Hearing

Question 1

What are the parts of the ear?= and what do they consist of?

Answer 1

Eternal ear - pinna/auricle and external auditory tube
Middle ear - auditory ossicles (malleus, incus and stapes) run from tympanic membrane to oval window, connected to the nasopharynx via Eustachian tube
Inner ear - bony labyrinth, membranous labyrinth, organ of Corti

Question 2

How is sound amplified in the middle ear?

Answer 2

Pressure is transmitted to the oval window via the ossicles (stapes attaches to oval window via annular ligament). Amplifaction is due to eh area of the tympanic membrane being much greater than that of the oval window and due to the mechanical efficiency of the ossicles.

Question 3

What is the bony labyrinth?

Answer 3

Complex series of cavities in the petrous part of the temporal bone and surrounds the membranous labyrinth. It makes up the vestibule, three semicircular canals, and the cochlea, yet it is the cochlea that is responsible for hearing in response to the vibrations.

Question 4

What does the bony labyrinth contain?

Answer 4

perilymph, which runs throughout all the bony cavities

Question 5

Describe the cochlea

Answer 5

The cochlea is shaped like a conical snail shell and the cochlea canal is divided into two sections of scala vestibule and scala tympani. The vibrations from the middle ear pass through the oval window into the scala vestibule and then out via the round window.

Question 6

Describe the cochlear duct. What does it contain?

Answer 6

Within the bony spiral canal of the cochlea lies the cochlear duct (or the scala media), which is part of the membranous labyrinth; the cochlear duct runs along the outer wall of the bony cochlea. The scala media is filled with endolymph and is bound by vestibular (or Reissner’s) membrane and the Basilar membrane, which separate the endolymph in the membranous labyrinth from the perilymph in the bony labyrinth. The endolymph contains a high concentration of K+ ions and a low concentration of Na+ ions.

Question 7

What is the organ of Corti?

Answer 7

The organ of Corti is the sensory area of the cochlear duct and lies on the basilar membrane. It contains hair cells which are the sensory receptors for sound stimuli, and runs the entire length of the basilar membrane.

Question 8

What is the tectorial membrane?

Answer 8

The tectorial membrane runs the length of the organ of Corti and interacts with the hair cells

Question 9

What are the types of hair cells? Function?

Answer 9

• Inner hair cells’ stereocilia do not attach to the tectorial membrane and their movement is in response to movement of the endolymph. They are responsible for sensing the sound.
• Outer hair cells’ stereocilia are embedded in the tectorial membrane and movement of the basilar membrane relative to the tectorial membrane allows for generation of sound impulses. They are responsible for amplitude of sound.

Question 10

Describe where signals are transmitted from hair cells?

Answer 10

Signals produced by the hair cells will eventually conduct their signals to spiral (cochlear) ganglion which are bipolar neurones, forming at the base of the hair cell. These nerve endings eventually form cochlear division of the CN VIII.

Question 11

Describe the transmission of sound through the ear.

Answer 11

Vibrations are transmitted from the tympanic membrane to the ossicles, and then to the oval window. This displaces the perilymph in the scala vestibule, and the scala tympani causes the basilar membrane to resonate, producing oscillatory movements. This stimulates different groups of inner hair cells depending on the frequency of the sound.

Question 12

What is tonotopy.

Answer 12

The stimulation of different groups of inner hair cells depending on the frequency of sound.

Question 13

Describe the sensing and amplification of sound. How are hair cells tuned? How do they transmit sound?

Answer 13

The inner hair cells (of which there is one row) sense sound. The outer hair cells – three rows worth – serve as amplifiers. They are both mechanically tuned by their location along the cochlea, and electrically tuned by the expression of ion channels. They have stereocilia, which are mechanically linked – when they bend the potassium channels are opened, allowing influx of potassium. This causes depolarization of the hair cell, which opens voltage gated calcium channels, the resultant influx of calcium causes exocytosis of neurotransmitter onto the spiral ganglion neurons. 95% of the spiral ganglion neurons innervate inner hair cells, 5% innervate outer hair cells. The action potential will then propogate along the nerve fibres of the CN VIII.

Question 14

What hair cells respond to high and low frequencies?

Answer 14

The hair cells loacted at the base of the basilar membrane respond to high frequencies and the those located at the apical aspect of the membrane respond to low frequencies. This is known as tonotopic distribution of responding receptors

Question 15

What regulates the amplification of sound?

Answer 15

The amplification mediated by the outer hair cells is regulated by the olivocochlear system – the olivocochlear bundle provides efferent feedback to the cells.

Question 16

How is sound encoded? How is volume conveyed?

Answer 16

Sound is encoded as trains of action potentials – louder sounds are encoded with more frequent action potentials in more axons.

Question 17

How does the information in sound reach the brain? How is sound localised?

Answer 17

• The 8th nerve synapses onto the cochlear nucleus
• The neuron then decussates and innervates the medial nucleus of the trapezoid body
• It then innervates the medial and lateral superior olives, which help to localize the sound:
o Lateral superior olive is responsible for interaural volume differences
o Medial superior olive is responsible for interaural timing differences

Question 18

Describe the auditory pathway in the CNS.

Answer 18

The auditory pathway starts with the bipolar neurones in the vestibulocochlear nerve.
The vestibulocochlear nerve passes through the internal acoustic meatus to enter the posterior cranial fossa and divide into vestibular and cochlear branches at the brainstem level to the rostral medulla.

The bipolar neurones synapse with second order neurones in the cochlear nuclei of the medulla, projecting in a tonotopic manner.

These second order neurones vary in their pathways; some will pass and synapse in the ipsilateral or contralateral superior olivary complex whereas others will pass contralaterally and synapse directly in the inferior colliculus.

The superior olivary nuclei allows for localisation of sound in acoustic space, by discriminating differences in time of arrival or intensity of sound to each ear.
Neurones passing from the superior olivary nucleus to the inferior colliculus will pass though the lateral lemniscus.

From the inferior colliculus, axons are transmitted to the medial geniculate nucleus, which then are projected to the auditory cortex (Broadman’s Area is the primary site where the auditory inputs are received, found on temporal lobe).

Question 19

Give an overview of the auditory pathway.

Answer 19

• Cochlea
• Spiral ganglion
• Cochlear nucleus
• Superior olivary complex
• Inferior colliculus
• Medial geniculate nucleus
• Auditory cortex

Question 20

What are the types of hearing impairment?

Answer 20

Conductive: A conductive hearing loss is where there is a problem in the transmission of sound from the outer ear to the inner ear (i.e. in external auditory meatus, tympanic membrane, or the middle ear)

Sensorineural: A sensorineural hearing loss is caused by damage to the cochlea, cochlear nerve, or the central auditory pathway. Damage can occur to hair cells via ototoxic drugs, excessive noise etc., yet other sensorineural hearing losses can occur with spiral ganglion damage (such as acoustic neuroma), Tinnitus, or Auditory Neuropathy.

Question 21

What are causes of conductive hearing loss?

Answer 21

Glue ear (otitis media with effusion)
Ear wax
Foreign body
Otitis media
Perforate ear drum
Otosclerosis
Choleasteatoma
Malformation of the ear

Question 22

Give causes of sensorineural hearing loss.

Answer 22

Loud noises
Congenital
Rubella (viral infection of auditory nerve)
Ototoxins (e.g. ahminoglycosides)
Trauma - damage to temporal bone
Age
Meniere's disease
Meningitis
Acoustic enruoma
Auditory neuropathy (associated with hyperbilirubinaemia)

Question 23

How can hearing loss be investigated?

Answer 23

• Visual inspection with an otoscope
• Audiograms – plot sensitivity against frequency
• Otoacoustic emissions – measures sound generated by outer hair cells which is a measure of amplifier function
• Auditory brainstem response – can assess general hearing function in babies

Question 24

What is the treatment for hearing loss?

Answer 24

• Hearing aids
• Cochlea implants – direct electrical stimulation of spiral ganglion neurons, skipping the potentially damaged cochlea nerve all together and stimulating the cochlea nucleus accordingly
• Hair cell regeneration – not yet achieved
• Cochlear nucleus implants – directly stimulate neurons in the pathway

Question 25

What are possible congenital mutations that cause deafness?

Answer 25

• Changes to hair cells
• Changes in tectorial membrane proteins
• Changes in non-sensory cells (high prevalence, confers a survival advantage in wound healing)
• Mitochondrial proteins (oxidative phosphorylation)