How do the physical properties of the ears help the brain decode and interpret sounds?

by kundrotaite (student)

Sound travels as osccialtion through some medium. The function of ears is to transmit the sound into electric messages which are sent to the brain which perceives and interprets the sound. Ears consist of three parts the outer ear, middle and inner ears.

The outer ear consists of the pinna which collects the sound and transmits it to the ear canal, which amplifies the sound and transmits it to the tympanic membrane, the division between the outer ear and the middle ear. The middle ear consist of tympanic membrane, osiccles (three bones connected in sequence: hammer, anvil and stirrup) and oval window. When the sound vibration from the outer ear reaches the tympanic membrane it vibrates at the same rate as the vibration from the outer ear. The vibrating tympanic membrane causes ossicles to vibrate, which transmits the vibration to the oval window. The sound is amplified when transmitted as the tympanic membrane is much larger then the oval window, and the bigger force is needed to transmit the vibration from the air module to the fluid filled cochlea of the inner ear. This amplification is also aided by lever-like action of ossicles. This helps to permit the loss of sound energy. Two muscles connected to the ossicles (stapedius and tensor tympani) may damp down this amplification when tensed to protect the inner ear from the outgoing noise and self-generated sound.

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After sound passes through the oval window it reaches the inner ear, which is the innermost part of the ear. It consists of the fluid-filled cochlea, basilar membrane, organ of corti, tectorial membrane and hair cells. The cochlea consists of vestibular, middle and tympanic canals. After the sound reaches the cochlea, oscillation causes the fluid vibration, which in turn vibrates the basilar membrane, which separates the vestibular and middle canals and contains and supports the organ of corti, which contains outer and inner hair cells overlaid by the tectotial membrane. These hair cells send message to the cochlear nerve, the auditory pathway to the brain. The hair cells are tipped with cilia of different length, which bend in response to shearing forces created when the vibration of the sound is transmitted through the oval window along the length of cochlear bend the basilar membrane. The hair cell are surrounded by the liquid endorphin rich in potassium. Cilia are connected by tip links, when cilia is moving the longer cilia pulls the tip links the gates are open and the potassium enters the cell, increasing the likelihood of the action potential, which sends messages through cochlea nerve to the brain which the interprets the sound.

Basilar membrane varies in its thickness and wideness, making it sensitive to different frequencies of the sound. Higher frequencies sound bends it near the base and lower frequencies sounds make it bend at the end of apex. This is called the place code model for frequency discrimination. However the hair cells tend to generate action potential at one phase of their cyclic response to a sound wave, so the time can be used to estimate the frequency of the sound wave- this is known as temporal code used to determine the sounds of law frequency. Furthermore, the brain uses three cues to detect sound localisation: interaural time and intensity difference and filtering pinna properties which help to detect the location of the sound.