2. Introduction Photoreceptors Aims and objectives of the lecture
At the end of the lecture students should be able to:
identify different types of photoreceptors found among invertebrates &vertebrates,
discuss adaptation of animals to nocturnal and diurnal habits,
discuss the physiology of the basic units of compounds eyes,
discuss the physiology of the vertebrates eyes,
discuss how sharp images are formed in the retina,
name the five types of neurons in the retina,
discuss the physiology of the cones and the rods of the retina,
discuss how integration of visual information occurs in the retina.

3. Photoreceptors are group of sensory receptors that can respond to ligh t stimuli.
They transduce (convert) light energy into signals and serve as the sen sory receptors in eyespots and eyes.
Compound eyes of insects and crustaceans consists of visual units call ed ommatidia, which collectively produce a mosaic image.
Each ommatidium has a transparent lens and a crystalline cone that foc us light onto receptor cells called retinular cells.

4. The simplest light sensitive structures in animals are found in certain cnidarians and flatworms.
Eyespots, or ocelli, found in cnidarians and flatworms, detect light but do not form images.
They are eye-spots, called Ocelli that defect light but do not form images. Eyespots are often bowl-shaped clusters of light-sensitive cells within the epidermis.
They may detect the direction of the source of light and distinguish light intensity.
Effective image formation require a more complex eye, usually with a lens.
A lens is a structure that concentrate light on a group of photoreceptors.

5. Compounds eyes found in crustaceans & insects are structurally and functionally different from vertebrate eyes.
Compound eyes are prominent features of adult insects.
The eyes occupy a fairly large portion of the surface of insect heads, and they facilitate a rather wide field of vision.
Compounds eyes are not present in all insects.
They are reduced or absent in parasitic forms, many soil insects, and in some species that live in very dark places, such as caves.

6. The basic unit of compound eyes is Ommatidia (singular Ommatidium).
Ommatidia vary in size and number among different animals.
The sizes of ommatidia vary from about 5 to 40 microns in diameter
The workers of the ant species (Pomera punctatissima) have only one ommatidium in each eye.
There are 20 Ommatidia found in each eye of certain Crustaceans
28,000 Ommatidia in the eye of a Dragonfly.

7. Each facet of compound eyes consist of Ommatidia
In some dragonflies, for example, the dorsal units are considerably larger than the ventral ones.
The surface of compound eyes appears faceted, which means having more faces like a diamond.
Each facet is the convex cornea of one of the eye’s visual unit called Ommatidium.
The optical part of each ommatidium includes a biconvex lens and a crystalline cone.
These structures focus light onto photoreceptors cells called the reticular cells.
These cells have a light sensitive membrane made up of microvilli containing Rhodopsin.

8. Although the compound eye can form only coarse images, it compensates b y following flickers (momentary flash of light) to higher frequencies.
Flies can detect up to about 265 flickers per second. In contrast, the human eye can detect only 45 to 53 flickers/secs; for us, flickering lights fuse abov e these values, so we see light provided by an ordinary bulb as steady and th e movement in motion pictures as
smooth.
The insect’s high critical flicker fusion threshold permits immediate detecti on of even slight movement by prey or enemy.
The compound eye is an important adaptation to the arthropod’s way of life.

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10. Each ommatidium usually contains 8-9 photoreceptors known as retinula cells, where the photon-absorbing, visual pigments are arranged within microvilli (rhabdomeres) of the light-sensitive rhabdome.
These rhabdomeres can be either fused together as a rod- shaped rhabdome, or open (separated) throughout their length.
Rhabdome size plays a crucial role in the apposition eyes of nocturnal insects as enlarging its effective area directly enhances sensitivity.

11. In apposition eyes, there is no separation between the corneal layer and the photorece ptors.
Clear space exists between the corneal layer and the photoreceptors in superposition s eyes. Some authors use the expression "clear zone eyes" for the superposition eyes.
Superposition eyes are generally found in crepuscular and nocturnal insects, while apposition eyes occur in diurnal insects.
The crystalline cone couples the lens and photoreceptors in apposition eyes. Some hig her Diptera have pseudocone eyes.
Crystalline cones do not occur in pseudocone eyes, and optical coupling is by means of a gelatinous substance that is contained in a two-celled structure.
Some apposition eyes may lack solid cones or gelatinous pseudocones.
Visual pigments of all invertebrates such as insects, crustaceans & squids are all rhod opsins.

12. The cornea serves as a fixed lens that focuses light.
The lens of the eye is a transparent elastic ball just behind the iris. It be nds the light rays coming in and brings them to a focus on the retina. 
The lens is aided by the curved surface of the cornea and by the refract ive properties (the ability to bend light rays) of the liquids inside the e yeball.
The anterior cavity between the cornea and the lens is filled with a w atery substances, the aqueous fluid.
The larger posterior cavity between the lens and the retina is filled w ith a more viscous vitreous fluid.

13. Both aqueous & vitreous fluids are important in maintaining the shap e of the eyeball by providing an internal fluid pressure.
At its anterior margin, the choroid is thick and projects medially into t he eyeball to form the ciliary body, which consist ciliary processes and the ciliary muscles.
The ciliary process are gland like folds that project towards the lens an d secrete the aqueous fluid.
The amount of light entering the eye is regulated by the iris.
Iris is a ring of smooth muscles that appear as blue, green, grey, or bro wn depending on the nature of pigments present.

14. The eye is a fluid-filled sphere enclosed by three layers of tissue
The eye is a fluid-filled sphere enclosed by three layers of tissue. The outer la yer is composed of the sclera and the cornea.
The middle layer includes the iris, the ciliary body, and the choroid. The ir is contains two sets of muscles controlling the size of the pupil.
The ciliary body encircles the lens and contains a musculature that adjusts its refractive power.
The choroid is a capillary bed supplying the photoreceptors.
The innermost layer is the actual retina containing the photoreceptors.
En route to the retina, light successively travels through the cornea, the aqueo us humour (the clear and watery liquid within the anterior chamber that regul ates the intraocular pressure), the lens, and the vitreous humour (the thick gel atinous substance that accounts for the size and shape of the globe).

15. The retina corresponds to the light-sensitive film used in a camera.
Outside the retina is the choroid layer, a sheet of cells filled with black pigment that absorbs extra light and prevents internally reflected light from blurring the image.
(Camera are also black on the inside). The choroid is rich in blood ves sels that supply the retina.
The outer coat of the eyeball called the sclera, is a tough, opaque, curv ed sheet of connective tissue that protect the inner structure and helps t o maintain rigidity of the eyeball.
On the front surface of the eye, this sheet becomes the thinner, transpar ent cornea, through which light enters.

16. Here is a simplified summary of the visual pathway:
Light passes through Cornea to Aqueous fluid to Eye L ens to Vitreous body & the image forms on Photorece ptor cells in the Retina and then signal Bipolar cells sig nal, to Ganglion cells to Optic nerve transmits signals t o Thalamus integration by Visual areas of Cerebral cor tex

17. In the human visual pathway, after light passes through the cornea it
(a) stimulates ganglion cells
(b) passes through the lens
(c) sends signals through the optic nerve
(d) depolarizes horizontal cells
(e) hyperpolarizes rod cells

18. THE HUMAN EYE AND NEURAL PATHWAY FOR TRANSMISSION OF VISUAL INFORMATION

19. Light passes through the human eye to photoreceptor cells in the retina.
In this lateral view, the eye is shown partly sectioned along the sagittal plane to expose its internal structures.

20. Light must pass through several layers of connecting neurons in the retin a to reach the rods and cones.
This arrangement lets the rods and cones contact a layer of pigmented ep ithelium that provides retinal, a component of rhodopsin.
The retina consists of 5 main types of neurons  1. Photoreceptors (Rods and cones) synapse on 2. bipolar cells, which makes synaptic contact with 3. ganglion cells Two types of lateral interneurons are the  4. horizontal cells that receive information from the photoreceptors c ells and send it to bipolar cells 5. amacrine cells that receive messages from bipolar cells and send si gnals back to bipolar cells or to ganglion cells.

21. We focus the camera by changing the distance between the le ns and the film.
Power of accommodation, the ability to change the focus for near and far vision by changing the shape of the lens is accom plished by the ciliary muscles, a part of the ciliary body.
To focus on object that are near, the ciliary muscles contracts, causing the elastic lens to assume a rounder shape.
To focus on more distance object, the ciliary muscles relaxes, the lens assume a flattened (ovoid) shape.

22. The amount of light entering the eye is regulated by the iris, a ring of s mooth muscles that appear as blue, green, grey, or brown depending an d the nature of pigments present.
The iris is composed two mutually antagonistic set of muscle fibres.
One set is arranged circularly and contracts to decrease the size of the pupil.
Each eye has six muscles that extend from the surface of the eye ball t o various points in the bony sockets.
These muscles enable the eye as a whole to move and be oriented in a given direction.
Cranial nerves innervate the muscles in such a way that the eyes norm ally move together and focus on the same area. 

23. The formation of focused images on the photoreceptors depends o n the refraction of light by the cornea and the lens.
The refractive power of the former is unvarying but that of the former is adjustable.
The dynamic changes in the refractive power of the lens are referred to as accommodation.
The ability to focus an image on the retina also depends on the shape o f the eye globe.
Adjustments in the size of the pupil also contribute to the retinal image formation.
Narrowing the pupil reduces both spherical and chromatic aberrations. It also increases the depth of field, i.e., the distance within which objec ts are seen without blurring.

24. Vision also requires a brain that can interpret the action potential generated by the photoreceptors.
The brain must integrate information about movement, brightness, location, position and shape of the visual stimulus.
Rhodopsin are the photopigments found in the eyes of cephalopods, molluscs, arthropods and vertebrates.
This receptor called rhodopsin is activated by light.
Rhodopsin is part of a signal transduction pathway that leads to vision in dim light.
Light energy striking a light sensitive receptor cell containing these pigments trigger chemical changes in the pigment molecules.

25. The size, intensity, and location of light stimuli determine initial pro cessing in the retina, Rods and cones can send signals directly to bipol ar cells and bipolar cells can send signals directly to ganglion cells.
Photoreceptor cells can also transmit signal to horizontal cells, and bip olar cells can transmit signals to amacrine cells.
Ganglion cells are inhibited by amacrine cells and can be inhibited in directly by horizontal cells by their action on bipolar cells. Thus, the horizontal and amacrine cells integrate signals literally.
Bipolar, horizontal and amacrine cells combine signals from several p hotoreceptors, each ganglion cell has a receptive field, a specific group of receptors that light must strike for the ganglion cells to be stimulate d.

26. There are two types of photoreceptor, rods and cones, in the retina.
The rods contain the visual pigment rhodopsin sensitive to blue-green lig ht.
Rods are highly sensitive photoreceptors, exclusively active during scoto pic vision.
They are completely inactivated during photopic vision, when cones are f ully active.
Cones contain different visual pigments that are maximally sensitive to l ong (red light), medium (green light) or short (blue light) wavelengths of light.
Cones of different wavelength sensitivity are the basis of our colour perc eption.

27. Rods and cones also differ in the degree of convergence onto ganglion cells.
Rods and cones are unevenly distributed.
The density of rods exceeds that of the cones, except in the fovea whe re the cone density is highest.
The central region of the fovea (foveola) is even rod-free.
The high density of cones with their one-to-one relationship with bipol ar and ganglion cells allow the fovea to mediate high visual acuity.
The superior foveal acuity further benefits from reduced optical distort ion provided by the displacement of the inner nuclear and ganglion cel l layers.

28. Both types of photoreceptors have an outer segment that is composed of membranous disks that contain photopigment and lies adjacent to th e pigment epithelial layer, and an inner segment that contains the cell n ucleus and gives rise to synaptic terminals that contact bipolar or horiz ontal cells.
Absorption of light by the photopigment in the outer segment of the ph otoreceptors initiates a cascade of events that changes the membrane p otential of the receptor, and therefore the amount of neurotransmitter r eleased by the photoreceptor synapses onto the cells they contact.

29. Cones are type of photoreceptor cells most concentrated in the ganglion area
more numerous than rods , responsible for vision in bright light
The outer segments of the rods are cylindrical, while those of the cones are c one-shaped.
But shape is not the only feature that distinguishes cones from rods.
They also differ in the number and arrangement of the discs formed by the f olding of their membranes.
In the rods, there is a stack of about 900 of these discs, which become compl etely detached from the membrane and float freely inside it.
In the cones, there are far fewer discs, and instead of becoming detached fro m the outer segment membrane, they remain attached to it.

30. On the photoreceptor’s disks, light strikes photosensitive molecule s and triggers a molecular cascade whose objective is to control the cell’s cGMP concentration to modulate the photoreceptor’s release of neurotransmitter (glutamate).
In the dark, high cGMP concentration keeps Na+ channels open a nd generates the dark current: the photoreceptor is depolarized.
Light lowers cGMP concentration, which closes Na+ channel: the photoreceptor becomes hyperpolarized.

32. 1) A photon converts a rhodopsin molecule (11cis-retinal + opsin to all -trans-retinal + opsin)
2) This activates 100 molecules of the G-protein transducin.
3) Each of which activates a cGMP phosphodiesterase molecule.
4) Each causes the breakdown of 100’s molecules of cGMP.
5) Which close several hundred Na+ channels.
6) The photoreceptor hyperpolarizes and fewer transmitters are release d.

33. VERTEBRATE EYES FORM SHARP IMAGES
The position of the eyes in the front of the head of humans and certain other vertebrates permits both eyes to be focused on the same object.
The overlap in information they receive results in the same visual infor mation striking the two retinas (light sensitive areas) at the same time.
This binocular vision is an important factor in judging distance and de pth.
Variation of eye position in other vertebrates offer different advantages .
For example the eyes of grazing animals are positioned laterally, enabl ing them to spot a predator coming from behind. The vertebrate eyes can be compared to a camera.

34. COLOUR VISION DEPENDS ON THREE DIFFERENT TYPES OF CONES.
Many invertebrates and at least some animals in each invertebrates class has colour vision. Colour pigmen ts depends on cones.
Human and other primates have 3 different types of cones commonly referred to as blue, green and red co nes. Each cones has a slightly different photopigments.
Although the retinal portion of the pigment molecule is the same as in rhodopsin.
The opsin protein differs slightly in each type of photoreceptor. Each type of cone can respond to light wit hin a considerable range of wavelengths but is named for the ability of its pigments to absorb that wavele ngth more strongly than do other cones.
For example red light can be absorbed by all three of cones, but those cones most sensitive to red act as re d receptors.
By comparing the relative responses of 3 types of cones, the brain can detect colours of intermediate wav elengths.
Colour blindness occurs when there is a deficiency of one or more of the three types of cones. This is usu ally an inherited X-linked condition.

35. Cyclic GMP (Cyclic Guanosine MonoPhosphate, a molecule similar to cyclic AMP ) open nonspecific channels that permit passage of Sodium ion and other cations into rod cell.
This depolarises the rod cell and it releases the neurotransmitter glutamate.
The glutamate hyperpolarizes the membrane of the bipolar so that it does not transmi t messages. Note that the photoreceptor is different from other neurons in that the ion chan nels in its membrane are normally open; it is depolarized and continually releas es neurotransmitter.
The steady flow of ions into the cell when it is dark is referred to as the dark current.
Another unusual characteristic of photoreceptors and also bipolar cells is that t hey do not produce action potentials.

36. Their release of neurotransmitter is graded, regulated by the extent of de polarization.
When light strikes rhodopsin, light is transduced. This process can be considered in three stages.
Light activates rhodopsin.
Light transforms cis-retinal to trans-retinal.
This change in shape causes Rhodopsin to change shape and to break its components, opsin and retinal. This is the light-dependent process in visi on.
Rhodopsin is part of a signal transduction pathway.
When it changes shape, it binds with a G- protein called Transducin.
Transducin activates an esterase that hydrolyses cyclic GMP (cGMP) to GMP thereby reducing the concentration of cGMP.

37. When the concentration of cGMP in the rod decreases the Sodium ions channels begin to close.
Fewer cations pass into the rod cells and it becomes more negative (Hyperpolarized). The rod then releases less glutamate.
The light causes a decrease in neural signals from the rod cells.
The release of glutamate normally hyperpolarizes the membrane of the bipolar cell, so a decrease in glutamate release results in its depolarization.
The depolarized bipolar cell increases its release of a neurotransmitter, which typicall y stimulates the ganglion cells. (Some ganglion cells decrease their firing rate in respo nse to stimulation).
When we move form bright day light to a dark room, or from a dimly lit room to brig ht light, we experience blindness for a few moments while our eyes adapt. In the dim l ight, vision depends on the rods.
However, when we are exposed to bright light, rhodopsin in the rods may be complete ly activated. Dark adaptation occurs as enzymes restores rhodopsin to a form in which it can respond to light.

38. Rods function in dim light, allowing us to detect shape and movement.
They are not sensitive to colours. Human eye blinks 4 million times a year
Because the rods are more numerous in the periphery of the retina, you can see an object in dim light.
Cones respond to light at higher level of intensity, for example day light, and they allow us to perceive fine detail.
Cones are responsible for colour vision; and they are differentially sensitive to different wavelengths (colo urs) of light.
The cones are most concentrated in the fovea, a small depressed area in the centre of the retina.
The fovea is the region of sharpest vision because it has the greatest density of receptor cells and becaus e the retina is thinner in that area.
Light must pass through several layers of connecting neurons in the retina to reach the rods and cones.

39. These photoreceptors are actually nothing more than highly specialized ci lial cells whose outer and inner segments are joined by a connecting ciliu m.
In the inner segments, as in the outer ones, there are some notable anatom ical differences between rods and cones.
The distribution of rods and cones varies from one point to another on the retina's surface.
There are very few cones around the periphery, where rods predominate. I n contrast, in the central region of the retina, called the fovea, there are no rods at all.
That is why you turn your eyes to make an object that you want to look at fall within this area of greater acuity within your field of vision.

40. How is light converted into the neural signals that transmit informati on about environmental stimuli into pictures in the brain?
Rod cells are so sensitive to light that they can respond to a single ph oton.
Rhodopsin in the rod cells & some very closely related photopigmen ts in the cone cells are responsible for the ability to see.
Rhodopsin consists of opsin, a large protein that is chemically joi ned with retinal, an aldehyde of vitamin A.
Two isomers of retinal exist: the cis form, which is folded, and the tr ans form, which is straight.

41. The pattern of neuron firing in the retina appear to be very important.
The signals sent to ganglion cells depend on spatial pattern and timing of the l ight striking the retina.
Ganglion cells receive signals from specific type of visual stimuli such as col our, brightness and motion and transmit this information about the characteris tic of a visual image.
Ganglion cells produce action potentials in contrast with rods and cones and most other neurons of the retina that produce only graded potentials.
Axons of ganglion cells form the optic nerves that transmit information to the brain by way of complex, encoded signals.
The optic nerves cross in the floor of the hypothalamus, forming an X-shaped structure called the optic chiasm.

42. In the dark, opsin binds to retinal in the cis form
In the dark, opsin binds to retinal in the cis form. Cyclic GMP (cyclic guanosine mono phosphate, a molecule similar to cyclic AMP) opens nonspecific channels that permit passage of Na+ & other cations into the rod cell.
This process depolarizes the rod cell, which releases the neurotransmitter glutamate.
The glutamate hyperpolarizes the membrane of the bipolar cell so it does not transmit messages.
Note that the photoreceptor differs from other neurons in that the ion channels in its m embrane are normally open; it is depolarized & continuously releases neurotransmitter .
The steady flow of ions into the cell when the environment is dark is called the dark c urrent.
Another unusual characteristic of photoreceptors, & of bipolar cells, is that they do not produce action potentials.

43. Their release of neurotransmitter is graded, regulated by the extent of depolarization.
When light strikes rhodopsin, light energy is transduced.
This process can be considered in three stages:
Light activates rhodopsin. Light transforms cis-retinal to trans-retinal. Rhodopsin cha nges shape and breaks down into its components, opsin and retinal. This is the light-de pendent process in vision.
Rhodopsin is part of a signal transduction pathway. When it changes shape, it binds w ith transducin, a G protein. Transducin activates an esterase that hydrolyzes cyclic GM P (cGMP) to GMP, reducing the concentration of cGMP.
When the concentration of cGMP in the rod decreases, its Na+ channels begin to clos e. Fewer cations pass into the rod cell, and it becomes more negative (hyperpolarized). The rod cell then releases less glutamate. Thus, light decreases the number of neural si gnals from the rod cells.

44. The release of glutamate normally hyperpolarizes the membrane of the bi polar cell, so a decrease in glutamate release results in its depolarization.
The depolarized bipolar cell increases its release of a neurotransmitter, wh ich typically stimulates the ganglion cell.
When you move from bright daylight to a dark room, you experience “bli ndness” for a few moments while your eyes adapt.
During exposure to bright light, rhodopsin breaks down, decreasing the se nsitivity of your eyes.
Dark adaptation occurs as enzymes restore rhodopsin to a form in which it can respond to light. In dim light, vision depends on the rods.
Conversely, when you move from the dark to bright light, your eyes are v ery sensitive and you may feel “blinded” by the light.

45. Many invertebrates and at least some animals in each vertebrate class hav e color vision.
Colour perception depends on cones.
Most mammals have two types of cones, but humans and other primates h ave three types: blue, green, and red. Each contains a slightly different ph otopigment.
Although the retinal portion of the pigment molecule is the same as in rho dopsin, the opsinprotein differs slightly in each type of photoreceptor.
Each type of cone responds to light within a considerable range of wavele ngths but is named for the ability of its pigment to absorb a particular wav elength more strongly than other cones do.

46. For example, red light can be absorbed by all three types ofcones, but thos e cones most sensitive to red act as red receptors.
By comparing the relative responses of the three types of cones, the brain can detect colors of intermediate wavelengths.
Colour blindness occurs when there is a deficiency of one or more of the t hree types of cones.
This is usually an inherited X-linked condition