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SHB Neuroanatomy > Auditory > Flashcards

Flashcards in Auditory Deck (31)
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1

How is sound produced?

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

2

What is the Intensity of human speech? The frequency?

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.

3

What is the middle ear?

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

4

Describe where the cavity of the middle ear lies.

lies medial to the tympanic membrane

5

How does the middle ear cavity communicate with the nasopharynx?

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

6

What is otitis media?

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

7

What does the stapedius do?
What does it prevent?

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.

8

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

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.

9

What can happen in people with hyperacusis?

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)

10

What is the inner ear composed of?

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)

11

Describe the chemical composition of the endolymph.

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+ >> Na+; It carries a charge of roughly +80mV.

12

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

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

13

What is the "hearing" structure? Where is it?

cochlea
contained within the modiolus

14

What does the cochlea consist of?

Scala vestibuli
helicotrema (apex) and scala tympani

15

Where does the cochlear duct lie?

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

16

Where is the basilar membrane the widest and stiffest?
What is sitting on the basilar membrane?

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)

17

Describe the orientation of hair cells.

(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

18

Which are the only receptors that adjust themselves?

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)

19

Describe the inner hair cells.

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

20

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

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.

21

Where is the first action potential?

in BIPOLAR cells NOT in hair cells

22

Describe the way the basilay membrane resonates.

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

23

Describe the tonotopic organization of basilar membrane.

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.

24

Describe the functional auditory pathways to the CNS.

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)

25

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

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.

26

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

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).

27

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

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.

28

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

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.

29

What would occur with a lesion of the arcuate fasiculus?

Lesions of the arcuate fasciculus will also disrupt verbal output.

30

What other areas are important for language?

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.