Introduction to Sound and Hearing

The Nature of Sound

• Sound is an audible variation in air pressure.
• A cycle is the distance between successive compressed patches of air.
• Sound frequency is the number of cycles per second, measured in hertz (Hz).
• Amplitude represents volume.

Audible Sound

• The audible range is 20 Hz to 20,000 Hz.
• Amplitude determines volume.
• Frequency determines pitch.

Auditory Pathway

• Sound waves enter the ear.
• The tympanic membrane vibrates.
• Ossicles (three tiny bones) transmit vibrations.
• The oval window transmits vibrations into the cochlea.
• Cochlear fluid transmits vibrations.
• Sensory neurons respond to vibrations.
• Vibrations turn into sensory responses in the cochlea.

The Middle Ear

• Sound force amplification by the ossicles, important for sound amplification.
• The attenuation reflex protects the inner ear from loud sounds.
• Onset of loud sound causes tensor tympani and stapedius muscles to contract.
• This function helps adapt to loud sounds and protects the inner ear, allowing speech understanding.

The Inner Ear/The Cochlea

• Perilymph: fluid in the scala vestibuli and scala tympani.
• Endolymph: fluid in the scala media.
• Endocochlear potential: electrical potential in endolymph, higher than perilymph.
• Motion at the oval window pushes perilymph, and this makes the round window membrane bulge.
• Potassium causes depolarization for hearing.

The Basilar Membrane

• Traveling Waves: the response of the basilar membrane to sound, through creating waves in the membrane, with differing frequencies.
• The base is narrow and stiff, while the apex is wide and floppy. Higher frequency sounds have peaks at the base and lower frequencies toward the apex. The movement of (traveling wave) along the membrane stimulates hair cells for hearing.
• Fluid causes membrane vibration

The Organ of Corti

• Outer hair cells and inner hair cells transmit info via sensory nerve fibers.
• Stereocilia are stimulated and cause movement in the fluid to cause signals to the brain.

Bending of Stereocilia

• When the tectorial membrane moves, the hair cells move with it.

Transduction by Hair Cells

• Sheering force opens K+ channels.
• Cells depolarize.
• Voltage-gated Ca2+ channels open.
• Depolarization is further caused by Ca2+.
• Exocytosis releases neurotransmitters.

Hair Cells

• Innervation of hair cells: one spiral ganglion fiber synapses with one inner hair cell, and many outer hair cells.
• Amplification by outer hair cells (cochlear amplifier): amplifies sound.
• Function: sound transduction, changing lengths of outer hair cells.
• Prestin: protein required for outer hair cell movements.

Auditory Pathways

• Sound travels from cochlea -> Cochlear Nucleus-> Superior Olivary Complex (SOC)->Inferior Colliculus(IC)->Medial Geniculate Nucleus (MGN) -> Auditory Cortex
• Sound from both ears combines in the superior olive.

Properties of Auditory Neurons

• Characteristics of frequency (most responsive frequency).
• Binaural neurons are present in the superior olive.
• Receives info from both ears, allowing comparison.
• High-intensity sounds cause more frequent APs.

Encoding Sound Intensity

• Membrane potential correlates with depolarization, or hyperpolarization of hair cells.
• Firing rates of neurons relate to perceived loudness. More neurons firing means the sound is perceived as louder.
• Tonotopic maps from the basilar membrane & cochlear nucleus show how different sounds are encoded.

Tonotopy

• Tonotopic maps are on the basilar membrane, spiral ganglion, and cochlear nucleus.
• Frequency increases from base to apex of basilar membrane.
• Tonotopy is preserved in the auditory nerve and cochlear nucleus.
• Bands of cells in the cochlear nucleus respond to similar frequencies (higher freq toward the anterior, lower towards the posterior).

Phase Locking

• Low frequencies: phase locking is on every cycle or some fraction of the cycles.
• High frequencies: the phase locking is not fixed.
• Neurons fire at specific phases, which combine to complete the sound picture.

Mechanisms of Sound Localization

• Interaural time delay determines the horizontal sound plane localization.
• Interaural intensity difference is a measure of head's sound shadow.
• The two methods are used to detect the horizontal sound plane.
• Pinna, helps with localization from front/back.

Delay Lines and Neuronal Sensitivity to Interaural Delay

• Sounds from the left and the right side are compared at the superior olive of the brain stem.
• This comparison is used to determine the location of sound in the horizontal plane.

Localization of Sound in the Vertical Plane

• Sounds from the vertical plane are localized by differences in reflections from the pinna (outer ear).

Primary Auditory Cortex

• Axons from the MGN project to the auditory cortex through the internal capsule.
• Structure of the auditory cortex is similar to the visual cortex.

Principles of Auditory Cortex

• Tonotopy, columnar organization of cells with similar binaural interaction.
• Frequency tuning in neurons, similar characteristic frequencies
• Unilateral lesion in the auditory cortex, almost normal auditory function.
• Different frequency bands are processed in parallel.

The Vestibular System

• Balance, equilibrium, posture (head, body, & eye movements).
• Otolith organs: gravity and tilt (head movement).
• Semicircular canals: head rotation.
• Use hair cells like the auditory system to detect changes.

The Otolith Organs

• Detect changes in head angle and linear acceleration.
• Macular hair cells respond to tilting and activation.

Semicircular Canals

• Three canals on each side of the head.
• Help sense all possible head rotation angles.
• Paired on opposite sides of the head.
• Push-pull activation of vestibular axons.
• Endolymph lag causes depolarizations/hyperpolarizations in opposite directions in each ear.

Cupula

• Fluid doesn't move as quickly as the head.
• Pushing the cupula in the opposite direction deflects stereocilia and creates a nerve impulse.
• Causes depolarization of hair cells on one side of the body and hyperpolarization on the other side.
• Comparison establishes movement within three-dimensional space.

Central Vestibular Pathway

• Impulses reach the brain via pathways, and eventually eye muscles for movement.

The Vestibulo-Ocular Reflex (VOR)

• Function: fixate line of sight on a visual target during head movement.
• Mechanism: senses head rotations, and compensates for movement of eyes.
• Connections from semicircular canals, vestibular nucleus and the cranial nerves excite extraocular muscles.