How does the nervous system let you detect a stimulus and respond, sometimes without thinking?
The central nervous system, sensory, relay and motor neurones, synapses and neurotransmitters, the reflex arc as a fast automatic response, and reaction time and the factors affecting it.
A focused answer to the OCR Gateway GCSE Biology A topic B3 on the nervous system, covering the central nervous system, the three types of neurone, synapses and neurotransmitters, the reflex arc, and reaction time.
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What this dot point is asking
OCR wants you to describe the central nervous system, name the three types of neurone, explain how impulses cross a synapse, describe the reflex arc as a fast automatic response, and discuss reaction time and what affects it.
The central nervous system
The nervous system works with these key parts:
- A stimulus is a change in the environment (such as heat, light or sound).
- A receptor detects the stimulus (for example, receptors in the skin, eyes or ears).
- An effector is a muscle or gland that carries out the response. Muscles contract; glands release substances such as hormones.
The three types of neurone
Neurones (nerve cells) carry electrical impulses. There are three types you must know:
- Sensory neurone. Carries impulses from a receptor to the CNS.
- Relay neurone. Found within the CNS; passes impulses from a sensory neurone to a motor neurone.
- Motor neurone. Carries impulses from the CNS to an effector.
The general pathway for a response is: stimulus, receptor, sensory neurone, CNS (relay neurone), motor neurone, effector, response.
Synapses
Because the neurotransmitter has to be released and then diffuse across the gap, a synapse slightly slows the impulse. Synapses also make sure impulses travel in only one direction.
The reflex arc
A reflex action is a fast, automatic response that does not involve conscious thought. It protects the body from harm, for example pulling your hand away from something hot, or blinking when something flies towards your eye.
In a reflex arc, the impulse takes a short route through the spinal cord rather than the conscious parts of the brain:
- A receptor detects the stimulus.
- A sensory neurone carries the impulse to the spinal cord.
- A relay neurone in the spinal cord passes it on (across synapses).
- A motor neurone carries the impulse to the effector.
- The effector (a muscle or gland) produces the response.
Reaction time
Reaction time is the time taken to respond to a stimulus. It can be measured with simple tests, such as catching a dropped ruler (the distance it falls before you catch it shows your reaction time). Reaction time can be increased (slowed) by tiredness, distraction, alcohol and some drugs, and tends to increase with age; it can be improved by practice and caffeine.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20196 marksDescribe the path taken by a nervous impulse in a reflex arc when a person touches a hot object and pulls their hand away, and explain why a reflex action is fast and does not involve conscious thought.Show worked answer →
A 6-mark extended response. Mark it for an ordered pathway plus the explanation.
Path: a receptor in the skin detects the stimulus (heat) and sends an electrical impulse along a sensory neurone to the spinal cord (the central nervous system). At a synapse, the impulse is passed to a relay neurone, then across another synapse to a motor neurone. The motor neurone carries the impulse to an effector (a muscle in the arm), which contracts to pull the hand away (the response).
Fast and unconscious: a reflex arc does not involve the conscious parts of the brain, so the impulse takes a short, direct route through the spinal cord. This makes the response faster and automatic, which protects the body from harm. Reward the correct order (stimulus, receptor, sensory neurone, relay neurone, motor neurone, effector, response) and the link that bypassing the brain makes it fast and protective.
OCR 20214 marksExplain how an impulse passes from one neurone to the next at a synapse, and suggest one factor that could increase a person's reaction time.Show worked answer →
A 4-mark question on synapses plus application (Suggest).
Synapse: there is a small gap (the synapse) between two neurones. When the impulse reaches the end of the first neurone, it causes a chemical (a neurotransmitter) to be released. The neurotransmitter diffuses across the gap and binds to receptors on the next neurone, which starts a new electrical impulse in that neurone.
Increasing reaction time: tiredness, distraction, alcohol or certain drugs all slow the response. Age can also increase reaction time. Reward the diffusion of a neurotransmitter across the synapse, and any one sensible factor that slows the response.
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