-eyelashes are richly innervated and triggers reflex blinking

Size: px

Start display at page:

Download "-eyelashes are richly innervated and triggers reflex blinking"

Kerry Harvey
6 years ago
Views:

1 The Eye and Vision -vision is the dominant sense -70% of all sensory receptors in the body are in the eyes -half of the cerebral cortex is involved in some aspect of visual processing -accessory structures protect the eye or aid its function: 1) Eyebrows -short, coarse hairs that overlie the supraorbital margins of the skull -help to shade the eyes from sunlight -prevent perspiration fro reaching the eyes -contraction of the orbicularis muscle depresses the eyebrow -contraction of the corrugator muscle moves the eyebrow medially 2) Eyelids -known as the palpebrae -protect the eyes anteriorly -eyelids are separated by the palpebral fissure and meet at the medial and lateral commisures -tarsal plates support it internally and anchor the orbicularis oculi and the levator palpebrae superioris muscles that run within the eyelid -the upper eyelid is more motile because the levator palpebrae superioris raises it to open the eye -eyelashes are richly innervated and triggers reflex blinking -tarsal glands are embedded in the tarsal plates and produce an oily secretion that lubricates the eyelid and the eye and prevents the eyelids from sticking together 3) Conjuctiva -a transparent mucous membrane that lines the eyelids (palpebral conjuctiva) and folds back over the anterior surface of the eyeball (bulbar conjuctiva) -the bulbar conjunctiva covers only the white of the eye and not the cornea -functions to produce a lubricating mucous that prevents the eyes form drying out -conjuctivitis is the inflammation of the conjucta and results in reddened, irritated eyes 4) Lacrimal Apparatus -consists of the lacrimal gland and the ducts that drain excess lacrimal secretions into the nasal cavity -the lacrimal gland lies in the orbit above the lateral end of the eye

2 -it continuously releases a dilute solution called lacrimal secretion (tears) -blinking spreads the tears downward and across the eyeball to the medial commisure, where they enter the paired lacrimal canaliculi via the lacrimal puncta (the red dot in the corner of your eye) -the tears drain into the lacrimal sac and then into the nasolacrimal duct, which empties into the nasal cavity at the inferior nasal meatus -lacrimal fluid contains mucus, antibodies, and lysozyme which is an enzyme that destroys bacteria -it helps cleanse and protect the eye surface, as well as moistening and lubricating it Extrinsic Eye Muscles -six strap-like extrinsic eye muscles control the movement of the eyeball -allows the eyes to follow a moving object and help maintain the shape of the eyeball and hold it in orbit -the 4 rectus muscles originate from the common tendinous ring (annular ring) located at the back of the orbit and run straight to their insertion on the eyeball -movements are indicated by their names: superior, inferior, lateral, and medial rectus muscles -the 2 oblique muscles move the eye in the vertical plane -the superior oblique muscle depresses the eye and turns it laterally -the inferior oblique muscle elevates the eye and turns it laterally -extrinsic eye muscles are the most precisely and rapidly controlled skeletal muscles in the body -reflected by their high axon-to-muscle fibre ration Structure of the Eyeball -the eyeball is an irregular sphere with an anterior pole and a posterior pole -its wall is composed of a fibrous layer, a vascular layer, and inner layers -the internal cavity is filled with fluids known as humors that help to maintain its shape -anterior portion = aqueous humor -forms and drains continuously -supplies nutrients and oxygen to the lens and cornea and carries away metabolic wastes -posterior portion = vitreous humor -helps transmit light, support the posterior surface of the lens and hold the neural retina firmly against the pigmented layer, and contributes to intraocular pressure (helps counteract the pulling force of the extrinsic eye muslces)

3 -the lens acts as the adjustable focusing apparatus of the eye and is supported vertically within the eyeball, dividing it into anterior and posterior segments Layers Forming the Wall of the Eyeball 1) Fibrous Layer -outermost coat of the eye -made up of the sclera (white of the eye) and the cornea (lets light enter the eye and is a major part of the light bending apparatus of the eye) 2) Vascular Layer -middle coat of the eyeball -known as the uvea -made up of the choroid, ciliary body, and the iris -the choroid is blood-vessel rich and supplies nutrition to all layers of the eyeball -contains brown pigment (produced by melanocytes) that helps absorb light, preventing it from scattering and reflecting within the eye -the ciliary body has ciliary muscles that control lens shape -the iris is the visible colored part of the eye and lies between the cornea and the lens -it has a round central opening known as the pupil which allows light to enter the eye -the iris is made up of 2 smooth muscle layers that act as a reflexively activated diaphragm to vary pupil size -in close vision and bright light, the sphincter pupillae (circular muscles) contract and the pupil constricts -in distant vision and dim light, the dilator pupillae (radial muscles) contract and the pupil dilates, allowing more light to enter the eye -pupillary dilation is controlled by sympathetic fibres -pupillary constriction is controlled by parasympathetic fibres -the iris contains only brown pigment; the amount of brown pigment determines eye color 3) Inner Layer (Retina) -two-layered -contains pigmented epithelial cells which makes up the outer layer -helps absorb light and prevent it from scattering -act as phagocytes to remove dead or damaged photoreceptor cells and store vitamin A needed by photoreceptor cells -contains the neural layer which makes up the inner layer -only the neural layer plays a direct role in vision -composed of photoreceptors, bipolar cells, and ganglion cells (from posterior to anterior)

4 ganglion -signals are produced in response to light and spread from the photoreceptors to the bipolar cells and then to the innermost cells, where action potentials are generated -ganglion axon cells run along the inner surface of the retina and leave the eye as the optic nerve -the optic nerve exits at the optic disc which lacks photoreceptors and is known as the blind spot -the brain "fills in" the information in order to deal with the absence of information -the retina contains photoreceptors that transduce / convert light energy, other neurons involved in the processing of light stimuli, and glia -rod cells are dim-light and peripheral vision receptors -more sensitive to light than cones and best suited for night vision -more numerous than cones -used for peripheral vision -absorb all wavelengths of visible light -perceived input is in gray tones only -the sum of all visual input feeds into a single ganglion cell -produces fuzzy, indistinct images since the visual cortex does not know which rod (out of the large number that influence a ganglion cell) -cone cells operate in bright light and provide precise, color vision -found in the macula lutea -concentrated in the fovea centralis -need bright light for activation and have low sensitivity -each cone in the fovea has a straight-through pathway to a ganglion cell -this accounts for detailed, high-resolution views of very small areas of the visual field provided by cones -because rods are absent from the foveae and cones do not respond to low-intensity light, we see dimly lit objects best when we do not look directly at them, and we recognize them when they move The Lens -biconvex, transparent, flexible structure -can change shape to allow precise focusing of light on the retina -avascular -made up of the lens epithelium and the lens fibres -the lens epithelium is confined to the anterior lens surface -consists of cuboidal cells that differentiate into the lens fibres -lens fibres make up the bulk of the lens

5 -contains transparent, precisely folded proteins called crystallins that form the body of the lens -the lens enlarges throughout life, becoming denser, more convex, and less elastic; this impairs its ability to focus light properly Physiology of Vision -electromagnetic radiation includes all energy waves, from long radio waves to very short gamma waves / X-rays -our eyes respond to the visible light portion of the spectrum -red waves are the longest waves and have the lowest energy -violet waves are the shortest and have the highest energy -objects have color because they absorb some wavelengths and reflect others -different cone receptor cells in the retina respond to different wavelengths of the visible spectrum -refraction involves bending of light rays as it meets the surface of a different medium at an oblique angle rather than a right angle Focusing Light on the Retina -light sequentially passes through the cornea, aqueous humor, the lens, the vitreous humor, and then passes through the entire thickness of the neural layer to excite the photoreceptors -light is refracted 3 times: 1) as it enters the cornea 2) entering the lens 3) leaving the lens -the lens curvature and elasticity can be actively changed to allow fine focusing of the image Focusing for Distant Vision -eyes are best adapted for distant vision -the far point of vision is the distance beyond which no change in lens shape (accommodation) is needed for focusing -for a normal (emmetropic) eye, the far point is 20 feet -during distant vision, ciliary muscles are completely relaxed and the lens is stretched flat -ciliary muscles are relaxed when sympathetic input to them increases and parasympathetic input decreases Focusing for Close Vision -light from close objects diverges as it approaches the eyes and it comes to a focal point farther from the lens

6 1) Accomodation -increases refractory power of the lens -ciliary muscles contract, providing a shorter focal length needed to focus the image of a close object on the retina -the closest point we can focus clearly on is known as the near point of vision and represents maximum bulge of the lens; near point of vision in the normal eye is 10 cm -a non-accommodating lens is known as presbyopia 2) Constriction -the sphincter pupillae muscles of the iris enhance the effect of accommodation by reducing the size of the pupil -this accommodation pupillary reflex prevents divergent light rays from entering the eye 3) Convergence -when we fixate on a close object, our eyes converge -convergence is the medial rotation of the eyeballs by the medial rectus muscles so that each is directed toward the object being viewed Myopia -occurs when distant objects are focused in front of the retina, rather then on it -distant objects are blurred -near-sightedness -corrected by concave lenses Hyperopia -far-sightedness -occurs when parallel light rays from distant objects are focused behind the retina -close objects are focused beyond the retina and appear blurry -corrected by convex lenses Photoreceptors and Phototransduction -photoreception is the process by which the eye detects light energy -phototransductiontion is the process by which light energy is converted into a graded receptor potential -photoreceptors are modified neurons The Chemistry of Visual Pigments -a light-absorbing molecule called retinal combines with proteins called opsin to form 4 types of visual pigments that absorbs different wavelengths of the visible spectrum -retinal can assume a variety of distinct 3-dimensional forms -when bound to opsin, it is known as 11-cis-retinal

7 -when 11-cis-retinal is struck by light and absorbs photons, it becomes all-transretinal -retinal twists into a new configuration and causes opsin to change shape and assume its activated form -rods and cones contain visual pigments (photopigments) -arranged in a stack of dislike infoldings of the plasma membrane that change shape as they absorb light 1) Excitation of Rods -the visual pigment of rods is a purple pigment called rhodopsin (11-cis-retinal + opsin) -rhodopsin forms and accumulates in the dark sequence of reactions -vitamin A is oxidized to 11-cis-retinal form and combines with opsin to form rhodopsin -when rhodopsin absorbs light, retinal changes to all-trans retinal; eventually the retinal-opsin combination breaks down and retinal and opsin separate -when the light-struck all-trans-retinal detaches from opsin, it is reconverted by enzymes within the pigmented epithelium to 11-cis-retinal -rhodopsin is regenerated when 11-cis-retinal is rejoined to opsin 2) Excitation of Cones -the breakdown and regeneration of pigments in cones is essentially the same as for rhodopsin -cones are less sensitive than rods and it takes a higher-intensity / brighter light to activate cones -cone opsins differ from rod opsins -naming of cones reflects the colors of light that each type of cone absorbs -blue cones, green cones, red cones -the perception of intermediate hues result from differential activation of more than one type of cone at the same time -when all cones are stimulated, we see white Light Transduction Reactions -when light triggers pigment breakdown, an enzymatic cascade occurs that results in closing cation channels that are normally kept open in the dark 1) light-activated rhodopsin activates a G protein called transducin -light energy first splits rhodopsin into all-trans retinal, releasing activated opsin -the freed opsin activates the G protein transducin

8 2) transducin activates PDE (phosphodiesterase) -PDE breaks down cgmp to GMP via hydrolysis -in the dark, cgmp binds to cation channels in the outer segments of photoreceptor cells, holding them open -this allows Na+ and Ca2+ to enter, depolarizing the cell to its dark potential (-40 mv)

9 -in the light, cgmp breaks down and the cation channels close -Na+ and Ca2+ stop entering the cell and it hyperpolarizes to about -70 mv; this stops inhibitory neurotransmitter from being released -no longer being inhibited, bipolar cells depolarize and release neurotransmitter onto ganglion cells, which is then converted into an action potential -the AP is transmitted to the brain along ganglion cell axons that make up the optic nerve -the hyperpolarization of photoreceptors is transmitted trough the retina and to the brain because photoreceptors / bipolar cells do not generate action potentials -photoreceptors and bipolar cells only generate graded potentials (i.e.: excitatory postsynaptic potentials and inhibitory postsynaptic potentials) -action potentials are used to carry information over long distances -retinal cells are small cells that are close together -graded potentials are adequate as signals that directly regulate neurotransmitter release at the synapse by opening or controlling voltagegated Ca2+

Light Adaptation -occurs when we move from darkness into bright light -rods and cones are strongly stimulated and large amounts of visual pigments are broken down instantaneously,

10 Light Adaptation -occurs when we move from darkness into bright light -rods and cones are strongly stimulated and large amounts of visual pigments are broken down instantaneously, producing a flood of signals that causes the glare -compensation occurs -all transducins move to the inner segment and this uncouples rhodopsin from the rest of the transduction cascade

11 -without transducin in the outer segment, the light hitting rhodopsin cannot produce a signal -as well, there is a switch from the rod to the cone system and other retinal neurons readily adapt, causing retinal sensitivity to decrease -the cones are now desensitized enough to take over -visual acuity and color vision improves -in general during light adaptation, retinal sensitivity (rod function) is lost and visual acuity is gained Dark Adaptation -the reverse of light adaptation -at first, we see all black because: -our cones stop functioning in low-intensity light (darkness) -our rod pigments have been bleached out by the bright light and the rods are still turned off -once we are in the dark: -rhodopsin accumulates -transducin returns to the outer segment -retinal sensitivity increases -dark adaptation is much slower than light adaptation -during light / dark adaptations, reflexive changes also occur in pupil size -bright light = pupil constriction -low light = pupil dilation -controlled by the pretectal nucleus of the midbrain and by parasympathetic fibres Visual Pathway to the Brain -axons of the retinal ganglion cells form the optic nerve -at the optic chiasma, fibres from the medial aspect of each eye cross-over (decussate) to the opposite side and continue via the optic tracts -each optic tract: -contains fibres from the lateral (temporal) aspect of the same eye and fibres from the medial (nasal) aspect of the opposite eye -carries all the information from the same half of the visual field -because the lens system of each eye reverses all images, the medial half of each retina receives light rays from the temporal part of the visual field and the lateral half of each retina receives an image of central part of the visual field

-paired optic tracts send most of their axons to synapse with neurons in the lateral geniculate nuclei of the thalamus -lateral geniculate nucleus maintains fibre separation and balances and combines

12 -paired optic tracts send most of their axons to synapse with neurons in the lateral geniculate nuclei of the thalamus -lateral geniculate nucleus maintains fibre separation and balances and combines the retinal input for delivery to the visual cortex -axons of the thalamic neurons project through the internal capsule to form the optic radiation of fibres in the cerebral white matter -these fibres project to the primary visual cortex in the occipital lobes, where conscious perception of visual images occurs -some nerve fibres in the optic tracts send branches to the midbrain -one set of fibres ends in the superior colliculi (visual reflex centers that control the extrinsic muscles of the eye and initiate visual reflexes) -another set of fibres project to the pretectal nuclei (mediates pupillary light reflexes) and to the suprachiasmatic nucleus (functions as the timer to set our daily biorhythms) -these fibres contain the visual pigment melanopsin (the circadian pigment) Depth Perception -achieved when both eyes view the same image from slightly different angles -the visual cortex fuses the slightly different images delivered by the 2 eyes -produces 3-D vision -depth perception depends on the 2 eyes working together -if only one eye is used, depth perception is lost and one must learn to judge position based on learned cues -animals have panamoric vision -eyes are placed more laterally so that visual fields overlap very little

13 Thalamic Processing -the lateral geniculate nuclei of the thalamus: -relays information on movement -segregates the retinal axons in preparation for depth perception -emphasizes visual inputs from the cones -sharpens the contrast information received from the retina Cortical Processing -there are 2 types of areas for processing retinal inputs on the visual cortex -the primary visual cortex (striate cortex) has processing neurons that respond to dark and bright edges (contrast information) and object orientation - the striate cortex provides form, color, and motion inputs to visual association areas (prestriate cortices) -anterior prestriate cortices continue the processing of visual information regarding form, color, and movement -complex visual processing extends forward into the temporal, parietal, and frontal lobes -the temporal lobe specializes in the identification of objects in the visual field -the parietal cortex and post-central gyrus uses information from the primary visual cortex to assess the spatial location of objects -this information is passed on to the frontal cortex to direct activities / guide movements

Coarse hairs that overlie the supraorbital margins Functions include: Shading the eye Preventing perspiration from reaching the eye

Coarse hairs that overlie the supraorbital margins Functions include: Shading the eye Preventing perspiration from reaching the eye SPECIAL SENSES (INDERA KHUSUS) Dr.Milahayati Daulay Departemen Fisiologi FK USU Eye and Associated Structures 70% of all sensory receptors are in the eye Most of the eye is protected by a cushion of fat