Neuro 1 Chapter 11: Auditory And Vestibular Systems

Front 1

Frequency

Back 1

Number of compressed or rarified patches of air that pass by our ears each second.

Measured in Hz, number of cycles/second

Front 2

Intensity

Back 2

Difference in pressure between compressed and rarified patches of air; deermines perceived loudness of sound

Front 3

Pinna

Back 3

Visible portion of ear, forming a funnel that plays key role in localizing sounds.

More sensitive to sounds from ahead than behind.

Front 4

Auditory canal

Back 4

Entrance to internal ear, extends 1 inch inside the skull

Front 5

Tympanic Membrane (eardrum)

Back 5

Moves the ossicles through vibration from sound waves

Front 6

Ossicles: name them and their function

Back 6

Malleus (hammer)--attached to tympanic membrane, forms connection with

Incus (anvil)--connects with

Stapes (stirrup)--flat footplate at bottom tranmits sound vibrations to fluids of cochlea

Front 7

Cochlea

Back 7

Snail-shell tube filled with fluid that exerts pressure at back of oval window; the fluid requires movement by the ossicles to amplify the vibration pressure

Front 8

Attenuation Reflex

Back 8

Triggered by onset of a loud sound;
Tensor tymmpani muscle and stapedius muscle contract and the ossicles become rigid, vastly decreasing sound conduction to the inner ear.

Front 9

Anatomy of the Cochlea: three fluid-filled chambers

Back 9

Scala Vestibuli
Reissner's membrane divides
Scala media
Basiliar membrane divides
Scala tympani

Front 10

Organ of Corti

Back 10

Sitting on basiliar membrane and covered by tectorial membrane: contains hair cells, the rods of Corti and supporting cells.

Front 11

Depolarization of a hair cell

Back 11

1) Basiliar membrane deflects up from sound, bending the stereocilia outward.
2)TRPA1 ion channels open to permit K+ when the tip links between the stereocilia are stretched
3)The depolarization by K+ activates voltage-gated calcium channels, triggering glutamate which activates spiral ganglion fibers postsynaptic to the hair cell.

Front 12

Why does the opening of K+ channels depolarize rather than hyperpolarize hair cells (like most other neurons)?

Back 12

The unusually high concentration of K+ in endolymph hields a K+ equilibrium potential of 0mV as opposed to -80mV in most neurons.
In addition, the 80mV endocochlear potential helps create a 125mV gradient across stereocilia membranes.

Front 13

Innervation of hair cells

Back 13

95% of spiral ganglions communicate with inner hair cells even though there are 3x as many outer hair cells.
One inner hair cell feeds about 10 spiral ganglion neurites.

Front 14

Amplification by Outer Hair cells

Back 14

Motor protein prestin ("presto") quickly changes outer hair cell length in response to low intensity stimuli. The outer hair cells on the basiliar membrane thus called the cochlear amplifier.
Without this amplification, the peak movement of the basiliar membrane would be 100fold smaller

Front 15

Anatomy of important auditory pathways

Back 15

1) Spiral ganglion afferents enter brain stem at auditory-vestibular nerve
2)Medulla axons innervate ventral cochlear nuclei IPSILATERAL to the cochlea where the axons originated
3) VCN sends axons that project to the superior olive on BOTH sides of the brain stem
4) Olivary axons ascend in the lateral lemniscus and innervate the inferior colliculus of the midbrain (all ascending auditory pathways converge on to inferior colliculus)
5)IC neurons project to medial geniculate nucleus of thalamus
6) and from there to the auditory cortex

Front 16

Tonotopy

Back 16

Systematic organization of characterisic frequency within an auditory structure.

Tonotopic maps exist on basilar membrane within auditory relays, the MGN and auditory cortex.

Only useful for medium and high frequencies.

Front 17

Phase locking

Back 17

Consistent firing of a cell at the same phrase of a sound wave.

Useful at low frequencies cause easier to map action potential to soundwaves.
Occurs with sound waves up to 4kHz.

Front 18

Localization of sound in the horizontal plane

Back 18

Measurement of interaural time delay (sound from right hits right ear first).
If distance between ears is 20cm and sound comes from right, it will reach left ear 0.6msec later. People can discriminate as little as a 2 degree difference in direction in horizontal plane.
Interaural intensity difference is measured for high frequencies since our head creates a sound shadow for the high ones.

Front 19

Duplex theory of sound localization

Back 19

From 20-2000Hz, interaural time delay is used by neurons; from 2000-20,000Hz interaural intensity difference is used by neurons. These two processes together constitute the duplex theory.

Front 20

Olivary neuron horizontal localization specialization

Back 20

Some neurons in the superior olive have systematic differences in the arrangement of axonal delay lines, using phase locking to pick location in horizontal plane.

Front 21

Localization of sound in the vertical plane

Back 21

Largely based on reflections from the convolutions of the pinna. The delays between the direct path and the reflected path change as a sound moves vertically.

Front 22

Acoustic radiation

Back 22

The projection from the MGN to the auditory cortex via the internal capsule

Front 23

Structure of auditory cortex

Back 23

Similar to corresponding areas of striate cortex:
Layer I contains few cell bodies
Layers II and III contain small pyramidal cells
Layer IV (where MGN axons terminate) is composed of densely packed granule cells
Layers V and VI contain pyramidal cells larger than those in the superficial layers.

Front 24

Effect of auditory cortical lesions

Back 24

There is greater preservation of function after lesions in auditory than in striate cortex because both ears send output to cortex in both hemispheres; only sound localization abilities are lost.