Auditory System Overview and Sound Transmission

The outer, visible part of the ear. Its shape helps localize sounds vertically, especially high-frequency sounds.

The canal leading from the pinna to the eardrum. It amplifies sound waves.

A thin membrane that vibrates in response to sound waves. The intensity of vibration depends on the loudness of the sound, and the frequency of vibration depends on the pitch.

The three small bones in the middle ear: Malleus, Incus, and Stapes. They transmit vibrations from the eardrum to the oval window.

A membrane at the entrance to the inner ear. The stapes vibrates against it, transferring sound vibrations into the fluid-filled inner ear.

Sound is created by vibrating objects and propagated as waves of compressed and rarefied air. The amplitude of these waves determines the loudness, while the frequency (speed of oscillations) determines the pitch.

The audible range for humans extends from 20 Hz to 20,000 Hz. This means we can hear sounds with frequencies between 20 cycles per second and 20,000 cycles per second.

The minimal sound pressure needed to detect a sound varies depending on the frequency. This means we're more sensitive to certain frequencies than others. The audible range also changes with age.

Most sounds we encounter are not pure tones, but a complex combination of multiple pure tones. These complex sounds can be broken down into their individual pure tones through mathematical analysis.

The human ear acts like a natural 'Fourier transform' by decomposing complex sounds into their constituent pure tones. This information about the different pure tones is then analyzed by the brain.

The pinna (outer ear) and external auditory canal serve to funnel sound waves towards the tympanic membrane (eardrum), enhancing their intensity.

The middle ear acts as a transducer, converting sound waves in air into vibrations within the fluid-filled inner ear. This is achieved through a series of small bones called ossicles.

The middle ear can adjust its sound transmission efficiency to protect the inner ear from damagingly loud sounds and to amplify quieter sounds.

The fluid that fills the scala vestibuli and scala tympani in the inner ear.

The fluid that fills the scala media in the inner ear.

A membrane that separates the scala tympani from the scala media.

A structure within the scala media that contains the sensory receptors for hearing.

Sensory receptors in the organ of Corti that have tiny hair-like projections called stereocilia.

Tiny hair-like projections on hair cells that are responsible for converting sound vibrations into electrical signals.

A gelatinous membrane that lies above the organ of Corti and is connected to the modiolus.

The bony core of the cochlea.

Depolarization of hair cells occurs when stereocilia bend towards the largest stereocilium, while bending away from the largest stereocilium causes hyperpolarization.

The basilar membrane's movement directly influences the bending of hair cell stereocilia, ultimately affecting their depolarization or hyperpolarization.

The receptor potentials in hair cells are fast enough to follow rapid sound changes, allowing us to perceive a wide range of frequencies.

Tip links are tiny connections between stereocilia that act as mechanical gates for cation channels, controlling the flow of potassium ions.

The endolymph surrounding hair cells has a high potassium concentration, which is crucial for the depolarization of hair cells upon opening of cation channels.

The opening of voltage-gated calcium channels within hair cells is triggered by membrane depolarization, leading to further signaling processes.

Hair cells do not generate action potentials due to the absence of voltage-gated sodium channels, the mechanism responsible for action potentials.

Hair cells act as mechanoreceptors, converting mechanical energy (sound waves) into electrical signals, the basis of our sense of hearing.

The process of a neuron's membrane potential becoming more positive, often due to the influx of positively charged ions like sodium (Na+).

Voltage-gated channels that open in response to a change in membrane potential, specifically when the membrane becomes more positive (depolarized).

Voltage-gated channels that open in response to a change in membrane potential, specifically when the membrane becomes more positive (depolarized), allowing potassium ions (K+) to flow out of the cell.

Small, membrane-bound sacs within neurons that store neurotransmitters like glutamate. During synaptic transmission, these vesicles fuse with the presynaptic membrane, releasing the neurotransmitter into the synapse.

A neurotransmitter that plays a crucial role in excitatory signaling in the central nervous system, particularly in the brain.

Sensory neurons that transmit information from the sensory organs to the central nervous system.

Afferent neurons that innervate the inner hair cells in the cochlea, responsible for transmitting auditory information to the brain.

The fluid that fills the space between the bony labyrinth and the membranous labyrinth of the inner ear.

Neurons located at the apex of the basilar membrane are responsible for detecting low-frequency sounds. However, they have a limited ability to distinguish between different low frequencies based on hair cell location alone.

Neurons tuned to low frequencies (<5kHz) fire at a rate too fast for phase locking to work reliably. Therefore, they rely mainly on tonotopy to encode frequency information.

Neurons responsive to intermediate frequencies do not fire on every sound wave cycle, but they are still phase-locked. The collective activity of a group of neurons with the same frequency dependence carries precise information about the sound frequency.

Neurons tuned to high frequencies (>2kHz) cannot maintain phase locking because the rapid firing rate of the neurons makes it impossible to synchronize with the incoming sound waves.

The superior olive is the first region in the auditory pathway to receive input from both ears. It plays a crucial role in sound localization by comparing the timing and intensity of sound signals from each ear.

For low-frequency sounds (≤ 2kHz), the brain compares the timing of sound waves arriving at each ear. The ear further from the sound source will receive the sound slightly later due to the distance.

For high-frequency sounds (>2kHz), the brain compares the intensity of sound waves arriving at each ear. The head acts as an obstacle, effectively casting a 'sound shadow' that reduces the intensity of the sound reaching the further ear.

The inferior colliculus generates a map of auditory space by integrating information from the superior olive (horizontal sound location) and the dorsal cochlear nucleus (vertical sound location).