The human eye

Visible light is electromagnetic radiation that has a wavelength within a range of about 380 to 800 nm. Light of 380 nm in wavelength is seen as violet, while light of 800 nm in wavelength is seen as red. Light is perceived by the eyes. The impulses caused by light in the eyes are transmitted to the brain by the optic nerve, also called Cranial nerve II. The optic nerves are partially crossed in the optic chiasma, therefore, impulses from the inner sides of each retina cross over to the opposite side of the brain. Impulses from the external side, on the other hand, stay on the same side. After entering the brain, optic nerve fibres run to the visual cortex found in the occipital lobe via the optic tract. The sense of light is formed in the cerebral cortex.

The amount of light entering the eyes is regulated by the pupillary light reflex. In strong light, the pupil is contracted by the smooth muscles of the iris, while in low light, it is dilated. The pupillary light reflex is an unconditioned reflex, its centre is located in the brain stem. Abnormal operation of the pupillary reflex therefore is an indication of injury to the brain stem. The eyeballs are moved by the extraocular muscles. These are striated muscles under voluntary control.

The vitreous chamber forms the main mass of the eye. The cross-section of the eye shows three main layers. The outermost is the sclera, a very durable layer of connective tissue, which continues in the transparent cornea. This is where light entering the eye is refracted by the greatest angle.
The second layer is the choroid, which contains blood vessels that supply the eye. Its continuation at the front of the eye is the ciliary body and the iris. The smooth muscles of the iris are responsible for the pupillary light reflex. The iris contains pigments which lend human eyes their colour.
The smooth muscles of the ciliary body ensure the accommodation of the eye lens to the distance of the object viewed by changing its curvature.
The lens is connected to the ciliary body through the ciliary zonules. The ciliary body is also responsible for producing the aqueous humour, the liquid that fills the anterior chamber. If the drainage of the aqueous humour is insufficient, the pressure increases in the eye, which causes glaucoma. In serious cases, it may lead to blindness.
The innermost layer is the retina. This is where an inverted miniature image is formed of the object viewed; this is created by the lens. Its receptors are called rod cells and cone cells. The area of the retina responsible for visual acuity is called the macula lutea: in its centre there are only cone cells, while around the edge there are more rod cells. The blind spot is the place where the optic nerve passes through the retina. There are no receptor cells here. The impulses produced by the receptors in the retina are transmitted to the brain by the nerve fibres in the optic nerve.

The receptors in the retina are called rod cells and cone cells. They transmit impulses to the bipolar cells, which stimulate ganglion cells. The axons of the ganglion cells form the optic nerve.
The light-sensitive pigment in rod cells is rhodopsin, which consists of a protein called ‘opsin’ and a Vitamin A derivative called ‘retinal’. Rhodopsin is sensitive to light of any wavelength, therefore rod cells cannot differentiate between colours.
The stimulation threshold of rod cells is low, a single photon is enough to stimulate them, thus they work in weak light.
The three types of cone cells are sensitive to red, green or blue light. Their photosensitive pigment is iodopsin, which is similar to rhodopsin but contains a different protein. The stimulation threshold of cone cells is higher than that of rod cells; they are not active in low light. This is why we lose colour vision at dusk.
We can see dim stars better using our peripheral vision, because this way their images are not formed on the macula, but on areas richer in sensitive rod cells. Colour blindness means that a type of cone cell is missing or does not work properly. The most common type of colour blindness is red-green dichromacy, difficulty in distinguishing between red and green colours. When all three types of cones are involved, total colour blindness or monochromacy occurs.

In darkness, cones and rods continuously release a neurotransmitter called glutamate, which blocks bipolar cells. Light causes the receptor cells to hyperpolarise, that is to produce an electric impulse. This stops the release of glutamate, therefore bipolar cells are unblocked and they produce action potentials.