Auditory Senses

Question 1

What are the middle ear ossicles from tympanic membrane to oval window? Why does the stapes have stirrup shape?

Answer 1

• malleus
• incus
• stapes (opening for stapedial artery)

Question 2

What are the openings in the middle ear? What do they “open” into?

Answer 2

• oval window (scala vestibuli)

- round window (scala tympani)

Question 3

What are the muscles of the middle ear? When do these muscles contract?

Answer 3

• tensor tympani (inserts on malleus and pulls handle upward)
• stapedius (inserts on stapes and pulls outward)

-contract reflexively in response to loud sounds in order to dampen the vibrations of auditory ossicles

Question 4

What is impedance matching?

Answer 4

• tympanic membrane surface area is 17 times that of the oval window
• increases the force from the ossicular level system to the cochlear fluid by 22x

Fluid has greater inertia than air and requires this increase in force in order to cause vibration in the fluid

-tympanic membrane and auditory ossicles provide the impedance matching between the air and fluid

Question 5

What are the major components of the cochlea? What three components are tubes?

Answer 5

Tubes:

• scala vestibuli
• scale media
• scala tympani

-Reissner’s membrane

Question 6

What is the function of Reissner’s membrane?

Answer 6

-separates scala media and scala vestibuli
+thinness of the membrane, media and vestibuli are considered a single chamber as far as sound conduction is concerned
+maintains a special fluid with the scala media

Question 7

What membrane separates the scala media from the scala tympani?

Answer 7

-basilar membrane

Question 8

The basilar membrane has free fibers that project from the modiolus of the cochlea to the outer wall. What can these fibers do?

Answer 8

Vibrate

Question 9

What is the apex of the cochlea called?

Answer 9

-helicotrema

Question 10

How do the length and diameter of the basilar fibers change from the oval window to the helicotrema?

Answer 10

• length increases from the oval window to the helicotrema
• diameter if the fibers decrease

-results in:
+stiff, short fibers near the oval window that beat at a high frequency
+long, limber fibers near the apex that beat at a low frequency

Question 11

What frequencies reach what parts of the cochlea?

Answer 11

• high frequency is proximal to the oval window
• medium frequency is in the middle
• low frequency travels to the helicotrema

Question 12

What does the organ of Corti do? What are the sensory receptors?

Answer 12

• generates nerve impulses in response to vibration of basilar membrane
• sensory receptors are hair cells

Question 13

How do the hair cells in the organ of Corti stimulate AP? Where are they? How are AP decided on?

Answer 13

• hair cells stimulate nerve fibers that lead to the spiral ganglion of Corti
• stereocilia from the hair cells touch or are embedded in the overlying tectorial membrane
• bending of hairs in one direction depolarize them and bending them in the opposite direction hyperpolarizes them

Question 14

How is the organ of Corti a rigid unit?

Answer 14

-outer ends of hair cells are tightly attached to reticular lamina, supported by rods of Corti which are attached to the basilar fibers

Question 15

Where are AP from the organ of Corti sent?

Answer 15

-neurons in the ganglion send axons via the cochlear nerve into the upper level of the medulla

Question 16

What happens whenever the basilar membrane moves?

Answer 16

-the hair cells become excited

Question 17

Describe the hair cells and how they are bent to create an AP.

Answer 17

• each hair cell has about 100 stereocilia on its apical border
• stereocilia are longer in the side farther from the modiolus
• when cilia are bent in direction of the longer ones a tensile force is created in the shorter cilia
• this creates a mechanical transduction that opens potassium channels resulting in a depolarization of the hair cell membrane

Question 18

What fluid fills each chamber?

Answer 18

• media -> endolymph

- vestibuli and tympani -> perilymph

Question 19

What are the contents of endolymph and perilymph? How does this effect potentials?

Answer 19

• endolymph contains high conc of K+ and low conc of Na+
• perilymph is the opposite (80mV difference between endolymph and perilymph)
• tops of hair cells are bathed in endolymph and lower bodies are surrounded by perilymph

Question 20

What is the place principle?

Answer 20

-nervous system detects sound frequencies by determining positions along the basilar membrane that are most stimulated

Question 21

What are the components of the nervous system pathway?

Answer 21

• spiral organ of Corti
• dorsal and ventral cochlear nuclei
• superior olivary nucleus
• lateral lemniscus’ nucleus
• medial geniculate nucleus (thalamus)
• auditory cortex

Question 22

What frequency is lost first with old age?

Answer 22

-high frequency for both air and bone conduction

Question 23

What is the conduction pathway sequence?

Answer 23

• tympanic membrane
• malleus
• incus
• stapes
• oval window