**The Inner ear

Cochlea - main structure -liquid filled - liquid is set into vibration by the

movement of the stapes against the o oval window

Upper part – scala vestibuleLower part- scala tympani

Organ of Corti- large structure in the cochlear partition

two separated by

cochlear partition

Main structures

Hair cells- Bending of cilia in the inner hair cells are responsible for transduction.

2 types of hair cellsinner hair cells- 3,500- bends, and does

transduction process.outer hair cells- 12,000- increase the

vibration of the basilar membrane

Basilar membrane- supports the organ of corti and vibrates in response to soundTectorial membrane- extends over the hair cells

Why inner hair cell bends?

Ans: because the in and out movement of the stapes creates pressure changes in the liquid inside the cochlea that sets the cochlear partition into up and down motion

Movement in one direction of the cilia opens channel in the membrane ions flow into the cell

Movement opposite of that direction ion channels close no electrical signals generated

** we have low threshold for hearing

Video Clip

Q: how do we perceive pitch?Bekesy’s Place Theory of Hearing- frequency of a sound us indicated by the place along the cochlea at which nerve firing is highest.Low frequency- maximum activity in the in the hair cells and auditory nerve fibers at the apex end of the basilar membraneHigh frequency- cause maximum activity in hair cells and auditory nerve fibers at the base of the membrane.

Tonotropic map- an orderly map of frequencies along the length of the cochlea.Frequency tuning curve- This curve is determined by presenting pure tones of different frequencies and measuring how many decibels are necessary to cause the neuron to fire. This decibel level is the threshold for that frequencycharacteristic frequency- frequency to which the neuron is most sensitiveAuditory masking- ability to hear a sound is decrease by the presence of other sounds

Q: Why does the basilar membrane vibrate more sharply in healthy cochleas?

The outer hair cells expand and contract in response to the vibration of the basilar membrane, and this expansion and contraction, which only occurs in live cochleas, amplifies and sharpens the vibration of the basilar membrane.

**Cochlear amplifier

Frequency- is signaled by which fibers in the cochlea fire to a tone as well as how the fibers are fired.Phase locking- property of firing at the same place in the sound stimulusTemporal coding- connection between the frequency of the sound stimulus and the timing of the auditory nerve firing; measurements of the pattern of firing for auditory nerve fibers indicate that phase locking occurs up to a frequency of about 4,000 Hz.

Causes of hearing loss

• Conductive hearing loss- Blockage of sound from reaching the receptors

• sensorineural hearing loss-Damage to the hair cells-Damage to the auditory nerve or the brainPresbycusis- caused by old age

- Greatest at higher frequencies-Prevalent on males than females

• Noise-induced Hearing loss-Occurs when loud noises cause

degeneration of the hair cells.

-damage to the organ of corti

WHERE AND WHAT STREAM

** a 45-year-old man with temporal lobe damage caused by a head injury

** a 64-year-old woman with parietal and frontal lobe damage caused by a stroke

Pathway of the Cochlea to the cortex

Inner hair cellsAuditory receiving area

in the cortex

Cochlear Nucleus Super Olivary Nucleus

Inferior colliculus

Medial Geniculate

Nucleus

Auditory receiving area (A1)

Temporal lobe of the Cortex

Cortical Processing

PITCH AND THE BRAIN

• Tonotropic map in the monkey cortex shows that neurons that respond best to low frequencies are located to the left, and neurons that respond best to higher frequencies are located to the right

• Studies found neurons in the brain that respond to both pure tones and complex tones that differ in ther harmonics but have the same pitch

***Auditory cortex indeed is important for the perception of pitch

Experience dependent plasticity in the auditory system shows that in training that involves a particular frequency,

increases in the space devoted to that frequency in A1 is evident.