Auditory

Question 1

How is sound produced?

Answer 1

by vibrations that cause alternating compression and decompression of the surrounding air, resulting in the formation of a pressure wave with associated peaks and valleys of varying amplitudes of intensity

Question 2

What is the Intensity of human speech? The frequency?

Answer 2

human speech is about 65 dB
frequency = hertz (10-20,000) below 10 you can feel but not hear, above 20,000 cannot hear

The human ear is preferentially “tuned” to the frequencies of the human voice - hears sounds best in the 3000-6000 Hz range.

Question 3

What is the middle ear?

Answer 3

the cavity within the temporal bone (petrous portion) where the air pressure waves (sounds) are converted to mechanical energy by means of tympanic membrane and connecting ossicles

site of mild to severe acute and chronic infections known as otitis media

Question 4

Describe where the cavity of the middle ear lies.

Answer 4

lies medial to the tympanic membrane

Question 5

How does the middle ear cavity communicate with the nasopharynx?

Answer 5

communicates with nasopharnx through the auditory (eustachian or pharyngotympanic) tube

Question 6

What is otitis media?

Answer 6

mild to severe acute and chronic infections in the middle ear cavity

Question 7

What does the stapedius do?

What does it prevent?

Answer 7

The stapedius dampens the vibrations of the stapes by pulling on the neck of that bone.

It prevents excess movement of the stapes, helping to control the amplitude of sound waves from the general external environment to the inner ear.

The stapedius muscle dampens the ability of the stapes vibration and protects the inner ear from high noise levels.

Question 8

What would paralysis of the stapedius cause?`
How would this injury occur?
What would it result in?
What is this injury called?

Answer 8

Paralysis of the stapedius, such as an injury to the facial nerve (CN VII) distal to the geniculate ganglion prior to its branch to stapedius muscle (which would also cause Bell’s Palsy), allows wider oscillation of the stapes, resulting in heightened reaction of the auditory ossicles to sound vibration.

This condition, known as hyperacusis, causes normal sounds to be perceived as overly loud.

Question 9

What can happen in people with hyperacusis?

Answer 9

In many people with hyperacusis, an increased activity develops in the tensor tympani muscle in the middle ear as part of the startle response to some sounds.

This lowered reflex threshold for tensor tympani contraction is activated by the perception/anticipation of loud sound, and is called tonic tensor tympani syndrome (TTTS). In some people with hyperacusis, the tensor tympani muscle can contract just by thinking about a loud sound.

Following exposure to intolerable sounds, this contraction of the tensor tympani muscle tightens the ear drum, which can lead to the symptoms of ear pain/ a fluttering sensation/ a sensation of fullness in the ear (in the absence of any middle or inner ear pathology)

Question 10

What is the inner ear composed of?

Answer 10

bony and membranous labyrinths

bony portion- filled with perilymph, which is continuous with the fluid surrounding the vestibular system

membranous portion- filled with endolymph, which is also continuous with the structures in the vestibular system (semicircular canals, etc)

Question 11

Describe the chemical composition of the endolymph.

Answer 11

The chemical composition of the endolymph is VERY different from normal extracellular fluid, and contributes to the formation of the receptor potentials.
It has very little protein and K+&raquo_space; Na+; It carries a charge of roughly +80mV.

Question 12

Describe the three routes by which transduction of pressure waves gets to the inner ear.

Answer 12

air- poor conduction; most (95 percent) reflected off of round window (largely ignored)

osseous- due to vibration of bones (mastoid and petrous) in skull; large loss of energy (typically referred to as “bone conduction” (vibrate bones of skull, can vibrate fluid inside, why your voice sounds different to you than everyone else)

ossicular- MOST efficient due to direct coupling between inner structures and outer ear canal (typically referred to as “air conduction” by physicians) moves fluid

Question 13

What is the “hearing” structure? Where is it?

Answer 13

cochlea

contained within the modiolus

Question 14

What does the cochlea consist of?

Answer 14

Scala vestibuli

helicotrema (apex) and scala tympani

Question 15

Where does the cochlear duct lie?

Answer 15

cochlear duct (scala media; membranous labyrinth) lies between these two bony channels, and is separated from them by Reisner’s membrane above and the Basilar membrane below

Question 16

Where is the basilar membrane the widest and stiffest?

What is sitting on the basilar membrane?

Answer 16

widest at the helicotrema and stiffest at the base

these properties allow it to respond selectively to different frequencies of sound, with high pitches near the base where it is the stiffest, and low pitches near the apex where it is most flexible

sitting on the basilar membrane is the Organ of Corti where transduction of these pressure waves takes place in the hair cells
(hair cells control the amplitude of sound)

Question 17

Describe the orientation of hair cells.

Answer 17

(functionally significant)
3 outer rows of hair cells and 1 inner row

outer hair cells are displacement sensitive, and highly convergent on the bipolar cells (10 hair cells per bipolar cell); they receive only 10 percent of the overall innervation of the cochlea. The outer hair cells are long and flexible, and are used to modulate the tectorial membrane.

These are the ONLY receptors which can be directly modified by the CNS, and can change their length and stiffness

Question 18

Which are the only receptors that adjust themselves?

Answer 18

outer hair cells…
These are the ONLY receptors which can be directly modified by the CNS, and can change their length and stiffness

(others have NS do it for them)

Question 19

Describe the inner hair cells.

Answer 19

velocity sensitive
each are innervated by up to 10 bipolar cells (10 bipolar hair cells per hair cell) they recieve over 90 percent of the overall innervation of the cochlea

Inner hair cells are short and stiff, used primarily to detect sound

Thy are NOT directly modified by the CNS
due to unique innervation ratio of the inner hair cells, each nerve fiber has a characteristic frequency of excitation based on its position on the basilar membrane

Question 20

Describe the bending of hair cells toward/away from the kinocilum.

Answer 20

The bending of the hair cells TOWARDS the kinocilium mechanically opens K+ channels in the hairs, and allows the K+ ions to depolarize the hair cell.

Movement of the basilar membrane in the opposite direction causes the stereocilia to bend AWAY from the kinocilium, resulting in the closure of the K+ channels and hyperpolariztion of the hair cells and a decrease in the release of neurotransmitter.

Question 21

Where is the first action potential?

Answer 21

in BIPOLAR cells NOT in hair cells

Question 22

Describe the way the basilay membrane resonates.

Answer 22

Due to its differences in stiffness and in width, the basilar membrane resonates at different frequencies along its length. These differences are what give the nerve fibers their characteristic frequency of excitation and the “tonotopic map” of the basilar membrane

Question 23

Describe the tonotopic organization of basilar membrane.

Answer 23

this tonotopic organization is then carried through to all other levels of the CNS, including the cochlear nuclei, inferior colliculus, medial geniculate and the cerebral cortex itself (Herschel’s gyrus, primary auditory cortex)

This tonotopic map is essential to the perception of different sounds and frequencies. At its fastest recovery time, a typical hair cell could only transduce signals at a rate of ~500Hz. Therefore all sounds above this would be lost. The hair cells only transduce information concerning intensity; the frequency is determined by the placement on the basilar membrane, and interpretted by the CNS as a different pitches or tones.

Question 24

Describe the functional auditory pathways to the CNS.

Answer 24

The extensive bilaterality of the auditory system above the cochlear nuclei contributes to the prevention of total deafness in general brain dysfunctions. Unless there is a lesion in CN VIII or the cochlear nuclei, an individual will most likely NOT be deaf due to CNS problems.

(see slide 24)

Question 25

Describe the perception of sound. How is temporal resolution achieved?

Answer 25

Temporal resolution is achieved by “phase locking” of the hair cells and bipolar cells; they fire a burst of APs at the initiation of the sound and cue in the CNS as to the onset of the new stimulus; they also hyperpolarize at the offset.

Question 26

Describe the 2 methods by which localization of sound is interpretted by the CNS.

Answer 26

Due to our bilateral hearing structures, sounds lateral to our ears will reach one ear before the other, with a maximal temporal delay of ~50μs. This delay is called the “interaural difference” and can be interpretted by the CNS to localize where a sound is coming from.

We can also localize sounds by changes in pitch and intensity. As sounds move toward us, the intensity and pitch increase; as they move away, the intensity and pitch decrease (think of a train whistle as it passes by).

Question 27

Describe what would happen with a lesion in Wernicke’s area.

Answer 27

Lesions in Wernicke’s area will result in failure to comprehend the auditory signals, and if the lesion extends posteriorly into the PTO (association areas), then visual input for language may also be affected. The patient will be unable to understand either visual or spoken language.

Question 28

What would happen if there was a lesion in Broca’s area?

Answer 28

Lesions in Broca’s area will not affect comprehension or formation of language, but will cause a major disruption of speech output and the verbal production of language, due to the loss of input to the motor cortex.

Question 29

What would occur with a lesion of the arcuate fasiculus?

Answer 29

Lesions of the arcuate fasciculus will also disrupt verbal output.

Question 30

What other areas are important for language?

Answer 30

In addition to these cortical areas, subcortical structures (the left thalamus & caudate nucleus) are also important for language. Also, the visual information is processed independently of the auditory information in the PTO, and auditory signals themselves are processed differently.

Question 31

How are nonsense sounds processed?

Answer 31

Nonsense sounds -those without specific meaning - are processed separately from meaningful sounds (phonemes) and words.