These automatic control systems may involve nervous responses or chemical…

The idea

Automatic control systems keep internal conditions steady without conscious thought, using two complementary routes. Nervous responses are electrical: receptors detect a stimulus, a coordination centre processes it, and effectors act. This is fast, short-lived and targeted, as when a sudden noise triggers a reflex. Chemical responses use hormones carried in the blood plasma; they are slower, longer-lasting and act on cells with matching receptors. Insulin lowering blood glucose after a meal is a clear example. When you answer, name the stimulus, receptor, coordination centre and effector, and state whether the pathway is nervous or chemical.

What earns the marks

• Nervous responses use electrical impulses travelling along neurones, so they are rapid and usually short-lived.
• Chemical responses use hormones secreted into the blood, so they are slower but often longer-lasting and widespread.
• A nervous pathway can be described as stimulus → receptor → coordination centre → effector → response.
• Hormones only affect target cells that carry the complementary receptor, giving chemical responses specificity.

Common mistakes and how to put them right

• Saying hormones travel along nerves.

  Put it right by stating hormones are secreted into the bloodstream and carried in plasma.

• Claiming nervous responses last longer than chemical ones.

  Put it right by noting nervous responses are rapid and short-lived, while hormonal effects persist.

• Confusing the speed of the two systems.

  Put it right by remembering that nervous impulses are very fast, whereas hormones must travel in the blood.

Exam tips

• Use the full pathway vocabulary: stimulus, receptor, coordination centre, effector, response.
• When comparing, structure your answer around speed, duration, route and target specificity.