1. Hearing Physiology

2. Auditory Physiology
Sense organ that responds to sound vibrations over a frequency range of 16-20,000 Hz
Middle ear- Mechanical
Inner Ear- Hydraulic
How do these pieces send coded messages to the brain?
Encoding frequencies & intensities
Brain assembles elements of sound (pitch, loudness and quality)

3. Hydraulic Process
Both frequency & intensity characteristics arrive at the oval window of inner ear as mechanical vibrations
Within the cochlea, the hydraulic waves that result correspond to these vibrations
Frequency is reflected in the # of waves of compression generated per second, intensity is reflected in their amplitudes.

4. Hydraulic Process How does the cochlea respond to these waves?
Basilar membrane shape
Short and stiff at the basal end near oval window
Wide and lax at the other end near helicotrema
“Tuned” membrane responds selectively to different frequencies
High frequencies at narrow end
Low frequencies at the wide end

5. Basilar Membrane “Tuning”
Vestibule
More Stiff
High Frequencies
Narrow Basal
End
Helicotrema
Wide Apex
Low Frequencies
Less Stiff

6. Traveling Waves
Vibration transmitted along the basilar membrane --ex. Shake a bed sheet
Fluid in cochlea moves with movement of stapes & round window
Tuning of wave also dictated by stapes
Wave crest= Frequency of that place on membrane

7. Frequency Analysis
Sound generating traveling wave- Tuning fork (vibrates single frequency)
Air-conducted energy delivered to stapes
Rocking in and out of perilymph in vestibule
greater sound, greater movement
Rocking creates compression wave; moves toward exit (round window)
Round window displaced outward
Rarefraction (bounce back) pushes footplate backwards and doing this sucks in the round window

8. Generation of Hydraulic Wave
Compression Wave
Vestibular Canal
Tympanic Canal
Rarefaction Wave
Basilar membrane

9. Frequency Low frequency (50 Hz) Mid Frequency (1,000 Hz)
Wave will travel to far end of basilar membrane before peaking (near apex)
Mid Frequency (1,000 Hz)
Wave will grow to maximum amplitude about half-way along basilar membrane (higher frequency=shorter distance traveled)
High Frequency (up to 20,000 Hz)
Crests near basal end of membrane
Higher frequency, the more resistance the perilymph offers to being moved by stapes

10. Traveling Wave Peaks at Different Frequencies
Low
Basilar Membrane
Mid
High

11. Neural Processes
How does the mechanical motion of the basilar membrane encode into neural auditory signals?
Organ of Corti mounted on the basilar membrane
Bending the cilia of hair cells
Key to bending action is the manner of attachment to basilar and tectorial membranes

12. Shearing Force Bending of Hair Cell Cilia
Tectorial Membrane
Pivot Point
Shearing force
Fluid Pressure
Shearing force
Basilar Membrane
Pivot Point
Fluid Pressure

13. Cilia Bending
When tectorial membrane is displaced downward, basilar membrane will move downward; these two membranes will also move upward together
Lateral movement of cilia = up & down movement of basilar membrane
Radial movement= shearing force of cilia
Result in complex bending of cilia

14. Directions of Cilia bending Cilia Hair Cell Traveling Wave Basilar
Membrane

15. Traveling Wave Another aspect of the complex motion: Wave Envelope
Summarizes amplitudes of vibration
Peak at about the same frequency
1,000 Hz

16. Generating the Auditory Signal
Base of hair cell in contact with auditory nerve end
Outer hair cell primarily responsive to lateral shear
Inner hair cells, do not drag against tectorial membrane, have different function, activated by basilar membrane movement rather than shearing
Base of hair cell makes a synaptic contact with auditory nerve ends when cilia move

17. Auditory Pathways to Brain
30,000 nerve fibers from organ of Corti join to form auditory nerve
Organized like two parallel railway systems between the same city, each having its own passenger terminals:
Neural traffic travels from one line to another at several terminals

18. Auditory Pathways to Brain
Auditory nerve feeds into cochlear nucleus (first terminal in auditory pathway)
From cochlear nucleus transfer to ascending pathways then to auditory cortex, one in each temporal lobe.

19. Auditory Pathways to Brain
Between cochlear nucleus and auditory cortex:
3 sets of terminals
Superior olive- lowest & smallest (auditory information can be matched with infor from other ear)
Lateral lemniscus- next highest level (Info from both ears provides a basis for a quick reflexive response)
Auditory projection fibers- last terminal in brainstem (transfer of auditory neural impulses from one side of brain to the other at three levels:
Cochlear nucleus
Superior olive
Inferior colliculus

20. Auditory Pathways to Brain
Input from both ears are well represented on both sides of the brain
permits:
Comparison of information about frequency, intensity and time of arrival of the acoustic signal to both ears
“Main line” contralateral auditory pathway does make it slightly easier to understand speech better with right ear (main line to temporal lobe)

21. Auditory Pathway Auditory Cortex Medial Geniculate Inferior Colliculus
Superior Olive
Cochlear
Nucleus
Lateral
Lemniscus
Cochlear
Nerve

22. Ascending-Descending Auditory Pathways
Afferent Pathways
Efferent Pathways
Middle Ear
Muscles

23. Descending Pathways Sensory nerve- Auditory nerve
98% of fibers carry afferent information from the cochlea to brain
500 nerve fibers carry efferent neural impulses from brain to ear
This information controls the operation of ear
Some goes to middle ear muscles (protection)
Most goes to or near the hair cells of the cochlea

24. Reading/Assignments
Seikel: Pgs