Sense organs: the human eye and vision

Learning objectives

By the end of this lesson, you should be able to:

• explain the role of receptors and sense organs
• identify the main structures of the human eye
• trace the path of light through the eye
• explain accommodation for near and distant vision
• describe common defects of vision and their correction

Sensory receptors and response

A sense organ contains specialised receptor cells that detect a particular type of stimulus. A stimulus is a change in the internal or external environment. Receptors convert stimulus energy into electrical signals that travel along sensory neurones to the central nervous system, where information is interpreted and an appropriate response may be coordinated.

The eye detects light. Its photoreceptors respond to light intensity and wavelength, allowing vision, colour discrimination and detection of movement.

Structure of the human eye

The eyeball is protected by the bony orbit, eyelids, eyelashes and tear fluid. The main structures include:

• Sclera: tough, white outer coat that protects the eye and maintains its shape.
• Cornea: transparent curved front surface; it refracts most of the light entering the eye.
• Choroid: pigmented, blood-rich layer that supplies the eye and absorbs stray light.
• Iris: coloured muscular structure that controls the size of the pupil.
• Pupil: opening through which light enters.
• Lens: transparent, elastic structure that fine-tunes the focus of light onto the retina.
• Ciliary muscles and suspensory ligaments: adjust lens shape during accommodation.
• Retina: light-sensitive layer containing rods and cones.
• Fovea (yellow spot): region with a high density of cones and sharpest colour vision.
• Optic nerve: carries impulses from the retina to the brain.
• Blind spot: point where the optic nerve leaves the eye; it has no rods or cones.
• Aqueous humour: watery fluid in the front chambers that helps maintain pressure and supplies nutrients to the cornea and lens.
• Vitreous humour: transparent gel that fills the large cavity behind the lens and helps maintain the shape of the eyeball.

How vision occurs

Light reflected from an object enters through the cornea, passes through the aqueous humour and pupil, and is focused by the lens through the vitreous humour onto the retina. The image formed on the retina is real, inverted and smaller than the object. Rods and cones convert light into electrical signals. These signals pass through retinal neurones and leave through the optic nerve. The brain processes the information so that the person perceives the object in its correct orientation.

Rods are very sensitive to dim light and are important for night and peripheral vision, but they do not provide colour vision. Cones function best in brighter light and are responsible for colour vision and high visual acuity. Different cone types respond most strongly to different ranges of wavelengths.

Accommodation

Accommodation is the adjustment of lens curvature so that objects at different distances are focused on the retina.

For a near object:

1. Ciliary muscles contract.
2. Suspensory ligaments become less taut.
3. The elastic lens becomes thicker and more convex.
4. Its refractive power increases, bending light more strongly.

For a distant object:

1. Ciliary muscles relax.
2. Suspensory ligaments become taut.
3. The lens is pulled thinner and less convex.
4. Its refractive power decreases.

The pupil also changes size to regulate the amount of light entering the eye. In bright light, circular muscles of the iris contract and the pupil becomes smaller. In dim light, radial muscles contract and the pupil becomes larger.

Common defects of vision

Cataract is clouding of the lens, which reduces the amount and quality of light reaching the retina. It is not corrected simply by changing lens power; treatment may involve surgery to replace the cloudy lens.

WASSCE examination focus

Learn the path of light and the functions of the cornea, iris, pupil, lens and retina. In accommodation questions, state what happens to the ciliary muscles, suspensory ligaments and lens. Do not reverse the lens corrections: a concave lens corrects myopia, while a convex lens is commonly used for hyperopia.

Quick check

1. Which part of the eye controls pupil size?
2. Where are the rods and cones found?
3. What happens to the lens when focusing on a near object?
4. Which lens corrects myopia?
5. Why is the blind spot insensitive to light?

Answers: 1. The iris. 2. The retina. 3. It becomes thicker and more convex. 4. A concave (diverging) lens. 5. It is the point where the optic nerve leaves the eye and contains no photoreceptors.