How does the nervous system let the body detect and respond rapidly to its surroundings, and how does a reflex protect us?
Stimuli, receptors and effectors, the central nervous system and neurons, the transmission of impulses, and the reflex arc as an automatic protective response.
A focused answer to the WJEC GCSE Biology section 2.5 topic on the nervous system, covering stimuli, receptors and effectors, the central nervous system, neurons and electrical impulses, and the reflex arc as a rapid automatic response that protects the body.
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What this dot point is asking
WJEC wants you to know that organisms respond to changes in their surroundings, that receptors detect stimuli and effectors bring about responses, the make-up of the central nervous system, how impulses travel along neurons, and the pathway and purpose of a reflex arc.
Stimuli, receptors and effectors
Organisms survive better if they can respond to changes in their environment. A stimulus is any such change, for example light, heat, sound, touch or a chemical.
- A receptor detects a stimulus. Receptors are often grouped in sense organs such as the eye, ear, skin, nose and tongue.
- An effector brings about a response. Effectors are muscles (which contract) or glands (which release a substance such as a hormone).
Receptors and effectors can form part of larger control systems, the nervous system and the hormonal (endocrine) system.
The central nervous system
The central nervous system (CNS) consists of the brain and the spinal cord. The CNS is connected to the sense organs and effectors throughout the body by nerves, which are bundles of neurons.
Information passes through the body in this overall pathway:
stimulus → receptor → sensory neuron → CNS → motor neuron → effector → response
Neurons and impulses
A neuron is a nerve cell. Information travels along a neuron as an electrical impulse. There are three types you need:
- A sensory neuron carries impulses from receptors to the CNS.
- A relay neuron (found in the CNS) connects neurons inside the spinal cord or brain.
- A motor neuron carries impulses from the CNS to effectors.
Reflex actions and the reflex arc
A reflex action is an automatic, rapid response to a stimulus that does not involve conscious thought. Reflexes often protect the body, for example pulling your hand away from a hot object or blinking when something approaches the eye.
The pathway of a reflex is the reflex arc:
| Stage | Structure | What happens |
|---|---|---|
| 1 | Receptor | Detects the stimulus |
| 2 | Sensory neuron | Carries the impulse to the spinal cord |
| 3 | Relay neuron | Passes the impulse across the spinal cord |
| 4 | Motor neuron | Carries the impulse to the effector |
| 5 | Effector | Muscle contracts or gland responds |
Because the impulse passes through the spinal cord and not the conscious brain, the response is very fast.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksDescribe the pathway of a reflex arc when a person touches a hot object and pulls their hand away.Show worked answer →
A 4-mark sequence question.
A receptor in the skin detects the heat (the stimulus) and sends an electrical impulse along a sensory neuron to the spinal cord. In the spinal cord a relay neuron passes the impulse to a motor neuron, which carries it to the effector, a muscle in the arm. The muscle contracts and pulls the hand away.
Markers reward the correct order: receptor, sensory neuron, relay neuron, motor neuron, effector (muscle), with the muscle contracting. Naming the brain in a spinal reflex, or leaving out the relay neuron, are common errors.
WJEC style2 marksExplain why a reflex action is faster than a normal voluntary response.Show worked answer →
A 2-mark explain question.
A reflex does not involve the conscious part of the brain, so the impulse travels a short pathway through the spinal cord only. This means fewer steps and less processing, so the response is rapid and automatic, which helps to protect the body from harm.
Markers reward: does not go to the brain (no conscious thought) and shorter pathway, so it is quicker. Saying it is "instant" without a reason gains nothing.
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Sources & how we know this
- WJEC GCSE Biology specification (from 2016) — WJEC (2016)