The Auditory System Overview

The Auditory System

• The ear contains miniature acoustical detectors packed in a space the size of a pea
• These detectors can transduce vibrations as small as the diameter of an atom
• Auditory system responds 1000 times faster than visual receptors
• Sound location and analysis of rapidly changing sounds (music, speech) are critical functions of the auditory system
• Hearing is a vital sensory modality

Nature of Sound

• Sound is variations in air pressure
• Waveform of sound plotted against time
• Cycle: Distance between successive compressed patches
• Sound frequency (pitch): Cycles per second (Hertz, Hz)
• Human hearing range is 20 Hz to 20,000 Hz
• Sound loudness corresponds to the overall amplitude of the wave (logarithmic decibel scale, dB)

Structure of the Auditory System

• The auditory system consists of outer, middle, and inner ear structures
• Outer ear: Pinna, auditory canal
• Middle ear: Ossicles (malleus, incus, stapes), tympanic membrane
• Inner ear: Cochlea, auditory-vestibular nerve, oval window

Structure of the Auditory System (Continued)

• Sound waves are gathered by the pinna, concha, and auditory canal
• Tympanic membrane (eardrum) vibrates
• Ossicles (malleus, incus, stapes) amplify the vibrations & transfer them to the oval window
• The oval window creates pressure waves in the cochlea
• Cochlea contains sensory neurons (hair cells) that transduce vibrations into neural impulses

Middle Ear

• Components of the middle ear amplify sound force
• Pressure is greater at the oval window than the tympanic membrane, causing fluids to move
• The attenuation reflex: A response to loud sounds, causing muscle contraction to protect the ear

Inner Ear: Anatomy of the Cochlea

• Cochlea is where sound energy becomes nerve impulses
• Contains oval and round windows
• Flexible structure bisects the cochlea from base to apex (basilar membrane & tectorial membrane)
• Fluid-filled chambers: perilymph (Scala vestibuli & Scala tympani; high sodium, low potassium) and endolymph (Scala media; high potassium)
• Endocochlear potential: 80 mV more positive than perilymph

Inner Ear: Physiology of the Cochlea

• Pressure waves in the cochlea push perilymph into scala vestibuli and force the round window membrane to bulge out
• Basilar membrane properties are tuned for different frequencies
• The base of the basilar membrane is tuned for high frequencies
• The apex is tuned for low frequencies

Inner Ear: Hair Cells

• Hair cells in the cochlea contain stereocilia that transduce mechanical energy into electrical signals (transduction)
• Hair cell resting potential between -45 and -60mV
• Graded in height & arranged in bilateral, symmetric manner (stereocilia)
• Connected by tip links
• Movement in opposite direction compresses tip links and close channels
• Eventual transmitter release causes action potential in Cranial Nerve VIII fibers.

Inner Ear: Innervation of Hair Cells

• One spiral ganglion fiber interacts with one inner hair cell and numerous outer hair cells
• Otoacoustic emissions (self-generated sound)
• Outer hair cells: contract and expand (receive descending brain signals), modulating response to sounds

Central Auditory Processes

• Auditory pathway: Axons leaving the MGNS project to the auditory cortex via internal capsule, organized in array of cells with similar tonotopic arrangement
• Auditory cortex cells respond more to both ear stimulation than one
• Lesion to the auditory cortex results in abnormal sound localization, but not total loss of auditory function

Encoding Sound Intensity and Frequency

• Sound encoding based on firing rates and number of active neurons
• Tonotopic maps on the basilar membrane—Frequency encoding
• Tonotopic maps on basilar membrane, spiral ganglion and cochlear nucleus (frequency) (location of firing on membrane corresponds to frequency)

Encoding Sound Intensity and Frequency (continued)

• Phase locking: Consistent firing matches sound wave phase (low frequencies)
• Volley principle: Intermediate frequencies coded by pooled activity of neurons firing in a phase-locked manner

Mechanisms of Sound Localization

• Techniques for horizontal (left-right) and vertical (up-down) localization
• Time taken (inter-aural time delay) from one ear to the other ear
• Differences in sound intensity reaching both ears (interaural intensity difference)
• Reflections from the outer ear (pinna) are used for vertical sound localization

Explore the intricate workings of the auditory system, including the essential structures and functions involved in hearing. Learn about sound characteristics such as frequency, amplitude, and how our ear detects sound. This quiz covers the nature of sound and its critical role in our sensory experience.