A&P2 Hearing and Equilibrium

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The Structures of the Ear

-the structures that make up the ear are responsible for the senses of hearing and equilibrium

Function in Hearing

-the external ear, middle ear, and the cochlea in the inner ear function in the process of hearing

Function in Equilibrium

the vestibule and semicircular canals of the inner ear are the organs of equilibrium

Sensory Receptors

-of both senses are hair cells with microvilli which, when physically deflected initiate nerve impulses to the brain

The Process of Hearing

[detecting sound waves]
-begins when the pinna directs sound waves into the external acoustic meatus
-from there, the sound waves strike the tympanic membrane and cause it to vibrate
-the auditory ossicles amplify and transmit these vibrations through the oval window to the cochlea

Perilymph

-the fluid that fills the inside of the cochlea
-it is set in motion by the vibrations from the ossicles, and produces movement in sensory detector called hari cells

Stereocilia

[hair]
-movement of the hair of the hair cells in the cochlea results in nerve impulses being sent to the proper areas of the brain where they are interpreted as sound

The Cochlea

-the inside of the cochlea consists of a coiled tube divided into three linear chambers
-the middle chamber houses the spiral Organ of Corti, which contains rows of hair cells that extend the length of the tube

Pitch of the Sound Perceived

-is determined by which hair cells in the cochlea are stimulated
-high pitch sound vibrations do not travel very far into the cochlea
-the hair cells in the first part of the cochlea [near the oval and round windows] have stiff stereocilia and respond to the high pitch sounds
-when they are activated, nerve impulses are sent to the part of the brain that interprets them as high pitches
-low pitch sounds travel deeper into the cochlea and stimulate hair cells with more flexible stereocilia
-these are hard wired to a part of the brain that causes us to perceive those stimulations as low pitches
-the nerve impulses sent to the brain via the cochlear nerve which joins the vestibular nerve to form the vestibulocochlear nerve [VIII]
-louder sounds are generated by stronger vibrations which produce more movement in the tympanic membrane, auditory ossicles, and perilymph
-this results in a greater deflection of the hair cells, activation of more neurons, and more frequent action potentials which are interpreted by the brain as sounds of greater intensity

Deafness

-any amount of hearing loss, no matter how slight
-hearing loss is placed into one of two categories depending on its cause

Conduction Deafness

-is a condition where something interferes with the transfer and/or amplification of sound waves from the outside to the perilymph in the cochlea
-examples are: blockage of the external acoustic meatus, damage to the tympanic membrane, and fusion of the ossicles to one another or to the oval window

Sensorineural Deafness

-is deafness resulting from damage to any neural structure or neuron
-one of the main causes of this type of deafness is damage to the stereocilia of the hair cells
-this can result from prolonged exposure to high intensity sounds or a single loud sound such as an explosion
-other causes include tumors in the auditory part of the brain and degeneration of the cochlear nerve

Sense of Equilibrium

-is awareness of the position of the hear
-when it is stationary and when it is moving

Static Equilibrium

-involves sensing the position of the head and is a function of the vestibule

Dynamic Equilibrium

-refers to how fast and in which direction the head is moving, and is the job of the semicircular calans

Maculae

-the sense organs of static equilibrium
-are located within two small chambers inside the vestibule
-these chambers are known as utricle and saccule and each contains a macula
-the maculae consist of hair cells covered with an otolithic membrane

Otolithic Membrane

-is a gel like substance embedded with tiny stones referred to as otoliths which ass weight
-when the head is moved, the otolithic membrane shifts and deflects the cilia of the hair cells
-movement of the cilia triggers nerve impulses to the CNS via the vestibular nerve to give the brain information about the position of the head

Crista Ampullaris

-the sensory organ of dynamic equilibrium
-is a patch of hair cells in an expanded portion of each semicircular canal called the ampulla
-the crista ampullaris is covered with a membrane known as the cupula that deflects the cilia of the hair cells when it is moved
-when the head is moved, the fluis in the semicircular canals flows and shifts the cupula
-shifting of the cupula deflects the cilia of the hari cells and nerve impulses are sent to the CNS via the vestibular nerve to inform the brain of the velocity and direction of the head movements

The Rinne Test

-a test used to detect hearing loss and determine which type it is

The Weber Test

-main purpose of this test is to determine if sounds are heard equally loud in both ears
-if a person hears sounds louder in one ear than in the other, it could indicate a hearing problem
-if an ear is known to have hearing loss, this technique can also be used to determine what kind of deafness it is

Auditory Acuity

-is a measure of the clarity or sharpness of ones hearing
-one way to measure auditory acuity is to determine the threshold of hearing [the slightest sounds that can be heard]

Sound Localization

-demonstrates our ability to determine the direction a sound is coming from
-the pinna helps with sound localization because it blocks sounds form behind, but directs sounds from the front in to the ear canal

Auditory Accommodation

-occurs when you get so accustomed to a sound that you no longer hear it and you "tune it out"

Pointing Tests

-demonstrates spatial awareness, spatial coordination, and functioning of proprioceptors

The Romberg Test

-can be used to detect spinal cord injuries in the dorsal white column
-the dorsal white column facilitates the transmittance of impulses from the proprioceptors to the brain
-proprioceptors are sensory receptors which send information to the CNS regarding the position of body parts
-they are located in muscles, tendons, and joints and are involved with posture
-they monitor the state of muscle contractions, tension in tendons, and the position of ligaments and joints

Nystagmus

[field nystagmus is used as a drunk driving test]
-is unusual eye movements due to a disturbance of the semicircular canals, as occurs after rapid rotation of the head or alcohol consumption
-this can be observed by having the subject follow a penlight as you move it slowly from one side of the face to the other
-watch for the inability to follow the light, or rapid involuntary horizontal, vertical or rotational eye movements

Balance and Spatial Perception

-another test for drunk driving
-this demonstrates how the body adjusts to maintain balance, and our perception of the distance of objects
-the test subject should have no problem locating and manipulating the coins, and the motions should be smooth

Mental Acuity

-this is a test of a person's mental clarity
-it is often used for a drunk driving test also
-it can show if someone has a depressed central nervous system
-if a subject has been drinking or has nervous system problems, they may concentrate so hard on counting backwards they lose their balance

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