Unit 2 part 2 (Auditory system)

Question 1

Sound perception

Answer 1

pressure wave –> sound;

• wavelength (frequency/pitch)
• power/energy of waves (intensity/loudness)

Question 2

Intensity - quantification

Answer 2

intensity sensitivity range for humans: 12 orders of magnitude
–> Intensity (I) = measured pressure/SPLo (a ratio)
measured in decibels (dB)
– range is 0 - 120, change in 20 = x10 increase
* intensity varies depending on frequency of sound

Question 3

3 mechanisms of frequency selection in hair cells

Answer 3

1. frequency separation by basilar membrane
2. morphological specializations of individual hair cells
   (stereocilia near apex = longer than those near base)
3. Cochlear Amplifier: adjusts sensitivity of inner hair cell
   *uses somatic motor (by outer hair cells)

Question 4

Somatic motor (for cochlear amplifier)

Answer 4

cell bodies of outer hair cells change size in response to membrane potential changes;

Question 5

frequency encoding

Answer 5

1. place-coding:
   specific axon only carries info from one location on BM –> will carry only signals re: that frequency
2. phase-locking:
   axon signals in synchrony with cycles that are multiples of the sound

Question 6

pitch

Answer 6

the sensation of frequency of sound waves.

human range: 20-20,000 Hertz

Question 7

External Ear parts

Answer 7

cavity open to air environment; Collects sound from environment.

1. pinna
2. External auditory meatus
3. Tympanic membrane (border to middle ear)

Question 8

Middle Ear

Answer 8

Semi-closed cavity (open when Eustachian tube = open);
transfers sound energy from air to liquid.
1. Ossicles (malleus, incus and stapes)
2. round and oval windows
3. Tensor tympani and stapedius muscles
4. Eustachian tube

Question 9

Inner Ear

Answer 9

completely closed cavity;

• where sound energy is converted to neural signal.
  1. membranous labyrinth (sacs filled w/ fluid)
  2. cochlea (for hearing and vestibular f(x))

Question 10

Cochlea structure and function

Answer 10

3 scalae coiled 2.5 times; IN temporal bone, only partially rigid.
1, 2. Scala vestibuli and tympani
3. Scala media;
selectively responds to sound by frequency (in continuum along length)

Question 11

organization of the 3 scala

Answer 11

(in cochlea)
- scala vestibuli and tympani both filled with perilymph; communicate w/ each other at apical end.
- scala media in middle, filled w/ endolymph,
*contains organ of corti
(vestibuli//Reissner’s membr//media//basilar membrane//tympani)

Question 12

organ of corti

Answer 12

sits on basilar membrane in scala media, runs length of cochlea;
w/ outer and inner hair cells –> stereocilia flush against tectorial membrane;
* stria vascularis = endothelium
O of C= where nerves for auditory sensation get signals from

Question 13

stria vascularis

Answer 13

the highly vascular epithelial layer on lateral side of organ of corti;

• secretes endolymph
• regulates [K+]

Question 14

innervation pattern of hair cells

Answer 14

Afferent: Outer: ~10 hair cells per axon
Inner: 1 hair cell per 10 axons
Efferent: signals from superior olivary nucleus to aud. hair cells

Question 15

Efficiency of sound transfer in middle ear comes from:

Answer 15

total increase in amplitude in middle ear = 60-70%

• ossicles as levers: increase 3-5 dB
• TM much larger than oval window size (25:1) –> increase 30 dB

Question 16

Basilar membrane

Answer 16

moves in response to pressure wave along cochlea,

• mechanically specialized so that BM responds to different (specific) frequencies in gradient along length
• -> stiff at base: high frequencies
• -> wide and flexible at apex: low frequencies

Question 17

hair cell function

Answer 17

• base of hair cells gives/receives aff/eff neural signals
• stereocilia cause…
  a) depolarization if displaced toward longest cilia
  b) hyperpolarization if displaced toward shortest cilia
  
  • degree of displacement matches size of signal

Question 18

Mechanical Electrical Transduction (MET) Channels

Answer 18

process of de/re-polarizing Hair Cells for auditory sensation.

• endolymph = rich in K+; stereocilia cytoplasm = low in K+; perilymph = lowest in K+
  1. tension from tip links opens K+ channels in stereocilia
• -> HC depolarizes, AP sent down axon
  2. -> open volt-dep./Ca-dep. K+ channels in hair cell
• -> K+ leaves HC into perilymph => REpolarize cell (QUICK!)