Physiological anatomy of eye
| SCLERA | Covering of the eye – Protective in function, Tough in nature. |
| CORNEA (stratified squamous) | Transparent Structure, anterior modification of sclera. |
| CHOROID | Vascular and Pigmented area of sclera in its posterior part. |
| RETINA | Innerside of choroid, has PHOTORECEPTORS. |
| DARK PIGMENT | Animal with day light vision, to absorb light which has to pass receptor without stimulating, present between CHOROID and PHOTORECEPTORS. |
| TAPETUM | Causes nightshine in nocturnal animal by reflecting the light viz REFLECTING PIGMENT. (allows use of optimum light by retina) |
| LENS | Causes accomadation, made up of elastic capsule filled with jelly like substance, suspended by suspensory ligament. |
| SUSPENSORY LIGAMENT | Muscular structure connecting lens and ciliary body. |
| CILIARY BODY | Convexity of the lens is altered to focus the images of varying distance and is achieved by ciliary muscles and its contraction increases convexity and focusses near objects. |
| ANTERIOR CHAMBER | Between CORNEA and LENS |
| POSTERIOR CHAMBER | Between IRIS and Suspensory Ligament |
| AQUEOUS HUMOR | clear watery fluid, from ciliary processes of ciliary body in the posterior chamber, flows through PUPIL to anterior chamber. Absorbed into veins, passes through CORNEA and IRIS. Accumulation of aqueous humor due to obstruction in absorption causes increased intraoccular pressure viz GLAUCOMA. |
| IRIS | Diaphragm separating Anterior and Posterior chambers. Pigmented structure has dilator and constrictor muscles (smooth) |
| PUPIL | Hole in the Iris, whose diameter can be modified by smooth muscles and allows the varying intensity of light to strike retina |
| VITREOUS HUMOR | HYDROGEL with mucopolysaccharide, hyaluronic acid and collagen fibers filling the chamber behind IRIS, nourishes retina. |
| NEURAL RETINA | Present behind vitreous humour. |
| OPTIC DISK | Area where axons of retinal ganglia passes to brain. Has optic nerve (cranial nerve), arteries and veins. Nutrition of retina is taken care by CHOROID VESSELS. |
| LACRIMAL GLANDS | Production of tears from this, near lateral canthus. Parasympathetic stimulation ends in tears and are drained into nose by NASO-LACRIMAL DUCT. |
| NICTITATING MEMBRANE | 3 EYE LID, Present in medial carithus, protects eye and also produce tears. |
| HARDERIAN GLANDS | Lacrimal gland, absent in carnivores, found in birds and mammals. |
Retina
| RETINA (part of CNS) | Has 5 cell types,Photoreceptor cells has two types RODS, CONES.BIPOLAR CELLS with INTERNEURONS – Connects rods and cones with ganglion cells by synapsing.GANGLION CELLS : Ganglion cell layer-single layer of multipolar ganglion cells with scattered neuroglia cells with some blood vessels.Transparent and Unmyelinated carry impulses to brain via optic nerve.AMACRINE CELLS and HORIZONTAL CELLS : are interneurons lie between the bipolar and the ganglion cells.The light has to pass all the other layers like the nerve fibre layer, ganglion cell layer, etc., to reach the layer of rods and cones. |
| FOVEA | Area present in the back of the retina – Light rays are not distorted in this area, devoid of ganglion and bipolar cells.Light rays directly fall on retinal photoreceptors. Provides sharp visual image.FOVEA IS ABSENT IN DOMESTIC ANIMALS. |
| CONES | They are present in dense in the central portion of the retina – ( in primates) cones are best in day light – PHOTOPIC VISION.They are color sensitive.Similarly to fovea more no. of cones are present in MACULA or CENTRALIS in other animals. |
| RODS | Extremly sensitive to light, present in the periphery of retina.Responsible for NIGHT/SCOTOPIC vision, Sensitive to blue – green light. |
| BLIND SPOT | No photoreceptors at the origin of optic disk which is viz Blind spot.(Free from Rods and cones). |
Structure of rods and cones
Rods and cones have 3 parts-
- Outer segment – Photoreceptor part, has stacked membrane discs with VISUAL PHOTOPIGMENTS.
- Inner segment – Has nucleus, mitochondria and cytoplasm.
- Synaptic Termial – Synapse with bipolar cells.
Biochemistry of rods and cones
- Photosensitive pigment – rhodopsin / visual purple : Present in rods
- Made up of proteins viz opsins
- In Rods it is called scotopsin which consist of an aldehyde of vitamin A called retinal.
- As light strikes, Rhodopsin is transformend from CIS to TRANS and by hydrolysis, gets splits into scotopsin and retinal.
- Retinal is enzymatically changed into retinol (alcholic form) and is stored as retinyl ester in the pigment cell
- Stability of rhodopsin and its functions depends on the presence of CIS form of vitamin A. Formation of Rhodopsin is continuous, in the presence of Isomerase, retinal joins with scotopsin to form rhodopsin.
- During night/darkness- rhodopsin accumulates in rods, and have maximum sensitivity to light, this is known as dark adaptation. When exposed to light concentration of Rhodopsin decreases in rods and so rods become insensitive to light and the vision is caused by cone stimulation known as light adaptation. A vitaminosis causes night blindness viz nictalopia.
- Cones: Colour sensitive photoreceptor responsive for day vision photopigment in cones are IODOPSIN consists of opsin and RETINAL IODOPSIN varies from SCOTOSIN, in their protein part.
- Cone opsins are of 3 types
- Blue cone (430 nm)
- Green cone (535 nm)
- Red cone (575 nm)
- Difference cones respond to different wavelength. The primary colours are specific with cone. Various colour sensation are made possible by the combination of different cones. Colour blindness endup is due to lack of different cones.
- Domestic animals has more rods and birds have more cones.
- ELECTRO RETINOGRAM – ERG : Recording of electrical changes in the retina by flashing the light. Has 3 waves a,b and c;
- a wave : originates from rods and cones.
- b wave: from glial cells, Amacrine and bipolar cells.
- c wave: from Pigment epithelium.