Sense organs and stimuli

A stimulus is a change in the surroundings that can be detected. Sense organs contain groups of specialised cells called receptor cells, which respond to particular types of stimulus.

Different sense organs detect different stimuli:

The eyes detect light.
The ears detect sound.
Receptors in the skin detect touch and temperature.
Receptors in the nose and tongue detect chemicals, allowing us to sense smells and tastes.

Receptor cells are specific: each type responds to a particular kind of stimulus. For example, light-detecting receptors in the eyes do not detect sound. This specificity allows different sense organs to detect different changes in the surroundings.

Structure and function of the eye

Light enters the eye through the cornea, a transparent surface that refracts (bends) the light. It then passes through the pupil, an opening surrounded by the iris. The iris changes the size of the pupil, controlling how much light enters the eye.

Behind the pupil, the lens focuses the light onto the retina at the back of the eye. The retina contains light receptors that respond to light; some are sensitive to different colours. Signals from the receptors are carried as nerve impulses along the optic nerve to the brain.

The blind spot is the point where the optic nerve leaves the eye. This part of the retina has no light receptors, so it cannot detect light.

Pupil reflex

The pupil is the opening that lets light into the eye. The coloured ring around it, the iris, changes the pupil’s diameter in response to light intensity. This response is called the pupil reflex.

In bright light, the iris makes the pupil narrower, so less light enters the eye. In dim light, the iris makes the pupil wider, so more light can enter.

Pupil diameter therefore changes in the opposite direction to light intensity: as light intensity increases, the pupil narrows; as light intensity decreases, it widens.

Antagonistic iris muscles

The iris is the coloured part of the eye. Its muscles change the diameter of the pupil, the opening through which light enters. A narrower pupil lets less light into the eye; a wider pupil lets more light in.

The iris has two sets of muscles: circular muscles, which run around the pupil, and radial muscles, which run outwards from it. They act antagonistically: when one set contracts, the other relaxes.

In bright light, the circular muscles contract and the radial muscles relax. The pupil narrows, so less light enters the eye.
In dim light, the radial muscles contract and the circular muscles relax. The pupil widens, so more light enters the eye.

Accommodation of the eye

The eye focuses light from objects at different distances onto the retina, the light-sensitive layer at the back of the eye. To do this, it changes the shape of its lens. This adjustment is called accommodation.

The shape of the lens determines how strongly it refracts, or bends, light. A thicker, more curved lens refracts light more strongly than a thinner, less curved lens. The ciliary muscles and suspensory ligaments change the lens shape by altering the tension on it.

Focusing on a near object

The ciliary muscles contract, reducing tension in the suspensory ligaments. The lens becomes thicker and more curved, so it refracts light more strongly. This brings light from the near object into focus on the retina.

Focusing on a distant object

The ciliary muscles relax, making the suspensory ligaments taut. The ligaments pull the lens thinner and less curved, so it refracts light less strongly. This brings light from the distant object into focus on the retina.

Rods and cones in the retina

The retina is a light-sensitive layer at the back of the eye. It contains two types of light receptor, rods and cones, which respond to light and help provide vision under different conditions.

Rods: vision in dim light

Rods are more sensitive to light than cones, so they support vision in dim conditions. They do not provide colour vision. Rods are found mainly away from the centre of the retina and are more numerous towards its outer parts.

Cones: colour and detailed vision

Cones enable colour vision. There are three kinds of cone, which absorb light of different colours. Comparing their responses allows the visual system to distinguish colours.

Cones are concentrated in the fovea, a small central region of the retina. The fovea contains cones rather than rods and provides the sharpest, most detailed vision. Cones are also found outside the fovea.