Sound Conduction and Transduction

Question 1

What is sound

Answer 1

A vibration that propagates as an audible wave of pressure

Question 2

What is frequency

Answer 2

The speed of vibration or number of wave cycles/sec
Determines the pitch heard
umans hear between 20 -20,000Hz

Question 3

That is amplitude

Answer 3

Intensity/loudness of a sound is determined by amplitude

Question 4

How is sound measured

Answer 4

Decibel scale

Logarithmic scale for measure the volume (x10)

Question 5

Summarise the anatomy of the ear

Answer 5

Ear is divided into 3 sections:
Outer ear - collects sound waves and conducts them to the tympanic membrane
Middle ear - tympanic membrane to oval window, contains the ossicles
Inner ear - contains semicircular canals and cochlea

Question 6

Describe the cochlea

Answer 6

System of coiled tubes that lies in bone

contains 3 chambers: scala vestibuli, scala media, scale tympani

Question 7

Compare the three chambers of the cochlea

Answer 7

scala vestibuli - perilymph
Scala media - endolymph
Scala tympani - perilymph

Vestibuli and media are separated by Reissner’s membrane (vestibular membrane)
media and tympani are separated by the basilar membrane

Question 8

Describe the organ of Corti

Answer 8

On the surface of the basilar membrane
Contains electromechanical sensitive cells, hair cells (inner or outer)
These cells help convert sound impulses to nerve impulses
Vibrates at different positions on its length in response to different frequencies

Question 9

Describe the hair cells

Answer 9

Sensory receptors of the organ of corti
Specialised hair cells that synapse with cochlear nerve endings
Inner (3500) or outer (12000)
Both types of hair cells respond to sound but it’s the inner cells that transmit signals to the brain
Most nerve ending synapse on inner hair cells (afferent 95%)
most efferent connect to outer hair cells

Question 10

Describe the stereocilia

Answer 10

Minute hairs on hair cells which touch the tectorial membrane (in Scala media)
Bending of the stereocilia causes depolarisation or hyper polarisation

Question 11

Explain the process of sound transduction

Answer 11

1. Basilar membrane vibrates to sound as stapes strike against the oval window
2. Upward movement displaces the stereocilia away from the modiolus
3. Potassium channels open, potassium enters from the endolymph and the hair cell depolarises
4. Depolarisation opens calcium channels in the hair cell bodies
5. Glutamate is released from the base, causing depolarisation of the axon of the spiral ganglion cell
6. AP generation
7. Downward movement displaces the stereocilia towards the modiolus
8. Potassium channels close and the cell hyperpolarises

Question 12

What are the functions of the middle ear

Answer 12

Amplification and protection

Question 13

Describe the amplification function of the middle ear

Answer 13

Focusing vibrations from large surface area (tympanic membrane) to smaller surface area (oval window). The change in surface area means the pressure is increased.
Ossicles use leverage to increase the force on the oval window

Question 14

Describe the protection function of the middle ear

Answer 14

Reflex contraction of tensor tympani and stapedius muscles reduces amplitude of vibrations passing through ossicles
Protects against natural sounds but maybe not against man-made sounds

Question 15

Describe the central auditory pathway

Answer 15

1. Sound from cochlea
2. Transmitted by cochlear nerve to cochlear nucleus unilaterally
3. Transmitted to superior olives bilaterally
4. Transmitted to inferior colliculus
5. Transmitted to medial geniculate body
6. Transmitted to auditory cortex

Question 16

What is tonotopic mapping

Answer 16

different parts of auditory cortex for different pitches

Question 17

Describe the superior olivary nuclei

Answer 17

projects back to the cochlea as well as forward to the central pathways

Question 18

Describe the function of the inferior colliculi

Answer 18

Reflexes e.g. startle, head turn

Question 19

Describe the collateral pathway destination, function of lateral inhibition and descending pathways

Answer 19

Collateral pathways to reticular formation and cerebellum
Lateral inhibition in ascending pathway enhances resolution of similar frequencies
Descending pathways provide feedback at all levels

Question 20

What are the causes of conductive deafness

Answer 20

Wax
Congenital bony deformity
Otosclerosis
Otitis media
Tympanic membrane perforation

Question 21

What are the causes of sensorineural deafness

Answer 21

Sensory
Presbyacusis
Exposure to loud noise
Ménière’s disease
Toxicity e.g. some antibiotics
Hereditary disorders

Neural
Acoustic schwannoma/neuroma
Virus infection (HSV)

Some rare central pathway lesions too

Question 22

Describe the active process

Answer 22

1. Amplification
2. Frequency tuning
3. Compressive nonlinearity
4. Spontaneous otoacoustic emission

Question 23

Compare the basilar membrane to the tectorial membrane

Answer 23

The basilar membrane (BM) is sensitive to different frequencies at different points along its length (high proximal, low distal)
The tectorial membrane is gelatinous and does not vibrate with sound.