Ear and hearing

Question 1

The inner ear

Answer 1

• Bony labyrinth (continuous with temporal bone) housing membranous labyrinth

– Perilymph: low [K+], high [Na+]
– Endolymph: high [K+], low [Na+]

• Three major regions:

– Cochlea
• Cochlear duct
– Organ of corti; sense of hearing

– Vestibule
• Saccule and utricle (otolith organs)
– Maculae; gravitational pull and head position

– Semicircular canals
• Anterior, posterior and lateral canals
– Ampulla; rotational/angular head movements

• Vestibular and cochlear nerves to brain (different locations!).

Question 2

Properties of sound

Answer 2

• Sound = waves of pressure
– Air molecules compress

• Sound frequency
– No. of waves/unit time
– High frequency = high pitch (e.g. Trumpet)
– Low frequency = low pitch (e.g.Bass)

• Sound amplitude
– Amount of energy in each wave, measured in decibels (dB)
– Loud sounds have more energy

Question 3

Overview of sound transmission & detection

Answer 3

• Pinna
– Amplification of sound

• Tympanic membrane & Auditory
ossicles
– Mechanically distorted, transmitting
sound waves into mechanical vibrations - oval window

• Cochlea
– Pressure waves through fluid-filled compartments
– Distortion of stereocilia in Organ of
Corti - NT release
– AP’s via cochlear nerve - Vestibulocochlear Nerve (VIII)

Question 4

Transmission of sound to inner ear

Answer 4

1. sound waves arrive at tympanic membrane
2. movement of the tympanic membrane causes displacement of the auditory ossicles
3. movement of the staples at the oval window establishes pressure waves in the perilymph of the Scala vestibuli

Question 5

Detection of sound

Answer 5

4. the pressure waves distort the basilar membrane on their way to the round window of the Scala tympani
5. vibration of the basilar membrane causes vibration of hair cells against the tectorial membrane
6. information about the region and the intensity of stimulation is relayed to the CNS over the cochlear branch of cranial nerve VIII.

Human ear can detect sounds between 20 Hz (apex) and 20,000 Hz (base)

Pitch: region of basilar membrane Loudness: amplitude of vibration

Question 6

Cochlear duct & sound detection

Answer 6

Organ of Corti contains receptor structures for hearing
– Hair cells sit on basilar membrane
– Stereocilia line tops of hair cells
– Tectorial membrane lies over stereocilia

Question 7

Stimulation of receptor cells

Answer 7

• Hair cells are mechanoreceptors

• Stereocilia arranged in rows of different heights,
connected by tip links

• Differential movement of basilar memb. distorts
steriocilia, compressed against tectorial memb.

• PivoYng toward tallest member - depolarisation
(MG K+ channels) - NT release

• Synapse with afferent cochlear nerve fibres

Question 8

The auditory pathway & processing

Answer 8

• Auditory cortex is tonotopically organised:
– Each region of basilar membrane
connected to a specific region of auditory cortex
– Specific cortical neurons activated by particular frequencies

• Higher-order auditory cortex integrates separate sounds into meaningful pattern

Vestibulocochlear nerve - medulla oblongata - midbrain - thalamus - auditory cortex

Question 9

Hearing loss/impairment

Answer 9

Can be temporary/permanent, congenital/acquired

• conductive deafness
• sensorineural deafness

Question 10

Conductive deafness:

Answer 10

Conductive deafness:
– Outer or middle ear
– Blocked ear canal, infection, perforated tympanum, otosclerosis
– Inhibits/impairs conduction of sound waves
– Otitis externa/media, ear wax build up, congenital defects
– Otosclerosis – during pregnancy may be acquired (e.g. measles) or exacerbated (e.g. oestrogen stimulates osteocytic activity); traditional treatment with sodium fluoride to be avoided in pregnancy (adverse foetal effects).

Question 11

Sensorineural deafness:

Answer 11

Sensorineural deafness:
– Inner ear
– Disturbance to vestibulocochlear nerve, loss of hair cells, disruption to
endolymph homeostasis
– Sound waves conducted but detection and processing impaired/inhibited
– Acoustic neuroma, ototoxicity (e.g. aspirin),
– Congenital defects due to
• maternal illness (e.g. intrauterine infections (rubella), hypoxic episodes)
• nutritional deficiencies (IDAàcochlear/nerve tissue damage, reduced auditory acuity and neural transmission)

Question 12

Overview of equilibrium & balance

Answer 12

• Vestibule
• Semicircular canals
• Signals travel via vestibular nerve to
  brain stem and cerebellum:
  – Stimulation of reflex responses to maintain posture, balance and equilibrium

Question 13

equilibrium & balance: Vestibule

Answer 13

• Vestibule:
– Otolith organs (utricle and saccule)
– Linear movement (gravitational pull) and head position relative to gravity e.g. taking off in a car
– Static equilibrium (gravity is constant)

Question 14

equilibrium & balance: Semicircular canals

Answer 14

Semicircular canals:
– Rotational (angular) movement in three planes e.g. spinning
– Dynamic equilibrium

Question 15

Monitoring static equilibrium

Answer 15

• Stimulus: linear acceleration/deceleration

• Maculae respond to changes in velocity of head movement
– ↑ velocity = depolarisation, ↑ NT release & AP firing
– ↓ velocity = hyperpolarisation, ↓ NT release & AP firing

Question 16

Crista ampullaris of semicircular canals

Answer 16

• Crista ampullaris = sensory cells at base of ampulla

• One in each semicircular canal
– Provides information in the three planes of space
• Semicircular canals in the same plane work in pairs
– Depending on direction of rotation, depolarisation in one ear and hyperpolarisation in other ear (and vice versa)

• Hair cells lined by stereocilia + kinocilium
• Tips of hair cells project into ampullary cupula (gelatinous strands)

Question 17

Monitoring dynamic equilibrium

Answer 17

• Stimulus: rotational acceleration/deceleration

• At rest, cupula stands upright.
• Rotational acceleration.
  Endolymph inside semicircular canals moves in direction OPPOSITE rotation. Endolymph flow bends the cupula and excites hair cells
  (increase NT release and AP firing)
• Rotational deceleration.
  Endolymph moves in SAME direction as rotation. Endolymph flow bends cupula in opposite direction and inhibits hair cells.
  (decrease NT release and AP firing)

No signalling if head is still OR continued rotation at constant speed - endolymph comes to rest i.e. moves at same speed as body

Question 18

The vestibular pathway & processing

Answer 18

Information from equilibrium receptors goes to reflex centers in brain stem + cerebellum

Integration of information:
– Balance (vestibular apparatus)
– Spatial awareness (vision)
– Posture & body position (proprioception)

Vestibulo-ocular reflex:
– involuntary eye movements to keep an image (point of focus)
steady

Somatic reflexes: – Cerebellum
– Coordination and regulation of
muscle tone to maintain head and
body position
– E.g. knee jerk reflex