Auditory Mechanisms

What are the major functions of the auditory system?

Detection of sound, Determination of sound location, Communication through speech and language, Recognition of environmental sounds, Formation of an auditory representation of the environment.

What is sound?

A pressure wave produced when a vibrating object causes alternating: Compression of air molecules, Rarefaction of air molecules. These pressure changes propagate through air away from the source.

What is frequency?

Number of sound cycles per second, Measured in Hertz (Hz).

What is the audible frequency range in humans?

20 Hz – 20,000 Hz.

What determines pitch?

Frequency: Higher frequency = higher pitch, Lower frequency = lower pitch.

What determines loudness?

Sound intensity, measured in decibels (dB).

What frequencies can other animals hear?

Dogs/rodents: up to ~40,000 Hz, Elephants: down to ~15 Hz.

What are the functions of the pinna?

Collects sound waves, Helps localise sound sources, Funnels sound into the auditory canal.

What is the function of the auditory canal?

Channels sound waves toward the tympanic membrane.

What are the functions of the tympanic membrane?

Separates outer and middle ear, Vibrates in response to sound, Converts sound waves into mechanical vibrations.

Name the three ossicles.

Malleus, Incus, Stapes.

What are the functions of the ossicles?

Transfer vibrations from tympanic membrane to oval window, Amplify sound transmission.

What is the cochlea?

A spiral-shaped fluid-filled organ where mechanical vibrations are converted into neural signals.

What is the modiolus?

The central bony core of the cochlea containing spiral ganglion neuron cell bodies.

Outline the complete conversion of sound into a neural signal.

Sound enters auditory canal, Tympanic membrane vibrates, Ossicles amplify vibrations, Stapes pushes oval window, Cochlear fluid moves, Basilar membrane vibrates, Stereocilia bend, Tip links stretch, Mechanically gated channels open, K⁺ enters hair cell, Hair cell depolarises, Voltage-gated Ca²⁺ channels open, Glutamate released, Spiral ganglion firing increases, APs travel via CN VIII.

What neurotransmitter is released by hair cells?

Glutamate.

Why does K⁺ entry depolarise hair cells?

Opening of mechanically gated channels allows K⁺ influx from endolymph, depolarising the hair cell.

What causes neurotransmitter release from hair cells?

Opening of voltage-gated Ca²⁺ channels following depolarisation.

Why is middle ear amplification necessary?

To overcome impedance mismatch between air and cochlear fluid. Without amplification: Most sound energy would be reflected, Very little energy would enter the cochlea.

What is impedance matching?

The process of increasing pressure so sound energy transfers efficiently from air to fluid.

What two mechanisms amplify sound in the middle ear?

Ossicular lever system, Area difference mechanism.

How does the ossicular lever system amplify sound?

The malleus, incus and stapes act as a lever, increasing force at the stapes.

How does the area difference mechanism amplify sound?

The tympanic membrane is much larger than the oval window. Pressure = Force ÷ Area. Same force on smaller area = much higher pressure.

By approximately how much is sound pressure amplified?

~20-fold.

What type of hearing loss can result if the middle ear fills with fluid?

Conductive hearing loss.

Name the three cochlear chambers.

Scala vestibuli, Scala media, Scala tympani.

Which cochlear chamber contains the Organ of Corti?

Scala media.

What membrane supports the Organ of Corti?

Basilar membrane.

What membrane lies above the hair cells?

Tectorial membrane.

Describe the structural gradient of the basilar membrane.

Base: Narrow, Stiff; Apex: Wide, Flexible (~100× more flexible).

How are travelling waves generated?

Sound moves tympanic membrane, Ossicles move oval window, Pressure waves form in cochlear fluid, Basilar membrane displaced, Travelling wave propagates from base to apex.

Why are high frequencies detected at the base?

The base is: Narrow, Stiff, High resonant frequency.

Why are low frequencies detected at the apex?

The apex is: Wide, Flexible, Low resonant frequency.

What is tonotopic organisation?

An orderly spatial arrangement where different frequencies are represented at different anatomical locations.

What is place coding?

Frequency is encoded by the location of maximal basilar membrane vibration.

Which frequencies activate the cochlear base?

High frequencies.

Which frequencies activate the cochlear apex?

Low frequencies.

Through which auditory structures is tonotopy preserved?

Cochlea, Auditory nerve, Cochlear nuclei, Superior olive, Inferior colliculus, MGN, Auditory cortex.

What is temporal coding?

Frequency coding based on the timing of action potentials.

What is phase locking?

Auditory nerve fibres firing at specific phases of the sound wave.

For which frequencies is temporal coding most important?

Low frequencies (<1 kHz).

How is sound frequency encoded?

Using both: Place coding, Temporal coding.

How many inner hair cells are present?

~3,500.

What are the functions of inner hair cells?

Primary sensory receptors, Auditory transduction, Sound perception.

How many outer hair cells are present?

~12,000.

What are the functions of outer hair cells?

Cochlear amplifier, Improve sensitivity, Sharpen frequency tuning, Enhance frequency discrimination.

What special property do outer hair cells possess?

They can contract and elongate in response to membrane potential changes.

What happens when stereocilia bend toward the tallest row?

Tip links stretch, Channels open, K⁺ enters, Hair cell depolarises.

What happens when stereocilia bend away from the tallest row?

Hair cell hyperpolarisation.

How does a louder sound affect hair cells?

Larger basilar membrane displacement, Greater depolarisation, More neurotransmitter release.

What is recruitment?

Activation of: More hair cells, More auditory nerve fibres with increasing sound intensity.

How is loudness encoded?

Firing rate, Number of activated neurons.

Outline the complete auditory pathway.

Hair cells → Spiral ganglion neurons → Auditory nerve (CN VIII) → Cochlear nuclei → Superior olivary complex → Inferior colliculus → Medial geniculate nucleus → Primary auditory cortex.

What is the first auditory synapse in the brain?

Cochlear nuclei.

What is the first site receiving input from both ears?

Superior olivary complex.

What is the major auditory processing centre of the midbrain?

Inferior colliculus.

What is the auditory relay nucleus of the thalamus?

Medial geniculate nucleus (MGN).

Why does auditory information ascend bilaterally?

To: Compare inputs from both ears, Localise sounds, Provide redundancy, Improve processing of complex sounds.

Why do unilateral auditory cortex lesions rarely cause complete deafness?

Because each auditory cortex receives information from both ears.

Where does auditory information become bilateral?

At projections from the cochlear nuclei to both superior olivary nuclei.

What is the Duplex Theory of Sound Localisation?

Horizontal sound localisation uses: ITD, IID.

What is ITD?

Interaural Time Difference, Difference in arrival time of sound between ears.

Which frequencies use ITD?

Low-frequency sounds (~20–2000 Hz).

Which nucleus detects ITDs?

Medial Superior Olive (MSO).

What is the maximum natural ITD?

~0.6 ms.

Humans can discriminate sound direction differences as small as what?

~2°.

What timing difference corresponds to a 2° directional change?

~11 microseconds.

What is IID?

Interaural Intensity Difference, Difference in loudness between ears.

Which frequencies use IID?

High-frequency sounds (~2000–20,000 Hz).

Which nucleus detects IIDs?

Lateral Superior Olive (LSO).

Why are IIDs produced?

The head creates an acoustic shadow.

How is vertical localisation achieved?

Using spectral cues generated by the pinna.

What sound locations can pinna cues distinguish?

Above, Below, Front, Behind.

What is external acoustic delay?

The physical delay caused by sound reaching one ear before the other.

What is internal neural delay?

Delay created by different neural conduction pathways.

What does the Jeffress Model propose?

MSO neurons act as coincidence detectors with different internal delays, creating a neural map of sound location.

Where is A1 located?

Temporal lobe, Superior temporal gyrus, Heschl's gyri.

What are the major functions of A1?

Conscious perception of sound, Frequency analysis, Complex auditory processing.

What are isofrequency bands?

Groups of neurons responding best to similar frequencies.

What types of neurons are found in A1?

Frequency-tuned, Intensity-tuned, Binaural neurons.

What does A2 process?

More complex sounds and word-like sounds.

What is the function of Wernicke's area?

Language comprehension.

What is the function of Broca's area?

Speech production and language planning.

How sensitive are stereocilia movements?

Nanometre range (~10⁻⁹ m).

How rapidly can mechanotransduction channels open?

Microseconds (<10⁻⁶ s).

What pressure changes can humans detect?

Less than one billionth of atmospheric pressure.

At 1000 Hz, hearing threshold corresponds to vibrations approximately the size of what?

An atomic diameter (~0.3 × 10⁻⁹ m).

What is an audiogram?

A graph assessing hearing sensitivity across frequencies.

What defines normal hearing?

Threshold ≤ 25 dB HL.

What is conductive hearing loss?

Impaired transmission of sound through the outer or middle ear.

Causes of conductive hearing loss?

Earwax, Infection, Tympanic membrane damage, Ossicle abnormalities.

What is sensorineural hearing loss?

Damage to: Hair cells, Spiral ganglion neurons, Auditory nerve.

Causes of sensorineural hearing loss?

Noise exposure, Ageing, Ototoxic drugs, Auditory nerve damage.

What is presbycusis?

Age-related hearing loss.

What is a common complaint in presbycusis?

Difficulty understanding speech in noisy environments.

What is tinnitus?

Perception of sound without an external auditory stimulus.