Hearing Physiology

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)

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.

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

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

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

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

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

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

Traveling Wave Peaks at Different Frequencies Low Basilar Membrane Mid High

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

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

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

Directions of Cilia bending Cilia Hair Cell Traveling Wave Basilar Membrane

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

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

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

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.

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

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)

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

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

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

Reading/Assignments Seikel: Pgs.565-588