Stimuli, receptors and the need to respond

1A gecko, a dog and a thermostat

A gecko on a classroom wall freezes when a shadow crosses it. A dog lifts its head at a whistle that no person in the room can hear. Inside your own body, a rise of a few tenths of a degree in blood temperature is enough to start sweating. Each of these is a response to a stimulus, and each begins with a receptor. In Senior 3 you studied the sense organs and the reflex arc. This unit asks the A-level questions: how a receptor turns light, sound or a chemical into a nerve impulse, why receptors stop responding to a steady stimulus, and how the structure of each sense organ suits the stimulus it detects.

2Why animals must respond

An animal that cannot detect and respond to change soon dies. Responding to the external environment lets an animal:

• find food and water, and recognise what is safe to eat;
• escape predators and avoid dangers such as heat, sharp objects and falls;
• find a mate, care for young and keep contact with its group;
• move to places where temperature, light and humidity suit it, as a woodlouse moves into damp, dark places.

Responding to the internal environment is just as important. Enzymes work only within narrow limits of temperature and pH, and cells shrink or burst if the water potential of tissue fluid changes. Receptors inside the body monitor blood glucose, temperature, carbon dioxide concentration, blood pressure and water potential, so that the body can correct any change. This is homeostasis, and it depends on the same chain of events as escaping a predator.

3From stimulus to response

4Types of sensory receptors and stimuli

Each receptor is specific: it responds strongly to one form of energy, its adequate stimulus, and weakly or not at all to others. Biologists group receptors by that stimulus.

Receptors are also grouped by where they are and by what they monitor:

• Exteroceptors detect stimuli outside the body: the eye, ear, nose, taste buds and skin receptors.
• Interoceptors detect changes inside the body: osmoreceptors in the hypothalamus, chemoreceptors in the carotid arteries, stretch receptors in the walls of the gut and blood vessels.
• Proprioceptors detect the position and movement of the body itself: muscle spindles in skeletal muscles and receptors in tendons and joints. They let you touch your nose with your eyes closed.

Finally, receptors differ in structure. A primary receptor is the modified ending of a sensory neurone itself, such as a Pacinian corpuscle or a free nerve ending. A secondary receptor is a separate, specialised cell that passes its signal across a synapse to a sensory neurone, such as a rod, a cone, a hair cell of the ear or a taste receptor cell.