Ear Anatomy and Physiology PDF

The ear - Mechanoreceptive organ that contains mechanoreceptors - It houses two senses  Hearing: responding to pressure waves (sounds) in the air  Equilibrium (balance): responding to inertial forces that affect the body - Sound waves  Waves of air pressure cre...

The ear - Mechanoreceptive organ that contains mechanoreceptors - It houses two senses  Hearing: responding to pressure waves (sounds) in the air  Equilibrium (balance): responding to inertial forces that affect the body - Sound waves  Waves of air pressure created by vibration of air molecules  The peak of sound wave = maximum compression  The trough of sound wave = maximum refraction  Can be transmitted through liquids, solids & gases  Can’t be transmitted in vaccum Qualities of sound - Pitch  Related to the frequency  Number of sound waves peaks that pass a fixed point in one second  Measured by cycles per second (Hz)  Human hearing range is 20-20,000 Hz  Hearing is most sensitive to sound frequencies 1000-4000 (human speech) - Loudness (intensity)  Related to the amplitude of sound waves  Difference between the maximum and minimum pressure of sound waves  Loudness = 20 Log (pressure of sound)/(pressure of standard)  Pressure of standard is equal to 0.0002 dynes/cm2  Measured in decibel  Zero decibel sound is the sound with same standard intensity  x-fold increase in sound intensity = 10 (ending dB value – starting dB value)/10  Example: 40 decibel sound is 100 times louder than 20 decibal sound  Notes  The softest audible sound is 0 db (absolute threshold)  Normal conversation is 60 db  Discomfort & damage to hearing receptors occurs at 140 db  Pain occurs at 160 db - Direction  Determined by spatial relationship between the source of sound and the subject Physiologic anatomy - Outer ear  Pinna: collects and directs sound waves  External auditory meatus - Middle ear  Tympanic membrane  Ossicles: malleus, incus & stapes  Two muscles are attached to ossicles:  Stapedius  Tensor tympani  Eustachian tube - Inner ear  The sensory organ of hearing (cochlea) Amplification of sound - The lever system of ossicles gives amplification = 1.3 times - The area of tympanic membrane = 0.55 cm2 - The area of oval window = 0.032 cm2 - The area of tympanic membrane is 17 times larger than that of oval window - Total amplification is 1.3 x 17 = 22 times - Important for efficient transfer of sound energy form the air to the much denser liquid medium in the inner ear Middle ear reflex - Tiny muscles of the middle ear  Tensor tympani  Innervated by the CN V  When contracts it pulls malleus inward away from the tympanic membrane  Stapedius  Innervated by CN VII  When contracts it pulls stapes outward away from the oval window  - Attenuation (acoustic) reflex  Loud sounds causes contraction of the previous muscles reflexly  Decreases transmission of sound waves especially low frequency  Latent period is around 100 ms during which reflex can’t protect the inner ear form the damage caused by the sudden loud sounds  Functions  Protection against loud sounds  Masking low frequency sounds to reduce background noise  Decreases hearing sensation of person during his own speech Cochlea - Coiled tube inside temporal bone - Divided to 3 compartments by 2 membranes  Reissner’s membrane  Basilar membrane - The three compartments  Scala vestibuli: contains perilymph  Scala tympani: contains perilymph  Scala media : contains endolymph (form stria vascularis) - Inside scala media there is the organ of corti which is composed of  Hair cells with stereocilia resting on the basilar membrane  Tectorial membrane covering the hair cells ( hair cells are embedded in it)  Afferent fibers of auditory (choclear) nerve - Perilymph is similar in composition to the ECF (↑ sodium & ↓ potassium) - Endolymph composed of fluid with ↑ potassium and ↓ sodium - Endolymph is +80 mV compared to perilymph - Inside hair cells is -70 mV compared to perilymph - Inside hair cells is -150 mV compared to endolymph Hearing mechanism - Sound waves push on the tympanic membrane - Chain of ossicles push stapes against the oval window - Wave of pressure is produced in scala vestibuli - Pressure is transmitted to the elastic basilar membrane - Vibration of the basilar membrane - Bending of the stereocilia of the hair cells - Opening of potassium channel & potassium influx to hair cells - Excitation (depolarization) of the afferent cochlear nerve fibers - Pressure waves finally reach the round window (pressure release valve) Type of hair cells - A single row of inner hair cells  3500 cells  Main cells of hearing  90-95% of afferent sensory fibers come from inner cells to brain  Damage causes complete hearing loss - Three rows of outer hair cells  12,500 cells  95% of them are supplied by efferent fibers from auditory nerve  Modulate the response of the inner hair cells  Stimulation of efferent fibers of auditory nerve causes  Shortening of outer hair cells  Change in the inner hair cells sensitivity  Damage causes large amount of hearing loss - Notes  Exposure to loud sounds damages the hair cells  More intense & longer exposure causes more damage Presbyacusis - Loss of hearing in old people - The loss affects the high frequencies - Due to loss of hair cells near the base of cochlea Pitch frequency discrimination - Depends on which part of basilar membrane vibrates - At basal end  Basilar membrane is narrow & stiff  Basilar fibers are short and thick  For high frequency sounds - At distal end  Basilar membrane is wider & more flexible  Basilar fibers are long and thin  For low frequency sounds Sound localization - To determine whether the sound is coming from left or right side - Difference in arrival time to each ear: to localize low frequency sounds < 3000 Hz - Difference in intensity of sound reaching each ear: to localize high frequency sounds - Sounds in the midline are localized by the small curves of the pinna of the ear Auditory pathway - S: superior olivary nucleus - L: lateral leminiscus - I: inferior colliculus - M: Medial geniculate body Notes - Because of presence of many cross connections between the two sides of the auditory pathway, destruction of one side of auditory cortex or fibers from superior olivary nucleus and above doesn’t cause complete deafness - At superior olivary nucleus the minute differences in timing and loudness of sound of each ear are compared Cortical auditory areas - Primary  The superior part of temporal lobe  Areas numbers 41 & 42  Stimulated by fibers from the medial geniculate body  Has maps for different frequencies & directions  Has an area to detect sudden loud sound  Lesion in one side  Affects the ability to localize sounds  Decreases hearing in the opposite ear - Secondary  Near the primary area  Receives impulses from the primary area and other sensory areas  Inter-relates sounds with other sensations  Wernicke’s area is one of its parts  More developed in the dominant hemisphere  Responsible for sounds interpretation Vestibular system - In the petrous portion of temporal bone - Two parts  Two chambers: utricle and saccule  Three semicircular canals - Responsible for  Maintaining normal position of eyes and head when displaced  Maintaining body balance  Controlling eye movement - The entire system is continuous - Connected to the inner ear through saccule - All parts together are called vestibular labyrinth - Labyrinth is composed of two parts  Outer osseous labyrinth  Inner membranous labyrinth - The space between the osseous and membranous parts is the perilymph - Inside the membranous part there is the endolymph - Semicircular canals are responsible for angular acceleration (rotation) - Utricle and saccule  Sense head position in relation to the gravity during linear acceleration  Utricle senses  Linear horizontal acceleration  Head position in upright status  Saccule senses  Linear vertical acceleration  Head position while lying down  The sensory part of the is the macula  Macula contains hair cells that have cilia and stereocilia embedded in gelatinous membrane that contains calcium carbonate (CaCO3)  The thickest and longest cilium is called the kinocilium  The rest of cilia are called stereocilia  During linear acceleration  (CaCO3) crystals stay in their place due to their inertia  Bending of cilia in the opposite direction of movement   If bending of cilia is toward the kinocilium  hair cell excitation  If bending of cilia is away from the kinocilium  hair cell inhibition  Excitation is by opening of potassium channels & potassium influx  Potassium influx causes depolarization & release of excitatory neurotransmitter that causes AP in the nerve fiber Semicircular canals - Three semicircular canals on each side (orthogonal) - Filled with endolymph and opens into utricle - Each canal has a swelling at its junction with utricle - Swelling is called ampulla & contains the capula - Capula  Contains hair cells embedded in gelatinous mass  Base of hair cells lies on a crest called crista ampullaris - During head rotation  Endolymph lags behind & moves to the opposite direction of head movement  Endolymph pushes the capula to the opposite direction of movement  Capula movement causes bending of cilia of hair cells  If bending is toward the kinocilium  excitation  If bending is away from the kinocilium  inhibition - They work in pairs  Left and right horizontal  Left anterior and right posterior  Left posterior and right anterior - Due to the direction of endolymph movement during rotation the canal on one side is excited, and the corresponding on the other side is inhibited  - Example: right head rotation  Excitation of right horizontal semicircular canal  Inhibition of left horizontal semicircular canal - AP from semicircular canals during rotation  Before rotation: APs are at basal level (100 impulses/sec)  During rotation and bending: excitation peak (400 impulses/sec)  If rotation is fixed at a constant speed, capula will retain its original position & APs go back to their basal level  If you stop suddenly, capula will bend to the same direction of movement  Inhibition continues for a certain period until capula return to its original position - Vertigo  Sensation of rotation in absence of actual rotation  May arise from vestibular apparatus abnormalities  Infection of endolymph causes  Impairment of movement of capula at one side  Disagreement between semicircular canals Caloric stimulation of vestibular apparatus - Irrigation of external ear by hot or cold water causes movement of endolymph - This movement will either excite or inhibit the horizontal canals at the irrigated side - Irrigation with cold water (30 C) will cause nystagmus to the opposite side - Irrigation with warm water (44 C) will cause nystagmus to the same side - Nystagmus has two phases  Slow movement  Quick movement: considered to be the direction of nystagmus - Causes nystagmus, vertigo and nausea Functions of the vestibular system - Provides a subjective sensation of movement in 3D space - Maintains the body posture and balance - Controls the extra-ocular eye muscles Vestibulo-ocular reflex (VOR) - Function: stabilization of the retinal image during rotation of the head - When the head rotates with certain speed and direction the eyes will rotate with the same speed but in the opposite direction - Reflexive eye movement that occurs when semicircular canal is excited - To fix the eye on a certain visual target - Pathway:  Semicircular canal  Vestibular nucleus  CN nuclei  Extra-ocular muscles stimulation - Example: rotating the head rightward  Stimulation of right horizontal canal  Inhibition of left horizontal canal  Stimulation of right vestibular nucleus  Inhibition of left vestibular nucleus  Stimulation of right CN III (contraction of right medial rectus muscle)  Stimulation of left CN VI (contraction of left lateral rectus muscle)  Inhibition of left CN III (relaxation of left medial rectus)  Inhibition of right CN VI (relaxation of right lateral rectus)  Movement of both eyes to the left (opposite direction of head rotation) - Physiologic nystagmus  During prolonged head rotation  The eyes move in opposite direction  Then the eyes reach an extreme position  Then they rapidly return to the starting point to continue another cycle  Has two phases  Slow movement: opposite to the side of rotation  Quick movement:  To the same direction of the body rotation  Considered to be the direction of nystagmus Vestibular nerve - Terminates in the vestibular nuclei in the brain stem - Vestibular nuclei give 2nd order neurons to  Flocculonodular lobe of cerebellum  Nuclei of cranial nerves III, IV & VI  Spinal cord: for contraction & relaxation of antigravity muscles