When sound signal strikes the tympanic membrane, the vibration is transmitted to the stapes footplate through a chain of ossicles. The movements of the stapes causes pressure changes in the labyrinthine fluids which move the basilar membrane. This stimulates the hair cells of the organ of corti which acts as a transducer.
MECHANISMS OF HEARING
Mechanisms of hearing can be broadly divided into:
1. Mechanical conduction of sound.
2. Transduction of mechanical energy to electrical impulses.
3. Conduction of electrical impulses to brain.
Mechanical Conduction of Sound (Acoustic Transformer)
A sound wave, on arriving at the boundary of its supporting medium, may be reflected or absorbed by the material of which the boundary is constructed. For example, if the medium is air and the boundary is water, 99.9 percent of the sound energy is reflected. The resistance to the passage of sound through a medium is its acoustic resistance or impedance. A similar situation exists in the ear when air conducted sound has to travel to cochlear fluids. So to compensate this loss of sound energy, nature has made middle ear to convert sound of greater amplitude, but lesser force, to that of lesser amplitude and greater force. This function of middle ear is called impedance matching.
The major contributors to the human acoustic transformer are the pinna, external auditory canal, and the middle ear sound conduction system.
The pinna, because of their location and shape, serve to gather sound arriving from an arc of 135° relative to the direction of the head. This pattern rejects sound arriving from the ear and serves to determine the origin of the sound. The hom shaped conchea then acts like a megaphone to concentrate the sound at the entrance of the auditory canal. This action increases sound pressure as much as 6 dB (2 times).
External Auditory Canal
Acting in concert with the effect of the pinna, can increase sound pressure at the tympanic membrane by 15 to 22 dB at 4000 Hz.
Middle Ear Transformer Mechanism
The transformer system of middle ear can be divided into three stages:
1. That provided by the eardrum (catenary lever.
2. That provided by the ossicles (ossicular lever).
3. Provided by the difference in surface area between the tympanic membrane and the stapes footplate (hydraulic lever).
1. Catenary lever
Helmholtz was first to propose a concept of a catenary lever to the action of the tympanic membrane. A familiar example of this type of lever is tennis net. The tighter a tennis net is stretched, the greater the force exerted on the posts holding it. Because the bony annulus is immobile, sound energy applied to the tympanic membrane is amplified at its central attachment, the malleus. It is estimated that even though the curvature of the tympanic membrane is variable, the catenary lever provides at least a two times (2x) gain in sound pressure at the malleus. Forces exerted on the stretched curved fibers of the tympanic membrane are amplified at its point of attachment, the annular bone and the malleus handle. The annular bone is immobile, so that the malleus is the recipient of this magnified energy, directing it into the ossicular chain for transmission to the perilymphatic fluid.
2. Ossicular lever
Handle of malleus is 1.3 times longer than long process of the incus, providing a mechanical advantage of 1.3. The catenary and ossicular levers, acting in concert provide an advantage of 2.3.
3. Hydraulic lever
Helmholtz’s third concept of impedance matching is referred as areal ratio. The effective vibratory area of tympanic membrane is 55 mm sq. whereas foot plate area is 3.2 mm sq. Hence effective areal ratio is 14: 1. This is a mechanical advantage provided by tympanic membrane. The product of areal ratio into lever ratio is known as transformer ratio. i.e., 14 x 1.3 = 18: 1.
In normal ear, sound pressure waves never reach the oval window and round window in the same phase, due to presence of tympanic membrane, middle ear and air cushions. If air waves reach round window and the oval window at the same time it cancel the effect of sound waves leading to stasis of perilymph. This reciprocal action at oval window and round window is called as phase difference. Therefore, loss of this phase difference (due to large perforation) may lead to deafness. However in normal case sound wave reaches oval window earlier than round window which is also an added advantage of hearing.
Transduction Function of the Cochlea
What is Transduction?
It is the conversion of mechanical energy of movement of sound to electrical energy, which is followed by electrical event in the cochlear nerve. Many theories have been put forward while exploring the mehanics and mode of encoding in the cochlea which have been modified with the present day knowledge.
When the stapes is pressed onto the oval window, pressure is exerted to the perilymph in scala vestibuli which is transferred to the scala media. This causes downward movement of the basilar membrane exerting pressure in the scala tympani. This is transmitted in turn to the round window which bulges into the middle ear. When the stapes and oval window move out, there is an upward movement of the basilar membrane. The elastic tension built up in the basilar fibers initiates a wave which travels towards the helicotrema. This wave is comparable to the movement of a pressure wave along the arterial walls.
Each wave is weak at the onset but becomes stronger as it reaches its natural resonant frequency.
High frequency waves travel a short distance and die. Low frequency waves travel a long distance and die. This is because the energy in the wave is completely dissipated. The nature of interaction between the membrane and the fluids is complicated and many theories were put forward based on experimental findings and hypothesis.
• Transduction by Hair Cells
Many theories were put forward regarding transduction by the hair cells. It is obvious now that auditory nerve endings are not only stimulated electrically but also by chemical transmitters.
Major steps involved in transduction are:
• The Basilar Membrane and the organ of Corti move up and down with sound stimulus. This causes a shearing action between the tectorial membrane and the reticular lamina causing the stereocilia to bend sideways.
• This bending of the hair bundles opens the channel to allow K+ to flow into the hair cell, resulting in depolarization.
• Depolarization spreads to the lower part of the cell causing Ca + channels to open.
• Ca+ causes transmitter vesicles to fuse with the basal part of the cell membrane. This fusion releases transmitter substance.
• The transmitter substance i.e. amino acid glutamate diffuses across the synaptic cleft to initiate action potential in the auditory nerve fiber.
Electrical responses of cochlear hair cells Using microelectrodes, four gross potentials have been extensively studied,
• Endocochlear potential: In relation to the perilymph, the endolymph in the scala media has a positive potential of +80mV and a high K+ concentration. This is known as endocochlear potential. It is dependent on adequate oxygen and is produced by stria vascularis. When the K+ is driven into the cell by this big potential gradient, large voltage is given triggering the natural impulses.
• Cochlear microphonics: Described by Wever and Bray are generated by the outer hair cells at the apical region. It produces the AC current wave form of stimulating sound and represents the K+ flow through the outer hair cells. Cochlear rnicrophonics is absent in any part of the cochlea where outer hair cells are damaged.
• Summating potential: It is a DC potential that follows the envelope of stimulating sound. Several origins have been cited; probably arising from inner hair cells with a small contribution from outer hair cells, this is a rectified derivative of sound signal.
• Auditory nerve action potential: It is neural discharge in the auditory nerve produced at the presence of stimulus. Each fiber has optimum stimulus frequency for which the threshold is lowest. Amplitude increases while latency decreases with intensity over 40 to 50 dB range.
Conduction of Electrical Impulses to the Brain
Physiology of the cerebral cortex
Area of auditory pathway to the cerebral cortex is illustrated as primary auditory cortex excited by medial body.
Secondary auditory cortex excited secondarily by impulses from primary auditory cortex, thalamic area adjacent to medial body.
Sound frequency perception in the primary auditory cortex
Six different tonotopic maps have been found in both the primary and secondary auditory cortex. Low frequency sounds are located anteriorly and high frequency sounds posterior. One of the large maps in the primary auditory cortex discriminates the sound frequency; another map detects direction of sound.