The anatomy and physiology of the eye are highly organized and effective despite being small.
Eye Anatomy and Physiology
Eyes are spheroid shape organs fitted into the two orbitals of the skull. There are three major parts in each eye like
- The sclera (fibrous layer)
- Choroid layer
It makes up the outermost part of eye anatomy. It is made of a dense, strong fibrous wall consisting of the sclera that is 5/6th and the cornea that is anterior 1/6th of the eyeball.
The sclera is the outermost layer and it gives a definite shape to the eye. The gap between the sclera and the orbitals of the skull is filled with adipose tissue.
This sclera in the anterior side extends as a transparent epithelium called the cornea. The junction of the cornea and sclera is called the limbus.
The choroid layer
This is the second layer forming the eyeball. It consists of a densely capillary-rich layer supplying blood to the eyeball.
The choroid layer also acts as a black screen which prevents extra reflections inside the eyeball so that we can get a perfect image.
Parts are the iris, ciliary body and ligaments.
This is the extension of the choroid layer towards the anterior side of the eye. This iris is pigmented, due to which there appears color in the eyes.
The pigment is genetically determined and can be black, brown, green, blue, etc.
The iris has a small orifice at the center, which is called the pupil.
This iris is supplied by both parasympathetic and sympathetic nerves.
It dilates due to sympathetic stimulation for distant vision.
It constricts by parasympathetic stimulation for short vision.
This iris separates the aqueous chamber into anterior and posterior chambers.
The lens is a biconvex, transparent and elastic structure that can alter its shape. It helps to focus the objects based on their distance and concentrate the refracted light onto the retina.
When it constricts, it becomes thicker and when it dilates, it becomes thin.
This is the innermost layer of the eye made of nerve cells. This neural coat of the eye is an essential part of the eye.
This retina is the only part of the eye that perceives the light and converts it into electrical energy.
It consists of photosensitive cells, namely the rods and cones.
The rods are light-sensitive and recognize the dark and bright light.
The cones are color sensitive and recognize the colors.
The light received from the lens is converted into a nerve impulse and carried backward as the optic nerve into the brain for further processing.
The point at which the nerves from the retina converge is called the optic spot.
The retinal chamber is filled with vitreous humor.
Accessory parts of the eye
These are the parts that help in the safety and smooth functioning of the eye. They are
- The eyebrows
- The eyelids and eyelashes
- The lachrymal bodies
These are arch-shaped ridges projecting from the frontal bone. They have hair that is bent to one side. These help to prevent sweat, dust, and other waste from falling over the eye and the head.
Eyelids and lashes:
Eyelids are delicate layers of the tissue which cover the eye. They protect the eyes from dust, insects, etc., by their reflection action.
The eyelashes are hair-like outgrowth projecting from the eyelids. They protect the eyes from insects and dust particles.
These are the glands that secrete tears. These tears are rich in salts, lysozyme, and immunoglobulins. They help
a) Protection from irritants and dust.
b) Prevent microbial growth
c) Keep the eye moist and the oil in it slows the evaporation.
Different parameters of the eyeball are given below:
- Anteroposterior diameter – 24mm
- Horizontal diameter – 23.5mm
- Vertical diameter – 23mm
- Circumference – 75mm
- Volume- 6.5ml
Segments and Chambers of Eyeball: Eyeball is divided into two segments, the anterior one and the posterior one.
- Anterior segment: The segmentation inside the eye is based on the position of the lens. An anterior segment consists of a crystalline lens hanging from the ciliary body by zonules and all structures in front of it, viz. iris, cornea, and two aqueous humor-filled spaces, i.e., anterior and posterior chambers.
- Anterior Chamber: The boundaries of the anterior chamber are anteriorly the back of the cornea and posteriorly the anterior surface of the iris and part of the ciliary body. It is 2.5mm deep in adults but varies in hypermetropes and myopes. It communities with the posterior chamber, located just behind it via pupil, i.e., opening through the iris.
- Posterior Chamber: It is a small triangular structure located just behind the anterior chamber guarded by the posterior surface of the iris and parts of the ciliary body anteriorly and by the crystalline lens and its zonules posteriorly.
- Posterior Segment: It consists of all structures that are posterior to the lens, i.e., vitreous humor, retina, and choroid optic disc.
Extraocular Muscles: Extraocular muscles are responsible for the continuous movement of the eyeball to increase the field of vision. They can be divided into two groups of muscles:
- The Recti: There are four recti muscles :
- Superior Rectus
- Inferior Rectus
- Medial Rectus
- Lateral Rectus
- The Oblique: There are 2 oblique muscles :
- Superior Oblique
- Inferior Oblique.
Physiology of Vision: Physiology of Vision consists of three main mechanisms, they are:
- Initiation of vision (phototransduction), which is a function of photoreceptors, i.e., Rods and cones
- Processing and transmission of visual sensation is a function of image processing cells of the retina and visual pathway
- Visual perception is a function of the visual cortex and related areas of the cerebral cortex.
Layers of the retina: Retina is the primary light perceiving the area of the eyeball covering the whole inner surface of the eyeball. They are:
- The inner limiting membrane (ILM);
- the nerve fiber layer (NFL);
- the ganglion cell layer (GCL);
- the inner plexiform layer (IPL);
- the inner nuclear layer (INL);
- the outer plexiform layer (OPL);
- the outer nuclear layer (ONL);
- the outer limiting membrane (OLM);
- the photoreceptor layer (PL),
- The retinal pigmented epithelium (RPE) monolayer.
All these layers have individual functions in the perception of light.
- Phototransduction: The rods and cones are the cells that are designed to cause chemical changes when light falls on them. They serve as sensory nerve ending for visual sensation. When the light falls on them, a cascade of visual reactions starts resulting in the generation of electrical changes. Rods contain a pigment called Rhodopsin or Visual Purple, while cones contain Iodopsin. Rods are responsible for dim light vision or monochromatic vision, while cones are responsible for color vision.
- Photochemical changes: The Photochemical changes include
- Rhodopsin bleaching
- Rhodopsin regeneration
Details about this are not discussed. But a picture is given for helping out to find how NAD is produced, which thereby helps in electricity generation.
- Processing and Transmission of visual Impulse
A receptor potential is developed in the photoreceptors, which is transmitted by electronic conduction, i.e., direct flow of current, not as an action potential, to other cells such as amacrine cells, horizontal cells, and ganglion cells. Now, these ganglion cells transmit electric impulses to an action potential to neurons of lateral geniculate bodies and later to the primary cortex.
- Visual perception
It consists of some thresholds, for example:
- Light sense: It refers to minimal brightness required to evoke a sensation of light, called light minimum. It should be measured after at least a dark adaptation for 20 to 30mins. The two types of visual adaptations are
- Light adaptation
- Dark adaptation
Rods are more sensitive to low light vision, i.e., scotopic vision. Therefore rods are more used in dim light vision, whereas cones are more used in bright light vision, i.e., photopic vision.
The dark adaptation depends on the presence of vitamin A. So, deficiency of Vit A causes night blindness.
The eye is the only organ of photoreception. It lets us see the world. Many diseases are caused to the eye, starting from minor ailments like hypermetropia, myopia, and cataracts to diseases of significant concern like glaucoma, retinopathy, etc.