The audio-vestibular system Flashcards
How were the vestibular organs formed?
-Evolved first from Statocyst (does not exist in humans but did in cumcumbers): a ball of cells, inside was lined with small hairs and had a calcified ball inside of it
- When the ball would move it would deflect the hair
- Deflection would correspond to gravity/ movement (plants knew which way to grow)
- The statocyst then evolved into a utriculus, then a sacculus then into our 2 vestibular organs:
1. Semicircular canals
2. Cochlea
(still use the same principle, something in the ear moves, deflects hairs, corresponds to gravity/ movement)
What is the difference in the role of the vestibular organs vs the hearing organs?
Vestibular organ: capture low frequency motion (movements)
Hearing organ: capture high frequency motion (sound)
What does the frequency of sound waves represent?
“pitch”
Frequency/pitch (Hz): Cycles per second, perceived tone
What does the Amplitude of sound waves represent?
“Loudness”
Amplitude/loudness (dB): Sound pressure, subjective attribute correlated with physical strength.
What is the normal human range of hearing?
Frequency: 20–20,000Hz
Loudness: 0 dB to 120 dB sound pressure level (SPL)
How is the ear split?
Made up of 3 parts:
1. Outer ear
2. Middle ear
3. Inner ear
What is the function of the outer ear?
- To capture sound and to focus it to the tympanic membrane (this is very fragile)
- Modest amplification (10DB) of upper range of speech frequencies by resonance in the canal.
- To protect the ear from external threats
What is the function of the middle ear?
- Mechanical amplification (can provide an additional 20-30dB)
- Does have some structures that reduce sound but overall, the same is amplified
How is sound amplified in the middle ear?
- Vibrations enter the middle ear from the air after travelling through the outer ear
- The vibrations encounter the tympanic membrane: sound waves converted from air into mechanical movement- the tympanic membrane moves at the exact same frequency as the vibrations (same frequency that we hear)
- The tympanic membrane is attached to the ossicles (3 bones: Malleus, Incus and Stapes) that are very sensitive to the movement of the tympanic membrane
- Therefore, they amplify the vibrations to a large extent (increasing our ability to detect low levels of sound)
- The stapes bone is finally attached to the oval window (end of the middle ear) and it is much smaller than the tympanic membrane (go from larger entry-smaller= increased pressure, again amplifying the sound)
- The tensor tympani and stapedius muscles are there to reduce sound if too loud
How do the Tensor tympani and Stapedius muscles carry out their role?
- Innervated by different muscles but have the same role (to reduce sound if too loud)
- Tensor tympani muscle is attached to the tympanic membrane: if sound is too loud it contracts= stiffens the tympanic membrane, reducing the sound
- Stapedius is attached to the Stapes (last bone): if sound is loo loud it contracts= stiffens the Stapes, reducing the sound
What is the function of the inner ear?
- The hearing part of the inner ear is the cochlea
- Here, we want to change the mechanical movement into movement in fluid
- Contains medium/ fluid that moves at the same frequency (and captures the same amplitude (loudness) as the Stapes (last bone of middle ear)
- Then change the movement in fluid into neural signals/ impulses (known as transduction)
Describe the structure of the cochlea
The cochlea contains 3 compartments:
1. The stapes is attached to the oval window which is attached to the Scala vestibuli (1st compartment)
2. This bone structure is connected to the Scala tympani (2nd compartment) which connects to the round window
- both bones contain perilymph (high in sodium)
3. These bone structure form the spiral- in the centre, between the 2 bones is the Scala media (3rd compartment) which is a membranous structure, contains endolymph (high in potassium)- this is where the organ of Corti (hearing organ) is located
Where is the organ of Corti located?
The basilar membrane inside the Scala media
What is the role of the basilar membrane? how does it carry it out?
“To capture different levels of frequency”
- Basilar membrane is arranged tonotopically (arranged based on frequency)
- It starts at the base very stiff and narrow and becomes longer and more loose towards the apex
- High frequencies tend to produce vibrations in the 1st part of the membrane/ base
- Low frequencies end to produce vibrations in the last part/ apex
- Membrane is covered with hairs that are attached to nerves- send signals that correspond to the different frequencies
What is the structure of the hearing organ?
- The organ of Corti/ hearing organ contains 2 types of hair cells (not really hair):
1. Inner hair cells
2. Outer hair cells - Hair cells spikes =”stereocilia”
- Stereocilia of inner hair cells are arranged in single lines
- Stereocilia of outer hair cells are arranged in rows of 3
- Above the hair cells is the tectorial membrane: rubs against the hairs, causes hair deflection, which will depolarise the cell
How does the tectorial membrane affect hair cells?
“deflects the hair cells”
- If the tectorial membrane moves up- the hairs move forward
- If the tectorial membrane moves down- hairs move back
What are the roles of the 2 different hair cells?
Inner hair cells:
- Main ones that carry 95% of the afferent information of the auditory nerve.
- Function= The transduction of the sound into nerve impulses
- Receive the signals from the vibrations of the different sound frequencies
- Send the signals to the spiral ganglion cells
- Signals then sent through the auditory nerves to the brain
Outer hair cells:
- carry 95% of efferents of the auditory nerve (feedback from the brain)
- Function= modulation of the sensitivity of the response- if the sound send to the brain was too much or not it modulates it (protective)
