Detecting the world
A stimulus is any change in the environment that a living thing can detect, such as light, sound, heat, touch or a smell. A response is the action the body takes because of that stimulus. Special cells or organs called receptors detect stimuli. The five sense organs are receptors: the eyes detect light, the ears detect sound, the nose detects smell, the tongue detects taste and the skin detects touch, heat and pressure.
Key idea
Stimulus → receptor → sensory neuron → CNS → motor neuron → effector → response. An effector is a muscle or a gland that carries out the response.
Voluntary and involuntary actions
A voluntary action is a response you choose and control, like picking up a pen. It is directed by the brain. An involuntary action happens automatically without you deciding, like the heartbeat. A special involuntary response is the reflex action — a very fast, automatic response that protects the body, such as pulling your hand away from a hot pan.
The reflex arc
A reflex is fast because the impulse takes a short path called the reflex arc. The impulse travels from the receptor, along a sensory neuron, to a relay neuron in the spinal cord, then along a motor neuron to the effector. The brain is not needed to start the movement, so the action is quick and protective.
Example
Touching a sharp pin: the skin (receptor) sends an impulse through the reflex arc, and the arm muscle (effector) pulls the hand back before you feel the pain.