Anatomy and Physiology: Special Senses – The EAR
What Do We Hear?
All sounds (music, voice, a mouse-click, etc.) send out vibrations, or sound waves. Sound waves do not travel in a vacuum, but rather require a medium for sound transmission, e.g. air or fluid. What actually travels are alternating successions of increased pressure in the medium, followed by decreased pressure.
Hearing starts with the outer ear. When a sound is made outside the outer ear, the sound waves, or vibrations, travel down the external auditory canal and strike the eardrum (tympanic membrane). The eardrum vibrates. The vibrations are then passed to three tiny bones in the middle ear called the ossicles. The ossicles amplify the sound and send the sound waves to the inner ear and into the fluid-filled hearing organ (cochlea).
Once the sound waves reach the inner ear, they are converted into electrical impulses which the auditory nerve sends to the brain. The brain then translates these electrical impulses as sound.
The mechanism of hearing is summarized below:
- Pinna collects the sound heard
- External auditory canal
- Vibration in the ear drum is produced
- Amplified by the ossicular chain. The sound is then transmitted to the inner ear.
- The cochlea converts the sound vibration to electrical impulses. Within the cochlear duct, the endolymph-containing membranous labyrinth of the snail-like cochlea is the organ of Corti, which contains the hearing receptors or hair cells. The chambers above and below the cochlear duct contain perilymph. Sound waves that reach the cochlea through vibrations of the eardrum, ossiscles and oval window set cochlear fluids into motion.
- The hair cells transmit impulses along the cochlear nerve or auditory nerve (a division of cranial nerve VIII – the vestibulocochlear nerve) to the auditory cortex in the temporal lobe.
- Auditory cortex interprets the sound, or hearing occurs.
Since sound usually reaches the two ears at different times, a person can hear “in stereo,” which functionally helps humans differentiate where sounds are coming from the environment. In cases where the sounds or tones keep reaching the ears, the auditory receptors tend to adapt or stop responding to these sounds, thus the person becomes no longer aware of them. Important information about hearing is the fact that it is the last sense to leave the awareness when a person falls asleep or receives anesthesia or dies. As a person awakens from sleep, it is the first sense to return.
image courtesy of umm.edu