulting value is referred to as a sound pressure level (SPL) and has units of decibels (db SPL). C. Audibility Curve The minimum sound pressure amplitude that a subject can perceive is called the auditory threshold. A graph of auditory threshold over a range of frequencies or tones is called the audibility curve. Figure 2: Audibility Curve and Equal Loudness Contours (From Schmidt, Fundamentals of Sensory Physiology, 1986) The normal audibility curve shows that threshold for a 1000 Hz tone is about 0 decibels Sound Pressure Level (0dB SPL). Human auditory threshold is at its
lowest level however, between 2,000 and 4,000 Hz. The normal range for hearing is 0 to 120 db SPL. Above 120 db sounds become unpleasant and even painful. This range of sound pressure levels represents a million-fold change in sound pressure (10 6 or 6 log units). From an engineering point of view the auditory system responds over a very large dynamic range of stimulus intensities. II. THE OUTER AND MIDDLE EAR Figure 3: Diagram of the Ear, (from Costanzo, 2006, Fig. 3-19) A. Outer Ear The outer ear collects airborne auditory vibrations and directs them to the tympanic membrane. It consists of the pinna, a structure that helps direct or funnel sound waves and the external auditory meatus (or auditory canal). The external auditory meatus has a resonant frequency of about 3,000 Hz and in part explains why human thresholds are very low at this frequency. This structure also acts as a buffer to changes in the external environment such as temperature, humidity, etc.
B. Middle Ear The middle ear functions to match the acoustical impedance of airborne sound vibrations being transferred to the fluid-filled chambers of the inner ear. The middle ear consists of a small air-filled chamber connected to the pharynx by a narrow tube called the eustachian tube. 1. Eustachian tube Figure 4: The Middle Ear (From Ganong, 19952) This narrow passageway to the pharynx serves to equalize pressure between the middle ear and the outer ear. Sudden changes in external pressure (such as climbing to high altitudes) cause the eardrum to stretch, altering its response characteristics and in some cases causing discomfort or pain. By swallowing, however, the eustachian tube reflexively opens and permits air to equilibrate, thereby eliminating pressure differences across the tympanic membrane. 2. Malleus, incus, and stapes Within the middle ear there are 3 bones or ossicles. They are the malleus (hammer), incus (anvil) and stapes (stirrup). These bones transmit airborne sound vibrations from the tympanic membrane to the oval window. 3. Impedance matching Opposite the oval window is the fluid-filled auditory organ, the cochlea. The middle ear must transmit airborne sound vibrations to the fluid environment of the cochlea. Energy losses due to reflection are not insignificant and can be as high as 99% in some cases. The middle ear compensates for some of these losses in two ways:
Table 1 TYMPANIC MEMBRANE OVAL WINDOW DIFFERENCE AREA 55 mm 2 3.2 mm 2 (17 times) FORCE 1 unit 1.3 units (1.3 times) DISPLACEMENT 1 unit 0.75 units (0.75 times) i. Concentration of force on a smaller area The sound pressure impinging on the tympanic membrane is distributed over approximately 55 mm 2. These forces are transferred by the bones of the middle ear to the oval window (3.2 mm 2 ); the pressure is effectively increased some 17 times. Pressure = Force/Area ii. Increased force due to lever action of ossicles The lever action of the ossicles provides additional amplification of force. This is not, however, due to an increase in displacement at the oval window (the oval window is displaced only 3/4 that of the tympanic membrane) but rather by an increase in the force at the oval window. The total force at the oval window is 1.3 times that at the tympanic membrane. The overall result is that the sound pressure at the tympanic membrane is amplified approximately 22 times (17 x 1.3). This extra force is used to overcome the inertia of the fluid-filled cochlea and permits vibrations to be transmitted to the basilar membrane. Matching of the acoustical impedance of the air-fluid interface is an important function of the middle ear. III. INNER EAR The inner ear contains the sensory cells of the auditory organ (the cochlea) which transforms vibrations into neural impulses. A. The Cochlea
Figure 5: Structure of the Cochlea and the Organ of Corti (From Costanzo,, 2006, Fig. 3-20) Consists of three tubular canals: (1) the scala vestibuli, (2) the scala media (cochlear duct) and (3) the scala tympani. The human cochlea is a helical structure (approx. 2-1/2 turns) embedded in the temporal bone. 1. Scala vestibuli - upper canal- contains perilymph and at the basal end of the cochlea is terminated at the oval window. 2. Scala tympani - lower canal- contains perilymph and is connected to the scala vestibuli by a small opening at the top (apex) of the cochlea called the helicotrema. The basal end is terminated by the round window. 3. Scala media - located between the scala vestibuli and the scala tympani and is sometimes referred to as the cochlear partition. It is bordered by the vestibular (Reissner's) membrane above, the stria vascularis laterally, and the basilar membrane below. The scala media contains endolymph which is rich in potassium. B. Organ of Corti Located on the basilar membrane is the Organ of Corti which contains the receptor cells for auditory stimuli. 1. Receptor cells - do not have axons and therefore, are referred to as secondary sensory cells. There are two types: inner hair cells and outer hair cells. Inner hair cells form a single row and are fewer in number than outer hair cells, which are arranged in three parallel rows. 2. Auditory (8th) nerve - Cell bodies of the 8th nerve are located in the spiral ganglion. Individual nerve fibers connect to only a single inner hair cell even though a single inner hair cell may have several different fibers connected to it.
Nerve fibers innervating outer hair cells are branched and connected to several different cells. C. Transduction Process (shearing forces) The stereocilia which protrude from the hair cells are embedded in the tectorial membrane which is anchored to the medial wall of the scala media. The basilar membrane is more elastic than the tectorial membrane so that deflections in the cochlear partition produce a greater displacement of the basilar membrane than the tectorial membrane. This results in a medially directed shearing force on the stereocilia which in turn causes conductance changes in the receptor cell membrane. These conductance changes lead to the release of transmitter at the basal end of the cells which stimulate the terminal ends of the auditory nerve fibers. D. Cochlear Dynamics The physical characteristics of the basilar membrane make it an excellent frequency analyzer. Sound is encoded by the relative amplitude of vibration of different regions along the basilar membrane. Figure 6: Diagram of the Basilar Membrane (From Costanzo, 2006, Fig. 3-21) 1. Base - is narrow (.1 mm) and very stiff. Responds best to high frequencies
2. Apex (tip) - is widest part of basilar membrane and very compliant. Responds best to low frequencies. 3. Traveling waves - Vibrations are introduced to the cochlea at the oval window and are transmitted down the cochlea from the base to the tip. Due to differences in the elastic properties along the basilar membrane, vibrations differ in amplitude and phase. This results in a complex set of "traveling waves" and even though each point along the basilar membrane may be vibrating at the same frequency, these amplitude and phase differences produce an apparent movement of waves along the surface. E. Place Theory The envelope of maximal amplitudes of oscillation for a set of traveling waves changes for tones of different frequencies. Figure 7: Envelope of Maximal Amplitude Oscillations along the Basilar Membrane (From Schmidt, Fundamentals of Sensory Physiology, 1986) High frequencies produce maximal oscillations near the stapes (basal end) and with decreasing frequency there is a shift in maxima towards the apex (tip) of the cochlea. For each frequency component there is a point of maximal displacement along the cochlea which excites receptor cells in that region and thereby encodes that particular frequency. Cells in different regions of the basilar membrane encoding different frequencies. This is known as the Place Theory of Hearing. Evidence that the cochlea acts as a frequency analyzer comes from microelectrode recordings from the cochlea. Since it is extremely difficult to stay in cells while they are vibrating, electrodes have been placed in the perilymph of the scala tympani to monitor potential changes. These potential changes fluctuate and are identical to the stimulus waveform. Since this is similar to what happens in a microphone, the potentials are called cochlear microphonics. Measurements of cochlear
microphonics at different points along the basilar membrane give the same maximal excitation as predicted by the Place Theory. IV. TUNING CURVES Although a given nerve fiber innervates a particular region of the cochlea and is maximally excited by the characteristic frequency of that region, it can also be excited by other frequencies, although to a lesser extent. A. The frequency requiring the least intensity to excite a fiber is called its characteristic or best frequency. B. Frequencies other than the best frequency require more intense stimuli to excite the fiber. A larger range of frequencies below the best frequency than above are capable of exciting the unit. Figure 8: (From Schmidt, Fundamentals of Sensory Physiology, 1986) V. AUDITORY PATHWAYS A. Anatomical Structures 1. 8th nerve 2. cochlear nucleus (dorsal and ventral) 3. superior olive, accessory nucleus 4. lateral lemniscus 5. inferior colliculus 6. medial geniculate body 7. auditory cortex B. Bilateral Representation - occurs at most levels of the auditory system. As a consequence many ipsilateral lesions do not produce a significant loss of hearing.
C. Tonotopic Organization - There is a spatial mapping of frequencies at all levels of the auditory system. Figure 10. Netter Presenter Image III.15 D. Complex Pattern Recognition - The higher in the auditory system you go, the more complex the stimulus required to excite a cell. VI. CLINICAL DISORDERS A. Conduction and Nerve Deafness Auditory disorders are usually due to problems in sound transmission (conduction deafness) or damage of auditory pathways (nerve deafness). Conduction deafness can occur due to blockage of the external auditory canal with wax, inflammatory processes such as acute or chronic otitis media, thickening of the tympanic membrane, otosclerosis and stiffening of the attachment of the stapes to the oval window. Nerve deafness can occur as a result of trauma, tumors of the acoustic nerve, spread of infections originating in the middle ear, lesions of hair cells and degeneration of the acoustic nerve by toxic drugs. Streptomycin causes deafness by damaging the cochlear organ.
B. Clinical Tests Tests are available to differentiate between lesions of the conducting system in the middle ear (conductive deafness) and sensorineural lesions (nerve deafness). In the Weber Test a tuning fork is applied to the forehead and the patient is asked whether the sound is heard in the midline or localized to one ear. In conductive deafness the tone sounds louder in the affected ear (due to absence of background environmental noise). In nerve deafness it is louder in the normal ear (transmission from the affected ear is impaired). In conductive deafness normal air conduction though the external and middle ear is reduced, but bone conduction (direct stimulation of cochlea) is relatively enhanced. In the Rinne test a tuning fork is applied to the patient's mastoid process. When bone conduction is no longer audible the tuning fork is placed near the external meatus to determine if sound can be detected via air conduction. In conductive deafness sound cannot be heard by air conduction following bone conduction. In normal individuals and in nerve deafness the reverse is true. Audiometry uses a range of pure tones to determine threshold values for each ear using both bone and air conduction. Characteristic patterns in the audibility threshold curve can be used to determine nerve deafness (for high and low frequencies), conduction deafness and mixed types of auditory dysfunctions. Table 2: Clinical Tests for "Conductive" and "Nerve" Deafness Procedure Normal Conductive Deafness Nerve Deafness Weber Test Vibrating tuning fork placed on vertex of skull Hears the same in both ears Louder in diseased ear (no masking environmental noise) Louder in normal ear Rinne Test Vibrating tuning fork placed on mastoid process until subject no longer hears it, then held in air next to ear. Hears vibration in air after bone conduction is over. Vibration not heard in air after bone conduction May be detected in air after bone conduction ADDITIONAL REFERENCES Costanzo, L.S. Physiology, 3 rd Edition, Saunders Elsevier, 2006, pp. 86-90. Klinke, R. Physiology of Hearing. In Fundamentals of Sensory Physiology, 3rd ed., R.F. Schmidt Ed. Springer-Verlag 1986, p. 199-223. * Netter Presenter Image Copyright 2004 Icon Learning Systems. All rights reserved.