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Renin Angiotensin Mechanism.

Special Sense Organs: Eye

 Special Sense Organs: Eye.

Introduction:

 Vision

  • The eye is the organ of vision (sight) present in the orbital cavity. 

  • The adult eyeball measures about 2.5 cm in diameter. 

  • The wall of the eyeball consists of three layers.

    1. The superficial fibrous layer.

    2. The middle vascular layer

    3. The inner nervous layer

1) The superficial fibrous layer:

  • It consists of the anterior cornea and posterior sclera. 

  • Cornea is a transparent layer which covers the coloured iris. 

  • The sclera is the ‘white’ of the eye. 

  • The sclera covers the entire eyeball except the cornea and gives shape to the eyeball. 

  • An opening is present at the junction of the sclera and cornea called scleral venous sinus. 

  • A fluid called aqueous humour drains into this sinus.

  • The lacrimal caruncle is the small, pink, globular nodule at the inner corner of the eye. It is made of skin covering sebaceous and sweat glands.

  The front view of Eye

2) The middle vascular layer

  • It consists of three parts: choroid, ciliary body and iris.

  • Choroid consists of various blood vessels which gives nutrients to the posterior surface of the retina. 

  • It also contains melanocytes that produce the pigment melanin which prevents reflection and scattering of light within the eyeball.

  • The anterior portion of the choroid becomes the ciliary body. 

  • The ciliary body consists of ciliary processes and ciliary muscle. 

  • The ciliary processes contain blood capillaries which secretes aqueous humour.

  • The ciliary muscle is a circular band of smooth muscle. 

  • Contraction and relaxation of the ciliary muscle changes the shape of the lens for near and far vision.

  • Iris is attached to the ciliary processes, present in between the cornea and the lens. 

  • Iris consists of melanin which determines the colour of the eye. 

  • The opening in the centre of the iris is called the pupil.

Section of Eyeball

3) The inner nervous layer

  • The inner layer of the eyeball is the retina which covers the posterior part of the eyeball. 

  • It is a very delicate structure and responsible for stimulation by light rays.

  • It consists of several layers of retinal neurons. 

  • The light sensitive layer consists of sensory receptor cells namely rods and cones. 

  • The rods and cones are specialised cells which convert light rays into nerve impulses. 

  • Rods stimulate in dim light and do not produce colour vision while cones stimulate in bright light which produce colour vision.


Interior of the eyeball:

  • The lens divides the interior of the eyeball into two cavities: the anterior cavity and vitreous chamber.

  • The anterior cavity consists of two compartments namely the anterior and posterior chamber. 

  • The anterior chamber present in between the cornea and the iris. 

  • The posterior chamber present in between the iris and lens. 

  • Both the chambers are filled with a watery fluid that nourishes the lens and cornea called aqueous humour.

  • Vitreous chamber is present in between the lens and the retina. 

  • It consists of a colourless, transparent, jelly-like substance called vitreous body.

  • It maintains intraocular pressure and prevents the wall of the eyeball from collapsing.

Physiology of vision:

  • Physiological events of vision consists of following;

    1. Refraction of light entering the eye.

    2. Focusing of image on the retina by accommodation of lens.

    3. Convergence of image.

    4. Photo-chemical activity in the retina and conversion into neural impulse.

    5. Processing in the brain and perception.

  1. Refraction of light entering the eye:

    • Light waves travel parallel to each other but they bend when passing from one medium to another. This phenomenon is called refraction.

    • Before light reaches the retina it passes through the cornea, aqueous humor, lens vitreous humor, so refraction takes place in every medium before it falls on the retina.

    • In the normal eye, light waves focus on the retina.

    • However in myopic eye (short sightedness) light focused in front of retina. So this defect can be treated by using a concave lens.

    • In case of farsightedness, light is focused behind the retina, so no image is formed. This defect can be treated by using convex lens.

  2. Accommodation of lens to focus image:

    • Accommodation is a reflex process to bring light rays from an object into perfect focus on the retina by adjusting the lens.

    • When an object lying less than 6 meter away is viewed, an image formed behind the retina. But due to the accommodation of lens images formed in the retina, we can see the object.

    • For accommodation to view closer objects, the ciliary muscle contract and lens become thick which causes focus on closer objects.

    • Similarly, when a distant object is viewed, the ciliary muscles relax, so the tension of ligament becomes greater which pulls lens and lens become thinner, due to which image forms on retina.

  1. Convergence of image:

    • Human eye have binocular vision, it means although we have two eye, we perceive single image

    • In binocular vision, two eyeballs turn slightly inward to focus a close object so that both images fall on corresponding points on the retina at same time. This phenomenon is called convergence.

  2. Photo-chemical activity in retina and conversion of light into neural impulse:

    • 1. Photochemical activity in rod cells of retina:

    • Each eye contains 125 million rods which are located in neuro-retina.

    • Rods contain light sensitive pigment-rhodopsin.

    • Rhodopsin is a molecule formed by a combination of a protein scotopsin and a light sensitive small molecule retinal (retinene).

    • Retinene (retinal) is a carotenoid molecule and is derivative of vitamin A (retinol).

    • Rods upon interaction with light releases nerve signals.

    • Rods deal with a black and white vision due to rhodopsins inability to interact colored light

    • 2. Photochemical activity in cone cells of retina:

    • Each eye contains 7 million cone cells.

    • The neural activity in cone cells is similar to that of rod cells but there are three different types of cone cells and each cone cell contains different photo-pigment and is sensitive to colored light: red, green and blue light .

    • Cone cell contains iodopsin as photo-pigment which is composed of 11 cis-retinal and photopsin.

    • The perception of color depends upon which cones are stimulated.

    • The final perceived color is a combination of all three types of cone cell stimulated depending upon the level of stimulation.

    • The proper mix of all three colors produce the perception of white and absence of all colors produce perception of black.

  3. Processing of image in brain and perception:

    • All visual information originates in the retina due to stimulation of rods and cones are conveyed to the brain.

    • Photoreceptor cells, bipolar cells and ganglion cells transmit impulse directly from the retina to the brain.

    • The nerve fiber of ganglion cells from both eyes carries impulse along two optic nerves.

    • The optic nerves meet at optic chiasma where fibers from the nasal half of each retina cross-over but fibers from temporal half of each retina do not cross-over.

    • The optic nerve after crossing the chiasma is called an optic tract.

    • Each optic tract continues posteriorly to primary visual cortex in occipital lobe of cerebrum and image is perceived.

Commonly Asked questions:

  1. With a well labelled diagram describing the structure of the eye.

  2. Write a short note on “Physiology of Vision”.
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