The eye and the brain

The eye is a sense organ that responds to light. Light is refracted by the cornea and the lens and focused onto the retina, where receptor cells send impulses along the optic nerve to the brain. The brain is a complex organ of billions of neurones that controls our behaviour, and different regions do different jobs.

Key facts

• The cornea refracts light into the eye. The iris controls the size of the pupil, the hole that lets light in. The lens focuses light onto the retina.
• The retina contains light receptors: rods work in dim light, and cones detect colour in bright light. The optic nerve carries impulses from the retina to the brain.
• The ciliary muscles and suspensory ligaments change the shape of the lens. This is accommodation. The sclera is the tough white outer layer.
• Near object: the ciliary muscles contract, the suspensory ligaments loosen, and the lens becomes thicker, so it refracts light more strongly. Distant object: the ciliary muscles relax, the ligaments are pulled tight, and the lens is pulled thin, so it refracts light only slightly.
• In bright light, the circular muscles of the iris contract and the pupil gets smaller, protecting the retina. In dim light, the radial muscles contract and the pupil gets bigger. This is a reflex.
• Short sight (myopia): distant objects are blurred because the image forms in front of the retina; a concave lens corrects it. Long sight (hyperopia): near objects are blurred because the image forms behind the retina; a convex lens corrects it. Contact lenses, laser surgery and lens replacement are other treatments.
• The cerebral cortex deals with consciousness, intelligence, memory and language; the cerebellum coordinates muscle movement and balance; the medulla controls unconscious activities such as the heartbeat and breathing. Scientists study the brain using patients with brain damage, electrical stimulation and MRI scans.

Worked examples