12. Organs of Special Senses – vi

Middle Ear

  • The middle ear lies between the outer ear and the inner ear. 
  • It consists of an air-filled cavity called the tympanic cavity and includes the three ossicles and their attaching ligaments; the auditory tube; and the round and oval windows. 
  • The ossicles are three small bones that function together to receive, amplify, and transmit the sound from the eardrum to the inner ear. 
  • The ossicles are the malleus (hammer), incus (anvil), and the stapes (stirrup). 
  • The stapes is the smallest named bone in the body. 
  • The middle ear also connects to the upper throat at the nasopharynx via the pharyngeal opening of the Eustachian tube.
  • The three ossicles transmit sound from the outer ear to the inner ear. 
  • The malleus receives vibrations from sound pressure on the eardrum, where it is connected at its longest part by a ligament. It transmits vibrations to the incus, which in turn transmits the vibrations to the small stapes bone. 
  • The wide base of the stapes rests on the oval window. 
  • As the stapes vibrates, vibrations are transmitted through the oval window, causing movement of fluid within the cochlea.

Inner Ear

  • The inner ear sits within the temporal bone in a complex cavity called the bony labyrinth. 
  • A central area known as the vestibule contains two small fluid-filled recesses, the utricle and saccule. 
  • These connect to the semicircular canals and the cochlea. 
  • There are three semicircular canals angled at right angles to each other which are responsible for dynamic balance. 
  • The cochlea is a spiral shell-shaped organ responsible for the sense of hearing.
  • The inner ear structurally begins at the oval window, which receives vibrations from the incus of the middle ear. 
  • Vibrations are transmitted into the inner ear into a fluid called endolymph, which fills the membranous labyrinth. 
  • The endolymph is situated in two vestibules, the utricle and saccule, and eventually transmits to the cochlea, a spiral-shaped structure. 
  • The cochlea consists of three fluid-filled spaces: the vestibular duct, the cochlear duct, and the tympanic duct.
  • Hair cells responsible for transduction—changing mechanical changes into electrical stimuli are present in the organ of Corti in the cochlea.

Physiology of Ear

  • The pinna is like a funnel, which directs sound to the ear canal and unto the eardrum. 
  • The ear canal has wax in it that helps trap infections and particles from getting deeper into the more sensitive parts of the organ.
  • Sound waves traveling through the ear canal reach the eardrum and bounces off, setting the eardrum to vibrate too. 
  • This sets off more vibrations of the ossicles (the three small bones called Hammer, Anvil and Stirrup) inside of the middle ear. 
  • The ossicles are connected to the cochlea. 
  • The cochlea is a long chamber, shaped like a garden snail, filled with a liquid.
  • The chamber has a hair-like lining. 
  • As the vibrations travel through the liquid in the chamber, the tiny hairs are stimulated, picking up the vibration signals from the liquid and send them to a special part of the brain for interpretation. 
  • The entire process happens within millions of a second.

This sets off more vibrations of the ossicles (the three small bones called Hammer, Anvil and Stirrup)

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