Specialised cells

Your body would be useless at reacting if it got no reliable updates. You need specialised cells to notice what is changing around you and inside you, then pass that information on so your nervous system or hormone system can respond.

When a change in your surroundings or internal conditions is picked up, that change is called a stimulus.

Receptors do not choose the response themselves. They are more like the person in your group chat who spots the problem first and sends the alert.

You have receptors for events outside your body, like light, sound, touch, and chemicals in food or air. You also have receptors checking internal conditions such as temperature, pressure, oxygen level, and carbon dioxide level. Without that constant stream of updates, your body would not know when to speed up breathing, release hormones, sweat, shiver, or make other adjustments.

One receptor type cannot do everything well. Your body uses specialists because a cell built to detect light would be terrible at sensing pressure, and a cell tuned for chemicals would miss vibrations.

That is why different receptor cells have structures matched to one kind of stimulus.

In your eyes, photoreceptors work because they contain pigments that react to light. In places that sense movement or pressure, mechanoreceptors have membranes that respond when they are bent or stretched.

You should know the main groups. Photoreceptors respond to light and are found in your retina. Chemoreceptors pick up chemicals, so you find them in your nose, tongue, and even in blood vessels. Mechanoreceptors detect pressure or vibration in places like your skin and inner ear. Thermoreceptors monitor temperature in your skin and in the hypothalamus. Nociceptors signal pain and are spread through much of your body.