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VISION AND OPTIC REFLEXES

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The eyes are truly remarkable organs. Their light-sensitive retinas are able to convert light rays from objects in the visual field to impulses on the optic nerves, which ultimately give rise to images in the visual cortex of the brain. These images are usually sharp and clear because of the focusing power of the lens. As if the ability to perceive images isn't enough, the eyes are also able to function under widely varying conditions. For example, they are able to adjust to viewing near and far objects, large and small objects, moving and stationary objects as well as objects in bright daylight and under the poor light conditions of night vision. A number of optic reflexes enable the eyes to make these adjustments.

THE EYE AND THE PATH OF LIGHT

Light from an object in the visual field must pass through the cornea, the aqueous humor, the pupillary aperture, the lens, and the vitreous humor before reaching the light-sensitive retina (Fig-1). When we look directly at an object, the light rays from that object are focused on an especially sensitive area of the retina, the fovea. Items in the peripheral visual field are focused on the remainder of the retina. Both kinds of photoreceptors (rods and cones) are located throughout the peripheral retina, while the fovea contains only cones. The optic disk. formed by the confluence of the optic nerve fibers from the nasal (medial) and temporal (lateral) portions of the retina, is devoid of any photoreceptors and is called the blind spot.

Refractive Power and Accommodation of the Lens

Light rays entering the eyes are selectively bent as they pass through the cornea, aqueous humor, lens, and vitreous humor on their way to the retina. While each of these contributes somewhat to bending the entering light rays, it is nevertheless the lens which is responsible for bending the rays sufficiently to focus them on the retina since it is the only refractive surface which can change its light bending (refractive) capability. The other three are all fixed values which give rise to the same amount of bending whether the eye is focused for distant or near vision.

Fig-1

The refractive power L of the lens is measured in diopters and is equal to the reciprocal of the focal length F expressed in meters. The natural tendency for the lens is to assume a curved shape, giving it a high refractive power and dioptric strength. Now when the eye is focused for distant vision (any distance greater than about 20 ft), the lens is pulled relatively flat and has only minimal refractive power. Nevertheless, even in this condition it does bend light and has a refractive power equal to 18 diopters. When the eye focuses on objects closer than 20 ft, the refractive power of the lens increases in order to focus the light rays on the retina. This increase in refractive power in caused by increasing the curvature (and hence the dioptric strength) of the lens. When very young children focus on an extremely close object they can increase their dioptric strength from 18 to nearly 32 diopters. This represents an accommodation of 14 diopters in adjusting from distant to near vision.

Accommodation is accomplished by contraction of the ciliary muscles of the eye. When they contract, the suspensory ligaments to which the lens is attached begin to slacken. This allows the lens to assume its more naturally curved (and hence more refractively powerful) shape (Fig-2).With advancing age our ability to accommodate begins to deteriorate. The lens begins to take on an inelastic and somewhat permanently flattened shape. Consequently the eye is less able to focus on near objects. Accordingly, the nearest point in front of the eye at which an object can be brought into focus (the near point) begins to increase. These relationships are illustrated in Figs-3 and 4.

Fig-2 Fig-3 Fig-4

Notice that accommodation is greatest in the very young child (about 14 diopters), decreases to about 11.5 in the young adult, and is not much better than 2 or 3 in the elderly. Accordingly, the near point increases with age. It is typically about 12 cm in the young adult and often reaches 100 cm in the elderly. Thus we see the familiar pattern of the aging person holding reading material farther and farther away from the eyes in order to be able to focus on it. Of course since 100 cm is beyond the reach of the arms, reading glasses are often required.

Emmetropia, Hypermetropia, and Myopia

In normal vision (emmetropia) parallel light rays from a viewed object are brought to a focus exactly on the retina. The individual perceives the image as sharp, clear, and in focus. However, if the refractive surfaces of the eye can't focus the parallel rays on the retina, the image is blurred and corrective glasses or contact lenses are required. If insufficient light bending occurs, parallel rays aren't sufficiently refracted to be brought to a focus on the retina. This condition is called hypermetropia and the individual is said to be far-sighted. That is, he can focus well on distant objects which don't require much light bending, but can't focus well on objects up close. On the other hand, if the refractive power of the eye is so great as to focus parallel rays in front of the retina, the image is also blurred and the condition is called myopia (near-sightedness).

Neural Control and Accommodation of the Lens

The ciliary muscle is innervated by both the somatic and autonomic nervous systems. Through the former, we are able to voluntarily change the focus from near to far vision by altering the thickness and curvature of the lens. The geometric orientation of the ciliary muscle is such that when it is relaxed, the suspensory ligament is taut and the lens is pulled flat, setting it for distant vision (Fig-2). Since the eyes are set for distant vision most of the time, it follows that the ciliary muscle is usually relaxed. Contraction causes the suspensory ligament to become less taut and allows the lens to assume the more spherical (and hence more powerful) shape.

The shape of the lens is also automatically adjusted as the gaze shifts between near and far vision. The autonomic nervous system regulates automatic adjustments. Parasympathetic stimulation contracts the ciliary muscle and thereby increases the refractive power of the lens. Sympathetic stimulation appears to relax the muscle, decreasing the strength of the lens (Fig-5).

Depth of Focus and Accommodation of the Pupil

Further examination of Fig-5 will show that pupil diameter is also under au­tonomic control. GVE fibers of the oculomotor nerve (III) supply parasym­pathetic innervation to the sphincter muscle of the iris. while sympathetic fibers innervate the radial muscles. Contraction of the former causes the pupils to constrict, while contraction of the latter produces pupillary dilation.

The amount of ambient light to a large extent determines the size of the pupillary aperture. In low-light situations the pupils dilate to allow the available light to reach the retina. In bright daylight the pupils are constricted in order to limit the amount of light entering the eyes. The pupils also automatically constrict when viewing objects at very close range and dilate when the gaze shifts to distant vision. The depth of focus is greatest in bright light when the pupillary aperture is small (i.e., 2 mm). On the other hand, it is minimum in dim light when the aperture is large (i.e., 8 mm). All things being equal, a large depth of focus means that a viewed object can move back and forth a slight distance without going out of focus. On the other hand, if the depth of focus is very small, even the slightest movement will put the object out of focus (Fig-6).

Fig-5 Fig-6

OPTIC REFLEXES

Pupillary Light Reflex

This is the well-known response in which the pupils constrict in bright light. The reflex arc employed in this response is illustrated in Fig-7. If an equal amount of light shines into both eyes, the degree of constriction is generally equal. However, if the light is directed primarily into one eye (i.e., with a flashlight), the pupil of that eye greatly constricts (direct reflex) while the pupil of the other eye shows a much smaller degree of constriction (consensual reflex).

Notice that some of the fibers of the optic tracts pass to the colliculi of the upper midbrain rather than continuing on to the lateral geniculate bodies. These fibers project to both the ipsilateral and contralateral superior colliculi. Short neurons project from the colliculi to the Edinger-Westphal nucleus (an accessory nucleus of III) in the midbrain, which serves as the origin of preganglionic parasympathetic fibers (GVE) of the oculomotor nerves (III). The GVE III fibers, in turn, project to the ciliary ganglia from which postganglionic fibers innervate the sphincter muscles of the iris. If the light is directed evenly into both eyes, the pupillary change is uniform. However, if it is directed primarily into one eye, the neural firing is "weighted" toward that side and the greatest constriction is observed on that side.

Accommodation Reflexes As an Object Is Brought Closer to the Eyes

When an object is brought closer to the eyes we must make visual adjustments (accommodations) in order to keep it in sharp focus. These accommodations include (l) convergence of the eyes, (2) thickening of the lenses, and (3) pupillary constriction. Convergence is necessary in order to keep the viewed object lined up with the visual axis of each eye. This keeps the object focused on the fovea for maximum visual acuity. As a viewed object is brought closer, light rays from any single point source on the object become less parallel, and the refractive power of the lens must be increased in order to focus the image on the retina. The lens accomplishes this by becoming thicker and more spherical, thereby increasing its refractive power. Finally, in order to increase the depth of focus (always a problem at short distances), the pupils constrict. It should be pointed out that one can consciously "override" the accommodation reflexes and prevent their occurrence. However, lacking such conscious effort, they proceed automatically.

Convergence of the eyes is brought about by the following reflex pathway.

GSE fibers of the oculomotor nerves (III) in the nucleus of III in the midbrain tegmentum become stimulated via an undefined route from the visual cortex. These fibers then project to the medial rectus muscles of the eyeballs, causing them to contract. This produces an inward turning of the eyes, keeping the viewed object focused on the fovea for maximum visual acuity. The oculomotor nerves also innervate the superior oblique and superior and inferior rectus muscles of the eyes as well. Thus, by selective stimulation of the appropriate GSE III fibers, specific muscles can be activated, causing appropriate degrees and angles of convergence.

Thickening of the lens and constriction of the pupils are both produced through reflex pathways involving the parasympathetic (GVE) fibers of the oculomotor nerve. The pathway for pupillary constriction is identical to that of the pupillary light reflex, starting from the Edinger-Westphal nucleus. However, it appears likely that signals are relayed to this nucleus via undefined routes from the visual cortex in the occipital lobe. Some of the GVE III fibers which enter the ciliary ganglia pass right through without synapsing to enter the episcleral ganglia (Fig-5). Postganglionic fibers project from here to the ciliary muscle, causing it to contract, thereby thickening the lens and increasing its refractive power.

Fig-7

 

THE CONSCIOUS VISUAL PATHWAY

We previously mentioned that impulses on the optic nerve give rise to both conscious visual images and a variety of purposeful optic reflexes. The conscious visual pathway, which we will examine now, is illustrated in Fig-8. In order to give rise to a conscious image, impulses generated by light stimulation of the retina must be transmitted to areas 17, 18, and 19 of the optic lobe. Area 17 is the primary visual area and initially receives the signals from the optic radiations. However, the visual association area (areas 18 and 19) helps to "make sense" out of the signals reaching area 17.

The retina of each eye is divided into a medial half, the nasal retina, and a lateral half is called the temporal retina. Optic nerve fibers from the nasal retina of each eye cross over in the optic chiasm and terminate in the lateral geniculate body on the contralateral side (Fig-8). Those from the temporal retina do not cross over in the chiasm but continue instead on the same side, terminating in the ipsilateral lateral geniculate body. The optic nerve is composed of that portion of nerve fibers between the eye and the optic chiasm. The continuation of the fibers from the optic chiasm to the lateral geniculate bodies are collectively called the optic tracts. Thus the optic nerves contain fibers from only one eye, while the optic tracts are composed of fibers from both eyes. The optic chiasm lies just anterior to the pituitary gland.

Fibers carrying visual signals from each lateral geniculate body project posteriorly as the optic radiations, terminating in the visual cortex of each occipital lobe. Some of these fibers terminate in the cuneus above the calcarine fissure, and some terminate below it in the lingula. Figure-9 illustrates the conscious visual pathway when a single quadrant of the retina is stimulated. Notice that light from the left visual field of each eye stimulates the nasal retina of the left eye and the temporal retina of the right eye. Further, light from the lower left visual field of both eyes stimulates the upper right quadrants of the nasal retina of the left eye and the temporal retina of the right eye. A little thoughtful examination of Fig-9 will enable you to appreciate the relationships between a point source of light in the visual field and the retinal quadrant which it stimulates.  The image focused on each retinal quadrant is represented on a specific area of the visual cortex. The upper right quadrant of each eye projects to the right cuneus while the upper left quadrant of each eye projects to the left cuneus. Similarly, the lower right quadrant of each eye projects to the right lingula while the lower left quadrants project to the left lingula. Thus the fibers of the optic radiation are spatially oriented with the more superior half of the radiation projecting to the cuneus, while the inferior half projects to the lingula.

Fig-8

Fig-9

 

Injury to the Conscious Visual Pathway

Figure-10 illustrates the defects in viewing the visual field associated with lesions to several specific locations in the conscious visual pathway. A complete lesion in a single optic nerve causes total anopsia (blindness) in that eye. Vision in the opposite eye is unaffected. An interior-posterior lesion through the middle of the optic chiasm interrupts only the crossing fibers (those from the nasal retinae) while leaving those from the temporal retinae intact. Now because the nasal retinae are stimulated by light from the lateral (temporal) vi­sual fields, the visual field loss is called heteronymous bitemporal hemianopia. Hemianopia means that the loss is to one-half of the visual field. Heteronymous means that the loss is to different visual fields for each eye. However, a lesion to the optic tract eliminates visual signals from the nasal retina of the contrala­teral eye as well as the temporal retina of the ipsilateral eye. This condition is also described as hemianopia since one-half of the visual field is eliminated from each eye, However. the hemianopia is homonymous as the loss is to the same visual field of each eye. Lesions of the optic radiation might cause either hemianopia (entire radiation affected) or loss in only one quadrant if cuneal or lingular radiation fibers are selectively damaged. Of course partial loss can occur if the damage to the superior or inferior part of the radiation is only partial.

Fig-10 Fig-11
 

THE RETINA: THE LIGHT-SENSITIVE PORTION OF THE EYE

Photons of light entering the eye must reach the light-sensitive rods and cones of the retina before neural signals are generated in the visual system. Figure­11 diagrammatically illustrates the 10 characteristic layers of the retina. Notice that the direction of light is opposite to the direction of the generated impulses.

Light first reaches the retina by passing through the inner limiting membrane. The impinging light rays next encounter the optic nerve layer which is composed of the fibers of the ganglion cells (optic nerve cells), whose cell bodies comprise the ganglion cell layer. Deeper bipolar neurons synapse with these cells in large dendritic arborizations forming the inner plexiform layer. The peripheral processes of these bipolar neurons receive synaptic input from the rods and cones in the outer plexiform layer. The cell body region of the bipolar neurons makes up the inner nuclear layer. The outer nuclear layer is the region of rod and cone cell bodies. The outer limiting membrane separates the outer nuclear layer from the enlarged photosensitive ends of the rods and cones in the rod and cone layer. Finally, the rods and cones are in close functional contact with pigmented epithelial cells, which comprise the tenth layer of the retina.

Photoreceptor Stimulation and Impulse Production

Light passing through the retinal layers stimulates the enlarged photosensitive portion of the rods and cones before being absorbed by the pigmented epithelium. Once activated, the rods and cones stimulate the bipolar neurons, which in turn excite ganglion cells, producing impulses in the optic nerve. It is likely that a receptor potential is generated on the photoreceptor cell membrane, which then generates impulses in the bipolar and ganglion cells. No impulses have actually been recorded in the rods and cones themselves.

The absorption of light by the pigmented epithelium is important for visual acuity as it prevents reflected light from stimulating rods and cones in other areas of the retina. By minimizing reflected light, the optical quality of the image detected by the array of photoreceptors in the retina is improved. The fovea is capable of generating higher visual acuity than any other part of the retina partly because it has more pigment in its tenth layer. Also, the cones in the fovea are more slender than elsewhere in the retina and thus produce a finer "grain" to their image.

The visual signals generated in the rods and cones converge considerably upon reaching the ganglion cells. Figure-11 gives the impression that each ganglion cell is in direct contact with a single bipolar neuron which is stimulated by a single rod or cone cell. Actually there are many more rods and cones than bipolar neurons, and many more bipolar neurons than ganglion cells. Each retina contains approximately 125 million rods and 5.5 million cones. Thus an average of 140 rods and 6 cones feed visual information into a single ganglion cell. This considerable convergence of the visual signal may possibly add "sharpness" to the visual image.

RHODOPSIN: THE PHOTOSENSITIVE CHEMICAL IN RODS AND CONES

Both rods and cones contain a photosensitive pigment, rhodopsin, which decomposes on exposure to light, releasing sufficient energy to establish a receptor potential on the photoreceptor cell membrane. Rods and cones appear to stimulate the bipolar neurons through chemical transmission. Whether the central state of the bipolar neurons is raised above the excitation threshold is probably a function of the quantity of chemical transmitter released, which itself is probably a function of the magnitude of the receptor potential developed on the photoreceptor.

Rods and cones have different characteristics. Rods are usually narrower (4 to 5 µm) than cones (5 to 8 µm). The photochemicals in the two kinds of photoreceptors are slightly different. Their functional capabilities are also different. Cones function best at high intensities of light such as that associated with daylight vision. Further, they are responsible for color vision and are characterized by high visual acuity, Rods, on the other hand, have higher sensitivity and are more suited for vision at night, when light intensity is low. They don't mediate color vision nor are they able to resolve fine detail. Most of the research on photoreceptor cells has been done on rods because of their relatively great number. Consequently, most of the functional information we have about the chemistry of rhodopsin has been obtained from rod studies. Nevertheless, there is evidence that cones probably function in a similar manner.

Rhodopsin is a complex chemical with a protein portion complexed with a caratinoid pigment, cis-retinine. The difference between rod rhodopsin and cone rhodopsin is in the protein portion. Rod rhodopsin (scotopsin) differs from cone rhodopsin (photopsin) by the number, type, and sequence of its amino acids. In either case, exposure of rhodopsin to light causes it to decompose with the release of sufficient energy to establish a receptor potential on the cell membrane.

The Rhodopsin Cycle

When rhodopsin is decomposed by light, releasing energy, the breakdown products of this decomposition subsequently recombine to synthesize more rhodopsin. The sequence is called the rhodopsin cycle (Fig-12).

Rhodopsin is a stable molecule. However, once exposed to light, it undergoes a configurational change becoming first lumirhodopsin and then metarhodopsin. meta-Rhodopsin is quite unstable and quickly decomposes to trans-retinine and scotopsin. This latter decomposition is accompanied by the release of sufficient energy to produce a receptor potential on the cell membrane. Most of the trans-retinine undergoes enzymatically catalyzed isomerization, becoming cis-retinine. Once formed, cis-retinine combines with scotopsin, reforming rhodopsin.

In dim light it is important that sufficient amounts of rhodopsin are available for decomposition so that the rods are maximally sensitive to any light which is present. On the other hand, when plenty of light is available, much of the trans-retinine is shunted into the pigmented epithelial cells which are in close contact with the photoreceptor cells (Fig-12). Here it undergoes conversion to trans-vitamin A. Of course, shunting trans-retinine into the pigment cells decreases the amount of rhodopsin available for decomposition by light, thus decreasing the sensitivity of the retina. This is certainly a desirable feature when abundant light is available.

trans-Vitamin A in the pigmented epithelial cells is in chemical equilibrium with its isomer, cis-vitamin A. The trans-retinine-trans-vitamin A and cis­retinine-cis-vitamin A interconversions are oxidation-reduction reactions. A constant supply of vitamin A is made available to the epithelial cells by capillaries of the choroid plexus, the vascular layer between the retina and the sclera (Fig-1).

Light Adaptation

A person in the dark for up to 30 min is said to be dark-adapted. That is, retinal sensitivity has increased to a sufficiently high level so that the eyes are sensitive to whatever minimal light is available. When the person subsequently moves into a well-lighted environment, everything is initially very bright due to the high sensitivity of the retina, and visual acuity is initially quite poor. So much rhodopsin is being decomposed that a "flash" of light rather than any fine detail is experienced. After a few seconds to a minute, the eyes adapt to the light and the retinal sensitivity is decreased. This is light adaptation. It is caused by the conversion of retinine to vitamin A and its subsequent storage in the pigment cells. Since the rate-limiting step for the reformation of rhodopsin is the availability of retinine, this effectively reduces the stores of rhodopsin in the photoreceptors and decreases their sensitivity.

Fig-12: The Rhodopsin cycle.

 

Dark Adaptation

A light-adapted person has low retinal sensitivity. That is, rod and cone rhodopsin stores have been reduced to a relatively low level by the conversion of retinine to vitamin A and its subsequent storage in the pigment cells. Now when the light-adapted person suddenly enters a very dark room, the available light is drastically reduced and it becomes necessary to increase the sensitivity of the retina in order to see. This process is called dark adaptation. Dark adaptation is a slower process than light adaptation. The adaptation occurs as stored vitamin A is converted to retinine, which immediately complexes with scotopsin to reform more rhodopsin. This obviously increases the rhodopsin stores and hence the sensitivity of the retina.

Insufficient dietary vitamin A intake can produce chronic low retinal sensitivity since it is a necessary precursor for rhodopsin production. The effects are more severe at night when high retinal sensitivity is necessary. The condition is called night blindness and is directly related to the lack of vitamin A. It takes weeks for a dietary deficiency to bring on symptoms, however, as the liver is capable of storing large amounts of vitamin A.

Figure-13 illustrates the increase in retinal sensitivity as dark adaptation progresses. The bimodal characteristic of the curve is due to the differences in rod and cone adaptability. Cones adapt more quickly than rods. That is, they resynthesize retinine from vitamin A at a greater rate. The initial small increase in sensitivity upon entering a dark room is due to activity in the cones, which start to adapt immediately. However, because of the relatively few cones compared to rods, the overall increase in retinal sensitivity due to cone adaptation is quite small. On the other hand, while rods adapt more slowly, they contribute much more to the overall increase in retinal sensitivity because of their relatively great numbers. Three-quarters of an hour is often required for full adaptation to the dark.

Fig-13:

Color Vision

While only one type of rod is found in the vertebrate retina, there are three types of cones. Each has its own color-sensitive pigment, making cones the photoreceptors responsible for color vision. The difference in the pigments of each type of cone probably lies in the opsin (protein) portion of the cone rhodopsin. Each type of cone responds maximally to a different wavelength of light. These three types are illustrated in Fig-14. Blue cones respond maximally to light of 430 nm (nanometers) wavelength, green cones to 535 nm, and red cones to 575 nm. Figure-14 is a spectral sensitivity curve for each of the three types of cones. Notice that while each cone is maximally sensitive to a specific wavelength, it nevertheless will respond (though to a lesser extent) to other wavelengths as well. Notice also that the response curves of the three types of cones significantly overlap.

The Young-Helmholtz theory of color vision states, in part, that the ratio of the relative responses of each different type of cone stimulated determines the color we see. If monochromatic light of 575-nm wavelength is presented to the eye and is focused on the retina, the response of the cone types in this area of the retina will be different. Examination of Fig-14 shows that the red cones will be maximally stimulated, the green cones will be stimulated to 50 percent maximum, and the blue cones will not be stimulated at all. Therefore the response ratio of the cones (red:green:blue) is 100:50:0. The brain decodes this signal and interprets it as the color yellow. Similarly, a monochromatic light of 535 nm would produce the ratio 65: 100:0 and be interpreted as green. A light of 502 nm would give rise to the response ratio 30:60:30 and would be seen as blue-green, and so on.

Fig-14: The spectral sensitivity of the the types of cones in the retina.

 

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