Sound conduction and transduction

Question 1

What is the hearing range in humans?

Answer 1

20Hz to 20kHz

Question 2

How does the ear detect the sound?

Answer 2

Vibration of the tympanic membrane, ossicles, into the cochlear where the hair cells are. The hair cells are sensory receptors in the internal ear arranged into rows of 4.

Question 3

What are the ossicles?

Answer 3

The 3 ossicles transmit the vibration of the tympanic membrane onto the cochlea, which is a snail-shaped organ filled with liquid. Their role is to match the impedance and reduce the loss in energy as the vibration goes form the air to the cochlea. (acts as a middle ground to match the impedance between the air and fluid filled cochlea)

Malleus and incus position can be adjusted by the tensor tympanic muscle and stapedius muscles to control the tension of the tympanic membrane. This protects the cochlea from loud sounds

Question 4

Define impedance?

Answer 4

Measure of reluctance of a system in receiving energy from a source. The more different the impendances of the two mediums are the less efficient the transmission (the more reflection) See diagram

The frequency at which the impedance of the system is minimal is called the resonant frequency.

Question 5

Define conductive hearing loss

Answer 5

When the ear is not capable of transmitting the vibration of sound waves onto the cochlea.

Question 6

Give some examples of conductive hearing loss?

Answer 6

• Infections - otitis media
• Fluid accumulation in the inner ear in children
• Perforated tympanic membrane
• Otosclerosis - abnormal bone growth that can obstruct the canal
• Barotrauma: Changing altitude

Question 7

What is the cochlea?

Answer 7

Is a liquid filled snail-shaped organ. The motion of the stapes generates a difference in pressure between the two liquid-filled chambers of the cochlea which in turns cause the vibration of the basilar membrane

It contains three chambers

• Scala vestibuli (contains a fluid called PERILYMPH)
• Scala media (has ENDOLYMPH)
• Scala tympani (PERILYMPH)
• Scala vestibuli and Scala media are separated by Reissner’s membrane (vestibular membrane)
• Scala media and Scala tympani are separated by the basilar membrane
• On the surface of the basilar membrane is the organ of Corti
• It contains electromechanically sensitive cells, hair cells (inner or outer)
• These cells help convert sound impulses to nerve impulses

Question 8

Define sound

Answer 8

Sound is a vibration that propagates as an audible wave of pressure

Question 9

Define intensity/loudness

Answer 9

The amount of energy delivered per second. Essentially determined by the amplitude of the sound wave

Question 10

Define frequency

Answer 10

The speed of vibration or the number of cycles per second (hertz).

The frequency also determines the pitch that we hear

Question 11

What is the decibel?

Answer 11

The decibel scale is a logarithmic scale for measuring the loudness of sound

Question 12

Describe hair cells

Answer 12

• Hair cells lie on the basilar membrane in the organ of Corti
• They are sensory receptors of the organ of Corti which synapse with cochlear nerve endings
• Hair cells can be inner hair cells (there are ~3500) or outer hair cells (~12,000)
• Most nerve ending synapse on inner hair cells
• Hair cells also have minute hairs called stereocilia which touch a membrane called the tectorial membrane (which is in the scala media).
• Bending of the stereocillia causes either depolarization or hyperpolarization. This process is called mechano-transduction
• Sterocilia are connected by filamentous linkages called tip links. They work as small springs stretched by the sterocilia sliding. Depolarisation is as a result of the opening of mechanosensitive ion channels activated by the stretching of the tip links.

Question 13

Describe sound transduction

Answer 13

• When the stapes strikes against the oval window it causes the base of the basilar membrane to vibrate
• Each sound wave will cause the basilar membrane to vibrate differently depending on the frequency of the wave.
• On the basilar membrane is the organ of Corti with the hair cells and stereocilia
• Movement of the basilar membrane upwards or downwards will ultimately displace the stereocilia
• This will open ion channels and cause depolarization or hyperpolarization of the nerve synapsing on the hair cells

Question 14

Describe the basilar membrane

Answer 14

Elastic structure of heterogenous mechanical properties that vibrates at different positions along its length in response to different frequencies.

Essentially it can break down complex sounds by distributing the energy of each component frequency along its length. Therefore receptors lie along the whole length of the basilar membrane in order to detect all frequencies.

Question 15

What are the active processes of hair cells?

Answer 15

Amplification

Frequency tuning

Compressive nonlinearity

Spontaneous acoustics emission

See graphs

A source of these active processes may be from the outer hair cells. Their cell bodies shorten and elongate due to the reorientation of protein prestin when their internal voltage is changed. This is called electromobility.

Question 16

What is sensorineural hearing loss?

Answer 16

Hearing loss caused by problems in the sensory apparatus of the inner ear of in the vestibulocochlear nerve.

Question 17

List some causes of sensorineural hearing loss?

Answer 17

Sensory

• Presbyacusis (aging)
• Exposure to loud noise
• Ménière’s disease (excess fluid in the cochlea)
• Toxicity e.g. some antibiotics
• Hereditary disorders (many affect the organ of Corti)

Neural

• Acoustic schwannoma/neuroma
• Virus infection (HSV)

Some rare central pathway lesions too

Question 18

What is the function of the middle ear?

Answer 18

AMPLIFICATION

• 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

PROTECTION

• 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 19

Describe the central auditory pathways

Answer 19

1) Sound from cochlea - IHCs form synapses with sensory neurones in the cochlear ganglion (spiral ganglion). There is constant NT being released but rate changes in response to the presynaptic voltage (as a result of mechanosensitive ion channel gating). Each ganglion cell responds best to specific frequency stimulations. Tonotopic map. There are many neurones attached to a single HC. This is because one can fail. Each axon is responsive to a single frequency.
2) Transmitted by cochlear nerve to cochlear nucleus unilaterally - Cochlear nucleus lies in the brain stem where different kinds of neurones are arranges tonotopically. High frequency are distrusted dorsally and low are ventraly.

3) Transmitted to superior olivary complex bilaterally in the pons- The SOC compares the bilateral activity of the cochlear nucleus.
- Medial superior olive: Interaural time difference is computed. Time difference between the sound reaching the nearest ear and then the other one. The brain can use this information to localise the sound in the horizontal plane. See video
- Lateral superior olive: Detects difference in intensity between the two ears - intraaural difference. The brain uses this to localise sounds in the horizontal plane.
SOC neurones sends feedback to the hair cells. This reduces the sensitivity of the cochlea
1) Neurones from the medial superior olive - IHCs bilaterally.
2) Neurones from the lateral superior olive - OHCs ipsilaterally

4) Transmitted to inferior colliculus - Here responses from different frequencies merge. It filters out background noise - precedence effect.
5) Transmitted to medial geniculate body
6) Transmitted to auditory cortex - ACN respond to complex sounds. The primary auditory cortex A1 is located in the superior bank of the temporal lobe. It is tonotopically mapped. Loudness, rate and frequency modulation also seem to be mapped in A1. Superior auditory cortex - we can identify what and where sounds are coming from.