Hearing

1. Hearing Maddie, Emma, Kelly, Meg

2. Inner Ear 4 layers Outer layer Bony Labyrinth – made up of dense bone 2ndlayer Perilymph – liquid in between the bony and membranous labyrinths 3rdlayer  Membranous labyrinth – delicate, interconnected network of fluid filled tubes (receptors of the inner ear are found within these tubes) Inner layer Endolymph – a fluid with electrolyte concentrations

3. KEY Semicircular canal Membranous labyrinth Anterior Bony labyrinth Semicircular ducts Lateral Cristae within ampullae Posterior Vestibule Maculae Endolymphatic sac Cochlea Perilymph Utricle (a) Bony labyrinth Saccule Endolymph Vestibular duct Membranous labyrinth Cochlear duct Tympanic duct Organ of Corti (b)

4. Inner Ear • Bony labyrinth – subdivided into… • Vestibule – consists of the saccule and the utricle (membranous sacs) • 3 semicircular canals – enclose semicircular ducts • Combination of vestibule and semicircular canals is called the vestibular complex • Cochlea – spiral shaped bony chamber that contains the cochlear duct

5. Inner Ear • Bony labyrinth consists of dense bone everywhere except the round windowand oval window

6. Equilibrium • Equilibrium sensations provided by receptors of the vestibular complex

7. Equilibrium • Semicircular Ducts (Anterior, Posterior, Lateral semicircular) • Sensory receptors in the semicircular ducts respond to rotation movements of the head • Each semicircular duct contains an ampulla (expanded region that contains the receptors) • Crista – region in the wall of the ampulla that contains the receptors • Bound to cupula • Each hair cell in the vestibule contains a kinocilium (single large cilium)

8. Equilibrium • Hair cells (receptors) are active during a movement, quiet when the body is motionless • Free surface of each hair cell supports 80-100 long stereocilia (resemble microvilli) • Hair cells provide information about the direction and strength of mechanical stimuli • Stimuli involved varies by hair cell’s location • Gravity or acceleration in the vestibule • Rotation in the semicircular canals • Sound in the cochlea

9. Equilibrium • Movement of receptors controlled by three rotational planes • Horizontal rotation (ex. Shaking your head no) stimulates the hair cells of the lateral semicircular duct • Vertical movement (ex. Nodding “yes”) excites the anterior duct • Tilting your head from side to side activates receptors in the posterior duct

10. The Utricle and Saccule • Function – provide equilibrium sensations • Utricle and Saccule are connected by a slender passageway that is continuous with the narrow endolymphatic duct, which ends in the endolymphatic sac

11. The Utricle and Saccule • Hair cells of utricle and saccule are clustered in oval structures called maculae • Hair cell processes are embedded in a gelatinous mass (contains densely packed calcium carbonate crystals known as statoconia) • Otolith Whole complex (gelatinous matrix + statoconia)

12. STEP Head in the anatomical position 1 Gravity (a) STEP Otolith Head tilted posteriorly 2 Gelatinous material Gravity Statoconia “Otolith moves downhill,” distorting hair cell processes Nerve fibers Receptor output increases (b) Structure of a macula (c)

13. Macula of Saccule • When your head is in the normal, upright position, the statoconia sit atop the macula (their weight pushes the hair cell processes down rather than one side or another) • When your head is tilted, the pull of gravity on the statoconia shifts them to the side, distorting the hair cell processes (alerts the central nervous system that the head is no longer level)

14. Macula of Saccule • Under normal circumstances, body can distinguish between sensations of tilting and linear acceleration through visual information (amusement park rides confuse your sense of equilibrium because of the change in position and acceleration with restricted/misleading visual information)

15. Pathways for Equilibrium Sensations • Sensory fibers contained within the vestibular nuclei  4 functions of the 2 vestibular nuclei • Integrating sensory information about balance and equilibrium that arrives from both sides of the head • Send information to cerebral cortex and cerebellum of brain

16. Pathways for Equilibrium Sensations • Reflexive motor commands issued by vestibular nuclei are distributed to motor nuclei for cranial nerves involved with eye, head, and neck movements • Automatic movements of eye that occur in response to sensations of motion • directed by the superior colliculiof the mesencephalon (in an attempt to keep your gaze focused on a specific point, despite changes in body position and orientation) • Nystagmus condition in which people have trouble controlling their eye movements

17. To ipsilateral superior colliculus and relay to cerebral cortex Red nucleus III Vestibular ganglion IV Semicircular canals Vestibular branch Vestibular nucleus VI To cerebellum Vestibule XI Cochlear branch Vestibulocochlear nerve (VIII) Vestibulospinal tracts

18. Hearing • Receptors responsible for hearing are hair cells in the cochlear duct • Auditory ossiclesconvert pressure fluctuation in the air into fluctuation in the perilymph of the cochlea (outside pressure to inside pressure)

19. Hearing • Frequency of sound determined from which part of cochlear duct is stimulated • Volume is determined from how many hair cells are stimulated

20. The Cochlear Duct • Cochlear duct is between perilymph ducts: vestibular duct and tympanic duct • Outer surfaces encased by bony labyrinth everywhere except bases of ducts • Ducts are connected and actually form one long duct

21. The Cochlear Duct • Hairs are located in the organ of Corti in longitudinal rows • When the basilar membrane (which the hairs are located on) bounces, the hair cells are distorted by pressing against the upper membrane (tectorial membrane)

22. An Introduction to Sound • Hearing is perception of sound • Sine waves: S-shaped curves created by high and low pressure, travel in cycles • Travel at about 768 mph: speed of sound

23. An Introduction to Sound • Wavelength inversely related to frequency (number of waves that pass through reference point for certain amount of time) • Pitch=sensory response to frequency • Amplitude=intensity of sound, energy content • Cycles per second=hertz, Hz • Sound energy reported in decibels

24. An Introduction to Sound • With the right combination of frequency and amplitude, object will vibrate at same frequency as sound: called resonance • To hear sound, tympanic membrane must vibrate in resonance with sound waves

26. The Hearing Process Sound waves arrive at the tympanic membrane… • Enter external acoustic canal and travel to tympanic membrane • Movement of the tympanic membranecauses displacement of the auditory ossicles • a) Tympanic membrane is the surface for sound collection • b) Resonate with frequencies ~20-20,000 Hz • c) When tympanic membrane vibrates, inner ossicles also vibrate= amplifythe sound

27. The Hearing Process • 3. Movement of the stapes at the oval window establishes pressure waves in the perilymph of the vestibular duct • a) Because liquid is incompressible, pressure can only be relieved at the round window • b) Stapes vibrate and creates pressure waves in the perilymph

28. The Hearing Process • 4. The pressure waves distort the basilar membrane on their way to the round window of the tympanic duct • a) Pressure waves travel around perilymph and reach round window • b) As they do this, they disrupt the basilar membrane • c) High frequencies vibrate the basilar membrane near oval window • d) Lower the frequency, longer wavelength and further from oval window is the maximum distortion • e) Frequency translated to position along basilar membrane • f) Amount of movement depends on force of sound

29. The Hearing Process • 5. Vibration of the basilar membrane causes vibration of hair cells against the tectorial membrane • a) Vibration of basilar membrane moves hair cells against tectorial membrane • b) Ion channels open, depolarizes hair cells • c) Leads to release of neurotransmitters/ stimulates sensory organs • d) Hairs are stimulated in rows • e) Number of cells responding indicates intensity of sound

30. The Hearing Process • Region and intensity of stimulated area is relayed to the CNS over the Cochlear branch of the vestibulocochlear nerve • a) Cell bodies of sensory neurons located in spiral ganglion • b) Vestibulocochlear nerve is responsible for transmitting sound and equilibrium to the brain for further distribution

31. Auditory pathways • Vestibulocochlear nerve formed by neurons • Info then goes to opposite side of brain to processing center which coordinates reflexes such as turning your head from a loud noise • Auditory cortex in temporal lobe maps out the organ of Corti

32. Auditory pathways • Frequency to position of basilar membrane is projected onto auditory cortex • Creates sensation of pitch • Damaged auditory cortex-responds to sound, but cannot interpret sounds or find patterns

33. Auditory sensitivity • Difficult to assess the absolute sensitivity of the system • We could, in theory hear air molecules, but full potential is never reached because of our own body and other peripheral sounds • We adapt to environment which affects hearing i.e. Relaxing in a quiet room

34. Auditory sensitivity • Young children have the greatest hearing range • Declineswith age due to damage or other accumulated injuries • Tympanic membrane is less flexible, articulations between ossicles stiffen and round window may begin to ossify • Result: older individuals exhibit hearing loss

35. Occupations associated with hearing • Audiologist- treat hearing and balance problems • Otoplasty surgeon- a plastic surgeon that corrects deformities in ears

36. Diseases/ Conditions associated with hearing • Deafness- loss of hearing • Tinnitus- perception of abnormal ear or head noises, ringing in your ears • Cholesteatoma-type of cyst found in the middle ear behind the eardrum, noncancerous tumor

37. http://www.youtube.com/watch?v=Jk-4YiiPwBc&safe=active