auditory nerve

• part of the CN 8
• contains about 30,000 fibers
• fibers begin at hair cells--> run through the habenula perforata; openings in the osseous spiral lamina
• go to rosenthals canal--> contains spiral ganglion cells
• From the modiolus; trunk of nerve fibers
• enter the internal auditory meatus
• Type 1 fibers--> larger more mylenated, more numerous, connect to inner hair cells
• Type II fibers--> cnnect to outer hair cells

• phase-locking
• fibers can respond only up to 1000 HZ
• Volley principle- used in response to frequencies higher than 1000 Hz
• each fibers responds less often
• others fibers recruited to help respond

higher intensities

• more auditory nerve fibers
• greater firing rates

Auditory brainstem response; has 5 primary waves

Waves 1 and 2- AN

Wave 3- CN

Wave 4-SOC

Wave 5- LL

• midbrain
• tonotopic
• phase-locking
  • • • • amplitude-modulated signals
    • gap detection

thalamus

has 3 main parts- ventral, medial, and doresal

major inputs form ipsi IC

Output to cortex--> venttral through internal capsule to A1

medial: through External capsule to insula

Dorsal: through internal capsule to secondary AC and insula

• • recieves input from the MGB
• thalamo cortical pathway
• thru the internal capsule)
  • gray matter- nucliei of nerve fibers; sulci(grooves or folds in cortex); gyri (bulges in neural tissue); sylvian fissure (hescl's gyrus; primary auditory area) planum temporale
  • tonotopic org
  • coding-intensity-more fibers, higher rates; excitatory and inhibitory changes; temporal coding-only up to 100hz
  • speech- greater left than rite hemisphere responses to speech
  • Late components of MLR; N1-P2 complex; mismatch negativity

• forms a feedback loop with the afferent system; rostral system-auditory cortex, MGB and iC; caudal system- Olivocochlear bundle and cochlea;
• impt for inhibitory activation; hearing in Noise; prtection from noise

small region above the malleolar folds

sparse fibers 

1. attaches between anterior wall and neck of the malleus
2. increases tension on the TM by pulling the manubrium inward
3. pulls mallueus in anterior and posterior directions
4. Innervated by the 5th CN

• head of the stapes
• pulls stapes rearward to fix footplate in the oval window
• innnervated by the 7th CN
  

Opposition to the flow of energy

Mass reacts directly with frequency

Stiffness refers inversely with frequency 

• 226 Hz probe tone at 80 dB SPL

Middle Ear Transformer

AREA 

DECRESES FROM tm TO OVAL WINDOW

PRODUCES A 17 FOLD INCREASE IN PRESSURE 

Middle Ear transformer

VELOCITY 

(buckling effect)

malleus manubrium moves less than the TM

Produces a two-fold increase in pressure 

Middle Ear Transformer

LEVER ACTION

Decreasing Length of the ossicles

produces a 1.3 fold increase in pressure  

Overall effect

17:2:1.3=44.2 

Sounds can vibrate bones of skull directly

Inertial vibrational forces: condensation and rarefaction; predominant for low frequency sounds

Compressional hearing: at higher frequencies of sounds; segmental differences in vibrational patterns 

Detected gamma rays from radioactive probe

could measure the velocity of the malleus and the BM

Frequency tuning curves: peak occurs at the place that matches frequency of stimulus

sharper tuning curves than von Beckesys

Decreases: -100 m/sec at the base

-2 sec at the apex

Amplitude of wave increases- is about 10 times greater at apex 

Laser velocimetry studies measured 2 things, velocity of the BM as a fxn of freq and intesity;gain-magnitude of displacement, normalized to stimulus pressure

1. Largest gain seen for the softest stimulus level of 0 dB SPL
2. Gain is approx 50 dB
3. Means that the BM is boosting the input signal at low signal levels
4. Amt of gain decreases as signal instensity increases 

OHCs contract when BM moves upward

OHCs expand when BM moves downward

Open pores to allow ion flow in

when cilia are deflected towards the tallest cilia

away from limbus

excitatory mode

essential for transduction

drives in potassium and calcium ions