Explain comparative audiometric curves.
Sounds important to different animals can vary. Impact how these animals have evolved and the ranges of sound that they can hear. Sensitivity can mean a sound can be at its quietest and still be heard by some species at a certain frequency. Depending how the species has evolved will depict where the sensitivity is.
What are the main roles of the inner, middle and outer ear?
Inner ear is sensory. and middle and outer ear are conductive. The middle ear does impedance matching.
Describe the importance of impedance matching.
- Ossicles have lever action
- Eardrum to oval window surface area = 20:1 for this
- Approximately x 26 pressure amplification
- Without impedance matching, only 0.1% energy transfer
- Unrelated to the cochlear amplifier
Describe the process of impedance matching.
- Tympanic membrane is vibrating and causes malleus to move and lever action of the incus and stapes.
- Stapes pressed up against the oval window, the window into the inner ear, behind this is the fluid of the canal in the inner ear.
- Energy in air to energy in fluid, which requires more energy.
Describe the structure of the mammalian cochlea spiral.
- 22mm in length spiral in rat
- Varies between species
- Partially developed in some mammals at birth in altricial species
- Compartments within the tube
What is the name of the mammalian auditory epithelium?
Organ of Corti
Describe the structure of the organ of Corti.
- Located on flexible basilar membrane
- Inner and outer hairs are sensory
- Spiral ganglion nerve cells
- Associated supporting and non-sensory cells
- Hair cells held rigidly while basilar membrane can move
- Endolymph has high potassium and high potential compared to perilymph with 0mV and low potassium ions. Perilymph bathes basolateral membranes.
- Tectorial membrane is a jelly like membrane on top of the hair cells.
What are the 3 compartments in the cochlea coil?
Perilymph and endolymph. Scala media bear the organ of Corti, which has the sensory hair cells within it.
Distinguish stereocilia in inner and outer hair cells.
Inner hair cell: mainly afferent innervation is 10-20 terminals per cell. Inward calcium channels and outward potassium channels. Does sound encoding.
Outer hair cell: little afferent innervation. Prestin motor protein in lateral membranes. Does amplification and tuning
How do sound induced movements cause hair bundle stimulation?
At organ of Corti, sound energy set up wave in basilar membrane that moves along the length of the cochlea. Takes hair cells with it and moves hair bundles.
What does hair bundle stimulation cause?
Opens ion channels:
- Tip links – gated springs
- Mechanically gated transducer channel
- Movements are tiny – perceptible sound at 0.3nm and saturation at 20nm
How does potassium activate afferent neurones?
Potassium moves into cell down concentration gradient via transducer channel. Cell depolarises and voltage gated calcium channels open. Calcium enters cell and neurotransmitter released, which activates afferent neurones.
Describe tonotopy for frequencies higher than 4kHz.
Basilar membrane has regional variation in fibrous structure, region of maximal displacement dependent on frequency of stimulus. Basilar separates out these frequencies. Tonotopic map produced from this by the basilar membrane. Neurones near the base have specific frequencies and bands of auditory cortex respond to specific frequencies.
What are the 3 populations of afferent nerves that encode amplitude?
High spontaneous rate fibres – easily excited at low sound levels, soon saturated.
Medium spontaneous rate fibres – excited at medium sound levels, will saturate.
Low spontaneous rate fibres – excited when sound levels are high, can detect changes in sound at very high sound levels.
How do outer hair cells act as a cochlear amplifier?
- Depolarisation activates the motor protein, prestin
- Rigidly held cells contract, amplifying basilar membrane movement
- OHC loss leads to 60 decibels hearing loss and poor frequency discrimination
- Amplify motion of basilar membrane and so stimulating the IHCs
List the primary central pathway from cochlea to brain.
- Spiral ganglion
- Ventral cochlear nucleus
- Superior olive
- Inferior colliculus
- MGN
- Auditory cortex
What is required for sound localisation?
The brain compares the timing and intensity of input to the ears.
How are horizontal, low frequency and high frequency sounds localised?
Horizontal – requires 2 ears and binaural processing.
Low frequency – comparison of the time at which the same phase of a sound arrives at each ear, interaural delay of the peak of the wave.
High frequency – interaural intensity difference
What is the role of the pinna?
Pinnae motile in most animals. But pinna produce direct and reflected sounds. But properties of the pinna help in sound localisation.
What are the conductive causes of deafness?
Outer or middle ear
Obstruction – wax, hair or dirt
Tumour
Infection/inflammation
Tympanic membrane damage
What are the sensorineural causes of deafness?
- Inner ear: hair cells and/or neurones
- Congenital - born or progressive. Often associated with lack of pigmentation due to a problem with maintenance of endolymph
- Noise damage
- Ototoxic drugs, such as aminoglycoside antibiotics
Name 3 ways of detecting deafness.
Owner’s observations
Preyer’s reflex
Brainstem auditory evoked response
What is Preyer’s reflex?
- Response to sound with movement
- Startle reflex in mice
- Elevation and rotation of ears in dogs
What is brainstem auditory evoked response?
- Electrodes placed on animals head used to detect neural activity in response to auditory stimulus
- Most objective measurement
- Referral – screening for breeding programmes
