The human nervous system: stimulus receptor coordinator effector response

The idea

A change in the surroundings is a stimulus. A receptor, such as a temperature receptor in the skin or a light receptor in the retina, detects it and generates an electrical impulse. The coordinator, typically the brain or spinal cord, processes the information and decides the response. The effector, a muscle or a gland, then carries out the action. For example, touching a hot beaker: thermoreceptors in the skin detect the heat, sensory neurones carry impulses to the spinal cord, which coordinates a rapid response, and the arm muscle contracts to pull the hand away. This sequence is the basis of both voluntary and reflex pathways, and you must be able to place each component in order and explain its role.

What earns the marks

• A stimulus is a detectable change in the internal or external environment, such as a change in temperature, light intensity or pressure.
• A receptor is a specialised cell or structure that detects the stimulus and converts it into an electrical impulse in a neurone.
• The coordinator is the brain or spinal cord; it receives impulses, processes information and determines the appropriate response.
• An effector is a muscle or gland that responds to the impulse, either by contracting or by secreting a substance.
• The response is the resulting change in the organism, such as movement away from a harmful stimulus.
• The sequence is stimulus → receptor → coordinator → effector → response, and each stage must be linked to a named example.

Common mistakes and how to put them right

• Writing the sequence as stimulus → effector → receptor → coordinator → response.

  Put it right: The receptor must detect the stimulus before the coordinator processes it, and the effector acts last.

• Confusing the coordinator with the effector.

  Put it right: The coordinator is the brain or spinal cord and processes information; the effector is a muscle or gland that carries out the response.

• Stating that receptors are only in the skin.

  Put it right: Receptors are found in many organs, including the eye, ear, tongue and internal organs, wherever stimuli need to be detected.

Exam tips

• Use the exact terms stimulus, receptor, coordinator, effector and response in your answer, and apply them to the example given in the question.
• When asked to complete a flow diagram, check that each arrow represents the direction of information flow from detection to response.
• If the question names a specific example, such as a bright light or a hot surface, name the receptor and effector involved rather than writing generically.