Sound Perception and Cochlea Function

Sound Perception in the Human Ear

• Sound is a wave of air movement. Higher pitch corresponds to higher frequency. Natural sounds are complex waves.
• Human ear consists of three main parts: external, middle, and inner ear.
• External Ear: Collects sound and amplifies frequencies around 3 kHz (important for human speech, 2-5 kHz).
• Middle Ear: Significantly amplifies sound (200-fold), focusing pressure from the large tympanic membrane onto the smaller oval window. Ossicles (malleus, incus, stapes) are involved in active amplification, transferring air pressure energy to the fluid-filled inner ear.
• Inner Ear: Houses the cochlea, responsible for sound transduction into neural signals, amplifying sound even further.

Cochlea Structure and Function

• Hair Cells: The cochlea contains hair cells (sound detectors)—movement of the basilar membrane causes stereocilia (hair-like structures) on these hair cells to bend.
• Organ of Corti: Situated on the basilar membrane within the cochlea, composed of hair cells covered by a tectorial membrane for added stimulus response. Fluid vibrations stimulate hair cells, triggering ion channel opening.
• Basilar Membrane: Vibrates at the frequency of the sound, a crucial element for sound processing.
• Sound Transduction: Hair cells convert mechanical motion into neural signals—this is a mechanoreceptor process.
• Compartments: The cochlea has two compartments (scala media and scala tympani) with differing ion concentrations (critical for hair cell function).
• Scala Media: Filled with endolymph, high in K+ and low in Na+.
• Scala Tympani: Filled with perilymph, high in Na+ and low in K+.

Mechanoelectrical Transduction

• Potential Differences: The endolymph has a high voltage (+80mV) while the perilymph, and the cell itself, has a significantly lower voltage (-45 to -60mV). This complex voltage differential allows for the transduction of sound into a nerve impulse.
• Tip Links: Stereocilia connect via tip links; fluid vibrations move the hair cells, opening ion channels and triggering K+ influx.
• Mechanoelectrical Transduction (MET): Fast process; K+ influx leads to depolarization. Depolarization triggers Ca^2+ channels opening, leading to neurotransmitter release—causing a hyperpolarization sequence.
• Ribbon Synapses: These specialized synapses allow the transmission of signals across a wide range of sound intensities, crucial for dynamic sound perception. They enable neurons to transmit signals accurately over a range of sound intensities.

Signal Amplification and Tuning

• Outer Hair Cells: Involved in signal amplification. They adjust the basilar membrane's motion via prestin (a motor protein).
• Cochlear Microphonic: An electrical response to sound, created by outer hair cells, amplifies sound signals significantly. Outer hair cells are also involved in tinnitus.
• Inner Hair Cells: The primary receptors for sound, responsible for 95% of the signal transmission to the brain.
• Basilar Membrane Tuning: The cochlea acts as a frequency analyser (tonotypy):
• Base of the cochlea: sensitive to high frequencies.
• Apex of the cochlea: sensitive to lower frequencies.

Auditory Pathway

• Crossing of Fibers: Significant crossing of auditory nerve fibers occurs at various points, facilitating processing of information (for sound localization).
• Binaural Cues: Inter-aural comparisons (between ears, time difference, and intensity differences) are crucial for sound localization in space.
• Superior Olivary Complex: Crucial for sound localization; medial superior olive detects difference in time of signal receipt (intramural time difference), while lateral superior olive detects differences in intensity (intramural intensity difference).
• Further Processing: The signal (crossing at various points in the auditory pathway) passes through intermediary nuclei like the cochlear nucleus, superior olivary complex, inferior colliculus, and medial geniculate body before reaching the auditory cortex in the temporal lobe. The pathway is highly organized, enabling specific sound processing and localization.
• Auditory Cortex Organization: tonotopic representation for frequency discrimination. The primary auditory cortex is highly complex.

Auditory Cortex and Beyond

• Belt Areas: Process more complex sounds.
• Cell Types: Specialized neurons (e.g., EE, Ei cells) respond to specific sound sequences.
• Hemispheric Asymmetry: Left hemisphere is dominant for language and speech, while the right hemisphere is more involved in perceiving environmental sounds and music.
• Dorsal/Ventral Streams: The dorsal stream (location) and ventral stream (object processing) separate processing of sound.