A stimulus is a detectable change, such as increased light or pressure. Receptors detect it and sensory neurons carry information towards the central nervous system, the brain and spinal cord. The central nervous system processes information; motor neurons carry instructions to effectors. Muscles can contract and glands can release substances. A useful pathway is stimulus → receptor → sensory neuron → central nervous system → motor neuron → effector → response.
Reflexes are rapid, automatic responses that help protect the body. A withdrawal reflex can be coordinated through spinal cord circuits before conscious awareness develops. The brain still receives information, but a deliberate decision is not required to initiate that response. Nervous signalling is generally fast and targeted, allowing quick coordination of particular muscles or glands.
Example 1
A hand touches a hot surface and withdraws. Identify stimulus, receptor and effector.
The stimulus is damaging heat. Skin receptors detect it. Arm muscles are effectors whose contraction withdraws the hand.
Example 2
Why is “the receptor contracts the arm” incorrect?
A receptor detects the stimulus; it does not produce the muscle response. Neurons carry signals and the effector muscle contracts.
Watch out:Reflex pathways can initiate automatic responses through spinal cord circuits.
Stimulus, receptor and nervous response practice | Year 9 Science | Ascent Arcade