14.2 Neurobiology of Hearing

Question 1

What is the purpose of the ear?

Answer 1

• Detect sounds
• Maintain balance

Question 2

What are the 3 main parts of the ear?

Answer 2

• Outer ear
• Middle ear
• Inner ear

Question 3

Label the diagram

Answer 3

Question 4

What are the main structures of the inner ear?

Answer 4

• 3 semi-circular canals
• Utricle
• Saccule

Are involved with equilibrium in the ear

Question 5

What makes up the external ear?

Answer 5

• Auricle (pinna)
  
  • Captures sound waves
• External auditory meatus (ear canal)
  
  • Sound waves travel through this
• Tympanic membrane (ear drum)
  
  • Moves in & out in response to sound

(tympanic membrane marks the beginning of the middle ear)

Question 6

What is the range of human hearing & the most sensitive range?

Answer 6

• The commonly stated range of human hearing is 20 to 20,000 Hz.
• The human auditory system is most sensitive to frequencies between 2,000 and 5,000 Hz

Question 7

What is the function of the pinna (auricle)?

Answer 7

• Gather and focus sound energy to tympanic membrane
• The one portion of the auricle that has no cartilage is the called lobule, the fleshy lower part of the auricle
• The concha is the ”shell-shaped” structure of the cavity of the external ear
• Pinna and concha are selective filter for different sound frequencies in order to provide clues about the elevation of the sound source

Question 8

What is microtia & what are the different grades?

Answer 8

Microtia is a congenital deformity where the pinna (external ear) is underdeveloped

Question 9

What is the function of the ear canal?

Answer 9

• Functions as an entryway for sound waves, which get propelled toward the tympanic membrane, known as the eardrum
• The concha and external auditory canal acts as RESONATOR, i.e. effectively enhance the intensity of sound that reaches the tympanic membrane (when look in the ear examination see this)

Question 10

Briefly describe the tympanic membrane (ear drum)

Answer 10

• Is thin and pliable so that a sound, consisting of compressions and rarefactions of air particles, pulls and pushes at the membrane moving it inwards and outwards at the same frequency as the incoming sound wave
• It is this vibration that ultimately leads to the perception of sound
• The greater the amplitude of the sound waves, the greater the deflection of the membrane
• The higher the frequency of the sound, the faster the membrane vibrates

Question 11

Talk about the 3 small bones in the middle ear

Answer 11

• Auditory ossicles are consisted of the following smallest three bones in human body to transfer the vibration of tympanic** **membrane to cochlea
  
  • Malleus (hammer): forms a rigid connection with the incus
  • Incus (anvil): forms a flexible connection with the stapes
  • Stapes (stirrup): connects to oval window
• The inward-outward movement of the tympanum displaces the malleus and incus and the action of these two bones alternately drives the stapes deeper into the oval windowand retracts it, resulting in a cyclical movement of fluid within the inner ear

Question 12

Explain what the eustachian tube(s) do

Answer 12

• Helps ventilate the middle ear and maintain equal air pressure on both sides of the tympanic membrane, inside middle ear and outside the body, via nasopharyx (the nasal part of the pharynx, lying behind the nose and above the level of the soft palate)

Question 13

What is the function of the middle ear?

Answer 13

• The overall function of middle ear is to amplify the vibrations of tympanic membrane to oval window
• It also matches low acoustic impedance of air to high acoustic impedance of fluid inner ear (impedance matching)

Question 14

What is the vestibular system?

Answer 14

• Is the sensory apparatus of the inner ear that helps the body maintain its postural equilibrium
• The information furnished by the vestibular system is also essential for coordinating the position of the head and the movement of the eyes

Question 15

What are the two sets of organs found in the inner ear & explain how they work

Answer 15

1. The semicircular canals, which respond to rotational movements (angular acceleration)
   
   • 3 semi-circular ducts at 90 degrees from each other to detect angular rotation of the head
   • Are bony passages that contain the fleshy, fluid-filled semicircular ducts
2. The utricle and saccule within the vestibule, which respond to changes in the position of the head with respect to gravity (linear acceleration)

The information these organs deliver is proprioceptive (dealing with events within the body itself) rather than exteroceptive (dealing with events outside the body) as in the case of the responses of the cochlea to sound

Organs functionally related to cerebellum, brainstem, reflex centres of the spinal cord - govern movements of eyes, neck & limbs

Question 16

How do semicircular canals work?

Answer 16

Semicircular canals mediate interactions between the vestibular system and eye muscles via cranial nerve so plays a smooth movement of the eyes toward the left and right, keeping the visual field stable as the head turns

Question 17

Briefly explain otoliths organs

Answer 17

• Otoliths organs consist of saccule and utricle which perpendicular each others (and are also called as gravity receptor as the respond to the gravitational forces
• The receptors, called maculae (meaning ”spot”), are patches of hair cells topped by small, calcium carbonate crystals called otoconia

With any position of the head, gravity will bend the cilia of one patch of hair cells, due to the weight of the otoconia to which they are attached by a gelatinous layer

This bending of the cilia produces afferent activity going through the nerve to the brainstem

Question 18

What are the membranes & compartmements in the cochlea?

Answer 18

1. Raissner’s membrane
2. Basilar membrane
3. Scala vestibuli (vestibular ducts)
   
   • Conducts sound vibrations to the cochlear duct
4. Scala tympani (tympanic ducts)
   
   • Together with vestibular duct to transduce the movement of air that causes the tympanic membrane and the ossicles to vibrate, to movement of liquid and the basilar membrane.
5. Scala media (cochlear duct)
   
   • Houses of organ corti that transform fluid vibration into nerve impulse

Question 19

Explain what the Raissner’s membrane does?

Answer 19

• Raissner’s membrane together with the basilar membrane it creates a compartment in the cochlea filled with endolymph, which is important for the function of the spiral organ of Corti

Question 20

Explain what the basilar membrane does?

Answer 20

• Basilar membrane forms the division between the scalamedia and tympani and cause different frequencies to reach maximum amplitudes at different positions

Question 21

What does the oval window do?

Answer 21

Receives vibration from stapes and transmit to base of basilar membrane

Question 22

What does the round window do?

Answer 22

• It vibrates with opposite phase to vibrations entering the inner ear through the oval window
• It allows fluid in the cochlea to move, which in turn ensures that hair cells of the basilar membrane will be stimulated and that audition will occur

Question 23

What does the organ of Corti do?

Answer 23

• Organ of corti transduces auditory signals and minimize the hair cells’ extraction of sound energy and consist of two hair cells and tectorial membrane:
  
  • Inner hair cells (IHC): detect the sound and transmit it to the brain via the auditory nerve
  • Outer hair cells (OHC): perform an amplifying role
  • Tectorial membrane: the function for human is not clear yet, but TM may be involved in the longitudinal propagation of energy in the intact cochlea

Question 24

What is endolymph?

What is its function?

Answer 24

Fluid contained in the membranous labyrinth of the inner ear

Their function is to regulate electrochemical impulses of hair cells

Resembles intracellular fluid in composition (potassium is the main cation)

2 other functions:

1. HEARING
   
   • ​​Fluid waves in the endolymph of the cochlear duct stimulate the receptor cells, which in turn translate their movement into nerve impulses that the brain perceives as sound
2. BALANCE
   
   • ​​Angular acceleration of the endolymph in the semicircular canals stimulate the vestibular receptors of the endolymph

Question 25

What is **perylymph**? What is its function?

Answer 25

Is extracellular fluid inside ***perylmphatic space*** Their function is to ***regulate electrochemical impulses of hair cells*** Perilymph resembles *extracellular* fluid in composition (**sodium** salts are the predominate positive electrolyte)

Question 26

Explain the process of **conversion of a sound wave to an ACTION POTENTIAL**

Answer 26

1. Sensory signal is the sound wave and the responsibility of converting vibrations into action potentials falls upon the inner row of ***hair cells*** in the ***cochlea*** 2. The *apical* end of the *hair cell* contains the ***stereocilia*** and that they are arranged in order of *ascending lengths* from one side of the cell to the other 3. The membranes of the *stereocilia* contain ***mechanically gated cation channels*** 4. Extending from the gate of the ion channel to the adjacent, taller, stereocilium is a fibrous protein called a ***tip link*** 5. When the *stereocilia* bend toward the longest stereocilium the tension in the tip link increases, pulling the ***gates on the ion channels open***, and when they bend in the opposite direction the tension *decreases* and the *gates close* 6. The stereocilia are bathed in the ***endolymph*** of the *cochlear duct* which is similar to intracellular fluid and has a ***high K+ concentration*** 7. When the gates on the cation channels open, ***K+ rushes*** into the cell, depolarizing the membrane and depolarization o***pens voltage gated Ca2+ channels*** on the *basal membrane* of the hair cell allowing ***Ca2+ to enter*** 8. The influx of ***Ca2+ stimulates*** the release of *neurotransmitter* by the *hair cell triggering an action potential* in the neuron that synapses with the hair cell 9. The axons of these neurons form the cochlear nerve that ***transmits the action potential*** to the ***auditory cortex*** of the brain 10. In hair cells at rest, about 10% of the K+ ion channels are open resulting in a low frequency of action potentials traveling to the brain when it is perfectly quiet 11. This allows for both an increase in ***action potential frequency*** when hair cells bend toward the longest stereocilium, and a *decrease* in frequency of action potentials when the *hair cells bend the other way*

Question 27

Explain the process of **perception of sound**

Answer 27

1. Once the *action potential* is generated and sent to the brain it is the function of the ***auditory cortex*** to convert that action potential into a *perception* and each region of the *cochlea* is hardwired to its own specific region of the auditory cortex 2. When that particular region of the brain receives input from the ear we perceive the unique pitch associated with that frequency of sound wave (like a piano where each key is like a different segment of the cochlea) 3. Each time an action potential reaches that specific segment of the ***auditory cortex*** we perceive the *_same sound_* 4. Therefore, ***pitch*** is determined by the *region of the brain* that receives *_input from the cochlea_*

Question 28

Explain the process of perception of sound INparticular **LOUDNESS**

Answer 28

* ***LOUDNESS***, on the other hand, is determined by the *number of action potentials that reach the brain* (Sound is a function of the amplitude of the sound wave) * Sound waves of *higher amplitude* cause the hair cells to *_vibrate_* more vigorously, which would cause *more ion channels to open* * This would result in a greater depolarization of the hair cell, *_more Ca2+ entry_* through the *voltage-gated ion channels* and more neurotransmitter release * The end result is a **greater frequency of action potentials** going to the auditory cortex, which is perceived as a ***louder sound*** * ***Frequency of action potentials ≠ frequency of the sound waves*** * The frequency of action potentials is a function of the amplitude of the sound wave whereas the ***frequency of the sound waves*** *determines* which portion of the ***auditory cortex*** receives the *action potentials*

Question 29

What are the main categories of **HEARING LOSS** or **DEAFNESS** & explain them

Answer 29

1. **Sensorineural** * Is the most common type of hearing loss; it is usually *due to the _loss of cochlear hair cells_* but can be result from *_damage to the eighth cranial nerve_* or within central auditory pathways**​** * It often *impairs* the ability to _hear certain pitches_ while others are unaffected 2. **C****onductive****​​** * Refers to *impaired sound transmission* in the ***external*** or ***middle ear*** and impacts *all sound frequencies* * Severe conductive deafness can result from ***otosclerosis*** in which bone is *resorbed* and replaced with ***sclerotic bone*** that grows over the *oval window*

Question 30

What things can cause **hearing loss**?

Answer 30

* ***Aminoglycoside antibiotics*** * ***​*** E.g. *streptomycin* and *gentamicin* obstruct the *_mechanosensitive channels in the stereocilia of hair cells_* (especially outer hair cells) and can cause the cells to degenerate, producing sensorineural hearing loss and abnormal vestibular function * ***Damage to the hair cells*** by *prolonged exposure to noise* is also associated with hearing loss * Other causes include *autoimmune disorders, traumatic injuries, acoustic neuromas, tumors of the eighth cranial nerve* and *cerebellopontine angle*, and *vascular damage in the medulla*

Question 31

What are the different **LEVELS** of hearing loss?

Answer 31

* Normal hearing * Mild hearing loss * Moderate hearing loss * Severe hearing loss * Profound hearing loss

Question 32

What do you look for with regards to **hearing loss**?

Answer 32

* ***Bilateral*** (both ears) versus ***unilateral*** (one ear) hearing loss * ***Symmetrical*** (same level/severity of hearing loss in both ears) versus ***asymmetrical*** hearing loss (different levels/severity of hearing loss in each ear) * ***High-frequency/pitched*** versus ***low frequency/pitched*** hearing loss * ***Progressive*** versus ***sudden*** hearing loss * ***Stable*** versus ***fluctuating*** hearing loss

Question 33

What is **pure tone audiometry**?

Answer 33

* This test assesses the hearing *_sensitivity across a range of frequencies_* (pitches), which are involved in speech perception * It involves listening to *sounds* via *headphones* and responding by pressing a button every time a sound is heard

Question 34

What is **speech** **testing**?

Answer 34

* This is a diagnostic test that assesses speech discrimination using ***single words*** * It involves listening to ***words*** and ***repeating*** what was heard to the audiologist who records the results

Question 35

What is **bone conduction testing**? (& what would happen in these tests)

Answer 35

This is another type of ***pure-tone*** test that measures your *inner ear’s* response to sound 1. ***Weber's test*** 2. ***Rinne test***

Question 36

What is **acoustic reflex testing**?

Answer 36

This test measures ***involuntary muscle contractions*** of the *middle ear* and is used to determine the *location of hearing problem* (theossicles, cochlea, auditory nerve, etc.) as well as the type of hearing loss

Question 37

What is **auditory brainstem response** testing? (ABR)

Answer 37

This type of testing is used to determine whether a specific type of hearing loss (*sensorineural*) exists (*common test in new borns*)

Question 38

What is **Schwabach** testing?

Answer 38

Is the same as **Weber's** testing but, you compare **bone conduction** of patient vs that of a healthy subject e.g. **examiner** **CONDUCTION** = bone conduction BETTER than examiner **SENSORINEURAL** = bone conduction is WORSE than examiner