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    the vestibular system relies on hair cells that are similar in structure and function to those found in the auditory system.


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    17.4 Hearing and Vestibular Sensation – Concepts of Biology – 1st Canadian Edition


    Learning Objectives

    By the end of this section, you will be able to:

    Describe the relationship of amplitude and frequency of a sound wave to attributes of sound

    Trace the path of sound through the auditory system to the site of transduction of sound

    Identify the structures of the vestibular system that respond to gravity

    Audition, or hearing, is important to humans and to other animals for many different interactions. It enables an organism to detect and receive information about danger, such as an approaching predator, and to participate in communal exchanges like those concerning territories or mating. On the other hand, although it is physically linked to the auditory system, the vestibular system is not involved in hearing. Instead, an animal’s vestibular system detects its own movement, both linear and angular acceleration and deceleration, and balance.


    Auditory stimuli are sound waves, which are mechanical, pressure waves that move through a medium, such as air or water. There are no sound waves in a vacuum since there are no air molecules to move in waves. The speed of sound waves differs, based on altitude, temperature, and medium, but at sea level and a temperature of 20º C (68º F), sound waves travel in the air at about 343 meters per second.

    As is true for all waves, there are four main characteristics of a sound wave: frequency, wavelength, period, and amplitude. Frequency is the number of waves per unit of time, and in sound is heard as pitch. High-frequency (≥15.000Hz) sounds are higher-pitched (short wavelength) than low-frequency (long wavelengths; ≤100Hz) sounds. Frequency is measured in cycles per second, and for sound, the most commonly used unit is hertz (Hz), or cycles per second. Most humans can perceive sounds with frequencies between 30 and 20,000 Hz. Women are typically better at hearing high frequencies, but everyone’s ability to hear high frequencies decreases with age. Dogs detect up to about 40,000 Hz; cats, 60,000 Hz; bats, 100,000 Hz; and dolphins 150,000 Hz, and American shad (Alosa sapidissima), a fish, can hear 180,000 Hz. Those frequencies above the human range are called ultrasound.

    Amplitude, or the dimension of a wave from peak to trough, in sound is heard as volume and is illustrated in Figure 17.12. The sound waves of louder sounds have greater amplitude than those of softer sounds. For sound, volume is measured in decibels (dB). The softest sound that a human can hear is the zero point. Humans speak normally at 60 decibels.

    Figure 17.12.  For sound waves, wavelength corresponds to pitch. Amplitude of the wave corresponds to volume. The sound wave shown with a dashed line is softer in volume than the sound wave shown with a solid line. (credit: NIH)


    In mammals, sound waves are collected by the external, cartilaginous part of the ear called the pinna, then travel through the auditory canal and cause vibration of the thin diaphragm called the tympanum or ear drum, the innermost part of the outer ear (illustrated in Figure 17.13). Interior to the tympanum is the middle ear. The middle ear holds three small bones called the ossicles, which transfer energy from the moving tympanum to the inner ear. The three ossicles are the malleus (also known as the hammer), the incus (the anvil), and stapes (the stirrup). The aptly named stapes looks very much like a stirrup. The three ossicles are unique to mammals, and each plays a role in hearing. The malleus attaches at three points to the interior surface of the tympanic membrane. The incus attaches the malleus to the stapes. In humans, the stapes is not long enough to reach the tympanum. If we did not have the malleus and the incus, then the vibrations of the tympanum would never reach the inner ear. These bones also function to collect force and amplify sounds. The ear ossicles are homologous to bones in a fish mouth: the bones that support gills in fish are thought to be adapted for use in the vertebrate ear over evolutionary time. Many animals (frogs, reptiles, and birds, for example) use the stapes of the middle ear to transmit vibrations to the middle ear.

    Figure 17.13.  Sound travels through the outer ear to the middle ear, which is bounded on its exterior by the tympanic membrane. The middle ear contains three bones called ossicles that transfer the sound wave to the oval window, the exterior boundary of the inner ear. The organ of Corti, which is the organ of sound transduction, lies inside the cochlea. (credit: modification of work by Lars Chittka, Axel Brockmann)


    Vibrating objects, such as vocal cords, create sound waves or pressure waves in the air. When these pressure waves reach the ear, the ear transduces this mechanical stimulus (pressure wave) into a nerve impulse (electrical signal) that the brain perceives as sound. The pressure waves strike the tympanum, causing it to vibrate. The mechanical energy from the moving tympanum transmits the vibrations to the three bones of the middle ear. The stapes transmits the vibrations to a thin diaphragm called the oval window, which is the outermost structure of the inner ear. The structures of the inner ear are found in the labyrinth, a bony, hollow structure that is the most interior portion of the ear. Here, the energy from the sound wave is transferred from the stapes through the flexible oval window and to the fluid of the cochlea. The vibrations of the oval window create pressure waves in the fluid (perilymph) inside the cochlea. The cochlea is a whorled structure, like the shell of a snail, and it contains receptors for transduction of the mechanical wave into an electrical signal (as illustrated in Figure 17.14). Inside the cochlea, the basilar membrane is a mechanical analyzer that runs the length of the cochlea, curling toward the cochlea’s center.

    Source : opentextbc.ca

    Vestibular System: Structure and Function (Section 2, Chapter 10) Neuroscience Online: An Electronic Textbook for the Neurosciences

    10.1 Vestibular System

    All living organisms monitor their environment and one important aspect of that environment is gravity and the orientation of the body with respect to gravity. In addition, animals that locomote must be able to adjust their orientation with respect to self generated movements, as well as forces that are exerted upon them from the outside world. The vestibular system performs these essential tasks. It engages a number of reflex pathways that are responsible for making compensatory movements and adjustments in body position. It also engages pathways that project to the cortex to provide perceptions of gravity and movement. The first section of the Chapter begins with a description of the components of the peripheral sensory apparatus and describes the ways in which specialized receptors transduce mechanical signals into electrical events. The second section describes the projections of the vestibular afferents to the vestibular nuclei, and projection pathways from the vestibular nuclei to other brain structures such as the cerebellum.

    The membranous labyrinth of the inner ear consists of three semicircular ducts (horizontal, anterior and posterior), two otolith organs (saccule and utricle), and the cochlea (which is discussed in the chapter on Auditory System: Structure and Function).

    The Semicircular Ducts

    Figure 10.1 shows the main action of the semicircular ducts. These sensory organs respond to angular acceleration. In Figure 10.1, press the "expand" button to see progressively finer views of the horizontal semicircular duct. This expansion proceeds from the inner ear as it sits in the head, to a sketch of the horizontal semicircular duct, to a detail of the ampulla. (In the outline of the single horizontal semicircular duct the angle has changed, and what was initially horizontal is now seen as a vertically-oriented duct on the computer screen.) The ampulla is a localized dilatation at one end of the semicircular duct. A patch of innervated hair cells is found at the base of the ampulla in a structure termed a crista (meaning crest). The crista contains hair cells with stereocilia oriented in a consistent direction. The cupula, a thin vane, sits atop this crest, filling the lumen of the semicircular duct. The stereocilia of the hair cells are embedded in the gelatinous cupula.

    By pressing the "play button" in Figure 10.1, the animation will show the effects of head rotation. As the head rotates in one direction, inertia of the fluid causes it to lag, and hence generate relative motion in the semicircular duct in the direction opposite that of the head movement. This moving fluid bends the broad vane of the cupula. The stereocilia of the hair cells are bent because they are embedded in the gelatinous cupula. Shearing of the hair cells opens potassium channels, as discussed at the beginning of the auditory section (See Figure 12.1).

    Figure 10.1

    The Semicircular Duct. Press EXPAND to see drawings of the horizontal semicircular duct. Then, press PLAY to watch the reaction to head movement.

    There are three pairs of semicircular ducts, which are oriented roughly 90 degrees to each other for maximum ability to detect angular rotation of the head. Each slender duct has one ampulla. When the head turns, fluid in one or more semicircular ducts pushes against the cupula and bends the cilia of the hair cells. Fluid in the corresponding semicircular duct on the opposite side of the head moves in the opposite direction.

    The basic transduction mechanism is the same in the auditory and vestibular systems (See Figure 12.1). A mechanical stimulus bends the cilia of the hair cells. Fine thread-like tip links connect to trap doors in the adjacent cilium. Bending the hair cells stretches the tip link, causing an influx of K+ ions and the generation of neural impulses in the VIIIth cranial nerve.

    Hair cells in the vestibular system are slightly different from those in the auditory system, in that vestibular hair cells have one tallest cilium, termed the kinocilium. Bending the stereocilia toward the kinocilium depolarizes the cell and results in increased afferent activity. Bending the stereocilia away from the kinocilium hyperpolarizes the cell and results in a decrease in afferent activity.

    The semicircular ducts work in pairs to detect head movements (angular acceleration). A turn of the head excites the receptors in one ampulla and inhibits receptors in the ampulla on the other side.

    Figure 10.2

    The Counteracting Influences of Bilateral Vestibular Stimulation Press EXPAND to see drawings of the horizontal semicircular duct. Then press PLAY to watch the reaction to head movement.

    Figure 10.2 is an extension of Figure 10.1. Begin by pressing "expand" to show details from the horizontal semicircular ducts on both sides of the head. Beneath the ampullae are new details, which highlight the orientation of the stereocilia in both cristae and their outputs. The kinocilia are oriented in the direction of the ampullae (ampullo fugal) within the ducts on both sides. The two sides are mirror images. There is a constant low level of ionic influx into the body of the hair cells, so there is a steady-state receptor potential and a spontaneous low-level discharge of afferent activity. These neutral neurophysiological properties are shown in graphs below each ampulla.

    Source : nba.uth.tmc.edu

    Exam 2: Specialized sense AUDITION AND BALANCE Flashcards

    Start studying Exam 2: Specialized sense AUDITION AND BALANCE. Learn vocabulary, terms, and more with flashcards, games, and other study tools.

    Exam 2: Specialized sense AUDITION AND BALANCE

    Our detection of sound, vibrations, gravity and acceleration is made by

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    The ear

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    The ear is composed of

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    outer auricle --> auditory meatus --> tympanic membrane --> ossicles --> round & ovals windows --> cochlea--> semicircular canals

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    Terms in this set (59)

    Our detection of sound, vibrations, gravity and acceleration is made by

    The ear

    The ear is composed of

    outer auricle --> auditory meatus --> tympanic membrane --> ossicles --> round & ovals windows --> cochlea--> semicircular canals

    The ear has 2 main goals:

    1) detect sound waves or changes in head position

    2) convey the stimulus into a form recognizable by the brain

    visible portion of the ear

    external ear

    -it includes the auricle (pinna) and the external acoustic meatus or ear canal


    What separates the external and middle ears

    tympanic membrane, most commonly called the EARDRUM


    sits in the inner ear and contains the cells and molecular apparatus responsible for transducing sound into an electrical signal that can be relayed to the brain

    TRUE OR FALSE: the inner ear only plays a role in audition


    The inner ear contains the ______________ for audition and the ______________, which are oriented in three dimensions for balance.

    cochlea, semicircular canals

    for humans, sound is transmitted by

    by the vibration of air molecules , which are measured in waves (these have peaks and valleys)


    vertical distance between peak and valley of a sound wave


    Perception of the frequency of a sound wave in Hz

    -Humans can detect pitch in the range of 20-20,000 Hz, but those we hear "best" are between 1000-4000 Hz.

    The taller the amplitude of a sound wave, the ______________ the sound; the shorter the amplitude, the ______________ the sound. Separate your answers with a comma.

    louder, softer

    The pitch of a sound is directly related to the ______________ of sound waves


    the middle ear contains

    auditory ossicles (malleus, incus, stapes)

    Within the cochlear duct sits the Organ of Corti, which is directly responsible for

    responding to sound vibrations.


    this is what the cochlear duct is filled with

    (high K+, low Na+)

    The ducts of the scala tympani and scala vestibuli are filled with

    perilymph (low K+, high Na+)

    transmission of sound from outer to inner ear

    Sound waves are captured by outer ear and sent to inner ear via tympanic membrane

    -Tympanic vibrations push on malleus incus stapes; moves oval window

    -Oval window transmits vibrations to scala vestibuli

    -Basilar membrane moves, as well as the hair cells in the Organ of Corti

    how do hair cells detect specific frequencies of sound

    the basilar membrane is frequency tuned

    what are the specialized cells of audition

    the hair cells within the organ of corti

    -These cells are specialized in that at one side they contain stereocilia, and at the other, they can release glutamate onto afferent neurons that form the cochlear branch of the vestibulocochlear nerve

    there are two types of hair cells categorized by where exactly in the Organ of Corti they sit:

    There is a single row of inner hair cells and three or four rows of outer hair cells whose stereocilia all make direct contact with the tectorial membrane (which sits on top of the Organ of Corti like a cap).


    long microvilli along the ear

    stereocilia are arranged by height, and are linked to one another by tip links. If the stereocilia move in the direction of the tallest member (the kinocilium), what happens?

    ion channels at the base of the stereocilia are pulled open and cations (in this case, mainly K+ ions because the endolymph is high in K+ rather than Na+) flow INTO the cell

    When the stereocilia move, this large electrochemical potential drives K+ into the hair cells, causing them to ________?


    -This depolarization leads to the opening of voltage-gated Ca2+ channels in the body of the cell and increased release of glutamate onto the afferent neurons.

    -The increased glutamate release onto the afferent neurons, if suprathreshold, will increase the frequency of action potentials via the vestibulocochlear nerve

    When the hair cells are bent in the opposite direction (towards the shortest stereocilia), the tip links slacken and the cation channels close This causes the cells to _______?


    -glutamate release is lessened or ceased altogether.

    -Thus, as the vibrations of the tectorial membrane change, the stereocilia sway back and forth and the release of glutamate onto the nerves changes accordingly.

    Within the inner ear, the location of the hair cells responsible for auditory transduction is the:

    Organ of Corti

    TRUE OR FALSE: When the ion channels in the stereocilia membrane are pulled open, Na+ flows into the cell and causes the cell to depolarize.


    Balance is managed by

    the vestibular system with the help of the visual and proprioceptive systems, and it is closely related to the auditory apparatus.

    Source : quizlet.com

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