The Ear: Physiology

1. The Ear: Physiology Balance and Hearing

2. Mechanoreceptors • Can respond to deformation (bending), resulting in a change in ion flow • Get a hyper/depolarization depending on the direction • Can differentiate between direction of bending • Often grouped • Often attached to a gelatinous mass, which is influenced by the environment’s movement

3. Inner Ear Anatomy

4. Semicircular canals • Surrounded by both a membranous labyrinth and a bony labyrinth • Can’t expand/change shape • Held still even when body is in motion • Movement of fluid causes a traveling disturbance whose force isn’t lost against an expanding wall • Tubular structure that contains both perilymphand endolymph • Each canal ends with an ampulla

5. Cristae ampullaris • Contain “tufts” of hair cells, called cristae • Affected by movement • Are in planes perpendicular to one another (able to interpret any possible movement) cupola

6. Vestibular Apparatus • Enlargements extend from the vestibular apparatus • Utriculusand sacculus • Gelatinous mass with CaCO3 “ear stones” = cupola • This extra mass helps increase density • A more efficient position receptor • Allows proprioceptors a reference point to which it can compare the rest of the body

7. Endolymph is continuous throughout the vestibular apparatus and semicircular canals • During rotation of canals • Inertia moves the walls relative to the fluid • Fluid gains inertia of its own • When the wall stops, fluid moves relative to the wall • Endoloymph is also continuous throughout the cochlea

8. The Cochlea

9. Cochlea – A Hearing Structure • 1 central canal, filled with endolymph • 2 adjacent canals, filled with perilymph • This fluid allows vibration of the walls of the central canal

10. Vibration Transmission • Tympanic membrane vibrates along with sound waves •  Translated into the motion of the bones of the inner ear •  Stapes attached to the oval window •  The oval window vibrates at the same frequency

12. Vibration of oval window causes disturbance in the fluid behind it • High surface area leads to an amplification of the sound • The pressure on the perilymph in the vestibular canal is great, causing pressure waves • Round window acts as a pressure release

15. Sound detection • Vibrations produced in the perilymph are translated into traveling waves along the basilar membrane • Frequency of the vibration determines how far it goes • High = proximal membrane • Low = distal end • A maximal response happens along the portion of the membrane that vibrates the most

17. Hearing in detail… • 2 groups of hair cells along basilar membrane • Single, inner row (closest to bony ridge) • Vibrate with basilar membrane • Communicate with auditory cortex of brain via a single nerve fiber in auditory nerve

18. Why does this matter? • Your brain can “tell” what type of sound was perceived • A fairly strong stimulation needed to stimulate the hair cells so close to the bony ridge • If this nerve fiber is stimulated, the sound must be loud

19. Outer, Triple Row • Sensitive to the same frequency as the inner, single row • Easier to stimulate, though • Brain can’t distinguish the specific frequency that stimulates these cells, though • Harder to identify quieter sounds • Turning up volume helps!