Describe the structure and function of the three divisions of the ear.
the pinna and concha function to gather and focus sound energy into external auditory meiatus and on to the tympanum. It also provides clues to the location of the sound. The external auditory meatus boosts sound pressure 30-100 times for frequencies around 3kHz.
the function is impedance matching which is the process of converting the air conducted pressure waves (low impedance) that enter the outer ear into liquid conducted pressure waves (high impedance) in the inner ear. It does this will minimal loss of information. The Tympanum vibrates in response to the air conducted sound waves in the external auditory meatus. The ear ossicles (malleus, incus and stapes) act as a system of levers linking the trympanum to the oval window of the inner ear. The arrangement increases the mechanical advantage where the pressure exerted by the footplate at the oval window is 22x that of the original sound pressure
the cocleha is the most important structure because the pressure waves created by the middle ear are converted to liquid conducted pressure waves and finally to neural impulses by the hair cells in the organ of corti. this process is called mechanoelectrical transduction. the inner ear is also where signal analysis begins as the basilar membrane and hair cells begin the process of frequency discriminating and amplitude detecting
Describe the structure and function of the cochlea.
begins at the oval window and is a long tube that is coiled like seashell. There is a superior and inferior section that is divided by the basilar membrane. the superior portion or continuation of the oval window is called the scala vestibuli and the inferior portion is called the scala tympani. at the very tip of the cochela the two halves meet up. perilympth is the fluid that is inside the hollow organ. So there is a continuation of perilympth from the oval window into the scala vestibuli all the the up to the tip called the heliocotrena, then back down in the scala tympani. the scala tympani terminates at the round window which is not an opening.
function of the cochlea
to convert liquid conducted pressure waves to neuronal impulses via the auditory hair cells
Describe the structure and function of the basilar membrane.
stretches the length inside the cochlea as it spirals up. Its structure is designed to pick up different frequencies along its path. It is flimsy and tapers off at a stiff end pivot.
keep the endolymph and perilymoth separate. as sound waves come the membrane has an upward phase and downward phase depending on the shear force of the sound induced vibration. So there is a particular place along the membrane for each frequency
Describe the structure and function of the organ of Corti.
organ of corti
is between the scala vestibuli and scala typani and this is where the inner and outer hair cells are located. It is on the base is the basilar membrane. the tops of the hair cells project into the enolympth.
function of the organ of Corti
to transduce auditory signals and maximize the hair cells’ extraction of sound energy
Describe the structure and function of an auditory hair cell.
auditory hair cell
are flask-shaped epithelia cells with 10s-100s sterocilia on the apical surface. They are arranged in two rows in the organ of corti which make up the inner and outer hair cells. 95% of the fibers in the auditory nerve innervate the inner hair cells and the the rest (5%) innervate the outer hair cells. The outer hair cells make up 3 rows and the inner hair cells just have one row of them. The Inner hair cells are innervated by the spiral ganglion and the outer are innervated with 10 or more other hair cells to the spiral ganglion.
to transduce the movement of the stereocilia to neural impulses by endolympth moving them at different frequencies. The inner hair cells respond to a narrow frequency and the outer hair cells respond to a broader range. The inner hair cells are less sensitive to lower intensity sounds and the outer hair cells are more sensitive to lower intensity sounds
the basilar membrane is designed so that it pick up different frequencies along its path. near the apex of the cochlea lower frequencies are picked up and near the base, higher frequencies are picke up. This is called tonotopy in which the nurons are a representation of the basilar membrane. This means there is a particular place along the basilar membrane for each frequency and a particular place in the auditory cortex
This is interpreted in the brain as intensity so it is determined by two ways: the number of neurons firing and the rate of firing
there is two strategies we use to differentiate the direction of a sound: the first is ineraural time differences which is the difference in the arrival of identical points on a wave train in the two ears. the second is the difference in the amplitude of identical points.
Trace the auditory pathway from a hair cell to the auditory cortex.
from the cochlea
the auditory nerve synapse in the spiral ganglion and then travels through the rostal medulla up to the mid pons and on to the lateral leminiscus. from there it travels to the inferior colliculus then to the medial geniculate complex of the thalamus. It terminates in the primary auditory cortex in area 41
There is a primary and secondary auditory cortex is located in the temporal lobe and part of the auditory system. the primary auditory cortex has a tonotopic map of the cocleha which means from base to the apex of the cochlea there are specific frequencies that are captured at specific areas of the basilar membrane. The same order of frequencies are found in the primary auditory cortex. The secondary auditory cortex is where wernicke’s area is located with its function of speech comprehension
this is damage to the auditory/hair cells or other structures that affect the function of the hair cells. there is a loss or reduction n sensitivity to both air conducted and bone conducted sound. common causes include repeated exposure to loud environment noise, ototoxic drugs, presbyacusis, or viral disease
is the loss/reduction in sensitivity to the air conducted sound. Sensitivity to bone conducted sound is normal. common causes include wax accumulation in the external meatus, explosive sounds that rupture the tympanic membrane, repeated infections that destroy the middle ear ossicles, or osteosclerosis of of the oval window
Describe tests used to distinguish between nerve and conduction deafness.
tests if the pt has normal conductive hearing because with conduction deafness sensitivity to bone conduced sound is normal. A tuning fork is struck and the stem is applied to the mastoid bone of the ear to be tested. The pt is asked to signal they can hear the signal. If they cant hear they have nerve deafness. If they can hear then they are asked to signal when they cant hear it anymore. The pt will always signal that they can no longer hear before the tuning fork has quit vibrating. at this point the tine of the tuning fork is moved close to the external auditory meatus. If the pt has normal conductive hearing they will signal that they can hear the tuning fork and the ability will last at least 15 sec. If the ability to hear the second time doesn't exist or last at least 15 sec, the pt has a conductive hearing loss
simiular to the rinne test. The tuning fork is struck and the stem is applied to the nasal bone. If hearing is equally then the pt will say the sound is coming from withing the head. If the sound seems louder in ear then there conductive hearing loss. The other ear will be the one that has the nerve deafness
Describe the symptoms of damage to the middle ear, inner ear, auditory nerve and central pathways.
all lesions up to the hair cells and/or the auditory nerve produce conduction deafness. Damage to hair cells, auditory nerve fibers and of course the brain produce sensorineural deficits or nerve deafness. Conduction deafness can be helped sometimes with hearing aids as long as the neural pathways are intact, nerve deafness is difficult to remedy if at all; this is where cochlear implants may help
Describe reflexes that involve auditory input as the afferent limb.
two types of auditory reflexes
the auditory attenuation (middle ear) reflex and the auditory-ocular (acoustic startle) reflex. In both reflexes the cochlear nerve is the afferent limb.
in the middle ear reflex the stapedius (CN7) and tensor tympani m (CN5) contract which stiffens the response of the ear drum and ossicles. this is a reaction to loud noises but its not fast enough to protect. It attenuates low frequency sounds mostly. a function of this reflex may be to dampen sensitivity to ones voice. affernt is the cochlear n.
acoustic startle reflex
we hear a loud noise that startles us and the reflex is to look in that direction. The afferent is the cochlear n. and the efferent the CN 3,4,6. involves the inferior and Superior colliculi, tectorbulbospinal tracts and medial longitudinal fasciculus
Describe the equilibrial triad with respect to balance and orientation.
a state in which opposing forces or influences are balanced. The brain determines this by integrating input from the vestibular apparatus, from vision and from structures such as muscle spindles, Golgi tendon organs, pressure receptors from the feet etc = proprioception. Thus a trio of input = equilbrial triad. Vision when possible is the major input as is proprioception. The vestibular system is of course involved with control of the eye and sensing movements that may disorient one, e.g. leaning over too far
Describe the structure and function of a vestibular hair cell.
vestibular hair cell
is embedded in a supporting cell with the tip of the hair located in the endolymph. the tip is made up of stereocilia with the tallest called the kinocilium. Each hair cell has an afferent nerve terminal and an efferent nerve terminal. when there is movement, the hair cells move according to the sliding of the otolith membrane because they are embedded within that membrane. Hair cells respond strongest when bent toward the kinocilium in the direction of the movement. This allows a response that can code for any direction of motion
Describe the origin and course of the vestibular nerve.
is one of two nerves from the vestibulocochlear nerve. It originates from the bipolar cells in the vestibular ganglion on the lateral floor and wall of the 4th ventricle in the pons and medulla. It innervates the utricle and in the ampullae and saccule
Describe the origin, course and function of the lateral and medial vestibulospinal tracts.
lateral vestibulospinal tract
originates from the lateral and inferior vestibular nuclei and projects ipsilateal to all spinal cord levels ending on alpha and gamma motor neurons and interneurons. Its function is to exerting an excitatory influence on the extensors muscles in the extremities.
medial vestibulospinal tract
originates from the medial vestibular nucleus and projects bilaterally to cervical spinal cord levels only. the fibers end on both flexor and extensor alpha and gamma motor neurons of neck muscles and exert both excitatory and inhibitory influence. this is also called the descending portion of the MLF
is normal when in response to stimulation of vestibular system by angular acceleration. It can be rotational, post rotational(continued movement of endolympth after the body has stopped), optokinetic (watching telephone poles on the highway) or caloric (clinic). the fast eye movement portion of nystagmus is always in the direction of the motion or perceived direction and gives its name to the nystagmus. So if the person rotates to the right, the fast eye movement is to the right
Describe the symptoms of vertigo and Ménière’s disease.
nausea and vomiting and a balance disorder. making walking or standing difficult. It is when the person experiences the perception of movement when in reality there is none. It is caused by a vestibular system failure.
Meniere’s disease symptoms
rotational vertigo, unilateral or bilateral temporally hearing loss, unilateral or bilateral tinnitus, a sensation of fullness or pressure in one or both ears
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