The eye: accommodation, rods and cones

Last time

Before we start: last time, in Neurones, reflex arcs and synapses, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

A living camera

Walk from a bright street into a dark cinema and for a minute you can hardly see. Then your eyes adjust. The eye controls how much light enters and focuses on near and far objects automatically. Today: the parts of the eye, the pupil reflex, accommodation, and rods and cones.

Sense organs

Sense organs are groups of receptor cells that respond to specific stimuli. The eye responds to light, the ear to sound, the tongue and nose to chemicals, and the skin to touch and temperature.

The eye

Here is the eye. The cornea and lens focus light onto the retina. The iris controls the size of the pupil. The ciliary muscles and suspensory ligaments change the shape of the lens. The optic nerve carries impulses to the brain.

What each part does

The cornea refracts, or bends, light. It does most of the focusing. The iris controls how much light enters through the pupil. The lens focuses light onto the retina. The retina contains the light receptors, rods and cones. The optic nerve carries impulses to the brain. The blind spot has no receptors.

The pupil reflex

In bright light, the circular muscles of the iris contract and the radial muscles relax, so the pupil gets smaller. Less light enters, which protects the retina from damage. In dim light, the radial muscles contract and the circular muscles relax, so the pupil widens and more light enters.

Accommodation

Accommodation is changing the shape of the lens to focus on near or distant objects. For a near object, the ciliary muscles contract, the suspensory ligaments go slack, and the lens becomes fatter. For a distant object, the ciliary muscles relax, the ligaments are pulled tight, and the lens becomes thinner.

Accommodation table

Here it is as a table. A fat lens bends light more, for near objects. The examiner usually asks for the ciliary muscles and ligaments together.

Rods and cones

Rods work in dim light and give black and white vision. They are spread over most of the retina. Cones work in bright light and give colour vision. There are three types, for red, green and blue light. The fovea is a small area packed with cones, giving the sharpest, most detailed colour vision.