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Sensory Organs Help (page 3)

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By — McGraw-Hill Professional
Updated on Aug 18, 2011

Structure and Function of the Ear

The ear is the organ of hearing and equilibrium. It consists of three principal regions: the outer ear, the middle ear, and the inner ear (Figure 12-3).

Sensory Organs

The outer ear is open to the external environment and directs sound waves to the middle ear. Structures of the outer ear include the auricle (pinna), the external auditory canal, and the tympanic membrane ("eardrum"). The auricle directs the sound waves to the external auditory canal, a 2.5 cm (1 in.) fleshy tube that fits into the bony external acoustic meatus. The thin tympanic membrane conducts sound waves to the middle ear

The middle-ear cavity or tympanic cavity is the air-filled space medial to the tympanic membrane. The structures of the middle ear are the auditory ossicles, the auditory muscles, and the auditory (eustachian) tube. The auditory ossicles are three small bones that extend from the tympanic membrane to the vestibular (oval) window: the malleus ("hammer"); the incus ("anvil"); and the stapes ("stirrup"). These small bones amplify the sound waves. The auditory muscles are two tiny skeletal muscles that function reflexively to reduce the pressure of loud sounds before it can injure the inner ear. The auditory (eustachian) tube connects the middle-ear cavity to the pharynx. It functions to drain moisture from the middle ear cavity and to equalize air pressure on both sides of the tympanic membrane.

The inner ear contains the organs of hearing (cochlea) and equilibrium and balance (vestibular apparatus). The structures of the inner ear are described below. The bony labyrinth is a network of cavities that consist of three bony semicircular canals, the ampulla at the base of each semicircular canal, a central vestibule, and the cochlea. The membranous labyrinth is an intercommunicating system of membranous ducts seated in the bony labyrinth. Its parts are co-named with those of the bony labyrinth. The membranous semicircular canals and their ampullae posses receptors sensitive to rotary motions of the head. The vestibule consists of a connecting utricle and saccule, which possess receptors sensitive to gravity and linear motions of the head. These structures make up the vestibular apparatus. The membranous labyrinth is filled with a fluid, endolymph, and to the outside of the membranous labyrinth is a fluid called perilymph. The vestibular (oval) window, a membrane-covered opening from the middle ear into the inner ear, is located at the footplate of the stapes where it transfers sound waves from the solid medium of the auditory ossicles to the fluid medium of the cochlea. Within the cochlea is the membranous cochlear duct and the spiral organ (organ of Corti), the organs of hearing. The cochlear window (round window) is directly below the vestibular window that reverberates in response to loud sounds.

Hearing

  1. Sound waves are funneled by the auricle into the external auditory meatus.
  2. The sound waves strike the tympanic membrane, causing it to vibrate.
  3. Vibrations of the tympanic membrane are amplified as they pass through the malleus, incus, and stapes.
  4. The vestibular window is pushed back and forth by the stapes setting up pressure waves in the perilymph of the cochlea.
  5. The pressure waves are propagated to the endolymph contained within the cochlear duct.
  6. Stimulation of the hair cells within the spiral organ of the cochlea causes the generation of nerve impulses in the cochlear nerve (CN VIII), which pass to the pons of the brain.

Practice problems for these concepts can be found at:

Sensory Organs Practice Problems

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