How does the nervous system carry information, and how does a reflex protect the body?
The structure of the nervous system, the pathway of a nervous impulse, and the reflex arc as a fast automatic response.
A focused answer to the WJEC GCSE Science Double Award Unit 4 topic on the nervous system, covering its structure, the pathway of a nervous impulse through receptor, neurones and effector, and the reflex arc as a fast automatic protective response.
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What this dot point is asking
WJEC Double Award Unit 4 wants you to describe the structure of the nervous system, the pathway of a nervous impulse, and the reflex arc.
The structure of the nervous system
The nervous system allows fast communication and responses to changes (stimuli) in the environment.
The pathway of an impulse
Between neurones there are tiny gaps called synapses, where a chemical passes the signal from one neurone to the next.
The reflex arc
In a reflex, the pathway is: receptor to sensory neurone to relay neurone (in the spinal cord) to motor neurone to effector. The relay neurone connects the sensory and motor neurones in the spinal cord, so the response happens without waiting for the brain.
Why reflexes are fast and protective
Reflexes are fast because the impulse takes a short, fixed route through the spinal cord and does not have to be processed consciously by the brain. This speed makes reflexes protective: you pull your hand off a hot object, or blink to protect your eye, before you have time to think about it. Examples include the knee-jerk reflex, the pupil reflex and pulling away from pain.
How impulses cross a synapse
Where two neurones meet there is a tiny gap called a synapse. The electrical impulse cannot jump the gap directly, so when it arrives, the neurone releases a chemical (a neurotransmitter) that diffuses across the synapse to the next neurone. This chemical then starts a new impulse in the next neurone, carrying the signal on. Synapses make sure impulses travel in one direction only, and they are where many drugs and medicines act. Knowing that a chemical, not the impulse itself, crosses the synapse is a common higher-mark point.
Comparing nervous and chemical control
The body has two communication systems that work together: the nervous system and the hormonal (chemical) system. The nervous system uses fast electrical impulses for quick, short-lived, precisely targeted responses (such as a reflex). The hormonal system uses slower chemical messengers in the blood for longer-lasting, more widespread effects (such as controlling blood glucose). Being able to say which system suits a given response, and why, links this topic to hormones and homeostasis.
Try this
Q1. Name the two parts of the central nervous system. [1 mark]
- Cue. The brain and the spinal cord.
Q2. What is an effector? [1 mark]
- Cue. A muscle or gland that carries out the response.
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 style5 marksDescribe the pathway of a reflex action when a person touches a hot object, naming the parts involved in order.Show worked answer →
A Unit 4 extended question worth 5 marks. Reward the pathway in order: a receptor in the skin detects the heat (stimulus) (1); a sensory neurone carries the impulse to the spinal cord (CNS) (1); a relay neurone passes it on within the spinal cord (1); a motor neurone carries the impulse to the effector (a muscle) (1); the muscle contracts to move the hand away (the response) (1). Markers credit the correct order: receptor, sensory neurone, relay neurone, motor neurone, effector. A common error is to route the impulse through the brain (a reflex bypasses conscious thought).
WJEC style3 marksExplain why reflex actions are fast and do not involve conscious thought.Show worked answer →
A Unit 4 explain question. Reward: a reflex follows a short, fixed pathway (the reflex arc) through the spinal cord rather than the brain (1); because it does not wait for the brain to process it consciously, the response is very fast (1); this speed protects the body from harm, for example pulling away from danger before you feel the pain (1). Markers credit the short pathway bypassing the brain, the speed and the protective role. A common error is to say the brain controls every reflex.
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