Sense Organs - Eyes

There are five sense organs in humans - eyes, ears, nose, tongue and skin.

Sensation is the conscious or unconscious awareness of external or internal stimuli. Sensing external environment is extroception.

Example: Reading a page

Sensing of bodys internal condition and position is introception.

Example: Dancers and sport persons maintaining their proper body position by using their internal sense of balance.

Conscious awareness and interpretation of sensations is called Perception.

Eyes

They are the visual sensory organs. They are equipped with the photoreceptors called the rods and cones.

In humans, there is a pair of eyes that are directed forward and give binocular vision. The eyes are situated in the eye sockets or orbits of the skull.

Human Eye and Image Formation

Structure

Each eye is a spherical structure that comprises of a lens and a three-layered wall.

Wall

The three layers of the wall are the outer fibrous coat, the middle vascular layer and the inner nervous layer.

Fibrous coat or tunic

It is the outermost layer that is made up of thick, fibrous connective tissue. This layer is divided into two parts - the posterior larger part called the sclera and the anterior smaller part called the cornea.

Sclera

- it is made up of white, thick and fibrous connective tissue. It is tough and opaque. It forms almost 5/6^th of the outer fibrous layer.

Cornea

- it is a thick, transparent layer made up of elongated cells. It forms the anterior 1/6^th part of the outer layer. It is covered by a thin transparent membrane called the conjunctiva.

Functions of fibrous tunic

i) It provides protection as it is a tough layer.

ii) It gives shape to the eye.

iii) It provides surface for the attachment of eye muscles.

iv) The anterior transparent cornea allows the light to pass through the lens. The bulging shape of the cornea also refracts the light that results in the light focussing properly on the retina.

Vascular coat or Uvea

- it is the middle layer of the eye wall. It richly supplied with blood vessels that supply nutrients to the eye. This layer is opaque and incomplete, occupying the posterior part of the eye. It is made up of three regions-

Choroid

- it is a thin fibrous connective tissue layer that is present inner to the sclera. It is darkly pigmented and highly vascular.

Ciliary body

- at the free end of choroid is present a ring of thick tissue. This tissue is located inner to the junction of sclera and cornea. It is less vascular and lightly pigmented. It has two sets of muscles - circular and meridional, arranged on radiating folds called the ciliary processes. The circular muscles control the size of the eye ball and the meridional muscles are attached to the choroid.

Iris

- it is a circular, muscular, lightly pigmented diaphragm with a hole in the centre. The hole is called the pupil. The iris gives colour to the eyes. The muscles in the iris contract and expand to increase and decrease the size of the pupil, respectively.

Functions of uvea

i) Choroid

a) It prevents reflection of light within the eye as it is pigmented.

b) It supplies nutrients to the eye as it is highly vascular.

ii) Ciliary body

a) The muscles control the size of the eye ball.

b) It provides for the suspension of the lens.

iii) Iris

a) It controls the amount of light passing through pupil.

b) It gives colour of the eye.

Retinal part

It is the innermost layer of the eye wall. It is also an opaque and incomplete layer, present inner to choroid. It is also pigmented layer. It consists of the photoreceptor cells, neurons, ganglions and the pigment cells. The neurons are bipolar. Because of the presence of the neurons, this layer is also called the tunica nervosa.

The different structures are arranged in three layers - photoreceptor layer, intermediate layer and internal surface layer.

Photoreceptor layer

It consists of the rods and cones. The rods and cones are the photoreceptor cells that contain the pigments that trap the incident light rays and convert their energy into action potentials. These action potentials are then transmitted as nervous impulses.

The pigments are present on the membranes that are folded to form vesicles. This portion is called the pigmented portion of the photoreceptors. Each rod and cone cell has mitochondria, nucleus and end bulbs that form the synaptic knobs.

Rods

contain a pigment called rhodopsin that is sensitive to dim light. Thus, this pigment is necessary for night vision (Scotopic vision). Rhodopsin is synthesized with the help of vitamin A. In rods the vesicles are numerous and surrounded by an outer membrane. The pigmented region is cylindrical in shape. Rods have no ability to detect colours.

Cones

contain the pigment called iodopsin (Cone pigments) that is sensitive only to bright light. The cones are sensitive to the colours. Not all animals are able to distinguish colours. Human beings, apes, monkeys, birds, lizards, turtles and some fishes are the only animals that are able to distinguish colours well. Most of the domestic animals are colour blind. In cones the vesicles containing the pigments are fewer and are formed by the outer membrane itself. In cones the pigmented region is cone-shaped. Cones are of three types:

i) Short wavelength - sensitive (blue) cones,

ii) Medium wavelength - sensitive (green) cones,

iii) Long wavelength - sensitive (red) cones.

Intermediate layer

It comprises of the nerve cells or the neurons. These nerve cells are bipolar. One end of the bipolar cells, synapses with the rods and cones and the other end synapses with the ganglions present in the next layer. There are certain nerve cells called the horizontal and amacrine cells that provide connections between rods and cones for the processing of visual information. This layer, therefore, contains synapses between the photoreceptors and the nerve cells and is called the synaptic region.

Internal surface layer

This layer comprises of the ganglions. The dendrites of the ganglions at one end are in contact with the bipolar nerve cells and at the other end, with the optic nerve axons.

Regions of retina

Depending on the distribution of rods and cones, there are two spots on the retina. They are the blind spot and the yellow spot.

Blind spot

is the point on the retina where the optic nerve leaves the eye. At this region, there are no rods or cones and therefore, no image is formed on this spot.

Yellow spot

or Macula lutea is also called the fovea centralis. It is the point that is opposite to the optical axis of the lens and most light rays are focussed on this part. It contains only cone cells and is, thus, most sensitive in bright light. The image formed at this point is the sharpest.

Lens

It is a biconvex, circular, transparent object located just behind the pupil. It serves to focus the light on the retina. It is an elastic structure and its shape can be varied to adjust to objects at varying distances. The adjustment is made with the help of muscles. The lens is held in place with the help of ligament called the suspensory ligament.

Suspensory ligament

- it is a yellow, fibrous,

circular ligament extending from the ciliary body. It holds the lens in place.

The lens and the suspensory ligament divide the eye ball cavity into two chambers.

Aqueous chamber

It is the anterior, smaller chamber between the lens and the cornea. It is filled with a watery secretion of the ciliary body called the aqueous humour. The fluid supplies nutrition to the lens and cornea. It is finally drained into the blood through canal of Schlemm.

Vitreous chamber

It is the posterior, larger chamber between the lens and retina. It is filled with a semi-solid substance called the vitreous humour.

Both the aqueous and vitreous humour serve to refract the light, give shape to the eyes, support the lens and maintain the intra-ocular pressure to keep the eye ball inflated.