Auditory System Overview

### Auditory System

Sound is a pressure wave that travels through air.
Loudness of sound is determined by the amplitude (height) of the wave.
Pitch of sound is determined by the frequency (number of waves per second) of the wave.
The auditory system is composed of the outer, middle, and inner ear.
The outer ear collects sound waves and directs them to the middle ear.
The middle ear amplifies sound through the interaction of three bones known as ossicles (malleus, incus, stapes).
The tympanic membrane (eardrum) vibrates in response to sound waves; these vibrations are transmitted to the ossicles.
The ossicles move the oval window, which is a membrane between the middle ear and the inner ear.
The inner ear contains the cochlea, a fluid-filled structure where sound is converted into nerve signals.
The middle ear acts as an amplifier because the oval window has a much smaller surface area than the tympanic membrane.
This difference in area leads to increased pressure on the oval window, resulting in a stronger wave in the cochlear fluid.
Pressure is calculated by Force/Area.
The organ of Corti is located within the cochlea and houses the hair cells.
Hair cells are not neurons but are responsible for converting mechanical energy into electrical signals.
Hair cells have stereocilia, which are tiny hair-like projections, arranged in rows of increasing height.
The middle chamber of the cochlea is filled with endolymph, which has a high potassium concentration (K+) and a positive electrical potential relative to the outer chambers (+80mV).
This potential difference is called the endocochlear potential.
Hair cells have a negative resting potential.
When fluid waves move through the cochlea, they cause the stereocilia to bend, resulting in the opening of mechanosensitive K+ channels.
The potassium channels in hair cells are depolarizing, meaning that potassium ions move from outside the cell to inside the cell.
As the stereocilia bend, the tip links connecting them to adjacent stereocilia open the channel.
Depolarization of the hair cell causes the opening of voltage-gated calcium (Ca2+) channels, leading to the release of glutamate.
Glutamate acts as a neurotransmitter and excites the primary auditory nerve.
The primary auditory nerve sends these signals to the brain, where they are processed and interpreted as sound.
The basilar membrane is a flexible structure within the cochlea that vibrates in response to fluid waves.
The location of maximal vibration along the basilar membrane depends on the frequency of the sound.
High-frequency sounds cause vibrations closer to the base of the membrane, while low-frequency sounds cause vibrations closer to the apex (tip) of the membrane.
The tonotopic organization of frequency representation is maintained throughout the central auditory pathway.
Inner hair cells are primarily responsible for transmitting sound information to the brain, while outer hair cells act as amplifiers and fine tune the response of the cochlea.
The central auditory pathway involves the following structures:
- Spiral ganglion: Cell bodies of the primary auditory nerve.
- Cochlear nucleus: Located in the brainstem.
- Inferior colliculus: Located in the dorsal midbrain.
- Medial geniculate nucleus (MGN): Located in the thalamus.
- Auditory cortex: Located in the temporal lobe.
- The central auditory pathway is similar to the central visual pathway in its organization and function.
- The spatial processing of sound information (e.g., localization) is achieved by integrating sound information from both ears.

Vestibular System

The vestibular system is located within the inner ear and is responsible for balance and spatial orientation.
The vestibular system consists of the semicircular canals and the otolith organs (utricle and saccule).
The semicircular canals detect rotational movements of the head.
The otolith organs detect linear acceleration, including gravity.
The hair cells in the vestibular system have a similar structure and function to those in the cochlea.
Stereocilia within the macula of the otolith organs respond to movement of fluid.
The otoliths are small crystals of calcium carbonate embedded within the macula that add mass to the hair cells.
This mass allows for gravity or acceleration to exert a larger force on the hair cells, causing them to bend and open potassium channels.
These channels depolarize hair cells, leading to the release of neurotransmitter and activation of the vestibular nerve.
The vestibular nerve then sends signals to the brain.
The central vestibular pathway involves a complex interaction between the vestibular nerve, vestibular nuclei in the brainstem, and the cerebellum.
The vestibular nerve carries information from the labyrinth to the brainstem.
The vestibular nuclei are responsible for processing this information and coordinating balance.
The cerebellum receives input from the vestibular nuclei and plays a crucial role in maintaining balance and coordinating movements.
The vestibular nuclei also project to the spinal cord, controlling muscles involved in balance and posture.
The vestibular system also sends signals to the cranial nerves that control eye movements, allowing for smooth visual tracking during head movement.
While the vestibular system typically functions unconsciously, it can be consciously overridden.
Information from the vestibular system is relayed through the thalamus to the motor cortex, allowing for conscious control of limb and torso movements to maintain balance.
The vestibular system is crucial for motor learning.
Through repeated experiences and feedback, the brain learns to adapt to changes in balance and movement.
The cerebellum plays a dominant role in this motor learning process.

Auditory System Overview

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Questions and Answers

What does loudness of sound relate to in terms of wave properties?

What structure connects the ossicles to the cochlea within the auditory system?

Which ion concentration is significantly higher in the endolymph of the cochlea compared to the outer chambers?

How do hair cells in the organ of Corti convert mechanical energy into electrical energy?

What is the primary function of outer hair cells in the inner ear?

Which part of the central auditory pathway is responsible for relaying sound information to the auditory cortex?

What function do the otoliths serve in the vestibular system?

Which part of the brain does the vestibular nuclei primarily communicate with for balance control?

What type of neurotransmitter is released by hair cells in the inner ear in response to depolarization?

What describes the representation of different sound frequencies on the basilar membrane?

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Vestibular System

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