The Nervous System: Receptors, Coordinators and Effectors
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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
The nervous system detects stimuli through receptors, processes information in the central nervous system (brain and spinal cord), and triggers responses through effectors (muscles or glands). Three types of neurone carry impulses: sensory (receptor → CNS), relay (within CNS), and motor (CNS → effector).
The nervous system is about communication and response. Every answer should follow the stimulus-response pathway: stimulus → receptor → sensory neurone → CNS → motor neurone → effector → response. This is the framework for both voluntary actions and reflexes.
The stimulus-response pathway
A stimulus is a change in the environment that is detected by a receptor. Receptors are specialised cells or groups of cells that detect specific stimuli: light receptors in the eye, temperature receptors in the skin, chemical receptors in the nose and tongue.
The receptor generates an electrical signal (nerve impulse) that travels along a sensory neurone to the central nervous system (CNS: brain and spinal cord). The CNS processes the information and sends impulses along a motor neurone to an effector: a muscle (which contracts) or a gland (which secretes a substance).
Types of neurone
Neurone type
Direction of impulse
Function
Sensory neurone
Receptor → CNS
Carries impulses from receptors to the brain or spinal cord
Relay neurone
Within the CNS
Connects sensory and motor neurones inside the brain or spinal cord
Motor neurone
CNS → effector
Carries impulses from the CNS to muscles or glands
Neurones have features adapted for impulse transmission: a long axon for carrying impulses over distance, a myelin sheath (fatty insulation) that speeds up transmission, and branched endings (dendrites) for connecting to other neurones or receptors.
Voluntary and involuntary responses
Voluntary responses involve conscious decision-making. The brain receives sensory information, processes it, and sends impulses to effectors. For example, deciding to pick up a pen involves sensory input (seeing the pen), processing (deciding to pick it up), and motor output (muscles in the hand contracting).
Involuntary responses (reflex actions) are rapid, automatic responses that do not involve conscious thought. They protect the body from harm. The nervous pathway for a reflex is shorter and faster because it bypasses the decision-making parts of the brain: impulses pass through the spinal cord via a reflex arc.
Try a focused question (2 marks)
State the function of a sensory neurone and a motor neurone.
Show the answer and marking guidance
A sensory neurone carries nerve impulses from a receptor to the central nervous system (brain or spinal cord).
A motor neurone carries nerve impulses from the central nervous system to an effector (muscle or gland).
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Confusing sensory and motor neurones: sensory goes toward the CNS, motor goes away from it.
Saying the brain is the only part of the CNS: the spinal cord is also part of it.
Describing effectors as only muscles: glands are also effectors.
Forgetting that relay neurones are found only within the CNS.