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

15 The Special Senses: Part BLightOur eyes respond to visible light, a small portion of the electromagnetic spectrum Light: packets of energy called photons (quanta) that travel in a wavelike fashion Rods and cones respond to different wavelengths of the visible spectrumFigure 15.10Wavelength (nm)Visible light(b)(a)Bluecones(420 nm)Rods(500 nm)Greencones(530 nm)Redcones(560 nm)X raysUVInfraredMicro-wavesRadio wavesGammaraysLight absorption (pervent of maximum)Refraction and LensesRefractionBending of a light ray due to change in speed when light passes from one transparent medium to another Occurs when light meets the surface of a different medium at an oblique angleRefraction and LensesLight passing through a convex lens (as in the eye) is bent so that the rays converge at a focal pointThe image formed at the focal point is upside-down and reversed right to leftFigure 15.12Point sources(a) Focusing of two points of light.(b) The image is inverted—upside down and reversed. Focal pointsFocusing Light on the RetinaPathway of light entering the eye: cornea, aqueous humor, lens, vitreous humor, neural layer of retina, photoreceptorsLight is refractedAt the corneaEntering the lensLeaving the lensChange in lens curvature allows for fine focusing of an imageFocusing for Distant VisionLight rays from distant objects are nearly parallel at the eye and need little refraction beyond what occurs in the at-rest eyeFar point of vision: the distance beyond which no change in lens shape is needed for focusing; 20 feet for emmetropic (normal) eyeCiliary muscles are relaxedLens is stretched flat by tension in the ciliary zonuleFigure 15.13aLensInvertedimageCiliary zonuleCiliary muscleNearly parallel raysfrom distant object(a) Lens is flattened for distant vision. Sympatheticinput relaxes the ciliary muscle, tightening the ciliary zonule, and flattening the lens.Sympathetic activationFocusing for Close VisionLight from a close object diverges as it approaches the eye; requires that the eye make active adjustmentsFocusing for Close VisionClose vision requiresAccommodation—changing the lens shape by ciliary muscles to increase refractory powerNear point of vision is determined by the maximum bulge the lens can achievePresbyopia—loss of accommodation over age 50Constriction—the accommodation pupillary reflex constricts the pupils to prevent the most divergent light rays from entering the eyeConvergence—medial rotation of the eyeballs toward the object being viewedFigure 15.13bDivergent raysfrom close object(b) Lens bulges for close vision. Parasympathetic input contracts the ciliary muscle, loosening the ciliary zonule, allowing the lens to bulge.InvertedimageParasympathetic activationProblems of RefractionMyopia (nearsightedness)—focal point is in front of the retina, e.g. in a longer than normal eyeballCorrected with a concave lensHyperopia (farsightedness)—focal point is behind the retina, e.g. in a shorter than normal eyeballCorrected with a convex lensAstigmatism—caused by unequal curvatures in different parts of the cornea or lensCorrected with cylindrically ground lenses, corneal implants, or laser proceduresFigure 15.14 (1 of 3)FocalplaneFocal point is on retina.Emmetropic eye (normal)Figure 15.14 (2 of 3)Concave lens moves focalpoint further back.Eyeballtoo longUncorrectedFocal point is in front of retina.CorrectedMyopic eye (nearsighted)Figure 15.14 (3 of 3)Eyeballtoo shortUncorrectedFocal point is behind retina.CorrectedConvex lens moves focalpoint forward.Hyperopic eye (farsighted)Functional Anatomy of PhotoreceptorsRods and conesOuter segment of each contains visual pigments (photopigments)—molecules that change shape as they absorb lightInner segment of each joins the cell bodyFigure 15.15aProcess ofbipolar cellOuter fiberApical microvillusDiscs containingvisual pigmentsMelaningranulesDiscs beingphagocytized Pigment cell nucleusInner fibersRod cell bodyCone cell bodySynaptic terminalsRod cell bodyNucleiMitochondriaConnectingciliaBasal lamina (borderwith choroid) The outer segments of rods and cones are embedded in the pigmented layer of the retina. Pigmented layerOuter segmentInnersegmentRodsFunctional characteristicsVery sensitive to dim lightBest suited for night vision and peripheral visionPerceived input is in gray tones onlyPathways converge, resulting in fuzzy and indistinct imagesConesFunctional characteristics Need bright light for activation (have low sensitivity)Have one of three pigments that furnish a vividly colored viewNonconverging pathways result in detailed, high-resolution visionChemistry of Visual PigmentsRetinal Light-absorbing molecule that combines with one of four proteins (opsin) to form visual pigmentsSynthesized from vitamin ATwo isomers: 11-cis-retinal (bent form) and all-trans-retinal (straight form)Conversion of 11-cis-retinal to all-trans-retinal initiates a chain of reactions leading to transmission of electrical impulses in the optic nerveFigure 15.15bRod discsVisualpigmentconsists of• Retinal• Opsin(b) Rhodopsin, the visual pigment in rods, is embedded in the membrane that forms discs in the outer segment.Excitation of RodsThe visual pigment of rods is rhodopsin (opsin + 11-cis-retinal)In the dark, rhodopsin forms and accumulatesRegenerated from all-trans-retinal Formed from vitamin AWhen light is absorbed, rhodopsin breaks down 11-cis isomer is converted into the all-trans isomerRetinal and opsin separate (bleaching of the pigment)Figure 15.1611-cis-retinal Bleaching ofthe pigment:Light absorptionby rhodopsintriggers a rapidseries of stepsin which retinalchanges shape(11-cis to all-trans)and eventuallyreleases fromopsin.1RhodopsinOpsin and Regenerationof the pigment:Enzymes slowlyconvert all-transretinal to its11-cis form in thepigmentedepithelium;requires ATP.DarkLightAll-trans-retinalOxidation2H+2H+ReductionVitamin A211-cis-retinalAll-trans-retinalExcitation of ConesMethod of excitation is similar to that of rodsThere are three types of cones, named for the colors of light absorbed: blue, green, and redIntermediate hues are perceived by activation of more than one type of cone at the same timeColor blindness is due to a congenital lack of one or more of the cone typesPhototransductionIn the dark, cGMP binds to and opens cation channels in the outer segments of photoreceptor cellsNa+ and Ca2+ influx creates a depolarizing dark potential of about 40 mVPhototransductionIn the light, light-activated rhodopsin activates a G protein, transducinTransducin activates phosphodiesterase (PDE)PDE hydrolyzes cGMP to GMP and releases it from sodium channelsWithout bound cGMP, sodium channels close; the membrane hyperpolarizes to about 70 mVFigure 15.1712 Light (photons)activates visual pigment. Visual pig-ment activates transducin(G protein).3 Transducin activates phosphodiesterase (PDE).4 PDE convertscGMP into GMP, causing cGMP levels to fall.5 As cGMP levelsfall, cGMP-gated cation channels close, resulting in hyperpolarization.VisualpigmentLightTransducin(a G protein)All-trans-retinal11-cis-retinalOpencGMP-gatedcation channelPhosphodiesterase (PDE)ClosedcGMP-gatedcation channelSignal Transmission in the RetinaPhotoreceptors and bipolar cells only generate graded potentials (EPSPs and IPSPs)Light hyperpolarizes photoreceptor cells, causing them to stop releasing the inhibitory neurotransmitter glutamateBipolar cells (no longer inhibited) are then allowed to depolarize and release neurotransmitter onto ganglion cellsGanglion cells generate APs that are transmitted in the optic nerveFigure 15.18 (1 of 2)1 cGMP-gated channelsopen, allowing cation influx;the photoreceptordepolarizes. Voltage-gated Ca2+channels open in synapticterminals. 2 Neurotransmitter isreleased continuously.34 Hyperpolarization closesvoltage-gated Ca2+ channels,inhibiting neurotransmitterrelease. 5 No EPSPs occur inganglion cell.6 No action potentials occuralong the optic nerve.7 Neurotransmitter causesIPSPs in bipolar cell;hyperpolarization results. Na+Ca2+Ca2+Photoreceptorcell (rod)BipolarcellGanglioncellIn the darkFigure 15.18 (2 of 2)1 cGMP-gated channelsare closed, so cation influxstops; the photoreceptorhyperpolarizes. Voltage-gated Ca2+channels close in synapticterminals.2 No neurotransmitteris released.3 Lack of IPSPs in bipolarcell results in depolarization.4 Depolarization opensvoltage-gated Ca2+ channels;neurotransmitter is released.5 EPSPs occur in ganglioncell.6 Action potentialspropagate along theoptic nerve. 7Photoreceptorcell (rod)BipolarcellGanglioncellLightCa2+In the lightLight AdaptationOccurs when moving from darkness into bright lightLarge amounts of pigments are broken down instantaneously, producing glarePupils constrictDramatic changes in retinal sensitivity: rod function ceasesCones and neurons rapidly adaptVisual acuity improves over 5–10 minutesDark AdaptationOccurs when moving from bright light into darknessThe reverse of light adaptationCones stop functioning in low-intensity lightPupils dilateRhodopsin accumulates in the dark and retinal sensitivity increases within 20–30 minutesVisual PathwayAxons of retinal ganglion cells form the optic nerve Medial fibers of the optic nerve decussate at the optic chiasmaMost fibers of the optic tracts continue to the lateral geniculate body of the thalamusVisual PathwayThe optic radiation fibers connect to the primary visual cortex in the occipital lobesOther optic tract fibers send branches to the midbrain, ending in superior colliculi (initiating visual reflexes) Visual PathwayA small subset of ganglion cells in the retina contain melanopsin (circadian pigment), which projects to:Pretectal nuclei (involved with pupillary reflexes) Suprachiasmatic nucleus of the hypothalamus, the timer for daily biorhythmsFigure 15.19aPretectal nucleusRight eyeLeft eyeFixation pointOptic radiationOptic tractOptic chiasmaUncrossed (ipsilateral) fiberCrossed (contralateral) fiber Optic nerveLateral geniculatenucleus ofthalamusSuperior colliculusOccipital lobe (primary visual cortex) The visual fields of the two eyes overlap considerably. Note that fibers from the lateral portion of each retinal field do not cross at the optic chiasma.SuprachiasmaticnucleusDepth PerceptionBoth eyes view the same image from slightly different anglesDepth perception (three-dimensional vision) results from cortical fusion of the slightly different imagesRetinal ProcessingSeveral different types of ganglion cells are arranged in doughnut-shaped receptive fieldsOn-center fieldsStimulated by light hitting the center of the fieldInhibited by light hitting the periphery of the fieldOff-center fields have the opposite effects These responses are due to different receptor types for glutamate in the “on” and “off” fieldsFigure 15.20Stimulus pattern(portion of receptivefield illuminated)No illumination ordiffuse illumination(basal rate)CenterilluminatedSurroundilluminatedResponse of off-centerganglion cell duringperiod of light stimulusResponse of on-centerganglion cell duringperiod of light stimulusThalamic ProcessingLateral geniculate nuclei of the thalamusRelay information on movementSegregate the retinal axons in preparation for depth perceptionEmphasize visual inputs from regions of high cone densitySharpen contrast informationCortical ProcessingTwo areas in the visual cortexStriate cortex (primary visual cortex)Processes contrast information and object orientationPrestriate cortices (visual association areas)Processes form, color, and motion input from striate cortex Complex visual processing extends into other regionsTemporal lobe—processes identification of objectsParietal cortex and postcentral gyrus—process spatial location