Mammalian eye: Structure and mechanism

IntroductionThe human eye is a complex part of the body that is used for

seeing. Eyes enable people to perform daily tasks and to learn about the world that surrounds them. Sight, or vision, is a rapidly occurring process that involves continuous interaction between the eye, the nervous system, and the brain. When someone looks at an object, what he really sees is the light reflected from the object.

This reflected light passes through the lens and falls on the retina of the eye. Here the light induces nerve impulses that travel through the optic nerve to the brain and then over other nerves to muscles and glands. Our eyes are actually limited to only a very narrow band of light within the electromagnetic spectrum, we see only a fraction of what is really there.

Structure

• The adult human eye ball is nearly a spherical structure. These are paired structures lodged in the sockets or orbits of the skull. It is about one inch in diameter. Only a circular window measuring about half inch across can be seen from outside and is termed as cornea.

• A compact nerve, the optic nerve, comes out from the posterior part of the eye ball and carries nerve impulses to the brain. The eye is lodged in a protective bony case known as orbital cavity. There is a thick layer of fat between the eye and the orbital bones which act as shock absorber.

• A set of six muscles attach the eye to the orbital bones. These muscles move the eye in the orbit Each eye is about one inch in diameter and consists of three concentric layers:-

• Sclera• Choroid • Retina

Structure of the human eye

Sclera or sclerotic: • The sclerotic is made up of hard coat of fibrous material.• It is the outermost protective covering layer. • It gives shape to the eye and prevents the inner parts from minor injuries.• The part of the sclerotic visible from outside is known as white of the eye. • In the center of the white of the eye, there is a window known as cornea.• The cornea covers the pupil and the iris which lie just behind. The cornea is

covered by a delicate, thin and transparent membrane called conjuctiva.

Choroid:• It is the middle coat of eye ball that lies beneath the sclera and is

incomplete anteriorly.• It is made up of highly vascular connective tissues, pigmented cells and

smooth muscles.• In front of the eye ball it separates from the sclera and forms a circular disc

called iris which is perforated by a rounded pupil.• Behind the pupil lies a biconvex lens. It is held in position by suspensory

ligaments.• The choroid and the retina along the peripheral margin of the iris jointly

forms a folded muscular band called ciliary body.

Retina:• It is the innermost coat of the eyeball.• It is the light sensitive screen of the eye and consists of two types of

photoreceptor cells called rods and cones. These are connected posteriorly with the optic nerves. The point from where the optic nerves leave the retina is called the blind spot.

• Near the blind spot, there is a slightly depressed area on the retina that contains cone cells only. This is called fovea centralis or yellow spot. The sharpest image is formed at the fovea.

Rods and Cones:

The rods and cones are modified nerve cells and are the visual receptors of the eye. When stimulated by light, they set up impulses in the nerve cells which are transmitted to the brain through the optic nerves. Rods cells have a visual pigment called rhodopsin and are sensitive to dim light. So, these help in twilight (scotopic) vision during night. Cones have a visual violet pigment called iodopsin. This is sensitive to bright light.The cone cells also contain some other pigments for color vision like erythrolabe – sensitive to red light, chlorolabe – sensitive to green light and cyanolabe – sensitive to blue light.

Mechanism of Vision• The light rays in visible wavelength focussed on the retina through the cornea

and lens generate potentials (impulses) in rods and cones.• The photosensitive compounds (photopigments) in the human eyes is

composed of opsin (a protein) and retinal (an aldehyde of vitamin A).• Light induces dissociation of the retinal from opsin resulting in changes in the

structure of the opsin. This causes membrane permeability changes.• As a result, potential differences are generated in the photoreceptor cells.

This produces a signal that generates action potentials in the ganglion cells through the bipolar cells.

• These action potentials (impulses) are transmitted by the optic nerves to the visual cortex of the brain, where the neural impulses are analyzed and the image formed on the retina is recognized based on earlier memory and experience.

Accommodation• Accommodation  is the process by which the vertebrate eye changes optical

power to maintain a clear image or focus on an object as its distance varies.• Accommodation acts like a reflex, but can also be consciously controlled.

Mammals, birds and reptiles vary the optical power by changing the form of the elastic lens using the ciliary body (in humans up to 15 dioptres). Fish and amphibians vary the power by changing the distance between a rigid lens and the retina with muscles.

• The ability of the eye to focus objects lying at different distances is called the power of accommodation of the eye.

• To focus on distant objects the ciliary muscles relax making the eye lens thin.As a result the focal length of the eye lens increases and we see the distant objects.

• But to focus on nearby objects the ciliary muscles contract making the eye lens thick. As a result the focal length of the eye lens decreases and we see the nearby objects.

• Near point or least distance of distinct vision is the point nearest to the eye at which an object is visible distinctly.

• Far point of the eye is the maximum distance up to which the normal eye can see things clearly. It is infinity for a normal eye.

Accommodation of human eye

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