Effectors, muscles or glands, which bring about responses which restore…

The idea

Effectors are the output side of every control system. A receptor detects a change away from the optimum; a coordination centre compares this with the set point; an effector then produces a response that restores the optimum. Two effector types are named: muscles and glands. Muscles respond by contracting or relaxing, changing movement or the diameter of passages. Glands respond by secreting a chemical, such as a hormone or enzyme. For example, when core temperature falls, skeletal muscle contracts rapidly as shivering, releasing extra heat; when blood glucose rises, the pancreas (a gland) secretes insulin. The response is corrective, not merely a change: it opposes the original stimulus, which is why it is called negative feedback.

What earns the marks

• Identifies effectors as the structures that carry out the response, not the structures that detect the stimulus.
• States that muscles respond by contracting or relaxing, producing movement or changing the width of a passage.
• States that glands respond by secreting substances such as hormones or enzymes.
• Explains that the response restores the optimum level by opposing the original change, giving a named example such as shivering or insulin secretion.
• Uses the sequence receptor → coordination centre → effector → response when describing a control pathway.

Common mistakes and how to put them right

• Calling the receptor an effector.

  Put it right by stressing that receptors detect change while effectors produce the response.

• Saying glands 'release electrical impulses'.

  Put it right by stating that glands secrete chemicals such as hormones, whereas muscles respond mechanically.

• Describing any response as restoring the optimum without checking direction.

  Put it right by stating that the response must oppose the original change, as in shivering when too cold.

Exam tips

• Name the effector type explicitly in each answer, for example 'muscle in the skin arteriole wall' or 'the pancreas, a gland'.
• Link each response to the direction of change using 'because the level was too high/low', so the corrective nature is clear.
• Practise writing one full pathway for temperature and one for blood glucose, then adapt the same sequence to other examples.