Biophysics of Hearing Ján Jakuš.
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Hearing – is very important human sense because through the ear we can receive sound, music or noise.Sound-is a kind ofmechanical (pressure) energy, that results from vibration of air molecules.Thus, in a vaccum there isno sound, because no molecules that could undergo thevibration. Velocity of sound in airis approx.330 m/ s. However, in liquids the velocity of sound is 5-timeshigher, in solids there is a velocity even 20-times higher, but its intensity (amplitude )is lower.
Frequency of the sound –is the number of waves that pass a given point, in a given time. The shorter is the wavelenght( λ ), the higheris the sound frequency, and vice versa.Most sounds are mixtures of several frequen-cies.Harmonic soundswith the same frequency enable the perception of music.There are 4 subjectively perceiving sound properties: Pitch,Color, Strenght,Duration of the sound.Watch!For physical properties of sound, also a light, electricity ormagnetism, look Nave &Nave
Among the subjectively percieved properties of sound, the most important is the Strenght. The human ear can dis-tinguish the sound within the frequency range from 16 Hz – 20 000 Hz, but it is the most sensitive for sound frequen-cies between 1500 – 4000 Hz. The higher is the frequency,the bigger is the pitch of the sound, and vice versa.The Loudness of the sound closely co-rrelates withthe Amplitude of the sound.The higher is theamplitude, the louder is the sound, andvice versa.
Intensityof the sound in created in environment is mea-sured in units named decibels (dB).The ear threshold for the minimal sound intensity coming from environment is 0 dB.Subjectively percieved soud intensity is measured in units named Phons (Ph). At frequency of 1 kHz the scale for sound intensity (given in dB ) equals to scale for subjectively percieved intensity (given in Phons)The perception of sound changes into the pain sensa-tion at the sound intensity around 130 dB (the pain threshold).The hearing thresholdis examined by a diagnostic method – AUDIOMETRY. Audiometer is an electronic generator of a sound frequencies at different intensities. Audiometryenables to obtain a graphical record of hea-ring loss for right and left ears.
EAR ANATOMY. Ear consists of: I. Outer Ear– composed of the Pinna (auricula), and the Auditorycanal(Ear canal)II. Middle Ear– composed of Tym-panic membrane (Ear drum) + 3 ossicles: Maleus(Hammer), Incus (Anvil), Stapes(stirrup)+Oval win-dow and the Round window + Two tiny musles (the stapedius muscle and tensor tymphani muscle)being connected with the ossicles+ Eustachian tube(3.5 cm long tube connecting the middle ear with pharynx, thus balancing the air pressures within the middle earand in the atmosphere.(Inflammation can enter from nasopharynx to the middle ear in children, because ET is short)III. Inner Ear - Cochlea with Organ of Corti
Mechanism of hearing:The pinna captures the sound energy from the envi-ronment and directs it through theauditory canal tothe eardrum. Then, the sound passesthree small ossic-les, being located within the midle ear. Next, when pas-sing the last ossicle – stapes - thesoundcomes throu-ght the Oval window into the Cochlea of the inner ear.The Cochlea isdivided into three parts: SCALA VESTIBULY (is filled in with a perilymph, that consists of a high concentration of Na+ cations, and Cl -anions and higher amount of proteins) , SCALA MEDIA (filled with endolymph consisting a high concentration of K+ cations and lower concentration of Na+ and Cl- ions) and SCA-LA TYMPANI (filledagain with perilymph).The different concentrations of ions (mainly K+) play a key role in production of local electrical changes (GP and AP).
Within the Scala Media there is the Corti´s Organ, placed on the basilary membrane, consisting ofone row of the INNER HAIR CELLS (they serve as proper receptors forhearing)+ three rows of OUTER HAIR CELLS(serving as pre-amplifier) both equipped with stereocilia. Entering the Inner Ear,the sound pushes both the perilymph and endolymph, resulting in the liquid movements. Such liquid oscillations bend the stereocilia of the hair cells within the Corti´s Organ of the Scala Media.One direction of bending causes the depola-rization of the Hair cells, an opposite bending results in their hyperpolarization. GP rises up within the filaments of Inner Hair cells. AP is created in the axons leaving the Inner Hair Cells.
These APs are transmitted throughtheCochlernerve ultimately reaching the Temporal lobe of the Auditory brain cortex, where they are processed and perceived as a sound.
According to BEKESY´S HEARING THEORY:we are able to perceive the high frequencies of sound at the base (at a botom)of the cochlea. Contrary, the sound with a low frequencies stimulates the hair cells near the top of the cochlea, named the helicotrema. Thus, the high pitch sound (e.g. 10,000 Hz) is detected by the hair cells placed on the basilary membrane at the baseof the Cochlea, whereas the low pich sound(e.g. 200 Hz) is detected by the receptors near the helicotrema.