Auditory System

The world contains all kinds of energy that translates into information about what we see, hear, smell, touch and taste. A sensory system is a part of the nervous system responsible for processing specific sensory information. The components of a sensory system include sensory receptors, neural pathways, and parts of the brain involved in sensory perception. To begin, energy from the environment stimulates the receptor cells in whichever sense organ is being used. If this information were auditory, the ear would convert sound waves in the air into electrical impulses that would further be interpreted by the brain as sound.

A sound wave first enters the pinna, the fleshy part of the ear on the outside of the body. It then travels through the external auditory canal where it then meets the eardrum, a thin membrane in the outer ear. The eardrum then vibrates in response to the sound wave. What we hear will depend on the wavelength and frequency of the wave. The eardrum is connected to a group of three small bones call “the ossicles” in the middle ear. This group includes the malleus, incus and the stapes. These three bones, the smallest in the human body, protect the eardrum from more intense sounds and also deliver the vibrations to the base of the stapes.

The stapes then sends the vibrations into the inner ear and interacts with the round window. The round window, a small membrane that allows liquid inside the inner ear to be displaced and receive the vibration. The vibration travels through the spiral structure of the inner ear called the cochlea and ends at the round window. Inside the cochlea there are three canals: the scala vestibuli, the scala media and the scala tympani. The scala vestibule leads up to the apex of the cochlea, the scala tympani leads down to the round window and the scala media sits in between the other two canals.

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As the basilar membrane vibrates, the hair cells are bents and push up against the tectorial membrane. This causes the hair cells to fire and send nerve impulses to the auditory cortex on each of the brains hemispheres through the cochlear nerve. How we determine pitch can be explained with two different theories. The Place Theory states that the entire basilar membrane does not vibrate at once so different parts of the basilar membrane respond to different frequencies of sound. Lower frequency sounds vibrate the basilar membrane near the apex of the cochlea while higher frequency sounds produce vibrations closer to the base.

The Frequency Theory states that the frequency of firing matches the frequency of the sound wave. Hearing loss can occur for a number of reasons. Damage to the eardrum due to age and prolonged exposure to loud noise may cause the hairs or nerve cells in the cochlea to wear out and become less effective. A buildup of earwax can block the ear canal and prevent of sound waves from entering the eardrum. Otosclerosis, a genetic form of hearing loss in which the stapes is fixed in place so sound cannot enter the inner ear.

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