Vision
The eyes’ photoreceptor cells are responsive to the electromagnetic frequencies with the wavelengths ranging from nearly 380 to 750 nm, the visible light to the humans, domestic animals and birds.
The ultraviolet (UV) rays and the rays of short wavelengths than the visible light are generally absorbed by the ozone, and the cloud absorbs rays of higher wavelengths. The frequencies of the visible light are 4 ? 1014 to 8 ? 1014 hertz (Hz). The generated electrical potential within the sensitised photoreceptor cells sends the stimulation to the brain’s visual centre and recognises the image developed by the combinations of various frequencies of light. About 96% of the nine million animal species, including all the vertebrates, have eyes with species-specific morphological characteristics and a diversified nervous system. Hence, different animals and birds have a unique world of vision. This chapter considers the basic similarities in morphology and mechanism of vision of the domestic animals, birds and wild animals.12.1.1 VisualSystem
The eyes are the sensory organs that receive visual information from the environment and transmit them to the visual sensory area of the brain for interpretation. The eyes, equipped with an adjustable pupil and a lens, capture the illumination patterns in the environment as an ‘optical picture’ on a layer of light-sensitive photoreceptor cells in the retina. The retina facilitates feature analysis of the image. It transmits visual signals through the steps of visual processing to the various structures of the brain, where it is finally perceived.
12.1.2 Structure of the Eye
The eye is a ball-shaped sensory organ enveloped by three layers: the outer fibrous tunic, the sclera modified anteriorly into a stratified squamous epithelial layer and the transparent cornea (Fig.
12.1). When preserved at a low temperature, the cornea absorbs water and expels water at a high temperature. Hence, corneal grafting is done immediately after expiration without any preservation process. The fibrous tunic provides mechanical support and protection to the eye. Inner to the sclera lies the vascular tunic consisting of the iris, ciliary body and choroid, which are vascularised and highly pigmented. The choroid provides nutrition to the ocular tissue. The melanocytes present within absorb light and prevent the light that escapes past the retina from being reflected into the retina, where it would burr the sharpness of the image. In nocturnal animals, these pigmented layers contain a reflecting pigment called tapetum lucidum (Latin; tapetum: carpet, lucidum: bright). It allows the retina to make optimal use of available light, but visual acuity reduces. The reflection of light from the tapetum causes the ‘night shine’ from the nocturnal animal’s eye. The innermost layer is a neuroepithelial tunic. The retina consists of the photoreceptor cells (rod and cone cells) and other visual processing cells, such as bipolar, ganglion, horizontal, amacrine and pigment epithelial cells. These specialised photoreceptors absorb light and transduce the light energy into neural signals. The processing of these neural signals that begins in the retina continues as it passes along the pathways to the brain’s occipital lobe, where the visual cortex processes these signals.The eye’s interior comprises two fluid-filled anterior and posterior cavities, separated by an elliptical lens. These structures are transparent to permit light to pass through the eye from the cornea to the retina. The lens, composed of jellylike crystalline unique proteinous substance, is the primary structure responsible for vision accommodation. Disturbances in the oxygenation of the lens-forming substances cause more metabolites, including carbon dioxide, leading to opaque in the lens, called cataracts.
The lens suspends from the suspensory ligament. The convexity of the lens can be altered by the ciliary muscles of the ciliary body. The retina facilitates focusing of images of objects located at varying distances from the eye, and ciliary muscle contraction increases the convexity and focuses near objects. The ability of the eye lens to adjust its focal length by changing curvature for focusing the image to the retina for both near and distant vision is called accommodation.The space between the cornea and the lens is called the anterior chamber and posterior chamber, which lies between the iris and the suspensory ligament. These chambers fill with a clear, water-like fluid called aqueous humour. The ciliary processes of the ciliary body in the posterior chamber produce aqueous humour, and it flows into the anterior chamber through the pupil. It is absorbed into the venous system at the cornea and the iris angle. Disturbances in drainage of the fluid or contraction of extraocular muscle, particularly retractor bulbi, increase intraocular pressure, called glaucoma. A diaphragm of varying sizes separates the anterior and posterior chambers called the iris. The iris is a pigmented structure containing a dilator and constrictor smooth muscles that alter the pupil’s diameter, which is the hole in the iris through which light passes to the retina. Behind the iris is the lens suspended in the eye by the suspensory ligaments. The suspensory ligaments attach to the lens and the ciliary body, a muscular structure located at the base of the iris. The intraocular fluid drains out through the trabecular network and canal of Schlemm.
Fig. 12.1 Internal structure of the eye
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Pupil’s World
The shape of a pupil and eye structure depends on the activities of animals. The round pupil appears in large and wild cats and lions, and also in humans, to find the prey animals from a large field, while domestic cats have an elliptical and vertical pupil to track close to the ground.
Elliptical and horizontal pupils present in prey animals, viz. goats, sheep, horses and deer, have wider and narrower horizontal pupil for a broad field of vision. Usually, large-sized daylight predators have round pupils and forward-facing eyes. In contrast, small cat-like animals that hunt during the day and night comprise vertical slit pupil facilitating depth perception and night vision.Behind the lens is a chamber filled with a hydrogel called vitreous humour that contains hyaluronic acid and collagen fibres to supply the retina’s nutrition. The eye equipped with a pupil adjusts aperture diameter and possesses a lens that focuses light on the retina, where photoreceptor cells receive images. However, the retina is not just converting the image into nerve impulses; it also facilitates feature analysis of the captured image. Feature analysis and visual information processing progressively occur as visual signals are passed to the thalamus, rostral colliculus of the midbrain and visual cortex.
Lacrimal glands near the lateral canthus of the eye produce the tear in response to parasympathetic nerve stimulation. Tears flow over the cornea and are drained into the nose by the nasolacrimal duct. The nictitating membrane or the third eyelid is found in the medial canthus, and it aids in protecting the eye, and its glands also produce tears. The tear is also produced by the harderian glands, one of the lacrimal glands found in many birds and mammals except in carnivores.
The photoreceptor cells are named due to their shape and have four main functional components: the outer segment, inner segment, nucleus and synaptic body. The light-sensitive outer segment contains photochemical rhodopsin in the rod cells and iodopsin in the cone cells, stimulated by low and high light intensity, respectively. These photopigments are incorporated in as many as 700-2000 discrete disc membranes in humans, which are invaginations of cell membranes in each rod or cone cell. The retina contains approximately 80-110 million rod cells and four to five million cones in humans.
The rod and cone cells differ in their distribution, with most cone cells present in the area centralis where the sharpest images are obtained. The inner segment of the photoreceptor contains mitochondria that play an important role in providing energy to the photoreceptor cells. The synaptic body, the terminal portion of the photoreceptor cell, connects with the bipolar cells that form the following linking structure in the vision chain. The bipolar cells are of two types: OFF and ON, which express different glutamate receptors and respond in opposite ways to the glutamate released by the photoreceptors. The OFF and ON bipolar cells synapse on OFF-centre and ON-centre ganglion cells, respectively. Ganglion cells are the only cells that fire action potentials and send visual information out of the retina. The ganglion cells fire in all lighting conditions, but the relative firing rate encodes information about light. A move from dark to light will cause OFF-centre ganglion cells to decrease their firing rate and ON-centre ganglion cells to increase their firing rate. In humans, the photoreceptors (nearly 100 million) transmit signals to bipolar cells (36 million), which send signals to the ganglion cells (one to two million cells per eye), indicating that as information flows centrally, the number of cells carrying the information decreases. The axons of ganglion cells unite to form the optic nerve and, along with blood vessels, leave the retina at the point known as the optical disc or blind spot, as no images are detected in this area.12.1.3 Photochemistry of Vision
The photoreceptor cells contain photosensitive pigments that decompose on exposure to light and, in the process, cause the excitation of the nerve fibre leading from the eye. Photopigments are made up of opsins, a protein (in rods, the opsin is scotopsin, and in cones, the opsin is iodopsin) and an aldehyde of vitamin A, the retinal (retinene). Cones are colour-sensitive photoreceptors associated with day vision, and there are three kinds of cone opsins—blue cone (sensitive at 430 nm), green cone (535 nm) and red cone (575 nm).
Rhodopsin and iodopsin are embedded in the disc membrane of the outer segment. Rod and cone cells contain a G protein called transducin, which gets stimulated by the light-activated photopigments, and transduction of visual signals occurs via opsins. The plasma membrane of the photoreceptor’s outer segment contains cyclic guanosine monophosphate (cGMP)-gated Na+ channels that open in the dark and close in response to light.Additionally, K+ channels present in the inner segment membrane of the rod and cone cells allow the leaking out of K+, thus helping in maintaining proper K+ levels. Na+/K+ ATPases located in the inner segments of the photoreceptor cells maintain intracellular concentrations of Na+ and K+. Generated electrical impulses in the photoreceptor and nerve cells can be measured by placing the electrode on the cornea and the skin, called electroretinography.
12.1.3.1 Photoreceptor Activity in the Dark
In the dark, the 11-cis-retinal fits into a binding site within the interior of the opsin portion of rhodopsin. With the concentration of cGMP being high, an inward Na+ leak depolarises the photoreceptors. This passive spread of depolarisation from the outer segment (where the Na+ channels are located) to the synaptic terminal (where the photoreceptor’s neurotransmitter is stored) keeps the synaptic terminal’s voltagegated Ca2+ channels open. Ca2+ entry triggers the release by exocytosis of the neurotransmitter glutamate from the synaptic terminal while in the dark.
Glutamate release from the photoreceptor terminal in the dark has opposite effects on the two types of bipolar cells (ON and OFF bipolar cells) because they have different types of receptors that lead to different channel responses on binding with glutamate. The ON bipolar cells express inhibitory metabotropic glutamate receptors, and the OFF bipolar cells express excitatory ionotropic glutamate receptors.
In the dark, glutamate released by the photoreceptor activates the ionotropic receptors, leading to sodium flow into the cell and depolarising the membrane potential in the OFF bipolar cells, whereas in the ON bipolar cells, glutamate released by the photoreceptor binds to the metabotropic receptors. The G proteins close cation channels in the membrane, stopping the influx of sodium and calcium and hyperpolarising the membrane potential. Thus, the glutamate hyperpolarises (inhibits) ON-centre bipolar cells and depolarises (excites) OFF-centre bipolar cells.
12.1.3.2 Photoreceptor Activity in the Light
When light strikes the photoreceptor, rhodopsin’s photopigment begins to decompose, leading to an instantaneous change of cis-retinal to all-trans-retinal, which starts to pull away from the scotopsin. The partially split combination of all-trans-retinal and scotopsin is bathorhodopsin, which decays to lumirhodopsin, which decays to metarhodopsin I, followed by metarhodopsin II (activated rhodopsin). The activated rhodopsin acts as an enzyme to activate many molecules of transducin. The activated transducin then activates the intracellular enzyme phosphodiesterase, which degrades cGMP. The hydrolysed cGMP thus moves away from the cyclic guanosine monophosphate (cGMP)-gated Na+ channels resulting in the closure of many Na+ channels. This channel closure stops the depolarising Na+ leak, thereby causing hyperpolarisation that passively spreads from the outer segment to the synaptic terminal of the photoreceptor. Here, the potential change leads to the closure of the voltagegated Ca2+ channels and a subsequent reduction in neurotransmitter glutamate release from the synaptic terminal. This reduction/absence of glutamate causes the ionotropic receptors to close, preventing sodium influx and hyperpolarising the membrane potential in the OFF bipolar cells. In the ON bipolar cells, the absence of glutamate results in the ion channels being open, allowing cation influx and depolarising the membrane potential. Thus, in the light exposure, the reduction of glutamate depolarises (stimulates) ON-centre bipolar cells and hyperpolarises (inhibits) the OFF-centre bipolar cells. The bipolar cells pass on the information about patterns of illumination to the subsequent neurons in the processing chain, the ganglion cells, by changing their rate of neurotransmitter release following their state of polarisation-increased neurotransmitter release on depolarisation and decreased neurotransmitter release on hyperpolarisation. Thus, the hyperpolarising potential and subsequent decrease in neurotransmitter release are graded according to the light intensity. The brighter the light, the greater the hyperpolarising response and the greater the reduction in glutamate release. Thus, photoreceptor, horizontal, bipolar and amacrine cells have graded membrane potential but do not produce action potentials. Ganglion cells, however, produce action potentials that travel along their axons down the optic nerve.
12.1.3.3 Reformation of Rhodopsin
The short-lived active form of the photopigment quickly dissociates into opsin and retinal. The all-trans-retinal is reconverted to 11-cis-retinal by retinal isomerase with energy expenditure. The 11-cis-retinal normally recombines with scotopsin to reform rhodopsin. In the dark, enzyme rhodopsin kinase, rejoins opsin with 11-cis retinal, restore the photopigment to its initial inactive conformation, and the entire cascade turns back to the normal state with open sodium channels.
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Cat’s Vision
The domestic cat has tapetum lucidum to amplify the available light into 130 times more than the human fundus, adapted minimum light detection threshold up to seven times lower than that in humans and large quantities of light-sensitive rod photoreceptors. The large-sized cornea allows more light to enter, and the pupil can dilate about 6 mm more than the human. A distantly located lens produces a smaller but brighter image. All such adaptation facilitates the domestic cat as an efficient adapted domestic animal in nocturnal vision. But it has visual acuity with a range of 20/100 to 20/200; that is, it can see any object at 20 ft distance that a human can see at 100 or 200 ft.
12.1.3.4 Darkand LightAdaptation
In the dark, rhodopsin accumulates in the rods, and in about 20-40 min in humans, the rods become maximally sensitive to light, called dark adaptation. Similarly, when exposed to light, within 5 min, the concentration of rhodopsin decreases in rods, and they become insensitive to light, and the vision is caused by cone stimulation known as light adaptation.
12.1.3.5 Processing of Visual Information in the Retina
The processing of visual information in the retina involves the formation of three images. The effect of light on the photoreceptors changes the first image in the bipolar cells to a second image and, in the ganglion cells, converts to a third image. The horizontal cells alter the signal during the second image, which is further modified by the amacrine cells in the formation of the third image. A little change occurs in the impulse pattern in the lateral geniculate bodies when the third image reaches the occipital cortex.
12.1.4 TypesofVision
Primates, birds, reptiles, amphibians and fish perceive colour to a greater extent than domestic animals.
Monocular vision or periscopic vision: In monocular vision, animals with laterally placed eyes view the objects with one eye at a time independently of the other eye. It occurs due to the wider visual angle between the optic axis and median eyeline, e.g. amphibians and reptiles.
Binocular vision: Primates, carnivores and birds have the power of converging the eyes, thus viewing the same object simultaneously with both eyes. It occurs due to a parallel optic axis and median line, which provide an overlap of the field of vision (Fig. 12.2).
Stereo vision: Primates, cats and other felines have a three-dimensional view because of the small angle between the optic axis and median line of the eye, with each eye viewing the same object at a different angle. The left and right eyes show dissimilarity in viewing an object. These two retinal images are fused in the brain’s visual centre, giving information about the object’s height, width and depth—the ‘3’-dimensional picture.
Animals have a wider peripheral vision than humans because the visual fields of each eye do not completely overlap. In the dog, about 50% overlap of the visual fields occurs, which perceives the middle half of the field of vision. This area of visual overlap provides a binocular vision for the judgment of distances. The field of vision outside the binocular zone is the monocular zone. Binocular vision varies significantly in different animals, reflecting the position of the eyes in front of the head. Both eyes move as a unit to maintain clear binocular vision. In dogs, the binocular field is 60°, and they can see about 240° around their nose, which is 65° and about 360° in horses and 140° and 200° in cats (Fig. 12.2). The binocular field in humans is about 120°.
In equines, the binocular overlap of the visual field is located down the nose of the horse and is limited to between approximately 65° and 80°. The lateral position of the eye provides the horse with a wide panoramic vision, about 340°-360°, that facilitates maximum detection of predators at the expense of the advantages of binocular overlap. Two blind spots have been identified within the visual field, one in front of the forehead and the other directly behind the horse.
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The Best Eyesight in the Animal Kingdom
The eagle has the best eyesight in the animal kingdom, with a visual acuity of 20/5, which means that the eagle can see an object at 5 ft distance while the human see at 20 ft. The four-time stronger visual acuity occurs in eagles due to two foveae having one million cones per millimetre, whereas humans have one fovea with 0.2 million. The eagle has a long-range focal length lens with the compatible structured fovea, which improves long-range vision.
12.1.5 DiversificationofVision
Humans can distinguish nearly one million colours as they have a different permutation of blue, green and red cones in the eyes. Dogs have two types of cone cells sensitive to the wavelength of yellow and blue. Hence, they are acquainted with the images of combinations of these two colours. Cats have three types of cone cells like humans but contain fewer combinations of these cells, so they cannot distinguish the colour combination as humans can do. The red colour appears as dark, and the green colour senses white or grey to the dog and cat.
Domestic animals usually cannot interpret the sharp image due to fewer cones and wide pupils, which make a blur vision resulting in reduced visual acuity than humans. The visual acuity in dogs is 0.2-0.4 times that of humans (20/50-20/ 100), 0.6 in horses (20/33) and 0.2 in cats (20/100), considering the human as 20/20.
Rod cells infer light. Hence, animals bearing more rod cells can see in dim light. Owls have the best night vision due to their larger eye and possession of almost one million rod cells in a square millimetre, about five times greater density than humans. The rod photoreceptors can finely detect the motion and shape of the object. Hence, nocturnal animals perceive movable objects nicely, particularly in dim light. A
Fig. 12.2 Visual field of domestic animals. (a) The dog’s visual field shows 600 binocular and 90° monocular fields with a blind area of 120°.
(b) Visual field of the cat has 140° binocular and small 30° monocular
fields and wider 160° posterior blind area. (c) The horse’s visual field presented 65° binocular and comparatively larger, about 146° panoramic monocular fields with a little 3° blind areas
dog can recognise a movable object at nearly 800 m and a stationary entity at about 500 m.
12.2