Neuroscience/Objectives/Lecture 33

From PhysioWiki

Auditory system

Know the function of the auditory system in the reception, transduction, and interpretation of sound stimuli.

Sounds are transmitted through the outer and middle ear and are finally received by the cochlea of the inner ear. Sound waves mechanically distort the basilar membrane of the cochlea, with low frequency sounds affecting the apex most, and high frequency sounds most affecting the base. Hair cells along the basilar membrane are tonotopically organized in a related fashion, such that hair cells associated with low frequency sound located at the apex, and high frequency-sensitive hair cells located at the base of the basilar membrane. Hair cells transduce sound waves, firing action potentials when excited. These action potentials travel along the vestibulococchlear nerve and into the primary auditory cortex, where sound is interpreted.

What is sound and what are its physical properties?

Sound is a mechanical wave that results from the vibration of particles (commonly air), causing periodic regions of compression and rarefaction. The amplitude of the wave determines its intensity, commonly measured in decibels (dB). The wave's frequency determines the pitch of the sound. Complex sounds are composed of waves of many different frequencies.

What is a cochlear implant and how does it function?

Employed to treat congenital deafness, cochlear implants carry out the reception and transduction steps normally carried out by the auditory system. Such implants consist of a sound processor, implant, and electrode array that work together to bypass the defective cochlear apparatus (hair cells in particular). The sound processor receives the sound, converts it into a digital signal, and then transmits it to the implant. The implant converts the digital signal into electrical signals, which are propagated via the electrode array and into the cochlea. There, the electrical signals directly stimulate cranial nerve VIII afferents, which carry the impulses to the brain for interpretation.

Describe the role of the outer, middle, and inner ear in the transduction of sound to nerve impulses.

Outer (external) ear

The pinna localizes complex cells (especially when the sources differ in elevation) and differentiate sound sources behind the individual from those in front. It collects and transmits sounds into the auditory canal and to the tympanic membrane.

Middle ear

The three bony ossicles (malleus, incus, stapes) amplify the movement of the tympanic membrane. Their effects may be dampened by muscles such as the stapedius, which contracts in response to high-intensity sound. The Eustachian tube equalizes air pressure on either side of the tympanic membrane.

Inner (internal) ear

Derived from the embryonic otic placode, the inner ear comprises the fluid-filled cochlea and semicircular canals. The oval and round windows of the cochlea transmit sounds via the stapes and provide for pressure equilibration, respectively. The cochlea contains the organ of Corti, the principal organ of sound transduction.

Describe the structure and function of the spiral organ of Corti, the peripheral receptor for the auditory system.

The organ of Corti sits between the basilar and tectorial membranes of the cochlea and is continuous throughout the scala media. It has one row of inner hair cells and three rows of outer hair cells. IHCs are the cells that transduce sound into electrical signals, while OHCs modulate the sensitivity of IHCs. Both are connected with afferent cells of the spiral ganglion. One spiral ganglion cell can innervate one IHC or up to 10 OHCs. Both IHCs and OHCs receive afferents from several individual spiral ganglion cells.

How are different sound frequencies represented within the cochlea and encoded within the nerve fibers?

The organ of Corti (and therefore the cochlea) is tonotopically organized. High frequency sounds cause the most vibration at the base of the basilar membrane, while low frequency sounds vibrate the apex most. Similarly, IHCs tuned to high-frequency sounds are located at the base of the basilar membrane, while low-frequency-tuned IHCs are near the apex.

What is the function of outer hair cells and how are they innervated?

Outer hair cells modulate the sensitivity of the inner hair cells. OHCs receive afferent input from cells of the spiral ganglion, each of which may innervate up to 10 OHCs.