Sense of Sight

Sense of Sight

The sense of sight (vision) is our dominant sense, with approximately 70% of all sensory receptors found in the eyes, which are the organs of sight. Almost half of the cerebral cortex of the brain is used to some degree for visual processing. The eyes work along with ­accessory organs, including the eyelids, eyebrows, con-junctiva, lacrimal apparatus, and extrinsic muscles. All these organs are housed within the orbital cavity (or orbit) of the skull. Each orbit also contains blood vessels, fat, connective tissues, and nerves.

Eyebrows

The eyebrows consist of coarse, short hairs overlying the supraorbital skull margins and help to shade the eyes from light and to trap perspiration from the forehead.

Eyelids

Each eyelid has skin, muscle, connective tissue, and conjunctiva layers that collectively protect the ante-rior portion of the eye. The clinical term for eyelid is palpebrae. The eyelid is the thinnest portion of skinon the body, covering the lid’s outer surface while being fused to its inner lining near the margin of the lid. The eyelids are moved by the orbicularis oculi muscle and the levator palpebrae superioris muscle. They are sep-arated by the palpebral fissure yet are connected at the medial canthus and the lateral canthus. Inter-nally, the eyelids are supported by tarsal plates, which are made of thin connective tissue. The tarsal plates also function to anchor the eyelid muscles. The orbi-cularis muscle encircles the eye and its contraction causes the eye to close. In each eye, the upper eyelid is more mobile than the lower eyelid, mostly because of the levator palpebrae superioris muscle, because it raises the upper eyelid to open the eye. The functions of the eyelids include protection as well as blinking, a reflex action that normally occurs every 3–7 seconds. Blinking causes secretions from the accessory struc-tures to spread across the surface of the eyeball, which moistens it. These secretions include mucus, oil, and saline solution.

The eyelashes are hairs that project from the free margin of each eyelid. Eyelash follicles are innervated by many hair follicle receptors (nerve endings). Reflex linking is, therefore, triggered by anything that touches the eyelashes, including wind, an insect, and various particles. Within the tarsal plates are tarsalglands, which have ducts that open at the edge of theeyelid, slightly posterior to the eyelashes. The tarsal glands are modified sebaceous glands and produce an, lipid-rich, oily secretion that has two functions: pre-venting the eyelids from sticking together and lubri-cating both the eye and the eyelid. The tarsal glands are also called Meibomian glands. At the medial canthus of the eyelids, the lacrimal caruncle contains glands that produce rheum, the gritty substance that is often present when we awaken. Commonly, rheum is known as “sleep.”

Conjunctiva

The conjunctiva, also known as the palpebralconjunctiva, is a clear mucous membrane lining theinner eyelids. It folds back, covering the anterior eye-ball surface (except for the central cornea). In the area where it folds back, it is described as the bulbarconjunctiva . The area where the palpebral conjunc-tiva becomes continuous with the bulbar conjunctiva is called the fornix. The primary function of the con-junctiva is to produce mucus that lubricates the eyes and prevents drying.

Lacrimal Apparatus

The lacrimal apparatus contains the lacrimalgland, which secretes tears. It also has a series of ducts carrying tears into the nasal cavity (FIGURE 15-8) . The lacrimal gland actually lies in the orbit over the eye’s lateral end and can be seen through the conjunctiva when the eyelid is everted. The diluted saline solution released by the lacrimal gland is known as lacrimalsecretion or collectively astears. Tears are actuallysecreted continuously, exiting through tubules flow-ing downward and medially across the eye. Tears moisten and lubricate the eye and eyelid linings and contain the hormone lysozyme, which is antibacte-rial. Tears also contain mucus and antibodies. Collec-tively, the components of tears clean and protect the surface of the eye while they lubricate and moisten it. Tears reduce friction, remove debris, and provide oxygen and nutrients to parts of the conjunctival epi-thelium. Blinking of the eye causes tears to spread down and across the eye to the medial commissure. Here, through the two tiny lacrimal puncta, they enter the paired ­lacrimal canaliculi. The lacrimal puncta appear as two tiny red dots on each eyelid’s medial margin. The tears then drain from the lacrimal can-aliculi into the lacrimal sac and nasolacrimal duct. This duct empties into the nasal cavity at the inferiornasal meatus.

Extrinsic Eye Muscles

The six extrinsic muscles move the eye in many directions, with each strap-like muscle associated with one primary action. The extrinsic muscles originate from the walls of the eye orbit. They insert into the eyeball’s outer surface, allowing the eye to follow mov-ing objects. These muscles also hold it within the orbit and help to maintain the eyeball’s shape. The extrinsic muscles are shown in FIGURE 15-9. TABLE 15-2 lists the functions of the extrinsic and eyelid muscles.

Originating from the common tendinousring orannular ringare fourrectus muscles. Thecommon tendinous ring is found at the back of the eye orbit, and the rectus muscles connect it to their insertion points on the eyeball. Each rectus muscle is named for its location and the move-ments it controls: superior, inferior, lateral, and medial. The superior oblique muscle originateswith the rectus muscles and lies along the medial wall of the eye orbit. However,­ it soon makes a nearly 90-degree (right angle) turn to pass through the trochlea, a loop of fibrocartilage on the superolateral eyeball.

From the medial orbit surface, the inferior obliquemuscle originates to run laterally and obliquely. Itinserts on the inferolateral eye surface. The two oblique muscles assist the four rectus muscles in pro-viding even more defined eye movements. The lateral rectus and superior oblique muscles are innervated by, respectively, the abducens and trochlear nerves. How-ever, all the extrinsic eye muscles are served by the oculomotor nerves.

Anatomy of the Eyeball

The eye is hollow, spherical, and about 2.5 cm (1 inch) in diameter. Also referred to as the eyeball, it is slightly irregular in shape. It has three distinct ­layers: the outer, middle, and inner layers. The internal cavity of the eye is filled with humors, which are fluids that help to maintain its shape. FIGURE 15 -10 shows a transverse section of the right eye, with all three layers.

Outer Layer

The fibrous, anterior layer or tunic bulges forward to form the transparent cornea (the “window of the eye”), which helps to focus entering light rays and is continuous along its circumference with the white sclera. The border between the cornea and sclera iscalled the corneal limbus. The sclera is opaque, pro-tective, and the attachment for the six extrinsic mus-cles, which blend their collagen fibers with those of the outer layer as a whole. It is pierced at the back by the optic nerve and certain blood vessels. The thick-est part of the outer layer is located over the poste-rior surface of the eye, near the exit point of the optic nerve. The thinnest part of the outer layer is over the anterior surface. The visible portion of the eye is only one-sixth of its anterior surface. The remaining parts of the eye are enclosed in the walls of the bony orbit and cushioned by a layer of orbital fat. This layer of the eye is made up of dense, avascular connective tissue. Close visual inspection of the sclera shows it to con-tain tiny red blood vessels over a white background of collagen fibers.

The sclera makes up most of the outer layer and is described as “glistening.” The cornea is completely clear and controls much of the refractory power of the eye on a constantly occurring basis. It has many pain receptors and other nerve endings, and is made up of a dense matrix of collagen fiber layers. Touch-ing the cornea causes reflexive blinking and tearing. It is the part of the eye that is primarily exposed and often damaged. Even so, it can partially regenerate and repair itself. The cornea lacks blood vessels and cannot be affected by the immune system activities. There-fore, it can be transplanted between humans with nearly no risk of being rejected by the recipient. How-ever, when the cornea is damaged, it may cause blind-ness, even though the eye’s photoreceptors and other functional components remain completely ­normal. Corneal damage should be treated treated immediately in order to prevent serious loss of vision.

Middle Layer

The vascular tunic or uvea includes the choroid coat, ciliary body, andiris. Thechoroid coatis covered bythe sclera, attached to the outermost part of the ret-ina, and has many pigment-producing melanocytes located in greatest numbers near the sclera. It is, there-fore, dark brown in color and rich in blood vessels. It separates the outer layer of the eye from the inner layer, posterior to the ora serrata. The choroid coat is also referred to simply as the choroid and forms the pos-terior five-sixth of the middle layer. Extensive blood vessels of the choroid supply oxygen and nutrients to all layers of the eye. The choroid helps to absorb light and prevent it from scattering and reflecting.

The ciliary body develops anteriorly from the choroid and forms a thick internal ring around the front of the eye, with radiating folds (ciliary processes) and bundles of smooth ciliary muscles. The ciliary processes secrete the fluid that fills the anterior cav-ity of the eyeball, and the suspensory ligaments of thelens attach to the tips of these processes Connective tissue fibers of these ligaments hold the lens, poste-rior to the iris and centered on the pupil. Therefore, light that passes through the pupil also passes through the lens. The ciliary body extends posteriorly to the ora serrata, which is the serrated anterior edge of the thick, inner portion of the eye’s inner layer.

The iris is the visible colored membrane of the eye, lying between the cornea and lens. It is visible through the transparent surface of the cornea and contains extensive blood vessels as well as two layers of pupil-lary muscles, which allow it to reflexively change shape,varying pupil size. The two types of pupillary muscles are called dilators and constrictors, which are con-trolled by the ANS. Parasympathetic activation due to bright light causes the pupils to constrict, as part of the consensual light reflex. Sympathetic activation due todim light causes the pupils to dilate. The anterior iris has no epithelial covering, but instead has an incom-plete layer of melanocytes and fibroblasts.

The iris is continuous with the ciliary body pos-teriorly and contains two smooth muscle layers with groups of sticky elastic fibers that randomly form pat-terns during gestation. The iris actually contains only brown pigment cells, even though the irises of differ-ent individuals appear to be different colors. Darker eyes simply contain more pigment. A newborn baby usually has blue or gray iris color because the iris pig-ment is not fully developed. Eye color is determined­ by genes influencing density and distribution of mela-nocytes on the anterior and interior surfaces of the iris, also because of the density of the pigmented epi-thelium. In an iris with few melanocytes, light passes through and bounces off the pigmented epithelium, making the eye appear blue. People with green, brown, or black eyes have larger numbers of melanocytes in the body and on the surface of the iris. An albino has eyes that often appear very pale gray or blue-gray.

The pupil is the round central opening of the iris and allows light to enter the eye. When the pupil contracts, less light enters, controlling the amount of light the eye needs to see in specific conditions. Close vision or bright light causes the circular sphincter pupillae muscles to contract, which constricts the pupil. Distant vision or dim light causes the radial dilator pupillae muscles to contract, dilating the pupil so more light can enter. Pupil-lary dilation is controlled by sympathetic nervous sys-tem fibers, whereas papillary constriction is controlled by parasympathetic nervous system fibers. The pupils commonly dilate because of an interesting sight, fear, or when we are trying to solve a problem. Pupil constric-tion often occurs because of unpleasant sights or bore-dom. FIGURES 15-11A and B shows the lens and ciliary body viewed from behind and the dilation of the pupil.

Inner Layer

The inner layer of the eye contains the retina and optic nerve. It has approximately 130 million visual receptor cells called photoreceptors located in its outermost layer. These cells convert light energy in a process called transduction. The thin, outer portion is called the pigmented part, while the thick inner layer is called the neural part. The pigmented part absorbs light passing through the neural part, which prevents light from bouncing back through the neural part and causing visual echoes. The pigment cells have critical biochemical interactions with the light receptors of the retina found in the neural part. The neural part also contains supporting cells and neurons that han-dle preliminary processing and integration of visual information. The two retinal layers are normally close together, but not interconnected very tightly. The pigmented part continues over the ciliary body and iris. The neural­ part extends anteriorly­ up to the ora ­serrata and forms a cup-like structure that creates the posterior and lateral boundaries of the posterior cavity. FIGURE 15 -12 shows the layers of the retina and how light waves enter. In total, only a small portion of the eye is involved in photoreception.

1. What happens if the cornea of the eye is touched?

2. What is the structure of the vascular tunic of the eyes?

3. What is the color of the iris in many albino patients?

The retina also contains other neurons that pro-cess light responses and glial cells (glia). The retina has a complex structure of distinct layers, with a central depression (the fovea centralis) in the portion of the retina that produces the sharpest vision and a yellow-ish spot (the macula lutea). The fovea centralis is only 0.4 mm in depth. The optic disc is the point where nerve fibers leave the retina and join the optic nerve in the posterior wall (fundus). This area is not strength-ened by the sclera. Because the optic disc area lacks receptor cells, it is referred to as the blind spot. Light focused on the optic disc cannot be seen. The blind spot of the eye is where the optic nerve leaves the eye.

The neural layer of the retina contains the rods and cones. Rods have long, thin projections and pro-vide black and white vision. Rods are hundreds of times more sensitive to light than cones, providing vision in dim light without color. Cones have short, blunt projections and provide color vision. Cones pro-vide sharper images, whereas rods provide more gen-eral outlines of objects. In the fovea centralis, the ratio of ganglion cells and cones is approximately 1:1. Gan-glion cells that monitor cones are called P cells. They provide information about color, fine detail, and edges of objects in bright light.

Up to 1000 rods may conduct information via bipolar cells to just one ganglion cell. The larger gan-glion cells that monitor information from the rods are called M cells. These are less numerous than the P cells found in the cones. In dim light, M cells function to provide information about object shapes as well as motion and shadows. Certain ganglion cells, known as on-center neurons, are inhibited by light striking the edges of their receptive field but are excited by light that arrives in the center of their sensory field. Off-center neurons function in the opposite way. Together,on-center and off-center neurons help to improve detection of the edges of objects in the visual field. Vision is the only special sense that is not fully func-tional at birth.

Each eye receives a slightly different visual image. This is because the fovea of each eye is 2–3 inches or 5–7.5 cm apart. The view of the opposite side is blocked by the nose and eye socket. By comparing the relative positions of objects within the images seen by each eye, depth perception is achieved. Depth perception is defined as the interpretation of three-dimensional rela-tionships among viewed objects. Visual images from both eyes overlap as we look straight ahead. Visual information of the left eye’s field of vision reaches the visual cortex of the right occipital lobe and vice versa.

Lateral to the blind spot of each eye is an oval region known as the macula lutea (yellow spot). In its center is the fovea centralis, which is the size of the head of a pin. The foveae have enough cone den-sity for detailed color vision. Focusing directly on an object causes its image to fall on the fovea centralis. If an imaginary line were drawn from the object’s center through the center of the eye lens to the fovea would establish the eye’s visual axis. Both rods and cones are located in the deep portion of the retina near a layer of pigmented epithelium. The epithelial pigment helps to keep light from reflecting off surfaces inside the eye. Visual receptors are only stimulated when light reaches them. The rods and cones synapse with approximately six million neurons that are known as bipolar cells. These cells then synapse inside layers of ganglion cells near the posterior cavity of the eye. A horizontal cell network continues across the outer ret-ina between photoreceptors and bipolar cells. Where the bipolar cells synapse with ganglion cells, there is a layer of amacrine cells, which are involved in cellu-lar communications and alter the retina’s sensitivity. FIGURE 15-13 shows the structures of rods and cones.

Rods and cones contain light-sensitive pigments. Rhodopsin is a light-sensitive biochemical in rodsthat is also known as visual purple. In the presence of light, rhodopsin breaks down into a clear protein called opsin and a yellowish pigment called retinal or retinine made from vitamin A. When rods exposed tointense light need time to generate rhodopsin, this is called the phenomenon of dark adaptation. The light-­ sensitive proteins in cones are made of retinal and three different opsin proteins. The three types of cones each contain one of three visual pigments: ­erythrolabe (sensitive­ mostly to red light waves), chlorolabe (sensitive­ mostly to green light waves), or cyano-labe (sensitive mostly to blue light waves). Therefore, the three types of cones are referred to as red cones,green cones, and blue cones. Color blindnessoccurswhen certain cone pigments are lacking.

The process of photoreception involves photons striking the retinal portions of rhodopsin molecules in the membranes of photoreceptor discs. Opsin is then activated, which in turn activates transducin, a G ­protein. This activates phosphodiesterase, an enzyme that breaks down cyclic guanosine monophosphate­. As cyclic guanosine monophosphate levels decline, gated sodium channels close. The dark ­current is reduced and the rate of release of neurotransmitter declines. Each adjacent bipolar cell then senses that the photoreceptor has absorbed a photon.

Visual nerve pathways begin as axons of the ret-inal neurons and leave the eyes to form optic nerves.

They then form the X-shaped optic chiasma with crossed fibers. The fibers from the nasal half of the left eye and temporal half of the right eye form the right optic tract, and fibers from the nasal half of the right eye and temporal half of the left eye form the left optic tract. Most fibers enter the thalamus and synapse in its lateral geniculate body, where visual impulses enter nerve pathways called optic radiations leading to the visual cortex of the occipital lobe.

 After you spend 30 minutes or more in the dark, nearly all visual pigments become fully receptive to stim-ulation, which is known as the dark-adapted state. The visual system is very sensitive at this time, with a single road hyperpolarizing in response to a single photon of light. If only seven rods absorb photons at one time, you experience a “flash” of light. Turning the room lights on at first seems excessively bright, but over a few minutes, sensitivity to light decreases as bleaching occurs. This is soon balanced by the speed at which the visual pigments reform, which is called the light-adapted state.

In the brain stem, visual processing is integrated with movements of the head and neck. This affects how other brain stem nuclei function. The pineal gland along with the suprachiasmatic nucleus use visual information to establish the circadian rhythm. This rhythm is the daily patterns of visceral activity that occurs because of the day–night cycle. This important cycle affects endocrine function, metabolism, blood pressure, digestion, the sleep–wake cycle, and many behavioral processes.

1. Explain the blind spot of the eye.

2. Compare rods and cones.

3. Explain rhodopsin and its location in the eyes.

Lens

Many suspensory ligaments hold the transparent, bicon-vex lens in position behind the iris and pupil. The sus-pensory ligaments pull to adjust the lens and help it to focus, controlled by the ciliary muscles. Contraction of the ciliary muscles is regulated by the parasympathetic fibers of the oculomotor nerves. When the lens is no longer stretched, its elastic fibers recoil and bulge. This provides a shorter focal length, so an object close to the retina can be seen more clearly. The lens is actually enclosed by a thin, elastic capsule and is avascular, like the cornea. The lens enlarges throughout life because new fibers are continually being added. It becomes less elastic, more convex, and denser. This gradually reduces its ability to focus light properly.

Refraction

The eye functions in a similar way to a camera. Like the camera lens, the lens of the eye focuses incom-ing images. As light waves enter the eye, the image of the object is focused on the retina. If not perfectly focused, the image appears blurry. There are normally two steps involved in focusing as light passes through the cornea, and then passes through the lens.

When light waves bend to focus, the phenomenon is called refraction. It occurs when light waves pass at oblique angles from one optical density medium to a different one. Because the lens has a convex surface, it causes light waves to converge (FIGURE 15-14). The lens differs from the cornea in that its high elasticity allows it to change shape and bend light actively instead of on a constant basis allowing fine focusing to occur. Most refraction occurs when light reaches the corneal tissues, which have a density similar to that of water, but additional refraction occurs when the light passes from the aqueous humor into the lens. This addi-tional refraction is required to focus light rays from an object toward a focal point, which is a certain point of intersection on the retina. The focal distance of the lens is the distance between its center and the focal point. Focal distance is determined by the distance of the object from the lens and the shape of the lens. The closer an object is to the lens, the greater the focal dis-tance. The more round the lens is in shape, the more refraction occurs.

Internal Chambers and Fluids

The iris divides the anterior cavity into an ante-rior chamber and a posterior chamber. Theanteriorchamber is located between the cornea and the iris.The posterior chamber can be better understood as being “between the iris and the lens.” A clear, watery fluid called the aqueous humor is secreted by the epithelium on the inner surface of the ciliary body. The entire anterior segment is filled with aqueous humor, which is similar to blood plasma in composition. The aqueous humor circulates from the posterior chamber through the pupil into the anterior chamber and vice versa at the rate of 1–2 L per minute. When aqueous humor leaves the anterior chamber, it filters through connective tissue fibers near the base of the iris to enter the scleral venous sinus or canal of Schlemm. This is a passageway that extends around the eye at the level of the corneal limbus. Collecting channels then bring the aqueous humor to veins in the sclera. In general, this movement is at the same pace as the rate of generation at the ciliary processes. Therefore, aqueous humor is recycled a few hours after it has formed.

The intraocular pressure of the eye can be measured in the anterior chamber, because the fluid pushes against the inner corneal surface. This is usually done byappla-nation tonometry, which involves a small, flattened diskbeing placed on the anesthetized cornea. Normal intra-ocular pressure is between 12 and 21 mm Hg.

The posterior cavity is filled with a clear, ­jelly-like fluid called vitreous humor, which along with collage-nous fibers makes up the vitreous body of the eye, giving it support and shape. The vitreous humor contains large amounts of water, contributes to intraocular pressure by helping to counteract the forces of the extrinsic eye mus-cles, supports the posterior lens surface, holds the neural retinal layer against the pigmented­ layer, and transmits light. The ­vitreous humor forms in the human embryo and lasts throughout life. Unlike the aqueous humor, the vitreous humor does not form or drain continually and is not always in motion.

Focusing Light on the Retina

Light is focused on the retina’s photoreceptors because of the actions of the cornea and the lens. The cornea differs from the lens in that it cannot be adjusted to focus on objects that are up close. The flexibility of the lens is controlled­ by the ciliary body muscles, which are attached via the suspensory ligament. FIGURE 15-15 illustrates the refraction of light by the cornea and lens. The lens becomes thicker yet shorter and more curved to focus on a nearby object. This process is called accommodation (FIGURE 15-16). When a nearby object is viewed, the eyes actually turn inward to con-verge, which focuses the image on each fovea. Conver-gence occurs so much during our daily lives that it cancause straining of the extrinsic eye muscles. Eyestrain and headaches commonly result.