The Auditory Syatem and the Hearing Mechanism

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- Inner Ear

The inner ear comprises of the bony and complex osseous labyrinth that consists of the cochlea (i.e. the hearing organ) and semicircular canals (i.e. superior, lateral, and posterior) and vestibule (i.e. the balance organ). The cochlea, a tube coiled two and a half times around a hollow pillar (30mm stretched) to resemble a snail shell is divided into three fluid ducts. The hollow pillar of the coil contains fibres of the cochlear nerve which is attached to the brain stem wherein the nerve impulses reach the brain.

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As shown in Figure 2, the scala media, sandwiched between the tympanic and vestibular ducts, contains the fluid, endolymph and the organ of corti which is anchored on the basilar membrane and lines the scala media, forming a sensory epithelium. There are 15 thousand hair cells in the organ of corti; one flask shaped inner hair cell and three rows of outer hair cells (imbedded into the tectorial membrane) with bunch of 100 to 200 stereocilia (i.e. hair) protruding from each cell. The hair cells are called mechanoelectrical transducers as they convert the middle ears mechanical energy to electrochemical nerve impulses. They are much more outer hair cells than inner hair cells, and since the inner hair cells have the uttermost nerve-fibre connection to the brain, they play a direct role in transduction. The outer hair cells amplify the resonance peaks of the inner hair cells to enable the transduction. These hairs are stimulated according to the tonotopic organization, following the displacement, and spatiotemporal vibration pattern of the basilar membrane wherein frequency is mapped to the spot of stimulation.

The nerve fibres in the cochlear nerve, (i.e. axons of afferent neurones) release spikes of actions potential in varying firing rate, depending on the frequency that requires the least amount of intensity to incite increase in firing rate of a nerve fibre (i.e. characteristic frequency). High characteristic frequency comes from the inner hair cell whereas low characteristic frequency, the outer hair cell. The cochlear nerve is also tonotopically organized, thereby maintaining the spatiotemporal pattern incited in the basilar membrane.

- Central Auditory System

The cerebral cortex is the processing and interpretation station of neural energy. According to Wikipedia, the cochlear nuclei are two heterogeneous collections of neurons found in the dorso-lateral sides of the brainstem. It preserves temporal information from the cochlear and projects into the superior olivary nucleus via the trapezoid body. The trapezoid body is an internal transverse pathway for decussating axons, and also aids in acoustic localization. The superior olivary nucleus encompasses the lateral superior olive (LSO) and the medial superior olive (MSO) which respectively processes interaural level differences (ILD) and interaural time difference (ITD). Lateral lemniscus is a tract of axons which relays signals to various parts of the brainstem nuclei. Inferior colliculi is where information is integrated and sent to the thalamus and cortex. The primary auditory cortex is situated in the left posterior superior temporal gyrus and processes the sensations of sounds (i.e. pitch, rhythm, etc.). Similar to the basilar membrane, lower frequencies occur at the front of the temporal lobe and higher frequencies at the back. The frontal lobe and parietal lobe attend to complex processing to reach sensation and perception of sound.

- Hearing Process

Hearing begins with the motion of an event that causes vibrations in the air. The auricle funnels the pulse of vibration and sends it into the canal. While it allows some high-frequency sounds inside, while frequency components of some others bounce off the contours of the auricle, consequently altering the sound spectrum before entering the canal, resulting in cues. The brain recognizes the characteristic patterns that distinguish whether the sound is originating from in front of you, behind you, your left (i.e. louder and arrives sooner) or right, above you or below you as well as the distance or velocity of moving object.

The sound waves travel through the ear canal in the form of compression pressure waves and impinge upon the tympanic membrane which then vibrates at a speed and distance dependent upon the frequency and amplitude of the sound waves. For instance, the eardrum will vibrate more rapidly at a high-pitched (i.e. short frequency) sound and at a greater distance at a louder (i.e. higher amplitude) sound. (Harris, n.d.). When the eardrum vibrates, the longer malleus connected to the eardrum moves a greater length, thus pushing against the smaller incus, which consequently moves with greater force and transfers the vibrations to the stapes. The vibrations travel an approximate 55 in area from the eardrum through the ossicular chain and exit the smaller and higher pressure stapes footplate before striking the oval window. The pressure and lever effect amplify the vibration 22 times at roughly 25 decibels. The stapes transfers the sound by inserting its footplate into the oval window. [pic 7]

If a signal of loud sound is detected, the brain triggers the acoustic reflex and the tensor tympani muscle and the stapedius muscle suddenly tense, respectively tugging the stapes away from the oval window and malleus back, thereby stiffening the eardrum and the ossicular chain. This reflex is common during vocalization as to dampen your own voice or to facilitate auditory attention in a noisy place. Therefore, we are able to communicate without our own voice drowning the voice of the other person. We are also able to concentrate on the sound we are attending to, like a particular song in the stadium.

As the stapes moves back and forth the oval window, it inadvertently causes the basilar membrane extending below the cochlea to be pushed and pulled. As a reaction, a wave forms along the basilar membrane, rippling from the base to the apex. The wave releases its energy at the fibre it resonates with, creating a burst of energy. Since the fibres increase in length along the membrane, higher-frequency waves vibrate at the base whereas lower-frequency waves vibrate at the apex. The energy stimulates the cilia on the hair cells at that exact resonance point of the energy burst. More cilia will be stimulated if a large amplitude energy is released. The outer hair cells augment the resonance peaks of the inner hair enabling the inner hair to be stimulated by weaker sound waves. The cilia move back and forth, bending against the tectorial membrane, thus releasing the hair cells’ electrical impulses, thereby converting mechanical

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