Y khoa, y dược - The special senses: Part D

15 The Special Senses: Part DProperties of SoundSound is A pressure disturbance (alternating areas of high and low pressure) produced by a vibrating objectA sound waveMoves outward in all directionsIs illustrated as an S-shaped curve or sine waveFigure 15.29Area ofhigh pressure(compressedmolecules)CrestTroughDistanceAmplitudeArea oflow pressure(rarefaction) A struck tuning fork alternately compresses and rarefies the air molecules around it, creating alternate zones of high and low pressure.(b) Sound waves radiate outward in all directions.WavelengthAir pressureProperties of Sound WavesFrequencyThe number of waves that pass a given point in a given timeWavelengthThe distance between two consecutive crestsAmplitudeThe height of the crestsProperties of SoundPitchPerception of different frequenciesNormal range is from 20–20,000 HzThe higher the frequency, the higher the pitchLoudnessSubjective interpretation of sound intensityNormal range is 0–120 decibels (dB)Figure 15.30Time (s)(a) Frequency is perceived as pitch.High frequency (short wavelength) = high pitchLow frequency (long wavelength) = low pitch(b) Amplitude (size or intensity) is perceived as loudness.High amplitude = loudLow amplitude = softTime (s)PressurePressureTransmission of Sound to the Internal EarSound waves vibrate the tympanic membraneOssicles vibrate and amplify the pressure at the oval windowPressure waves move through perilymph of the scala vestibuli Transmission of Sound to the Internal EarWaves with frequencies below the threshold of hearing travel through the helicotrema and scali tympani to the round windowSounds in the hearing range go through the cochlear duct, vibrating the basilar membrane at a specific location, according to the frequency of the soundFigure 15.31aScala tympaniCochlear ductBasilarmembrane1 Sound waves vibratethe tympanic membrane. 2 Auditory ossicles vibrate.Pressure is amplified. 3 Pressure waves created bythe stapes pushing on the oval window move through fluid in the scala vestibuli. Sounds with frequenciesbelow hearing travel through the helicotrema and do not excite hair cells. Sounds in the hearing range go through the cochlear duct, vibrating the basilar membrane and deflecting hairs on inner hair cells.MalleusIncusAuditory ossiclesStapesOvalwindowScala vestibuliHelicotremaCochlear nerve321RoundwindowTympanicmembrane(a) Route of sound waves through the earResonance of the Basilar MembraneFibers that span the width of the basilar membrane are short and stiff near oval window, and resonate in response to high-frequency pressure waves.Longer fibers near the apex resonate with lower-frequency pressure wavesFigure 15.31bFibers of basilar membrane(b) Different sound frequencies cross the basilar membrane at different locations.Medium-frequency sounds displacethe basilar membrane near the middle.Low-frequency sounds displace thebasilar membrane near the apex.Base(short,stifffibers) Frequency (Hz)Apex(long,floppyfibers)Basilar membraneHigh-frequency sounds displacethe basilar membrane near the base.Excitation of Hair Cells in the Spiral OrganCells of the spiral organSupporting cellsCochlear hair cellsOne row of inner hair cellsThree rows of outer hair cellsAfferent fibers of the cochlear nerve coil about the bases of hair cellsFigure 15.28c(c)Tectorial membraneInner hair cellOuter hair cellsHairs (stereocilia)Afferent nervefibersBasilarmembraneFibers ofcochlearnerveSupporting cellsExcitation of Hair Cells in the Spiral OrganThe stereociliaProtrude into the endolymphEnmeshed in the gel-like tectorial membraneBending stereociliaOpens mechanically gated ion channelsInward K+ and Ca2+ current causes a graded potential and the release of neurotransmitter glutamateCochlear fibers transmit impulses to the brainAuditory Pathways to the BrainImpulses from the cochlea pass via the spiral ganglion to the cochlear nuclei of the medullaFrom there, impulses are sent to theSuperior olivary nucleus Inferior colliculus (auditory reflex center)From there, impulses pass to the auditory cortex via the thalamusAuditory pathways decussate so that both cortices receive input from both earsFigure 15.33Medial geniculatenucleus of thalamusPrimary auditorycortex in temporal lobeInferior colliculusLateral lemniscusSuperior olivary nucleus(pons-medulla junction)Spiral organ (of Corti)Bipolar cellSpiral ganglion of cochlear nerveVestibulocochlear nerveMedullaMidbrainCochlear nucleiVibrationsVibrationsAuditory ProcessingImpulses from specific hair cells are interpreted as specific pitchesLoudness is detected by increased numbers of action potentials that result when the hair cells experience larger deflectionsLocalization of sound depends on relative intensity and relative timing of sound waves reaching both ears Homeostatic Imbalances of HearingConduction deafnessBlocked sound conduction to the fluids of the internal earCan result from impacted earwax, perforated eardrum, or otosclerosis of the ossiclesSensorineural deafnessDamage to the neural structures at any point from the cochlear hair cells to the auditory cortical cellsHomeostatic Imbalances of Hearing Tinnitus: ringing or clicking sound in the ears in the absence of auditory stimuliDue to cochlear nerve degeneration, inflammation of middle or internal ears, side effects of aspirinMeniere’s syndrome: labyrinth disorder that affects the cochlea and the semicircular canalsCauses vertigo, nausea, and vomitingEquilibrium and OrientationVestibular apparatus consists of the equilibrium receptors in the semicircular canals and vestibuleVestibular receptors monitor static equilibriumSemicircular canal receptors monitor dynamic equilibriumMaculaeSensory receptors for static equilibrium One in each saccule wall and one in each utricle wallMonitor the position of the head in space, necessary for control of postureRespond to linear acceleration forces, but not rotationContain supporting cells and hair cellsStereocilia and kinocilia are embedded in the otolithic membrane studded with otoliths (tiny CaCO3 stones)Figure 15.34Macula ofsacculeOtolithsHair bundleKinociliumStereociliaOtolithicmembraneVestibularnerve fibersHair cellsSupportingcellsMacula ofutricleMaculaeMaculae in the utricle respond to horizontal movements and tilting the head side to sideMaculae in the saccule respond to vertical movementsActivating Maculae ReceptorsBending of hairs in the direction of the kinociliaDepolarizes hair cells Increases the amount of neurotransmitter release and increases the frequency of action potentials generated in the vestibular nerveActivating Maculae ReceptorsBending in the opposite directionHyperpolarizes vestibular nerve fibersReduces the rate of impulse generation Thus the brain is informed of the changing position of the headFigure 15.35Otolithic membraneKinociliumStereociliaReceptorpotentialNerve impulsesgenerated investibular fiberWhen hairs bend towardthe kinocilium, the hair cell depolarizes, exciting the nerve fiber, which generates more frequent action potentials.When hairs bend awayfrom the kinocilium, the hair cell hyperpolarizes, inhibiting the nerve fiber, and decreasing the action potential frequency.DepolarizationHyperpolarizationCrista Ampullaris (Crista)Sensory receptor for dynamic equilibriumOne in the ampulla of each semicircular canalMajor stimuli are rotatory movementsEach crista has support cells and hair cells that extend into a gel-like mass called the cupulaDendrites of vestibular nerve fibers encircle the base of the hair cellsFigure 15.36a–bFibers of vestibular nerveHair bundle (kinociliumplus stereocilia)Hair cellSupportingcellMembranouslabyrinth CristaampullarisCristaampullarisEndolymphCupulaCupula(a) Anatomy of a crista ampullaris in a semicircular canal(b) Scanning electron micrograph of a crista ampullaris (200x)Activating Crista Ampullaris ReceptorsCristae respond to changes in velocity of rotatory movements of the headBending of hairs in the cristae causesDepolarizations, and rapid impulses reach the brain at a faster rateActivating Crista Ampullaris ReceptorsBending of hairs in the opposite direction causesHyperpolarizations, and fewer impulses reach the brainThus the brain is informed of rotational movements of the headFigure 15.36cFibers ofvestibularnerveAt rest, the cupula standsupright.Section ofampulla,filled withendolymph(c) Movement of the cupula during rotational acceleration and decelerationCupulaFlow of endolymphDuring rotational acceleration,endolymph moves inside thesemicircular canals in thedirection opposite the rotation(it lags behind due to inertia).Endolymph flow bends thecupula and excites the haircells.As rotational movementslows, endolymph keepsmoving in the directionof the rotation, bendingthe cupula in theopposite direction fromacceleration andinhibiting the hair cells.Equilibrium Pathway to the BrainPathways are complex and poorly tracedImpulses travel to the vestibular nuclei in the brain stem or the cerebellum, both of which receive other inputThree modes of input for balance and orientationVestibular receptorsVisual receptorsSomatic receptorsFigure 15.37CerebellumOculomotor control(cranial nerve nucleiIII, IV, VI)(eye movements)Spinal motor control(cranial nerve XI nucleiand vestibulospinal tracts)(neck movements)VisualreceptorsSomatic receptors(from skin, muscleand joints)Vestibularnuclei(in brain stem)Input: Information about the body’s position in space comesfrom three main sources and is fed into two major processingareas in the central nervous system.Output: Fast reflexive control of the muscles serving the eyeand neck, limb, and trunk are provided by the outputs of thecentral nervous system.VestibularreceptorsCentral nervoussystem processingDevelopmental AspectsAll special senses are functional at birthChemical senses—few problems occur until the fourth decade, when these senses begin to declineVision—optic vesicles protrude from the diencephalon during the fourth week of developmentVesicles indent to form optic cups; their stalks form optic nervesLater, the lens forms from ectodermDevelopmental AspectsVision is not fully functional at birthBabies are hyperopic, see only gray tones, and eye movements are uncoordinatedDepth perception and color vision is well developed by age fiveEmmetropic eyes are developed by year sixWith age The lens loses clarity, dilator muscles are less efficient, and visual acuity is drastically decreased by age 70Developmental AspectsEar development begins in the three-week embryoInner ears develop from otic placodes, which invaginate into the otic pit and otic vesicleThe otic vesicle becomes the membranous labyrinth, and the surrounding mesenchyme becomes the bony labyrinthMiddle ear structures develop from the pharyngeal pouchesThe branchial groove develops into outer ear structuresHuman Eye: Study Guide