Auditory System Flashcards
What is sound?
Definition: A vibration that propagates as an audible wave of pressure
- Essentially just know that sound is due to the vibration of particles
What is frequency? What does frequency determine?
The speed of vibrations (i.e. of the particles) or number of wave cycles per second
- NOTE: 1 Hz = 1 vibration/sec
Frequency determines the pitch that we hear
- Higher frequency = higher pitch
Define pitch.
The perception of frequency
Explain the production of sound in terms of compressions and rarefactions. (for understanding only)
EXAMPLE: Stereo speaker
- To produce sound, a thin surfaced cone, called a diaphragm, vibrates back and forth
- The forward motion of a diaphragm pushes air molecules horizontally to the right (high pressure) → compression
- The backward retraction creates a low-pressure area allowing the air particles to move back to the left → rarefaction
- These alternating compressions and rarefactions produce a wave
- One compression and one rarefaction is called a wavelength
- Different sounds have different wavelengths
NOTE: The concept is the same when you speak because essentially your vocal cords are moving back and forth
What is amplitude?
The loudness (or intensity) of sound
- Taller wave = greater amplitude
What is timbre?
Distinguishes two sounds at the same frequency and intensity
- Essentially the characteristic of a sound
- It is why a clarinet and violin sound different even if they are played at the same freqeuncy and intensity
What is the decibel scale?
A logarithmic scale for measuring the loudness of sound
It allows us to compact a large range into a more manageable scale
- Essentially you are comparing the logarithms x 10 of the intensity (I) of sound with the faintest perceivable intensity of sound (I0**)
What are the 3 sections that the ear is divided into? What is the function of each section?
OUTER EAR:
- Collects sound waves and conducts them towards the tympanic membrane (eardrum)
MIDDLE EAR:
- Tympanic membrane to oval window
- Oval window is a membrane connected to the stapes
- Oval window causes vibrations within the cochlea which stimulates nerves that sends signals to the brain for processing.
- Contains the ossicles
- Ossicles are tiny bones which transmit mechanical vibrations of the tympanic membrane to the oval window
- They amplify the sound (pressure - amplitude)
- There are 3 of them: malleus, incus and stapes
- Ossicles are tiny bones which transmit mechanical vibrations of the tympanic membrane to the oval window
INNER EAR:
- Contains semicircular canals and the cochlea
What is the cochlea? Describe its structure.
It is a system of coiled tubes that lies in bone (i.e. surrounded by bone)
It has 3 chambers:
- Scala vestibuli
- Contains the fluid perilymph
- Scala media
- Contains the fluid endolymph
- Scala tympani
- Contains the fluid 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
Explain how the cochlea is involved in hearing.
- On the surface of the basilar membrane is the organ of Corti
- Projects upwards into the scala media and endolymph
- It contains electromechanically sensitive cells, hair cells (inner or outer)
- These cells help convert sound impulses (i.e. vibrations) to nerve impulses
What are hair cells? Describe the difference between inner and outer hair cells.
Hair cells are the sensory receptors of the inner ear
Hair cells can be:
- Inner hair cells (IHCs) - approx. 3500 per human cochlea
- Outer hair cells (OHCs) - approx. 110000 per human cochlea
95% of afferent projections project from IHCs
- IHCs provide sensory transduction
- Sensory axons that carry signals from the cochlea towards the brain via the cochlear nerve
- 5% of afferent projection from OHCs
Most of the efferent projections (from the brain to the cochlea) connect to OHCs.
What key structure do hair cells have?
Stereocilia
- These are the modified microvilli on the surface of hair cells
- These form a hair bundle
- They are arranged in rows which go from tall to short
- They touch the tectorial membrane in the scala media
- Bending of the stereocillia causes either depolarization or hyperpolarization
Describe the process of sound transduction from the middle to inner ear.
- When the stapes strikes against the oval window it causes the base of the basilar membrane to vibrate
- Each sound wave causes 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) deflects the hair bundles of the hair cells
- The stereocilia bend towards the tallest stereocilium
- This opens ion channels causing depolarisation or hyperpolarisation, changing the internal voltage of the cell
- This ultimately produces an electric signal that travels towards the brain (i.e. generates an AP in synapsing nerve)
- This is known as mechanotransduction
COME BACK TO THIS - Explain in detail how mechanotransduction works
What are the functions of the middle ear?
Amplification
Protection
How does the middle ear amplify sound?
- They focus vibrations from large surface area (tympanic membrane) to smaller surface area (oval window)
- The change in surface area means the pressure is increased
- Increased pressure = increased amplitude/intensity
- Ossicles use leverage to increase the force on the oval window
- Essentially the ossicles act as levers
- Moment = force x distance
- When balanced: turning moment is equal on either side of the pivot
-
Levers work using this formular but turning moments are unbalanced
- To lift a large load (force) on the right, you keep it close to the pivot
- On the left you apply a small force but at a distance large enough from the pivot to overcome the turning moment on the right and result in lifting (rotation)
-
The malleus is longer than the incus
- This creates a lever action with the pivot in between the two bones
- Low pressure or force at the end of the malleus which moves the two bones together (input) is converted into high pressure at the end of the incus (output)
How does the middle ear protect the ear?
Reflex contraction of tensor tympani and stapedius muscles reduces amplitude of vibrations passing through ossicles
- The stapedius muscle pulls on the stapes bone when it contracts
- The tensor tymapni (muscle) pulls on the malleus when it contract
- This muscle contraction increases the rigidity of the ossicular system and reduces ossicular conduction to the inner ear
- This contraction is triggered by loud sounds so it is essentially a protective reflex to protect the inner ear from loud sounds
- The muscles are also involved in reducing the ossicular conduction of low frequency sounds (i.e. background noise)
Describe the central auditory pathway.
- Sound from cochlea
- Transmitted by cochlear nerve to cochlear nucleus unilaterally
- Transmitted to superior olives bilaterally
- Transmitted to inferior colliculus
- Transmitted to medial geniculate body
- Transmitted to auditory cortex
Superior olivary nuclei project back to the cochlea as well as forward to the central pathways
Inferior colliculi – reflexes eg. startle, head turn
Collateral pathways to reticular formation and cerebellum
Lateral inhibition in ascending pathway enhances resolution of similar frequencies
Descending pathways provide feedback at all levels
Where do the superior olivary nuclei project to?
- Forward to the central pathways
- Back to the cochlea
Inferior colliculi – reflexes eg. startle, head turn
Collateral pathways to reticular formation and cerebellum
Lateral inhibition in ascending pathway enhances resolution of similar frequencies
Descending pathways provide feedback at all levels
What is tonotopic mapping?
Different parts of the primary auditory cortex (A1) is mapped to different pitches (frequencies)
However, you only get tonotopic arrangement in other components:
-
Basilar membrane
- Different frequencies detected along its length
- Vibrates at different positions along its length in response to different frequencies
-
Cochlear (spiral) ganglion
- Made up of sensory nerve fibres of the cochlear nerve
- Each ganglion cell responds best to stimulations at a particular frequency
- Ganglion cells in a particular area of the spiral ganglion respond best to the resonant frequency of the basilar membrane in that same area
-
Ventral cochlear nucleus:
- Low frequencies ventrally, high frequencies dorsally
- NOTE: ventral and dorsal cochlear nuclei are where the afferent fibres of the cochlear nerve synapse
-
Central nucleus
- One of the 3 subdivisions of the inferior colliculus
NOTE: Primary auditory cortex in the temporal lobe
What are the two types of deafness? What do they each refer to?
Sensorineural
- Refers to a problem with the neural conduction of sound
- So generally problem with inner ear or cochlear nerve
Conductive
- Refers to a problem with the conduction of sound in the ear
- Sound waves are not being trasmitted to the inner ear to allow neural conduction
List some causes of sensorineural deafness.
Sensory
Presbycusis
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
