Auditory and Vestibular Systems

Question 1

Describe general anatomy of the ear

Answer 1

The ear is divided in three sections: outer, middle and inner ear. The organ is embedded in the petrous portion of the temporal bone, the hardest bone in the body.

Question 2

What are the functions of the outer ear?

Answer 2

1. To capture sound an to focus it the tympanic membrane.
2. To amplify some frequencies by resonance in the canal.
3. To protect the ear from external threats.

Question 3

What are 2 ways the middle ear carries out amplification?

Answer 3

1. Focusing vibrations from large surface area (tympanic membrane) to smaller surface area (oval window). The change in surface area means the pressure is increased.
2. Using leverage from the incus-stapes joint to increase the force on the oval window.

Question 4

What is the main functional part of the inner ear?

Answer 4

The hearing part of the inner ear is the cochlea. Its function is to transduce vibration into nervous impulses. While doing so, it also produces a frequency (or pitch) and intensity (or loudness) analysis of the sound.

Question 5

What are the 3 compartments of the cochlea?

Answer 5

Scala vestibuli and scala tympani: Are bone structures, contain perilymph (high in sodium)
Scala media: Membranous structure, contains endolymph (high in potassium). Here is where the hearing organ or Organ of Corti is located.

Question 6

What is the basilar membrane?

Answer 6

The structure where the organ of Corti lies in is the basilar membrane. Basilar membrane is arranged tonotopically, meaning it is sensitive to different frequencies at different point along its length.

Question 7

What is tonotopical arrangement?

Answer 7

Thicker at the base as compared to apex which creates properties as related to sound. High frequency sounds creates waves of greater amplitude towards the base whereas low frequency creates greater amplitude waves towards apex.

Question 8

What does the organ of Corti contain?

Answer 8

Contains inner hair cells and outer hair cells. Inner hair cells are arranged on one column while outer on three columns. The tectorial membrane is located above the hair cells and will allow the hair deflection, which in turn will depolarise the cell. Only OHC hair are in constant contact with the tectorial membrane, and these assist the contact with the IHC.

Question 9

What is the role of inner hair cells?

Answer 9

IHC carry 95% of the afferent information of the auditory nerve. Their function is the transduction of the sound into nerve impulses.

Question 10

What is the role of outer hair cells?

Answer 10

OHC carry 95% of efferents of the auditory nerve. Their function is modulation of the sensitivity of the response. Outer hair cells can also contract to move the tectorial membrane closer or further away from the inner hair cell. If moved closer, inner hair cell can tranduct sound.

Question 11

What are stereocilia and what is their role?

Answer 11

The hairs of the hair cells are called stereocilia. The deflection of the stereocilia towards the longest cilium will open K+ channels. The ionic interchange depolarises the cell and the neurotransmitter is liberated. Higher amplitudes (louder) of sound will cause greater deflection of stereocilia and K+ channel opening.

Question 12

Describe the auditory pathway

Answer 12

Spiral ganglions from each cochlea project via auditory vestibular nerve (VIII) to the ipsilateral cochlear nuclei (monoaural neurons). Auditory information crosses at the superior olive level. After this point all connections are bilateral.

Question 13

Define frequency and amplitude

Answer 13

Frequency/pitch (Hz): Cycles per second, perceived tone.

Amplitude/loudness (dB): Sound pressure, subjective attribute correlated with physical strength.

Question 14

Describe the human range of hearing

Answer 14

Frequency: 20–20,000Hz
Loudness: 0 dB to 120 dB sound pressure level (SPL)

Question 15

Why is the decibel scale useful?

Answer 15

The decibel scale (a log scale) is useful because the range of sensitivity is very large. This allows us to compress the scale on a graph, and reflect the fact that many physiological processes are non-linear (i.e. they can respond to both very low and very high values).

Question 16

How does hearing change with age?

Answer 16

Hearing acuity decreases with age, particularly higher frequencies. Medium and low frequencies could be affected with the progression of a hearing loss.

Question 17

What procedures are involved in a hearing assessment?

Answer 17

• Tunning fork
• Audiometry
• Central processing assessment
• Tympanometry
• Otoacustic Emission
• Electrocochleography
• Evoked potentials

ETACTOE

Question 18

What does a tunning fork test involve?

Answer 18

Used to establish the probable presence or absence of a hearing loss with a significant conductive component. They are used to provide early and general information, when audiometry is not available or possible. Weber test is when fork placed above head while Rhine test done bilaterally from the sides.

Question 19

What is pure tone audiometry?

Answer 19

Science of measuring hearing acuity for variations in sound intensity and frequency. An audiometer is the device used to produce sound of varying intensity and frequency.

Question 20

What is an audiogram?

Answer 20

The audiogram is where the hearing thresholds are plotted to define if there is a hearing loss or not. A normal hearing threshold is located between 0 – 20dB.

Question 21

What does a central processing assessment involve?

Answer 21

Assessment of hearing abilities other than detection. Verbal and non verbal tests. 
Examples: 
Sound localization
Filtered speech
Speech in noise

Question 22

What does tympanometry involve?

Answer 22

Examination used to test the condition of the middle ear and mobility of the eardrum (tympanic membrane) and the conduction bones by creating variations of air pressure in the ear canal.

Question 23

What are the most common results of tympanometry?

Answer 23

Normal curve – normal
Shifted towards -200 – negative middle ear pressure
Higher at negative end but low throughout:
Middle ear effusion
Perforation of tympanic membrane
Eustachian (pharyngotympanic) tube dysfunction
Occluded ear canal

Question 24

What are Otoacoustic Emissions?

Answer 24

The normal cochlea produces low-intensity sounds called OAEs. These sounds are produced specifically by the outer hair cells as they expand and contract. This test is often part of the newborn hearing screening and hearing loss monitoring.

Question 25

What are the 3 types of auditory evoked potentials?

Answer 25

Electrocochleography
– 0.2-4.0 ms, electrical activity from the cochlea and eighth nerve. Evoked by clicks or tone burst.

Auditory Brainstem Response (ABR)
– 1.5-10.0 ms, electrical activity from the eighth nerve and brainstem nuclei and tracts. Evoked by clicks.

Late Responses (N1-P2, P300, MMN, and more)
– 80-500+ ms, electrical activity from the primary auditory and association cortex. Evoked by tone burst and oddball paradigm.

Question 26

What is auditory brainstem response used for?

Answer 26

Electrical responses from the auditory pathway. ABR is more commonly used in clinic. Does not require attention from the patient. Alterations in the latency of waves can point to the location of the deficit. Objective measurement commonly used in babies and children.

Question 27

Why are cortical potentials useful?

Answer 27

Cortical potentials could be affected on neurological conditions or processing problems.

Question 28

What are the 3 types of hearing loss?

Answer 28

Conductive hearing loss: Problem is located in outer or middle ear.
Sensorineural hearing loss: Problem is located in the inner ear or the auditory nerve.
Mixed hearing loss: Conduction and transduction of sound are affected. Problem affects more than one part of the ear.

Question 29

What are the degrees of hearing loss?

Answer 29

Depending on where the hearing thresholds are located, hearing loss can be classified as mild, moderate, severe or profound.

Question 30

What are the causes of conductive hearing loss?

Answer 30

If in the outer ear, could be wax or foreign body. If in the middle ear, can be otitis or otosclerosis.

Question 31

What are the causes of sensorineural hearing loss?

Answer 31

If in inner ear, presbycusis and ototoxicity. If nerve, 8th nerve tumour.

Question 32

How do hearing aids work?

Answer 32

Depending on the type, degree and characteristics of the patient there are many options. Hearing aids amplify the sound, does not replace any structure.

Question 33

How do cochlear implants work?

Answer 33

The cochlear implant replaces the function of the hair cells by receiving sound, analysing it, transform it into electrical signals and sending an electric impulse directly to the auditory nerve. It needs a functional auditory nerve to function.

Question 34

How do brainstem implants work?

Answer 34

When the auditory nerves are the affected structures, the electrical signals can be send to a set of electrodes placed directly into the brainstem. It is very risky, then it is advised for people with bilateral important auditory nerve damage.

Question 35

What are the inputs of the vestibular system?

Answer 35

The vestibular system includes three main inputs: visual, proprioceptive and vestibular information.

Question 36

What are the ouputs of the vestibular system and the role of the CNS?

Answer 36

The outputs are mainly reflexes to maintain a stable posture and stable gaze. The CNS integrates this information and generates the responses.

Question 37

What are the parts of the ear?

Answer 37

The ear is separated into three parts: outer, middle and inner ear. The vestibular organ is in the posterior area of the inner ear. The inner ear contains hair cells for hearing and balance.

Question 38

Describe the vestibular organ

Answer 38

The utricule and saccule are located in the vestibule and are joined by a conduit. The saccule is also joined to the cochlea. There are three semicircular canals on each ear, anterior, posterior and lateral. The semicircular canals have an ampulla on one side, and they are connected to the utricle.

Question 39

What is the role of the skull labyrinth?

Answer 39

Superior projection of right bony labyrinth on base of skull. The location of the vestibular organ draws planes for anterior and posterior canals. These planes determine which structure will be stimulated with a specific head movement.

Question 40

What is the role of cilia on hair cells?

Answer 40

Vestibular hair cells have a kinocilium (the biggest cilium) and stereocilia. Cilia allows the cells to depolarise the cell with movement of the endolymph generated by head movement.

Question 41

What are the otolith organs?

Answer 41

Utricle and saccule are the otolith organs. Their cells are located on the maculae, placed horizontally in the utricle and vertically in the saccule. The maculae contain the hair cells, a gelatinous matrix and the otoliths on top. These otholiths are carbonate crystals that help the deflection of the hairs.

Question 42

What are semi-circular canals?

Answer 42

The hair cells in the canals are located in the ampulla. The rest of the canal only has a liquid high in potassium called endolymph. The ampulla has the crista, where the hair cells are located. The cells are surrounded by the cupula which helps the hair cell movement.

Question 43

What are the semi-circular canal planes?

Answer 43

The orientation of the canals in the head defines three planes. Anterior and posterior canals form a 90° angle. Lateral canals are horizontal to the other canals.

Question 44

What is the vestibular nuclei connect to?

Answer 44

Primary afferents end in vestibular nuclei and in the cerebellum. Vestibular nuclei have projections to:
Spinal cord
Nuclei of the extraocular muscles
Cerebellum
Centres for cardiovascular + respiratory control