Anatomy And Physiology Of Human Eye Pdf
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The lens is a transparent biconvex structure in the eye that, along with the cornea , helps to refract light to be focused on the retina.
Physiology of the Eye, Fourth Edition reviews major advances in the physiology of the eye, including improvements in photochemical and electrophysiological techniques. In particular, the successful application of modern microelectrode techniques to the recording of activity at all stages in the visual pathway is considered.
The material in this chapter is provided as an introduction to those aspects of the anatomy and physiology of both the eye and skin which should be familiar to those working extensively in the field of optical radiation safety. This chapter is neither a comprehensive nor a detailed treatment of these subjects. It is primarily intended for those who have not had specialized training in the life sciences and it can be conveniently bypassed by anyone who is familiar with the subject. Hopefully, it may also serve as a review of the anatomy and physiology of the skin and eye for those who have not studied this subject for some time.
Vision is the special sense of sight that is based on the transduction of light stimuli received through the eyes. The eyes are located within either orbit in the skull. The bony orbits surround the eyeballs, protecting them and anchoring the soft tissues of the eye Figure The eyelids, with lashes at their leading edges, help to protect the eye from abrasions by blocking particles that may land on the surface of the eye.
The inner surface of each lid is a thin membrane known as the palpebral conjunctiva. The conjunctiva extends over the white areas of the eye the sclera , connecting the eyelids to the eyeball.
Tears are produced by the lacrimal gland , located beneath the lateral edges of the nose. Tears produced by this gland flow through the lacrimal duct to the medial corner of the eye, where the tears flow over the conjunctiva, washing away foreign particles. Movement of the eye within the orbit is accomplished by the contraction of six extraocular muscles that originate from the bones of the orbit and insert into the surface of the eyeball Figure Four of the muscles are arranged at the cardinal points around the eye and are named for those locations.
They are the superior rectus , medial rectus , inferior rectus , and lateral rectus. When each of these muscles contract, the eye to moves toward the contracting muscle. For example, when the superior rectus contracts, the eye rotates to look up. The superior oblique originates at the posterior orbit, near the origin of the four rectus muscles. However, the tendon of the oblique muscles threads through a pulley-like piece of cartilage known as the trochlea.
The tendon inserts obliquely into the superior surface of the eye. The angle of the tendon through the trochlea means that contraction of the superior oblique rotates the eye medially. The inferior oblique muscle originates from the floor of the orbit and inserts into the inferolateral surface of the eye. When it contracts, it laterally rotates the eye, in opposition to the superior oblique. Rotation of the eye by the two oblique muscles is necessary because the eye is not perfectly aligned on the sagittal plane.
When the eye looks up or down, the eye must also rotate slightly to compensate for the superior rectus pulling at approximately a degree angle, rather than straight up. The same is true for the inferior rectus, which is compensated by contraction of the inferior oblique. A seventh muscle in the orbit is the levator palpebrae superioris , which is responsible for elevating and retracting the upper eyelid, a movement that usually occurs in concert with elevation of the eye by the superior rectus see Figure The extraocular muscles are innervated by three cranial nerves.
The lateral rectus, which causes abduction of the eye, is innervated by the abducens nerve. The superior oblique is innervated by the trochlear nerve.
All of the other muscles are innervated by the oculomotor nerve, as is the levator palpebrae superioris. The motor nuclei of these cranial nerves connect to the brain stem, which coordinates eye movements. The eye itself is a hollow sphere composed of three layers of tissue. The outermost layer is the fibrous tunic , which includes the white sclera and clear cornea. The transparent cornea covers the anterior tip of the eye and allows light to enter the eye.
The middle layer of the eye is the vascular tunic , which is mostly composed of the choroid, ciliary body, and iris. The choroid is a layer of highly vascularized connective tissue that provides a blood supply to the eyeball. The choroid is posterior to the ciliary body , a muscular structure that is attached to the lens by zonule fibers. These two structures bend the lens, allowing it to focus light on the back of the eye. Overlaying the ciliary body, and visible in the anterior eye, is the iris —the colored part of the eye.
The iris is a smooth muscle that opens or closes the pupil , which is the hole at the center of the eye that allows light to enter.
The iris constricts the pupil in response to bright light and dilates the pupil in response to dim light. The innermost layer of the eye is the neural tunic , or retina , which contains the nervous tissue responsible for photoreception. The eye is also divided into two cavities: the anterior cavity and the posterior cavity. The anterior cavity is the space between the cornea and lens, including the iris and ciliary body.
It is filled with a watery fluid called the aqueous humor. The posterior cavity is the space behind the lens that extends to the posterior side of the interior eyeball, where the retina is located. The posterior cavity is filled with a more viscous fluid called the vitreous humor.
The retina is composed of several layers and contains specialized cells for the initial processing of visual stimuli. The photoreceptors rods and cones change their membrane potential when stimulated by light energy. The change in membrane potential alters the amount of neurotransmitter that the photoreceptor cells release onto bipolar cells in the outer synaptic layer. It is the bipolar cell in the retina that connects a photoreceptor to a retinal ganglion cell RGC in the inner synaptic layer.
There, amacrine cells additionally contribute to retinal processing before an action potential is produced by the RGC. The axons of RGCs, which lie at the innermost layer of the retina, collect at the optic disc and leave the eye as the optic nerve see Figure Because these axons pass through the retina, there are no photoreceptors at the very back of the eye, where the optic nerve begins.
Note that the photoreceptors in the retina rods and cones are located behind the axons, RGCs, bipolar cells, and retinal blood vessels. A significant amount of light is absorbed by these structures before the light reaches the photoreceptor cells. However, at the exact center of the retina is a small area known as the fovea. At the fovea, the retina lacks the supporting cells and blood vessels, and only contains photoreceptors.
Therefore, visual acuity , or the sharpness of vision, is greatest at the fovea. This is because the fovea is where the least amount of incoming light is absorbed by other retinal structures see Figure As one moves in either direction from this central point of the retina, visual acuity drops significantly. In addition, each photoreceptor cell of the fovea is connected to a single RGC. Therefore, this RGC does not have to integrate inputs from multiple photoreceptors, which reduces the accuracy of visual transduction.
Toward the edges of the retina, several photoreceptors converge on RGCs through the bipolar cells up to a ratio of 50 to 1. The difference in visual acuity between the fovea and peripheral retina is easily evidenced by looking directly at a word in the middle of this paragraph.
The visual stimulus in the middle of the field of view falls on the fovea and is in the sharpest focus. Without moving your eyes off that word, notice that words at the beginning or end of the paragraph are not in focus. The images in your peripheral vision are focused by the peripheral retina, and have vague, blurry edges and words that are not as clearly identified. As a result, a large part of the neural function of the eyes is concerned with moving the eyes and head so that important visual stimuli are centered on the fovea.
Light falling on the retina causes chemical changes to pigment molecules in the photoreceptors, ultimately leading to a change in the activity of the RGCs. Photoreceptor cells have two parts, the inner segment and the outer segment Figure The inner segment contains the nucleus and other common organelles of a cell, whereas the outer segment is a specialized region in which photoreception takes place.
There are two types of photoreceptors—rods and cones—which differ in the shape of their outer segment. The rod-shaped outer segments of the rod photoreceptor contain a stack of membrane-bound discs that contain the photosensitive pigment rhodopsin. The cone-shaped outer segments of the cone photoreceptor contain their photosensitive pigments in infoldings of the cell membrane.
There are three cone photopigments, called opsins , which are each sensitive to a particular wavelength of light. The wavelength of visible light determines its color. The pigments in human eyes are specialized in perceiving three different primary colors: red, green, and blue. At the molecular level, visual stimuli cause changes in the photopigment molecule that lead to changes in membrane potential of the photoreceptor cell.
A single unit of light is called a photon , which is described in physics as a packet of energy with properties of both a particle and a wave. The energy of a photon is represented by its wavelength, with each wavelength of visible light corresponding to a particular color.
Visible light is electromagnetic radiation with a wavelength between and nm. Wavelengths of electromagnetic radiation longer than nm fall into the infrared range, whereas wavelengths shorter than nm fall into the ultraviolet range. Light with a wavelength of nm is blue whereas light with a wavelength of nm is dark red. All other colors fall between red and blue at various points along the wavelength scale.
Opsin pigments are actually transmembrane proteins that contain a cofactor known as retinal. Retinal is a hydrocarbon molecule related to vitamin A. When a photon hits retinal, the long hydrocarbon chain of the molecule is biochemically altered. Specifically, photons cause some of the double-bonded carbons within the chain to switch from a cis to a trans conformation. This process is called photoisomerization. This molecule is referred to as cis -retinal.
A photon interacting with the molecule causes the flexible double-bonded carbons to change to the trans — conformation, forming all- trans -retinal, which has a straight hydrocarbon chain Figure The shape change of retinal in the photoreceptors initiates visual transduction in the retina. Activation of retinal and the opsin proteins result in activation of a G protein. The G protein changes the membrane potential of the photoreceptor cell, which then releases less neurotransmitter into the outer synaptic layer of the retina.
Until the retinal molecule is changed back to the cis -retinal shape, the opsin cannot respond to light energy, which is called bleaching. When a large group of photopigments is bleached, the retina will send information as if opposing visual information is being perceived. After a bright flash of light, afterimages are usually seen in negative. The photoisomerization is reversed by a series of enzymatic changes so that the retinal responds to more light energy.
The opsins are sensitive to limited wavelengths of light. Rhodopsin, the photopigment in rods, is most sensitive to light at a wavelength of nm. The three color opsins have peak sensitivities of nm, nm, and nm corresponding roughly to the primary colors of red, green, and blue Figure
Anatomy of the Eye
Jump to content. The five senses include sight, sound, taste, hearing and touch. Sight, like the other senses is closely related to other parts of our anatomy. The eye is connected to the brain and dependent upon the brain to interpret what we see. How we see depends upon the transfer of light.
The eye is embryologically an extension of the central nervous system. It shares many common anatomical and physiological proper- ties with the brain.
Physiology of the Eye
The iris, the structure that gives the eyes color, works like a shutter in a camera. It has the ability to enlarge and shrink, depending on how much light the environment is sending into the pupil. This clear, flexible structure works much like the lens in a camera — shortening and lengthening its width in order to focus light rays properly. In a normal eyeball, after exiting the back of the lens, the light rays pass through the vitreous — a clear, jelly-like substance that fills the globe of the eyeball.
Vision is the special sense of sight that is based on the transduction of light stimuli received through the eyes. The eyes are located within either orbit in the skull. The bony orbits surround the eyeballs, protecting them and anchoring the soft tissues of the eye Figure
- Я позвоню Стратмору и попрошу прислать нам письменное подтверждение. - Нет, - сказала Мидж, - игнорируя сарказм, прозвучавший в его словах. - Стратмор уже солгал нам. - Она окинула Бринкерхоффа оценивающим взглядом. - У тебя есть ключ от кабинета Фонтейна.
Physiology of the Eye
- Он протянул конверт Беккеру, и тот прочитал надпись, сделанную синими чернилами: Сдачу возьмите. Беккер открыл конверт и увидел толстую пачку красноватых банкнот. - Что .
Извини, Сью, я пошутил. Сьюзан быстро проскочила мимо него и вышла из комнаты. Проходя вдоль стеклянной стены, она ощутила на себе сверлящий взгляд Хейла. Сьюзан пришлось сделать крюк, притворившись, что она направляется в туалет. Нельзя, чтобы Хейл что-то заподозрил. ГЛАВА 43 В свои сорок пять Чед Бринкерхофф отличался тем, что носил тщательно отутюженные костюмы, был всегда аккуратно причесан и прекрасно информирован. На легком летнем костюме, как и на загорелой коже, не было ни морщинки.
Затем она, наверное, вмонтирует алгоритм в защищенный чип, и через пять лет все компьютеры будут выпускаться с предустановленным чипом Цифровой крепости. Никакой коммерческий производитель и мечтать не мог о создании шифровального чипа, потому что нормальные алгоритмы такого рода со временем устаревают. Но Цифровая крепость никогда не устареет: благодаря функции меняющегося открытого текста она выдержит людскую атаку и не выдаст ключа. Новый стандарт шифрования. Отныне и навсегда. Шифры, которые невозможно взломать. Банкиры, брокеры, террористы, шпионы - один мир, один алгоритм.
This course is designed to provide eye care staff with a detailed, up-close look at the human eye, and help transfer that knowledge to enhance their ability to.
В ослепительной вспышке света коммандер Тревор Стратмор из человека превратился сначала в едва различимый силуэт, а затем в легенду. Взрывной волной Сьюзан внесло в кабинет Стратмора, и последним, что ей запомнилось, был обжигающий жар. ГЛАВА 106 К окну комнаты заседаний при кабинете директора, расположенной высоко над куполом шифровалки, прильнули три головы.