auditory system

What is sound, physically?

Audible vibrations (variations) in air pressure. Almost anything that can move air molecules can generate a sound.

Define sound frequency and its unit.

The number of sound waves (cycles of compression and rarefaction) per second, measured in hertz (Hz). Frequency is perceived as pitch.

How is high vs. low frequency perceived? Give examples.

High frequency is perceived as high pitch (e.g., a flute). Low frequency is perceived as low pitch (e.g., a bass).

Define sound intensity and its unit.

The difference in pressure between the peak and trough of a sound wave (its amplitude), measured in decibels (dB). Intensity is perceived as loudness: high intensity = louder, low intensity = softer.

What is the range of human hearing, and how does it change with age?

About 20 to 20,000 Hz. The upper (high-frequency) limit decreases as people age.

What is a pure tone, and how do natural sounds differ?

A pure tone has energy at a single frequency. Natural sounds contain energy distributed across a broad range of frequencies.

What is the difference between a sonogram and an oscillogram?

An oscillogram plots sound pressure (amplitude) over time. A sonogram (spectrogram) plots which frequencies are present over time, with intensity shown by shading.

What is special about the frequency structure of speech?

Speech has a lot of periodic structure, meaning its frequency components change over time. The auditory system transforms these stimuli into distinct patterns of neural activity.

What thalamic nucleus relays auditory information, and what is its visual counterpart?

The medial geniculate nucleus (MGN) relays auditory information. The visual counterpart is the lateral geniculate nucleus (LGN).

Name the 3 major components of the ear and the main job of each.

Outer (external) ear: collects and focuses sound waves. Middle ear: amplifies sound waves. Inner ear: hair cells transduce sound waves and send signals to the central nervous system.

What is the pinna and what does it do?

The cartilage and skin structure of the external ear. It gathers and funnels sound waves, and its convolutions are shaped to provide clues about the location of a sound (especially elevation).

What is the external auditory meatus, and where does it end?

The entrance to the ear, also called the auditory canal. It ends at the tympanic membrane (eardrum).

By how much does the external ear boost sound pressure, and for which frequencies? Why does this matter?

It selectively boosts sound pressure 30- to 100-fold for frequencies around 2 to 5 kHz. This matters because speech falls in this frequency range.

Describe the chain of events in the middle ear, from eardrum to inner ear.

Sound waves vibrate the tympanic membrane, which vibrates the three ossicles in order (malleus, incus, stapes). The stapes pushes on the oval window of the cochlea, moving the fluid inside.

Name the three ossicles in order from eardrum to cochlea.

Malleus (hammer), incus (anvil), stapes (stirrup).

What is the main job of the middle ear in terms of air and fluid?

It 'translates' air pressure deflections from the outer ear into movement of fluid in the inner ear.

Why is the middle ear called an impedance matching device?

Air has low impedance (resistance to movement) and the fluid-filled cochlea has high impedance. Without matching, most sound energy would reflect off the fluid. The middle ear boosts pressure so energy is transmitted efficiently into the cochlea.

How do the ossicles overcome the air-to-fluid impedance mismatch?

They transmit energy from a large surface (the tympanic membrane) to a much smaller one (the oval window). The same force on a smaller area produces a much larger pressure. The lever action of the ossicles adds further gain.

Which structure does the stapes push on?

The oval window, a membrane-covered opening on the cochlea. Moving it moves the fluid inside the cochlea.

Where is sound transduced into neural signals?

In the cochlea of the inner ear, by hair cells in the organ of Corti.

What two sensory functions is the inner ear specialized for?

Auditory transduction (cochlea) and vestibular transduction (balance and head movement).

Name the three fluid-filled compartments of the cochlea, top to bottom.

Scala vestibuli (top, near the vestibular nerve and oval window), scala media (middle), scala tympani (bottom).

What is the organ of Corti and where does it sit?

The sensory structure of the cochlea containing the hair cells. It sits on the basilar membrane, with the tectorial membrane above it as its roof.

What drives changes in hair cell membrane potential?

Movement of the basilar membrane.

Where along the basilar membrane are high frequencies detected, and why?

At the base, nearest the stapes, where the membrane is narrow and stiff.

Where along the basilar membrane are low frequencies detected, and why?

At the apex, farthest from the stapes, where the membrane is wide and floppy.

What is a traveling wave in the cochlea?

A wave of displacement that moves along the basilar membrane from base to apex after the stapes moves the oval window. It peaks at the place whose mechanical tuning matches the sound's frequency.

What is tonotopy?

An orderly map of sound frequency (tones) across a structure, where neighboring positions respond to neighboring frequencies. The auditory system is organized around tonotopy, starting at the basilar membrane.

Where does frequency sensitivity in the auditory system originally come from?

From the mechanics of the basilar membrane (its gradient of width and stiffness from base to apex).

What are hair cells?

The auditory transduction cells. They are epithelial cells (not true neurons) with a hairlike process (hair bundle) on top.

What makes up a hair bundle?

30 to 200 stereocilia arranged in rows of increasing height, plus 1 kinocilium. The stereocilia are connected by tip links.

What are tip links?

Fine filaments connecting the tips of shorter stereocilia to taller neighbors. When the bundle is deflected, tension on the tip links pulls mechanically gated ion channels open.

Which hair cells are attached to the tectorial membrane?

Outer hair cells. The tips of their stereocilia are embedded in the tectorial membrane.

What percentage of auditory nerve fibers come from inner hair cells?

About 95%. Inner hair cells are the main sensory cells.

How do traveling waves create a shearing force on hair bundles?

The basilar and tectorial membranes pivot around different points, so when the basilar membrane moves up or down they slide relative to each other. This differential movement creates a shearing force that bends the hair bundles.

What happens when the hair bundle is deflected toward the tallest stereocilium?

Mechanically sensitive, cation-selective channels near the stereocilia tips open, allowing K+ influx, which depolarizes the cell.

What happens when the hair bundle is deflected away from the tallest stereocilium?

Tip link tension decreases, the mechanotransduction channels close, and the cell hyperpolarizes.

After a hair cell depolarizes, how does it signal to the auditory nerve?

Depolarization opens voltage-gated Ca2+ channels near the soma (base). Ca2+ influx triggers neurotransmitter (glutamate) release onto auditory nerve endings.

Why is K+ influx depolarizing in hair cells, when K+ normally leaves neurons?

The hair cell tips sit in endolymph, which has a very high K+ concentration (and the scala media is positively charged). This creates a large electrochemical gradient driving K+ into the cell, so K+ flowing in depolarizes it.

What is endolymph, and where is it?

The fluid in the scala media. It is high in K+ and low in Na+. Hair cell tips (stereocilia) are bathed in it.

What is perilymph, and where is it?

The fluid in the scala vestibuli and scala tympani. It is low in K+ and high in Na+, similar to normal extracellular fluid. Hair cell bodies are bathed in it.

List the key ways the inner ear 'breaks the rules' of typical neurons.

1) K+ both depolarizes and repolarizes hair cells. 2) Hair cell resting potential is about -45 mV. 3) Hair cell tips are in endolymph. 4) Hair cell bodies are in perilymph. 5) Tips and bodies therefore have different Nernst potentials for K+.

How does K+ both depolarize and repolarize a hair cell?

At the tips (in high-K+ endolymph), K+ flows IN and depolarizes. At the base (in low-K+ perilymph), K+ flows OUT through K+ channels and repolarizes. Different Nernst potentials at the two ends make this possible.

What is the resting membrane potential of a hair cell?

About -45 mV.

Why is K+ entry an efficient transduction mechanism for hair cells?

The cell does not need to spend energy (ATP pumps) to clear the K+ that enters, because it can leave passively at the base into perilymph.

How do hair cell receptor potentials respond to low-frequency sounds?

The receptor potential closely follows the air pressure changes, oscillating in step with each cycle of the sound wave.

How do hair cell receptor potentials respond to high-frequency sounds?

The membrane cannot follow each cycle, so the oscillating (AC) component shrinks and the response becomes mostly a sustained depolarization (DC component). Timing of individual cycles is lost at high frequencies.

How do hair cells send their signals to the brain?

They synapse onto spiral ganglion cells, whose axons form the cochlear branch of cranial nerve VIII (the vestibulocochlear nerve).

What is cranial nerve VIII?

The vestibulocochlear nerve. Its cochlear branch carries auditory signals from spiral ganglion cells, and its vestibular branch carries balance signals.

What is the characteristic frequency of a cochlear nerve fiber?

The frequency at which that fiber responds maximally (lowest threshold). It reflects the place on the basilar membrane it innervates.

How is loudness encoded in the cochlear nerve?

Two ways: 1) increased firing rate in each nerve fiber, and 2) an increased number of responsive nerve fibers (louder sounds recruit fibers with neighboring characteristic frequencies).

Do cochlear nerve projections to the brainstem preserve tonotopy?

Yes. Projections to the brainstem cochlear nuclei maintain the tonotopic map of frequencies.

Contrast the function of inner and outer hair cells.

Inner hair cells are sensory: they perform auditory transduction and synapse onto spiral ganglion cells (about 95% of afferents). Outer hair cells are feedback: they control cochlear processing and sensitivity.

How do outer hair cells sharpen the frequency resolving power of the cochlea?

They actively contract and relax (electromotility), changing the stiffness of the tectorial membrane and amplifying basilar membrane motion at specific locations.

What happens to outer hair cells when they are depolarized? Hyperpolarized?

Depolarized: they contract (shorten). Hyperpolarized: they elongate. This electromotility is driven by the motor protein prestin.

What is 'active listening'?

Top-down control of the cochlea by the brain. Efferent fibers to outer hair cells change how they contract and expand, adjusting cochlear sensitivity to sharpen responses to the auditory environment.

What do cochlear efferents to outer hair cells do? (3 points)

1) Enhance responses to sounds. 2) Focus hearing based on attention. 3) Are required for normal hearing.

What is the difference between a passive and an active cochlea?

In a passive cochlea (no outer hair cell function) basilar membrane tuning is broad and weak. In an active cochlea outer hair cells amplify motion, making tuning sharp and sensitivity high.

What is the cochlear amplifier theory?

As the traveling wave deflects the basilar membrane, outer hair cells produce a secondary disruption of the membrane that amplifies the signal sent to the brain.

What are otoacoustic emissions?

Nearly inaudible sounds produced by the ear. They are a byproduct vibration of outer hair cell amplification that travels backward out of the cochlea and can be recorded by a microphone in the ear canal.

How are otoacoustic emissions used clinically?

As a common, non-invasive hearing screen (e.g., in newborns). Presence of emissions indicates a functional cochlear amplifier, healthy outer hair cells, and normal hearing.

What is conductive hearing loss? Give causes.

Sound cannot get into the inner ear. Causes: fluid in the ears from colds or allergies, ear infection, developmental defects in ear formation, a hole in the eardrum, ear wax, tumors, physical blockage.

What is sensorineural hearing loss? Give causes.

Damage to the inner ear (hair cells or nerve). Causes: illness, ototoxic drugs, hereditary hearing loss, head trauma, aging, exposure to loud noises or explosions, developmental defects.

What is mixed hearing loss?

A combination of conductive and sensorineural hearing loss.

How common is age-related hearing loss in the US?

More than half of people in the US older than 75 have some age-related hearing loss.

What is a cochlear implant?

A small electronic device that provides a sense of sound to a person who is profoundly deaf or severely hard of hearing, by directly stimulating the auditory nerve. First FDA approved in the mid-1980s.

List the main components of a cochlear implant.

1) Microphone: picks up environmental sound. 2) Speech processor: selects and arranges sounds. 3) Transmitter and receiver/stimulator: receive signals from the processor and convert them to electric impulses. 4) Electrode array: stimulates the auditory nerve.

Why does a cochlear implant electrode array work with tonotopy?

Electrodes placed at different positions along the cochlea stimulate nerve fibers with different characteristic frequencies, so the processor can send different frequency bands to different electrodes.

What is the major target of auditory nerve afferents in the brainstem?

The cochlear nucleus (cochlear nuclei). It is tonotopically organized.

List the 5 main stations of the central auditory pathway in order.

1) Cochlear nuclei (brainstem). 2) Superior olivary complex: MSO, LSO, MNTB (brainstem). 3) Inferior colliculus (midbrain). 4) Medial geniculate nucleus (thalamus). 5) Auditory cortex.

What is the function of the cochlear nuclei?

Receive auditory nerve input, maintain tonotopy, and start feature processing.

What is the function of the MSO, LSO, and MNTB together?

Sound localization (comparing input from the two ears).

What is the function of the inferior colliculus?

It combines superior olive (LSO and MSO) inputs to form a map of auditory space, with directional tuning and tone information.

What is the function of the MGN in hearing?

Thalamic relay of auditory information to the auditory cortex.

What is the function of the auditory cortex?

To identify and process complex sounds.

Why is sound localization important?

It allows animals to orient toward and identify the source of sounds, such as prey, predators, or a speaker, which is critical for survival and communication.

What are the two main cues for localizing sounds in the horizontal plane?

1) Interaural intensity (level) difference, processed by the LSO. 2) Interaural time difference (delay), processed by the MSO.

What is the interaural intensity difference, and why does it occur?

The difference in sound intensity between the two ears. The head blocks sound and creates a 'sound shadow,' so the ear farther from the source receives a softer sound.

Which brainstem nucleus detects interaural intensity differences?

The lateral superior olive (LSO).

Describe the excitatory and inhibitory inputs to the LSO.

Ipsilateral (same side) cochlear nucleus directly excites the LSO. Contralateral cochlear nucleus excites the contralateral MNTB, which inhibits the LSO. So the LSO receives excitation from the same-side ear and inhibition from the opposite ear.

What is the MNTB and what is its role in sound localization?

The medial nucleus of the trapezoid body. It receives excitation from the contralateral cochlear nucleus and sends inhibitory output to the LSO, converting the opposite ear's excitation into inhibition.

If a sound is louder in the left ear, what happens in the left vs. right LSO?

The left LSO gets strong ipsilateral excitation and weak inhibition, so it fires strongly. The right LSO gets weak excitation and strong inhibition (via its MNTB from the left ear), so it is suppressed. The LSO thus signals which ear the sound is stronger in.

What is the limitation of the interaural intensity difference cue?

The head shadow is only significant for high-frequency sounds (above about 3,000 Hz). Low-frequency waves are long enough to bend around the head, so a different mechanism is needed for them.

What is the interaural time delay (difference)?

The difference in arrival time of a sound at the two ears. Sound reaches the nearer ear slightly earlier. It is used to localize low-frequency sounds (below about 3,000 Hz).

Which brainstem nucleus detects interaural time differences?

The medial superior olive (MSO).

How does the MSO detect interaural time differences? (delay line model)

Axons from each ear's cochlear nucleus act as delay lines of varying length to MSO neurons. Each MSO neuron is a coincidence detector that fires maximally when excitation from both ears arrives at the same time. The neuron where the delays cancel out the time difference encodes that location.

When do MSO neurons fire maximally?

When they are excited by both ears simultaneously (coincident input).

Summarize which nucleus localizes which frequencies and using which cue.

MSO: timing difference, for frequencies below 3 kHz. LSO: level/intensity difference, for frequencies above 3 kHz.

Is tonotopy preserved in projections to the superior olive?

Yes. Projections from the cochlear nuclei to the LSO and MSO preserve the high-to-low frequency map.

What two inputs are combined in the inferior colliculus?

Projections from the superior olive: the LSO (intensity differences) and the MSO (timing differences).

What are the two major outputs of the inferior colliculus and their functions?

1) The superior colliculus (brainstem): controls orienting toward a sound. 2) The medial geniculate nucleus (thalamus): for perceptual awareness and further sound processing.

Where is the primary auditory cortex (A1)?

In the temporal lobe (superior temporal gyrus, in Heschl's gyrus).

What happens after damage to A1?

Loss of conscious awareness of sound, although reflexive reactions to sound remain.

How is A1 organized?

Tonotopically, and in columns like visual cortex. Within each frequency band there are columns preferring sound from the left, columns preferring sound from the right, and columns preferring sound from the midline.

How are language areas organized in the cortex?

They are distributed: low-level tonotopic signals in primary auditory cortex, speech processing in temporal areas (Wernicke's), speech production in frontal areas (Broca's), and motor control in motor cortex.

What is Broca's aphasia?

Patients can understand speech but cannot produce it (non-fluent speech). Caused by damage to Broca's area in the left frontal lobe.

What was Broca's famous patient case?

A severely affected patient could only say one word, 'tan.' After death, a lesion caused by syphilis was found destroying a small region in the left frontal lobe.

Who was Pierre Paul Broca?

A physician (1824-1880) who studied patients who could understand but not produce speech, identifying Broca's area in the left frontal lobe.

What is Wernicke's aphasia?

Loss of the ability to recognize and comprehend words. Linked to damage in Wernicke's area in the left superior temporal lobe.

Who was Carl Wernicke?

A neurologist (1848-1905) who discovered an area in the left superior temporal lobe correlated with loss of the ability to recognize words.