The Nervous System and Coordination

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1Summary 重點摘要

The nervous system consists of the brain, spinal cord and nerves. It receives information, processes it and sends out instructions. Neurones are the basic units, carrying messages as fast electrical impulses — that's how the body reacts instantly.

2Vocabulary cards Vocabulary

3Step by step Steps

1. Stimulus appearsA stimulus is any change in the environment that can set off a response in the body: light entering the eye, a needle pricking the finger, a bell ringing. Receptors around the body are specialised for different stimuli — light receptors in the eye respond to light, while touch and heat receptors in the skin respond to contact and temperature. Once stimulated, a receptor converts the stimulus into a nerve message and hands it to the nervous system; without receptors the body could not detect changes around it, let alone react.
2. Message inSensory neurones carry the message produced by a receptor towards the central nervous system — the spinal cord or brain — as electrical impulses. Impulses race along nerve fibres at speeds above 100 metres per second, so the body senses a stimulus almost the instant it appears. Rapid conduction is a key reason humans can respond to their surroundings at once; if a sensory neurone is damaged, the message never reaches the CNS and the sensation of that stimulus is lost.
3. CNS processesWhen a message arrives at the central nervous system, the brain or spinal cord analyses it and decides how to respond. The brain is the higher centre, handling thinking, memory, language and voluntary decisions; the spinal cord deals with some faster responses — pulling a hand away does not wait for an order from the brain, acting before you are aware and saving precious time. The CNS also integrates messages from all over the body so that organs act together: this is exactly what 'coordination' means in biology.
4. Instruction outOnce the CNS has decided, motor neurones carry the command from the brain or spinal cord to the effectors — muscles or glands; a muscle responds by contracting and a gland by secreting. Common error: the message must pass through the CNS — it cannot jump straight from receptor to muscle, and this is the most frequent mistake in exams. All five links — receptor, sensory neurone, CNS, motor neurone and effector — are indispensable, and answers should list them in order.
5. ResponseThe effectors carry out the command and produce a visible response: muscles contract to pull the hand back or move the bones, while glands secrete sweat, digestive juices and more. The whole journey from stimulus to response often takes under one second, letting the body react instantly — blinking at a flash, dodging a flying object. Fast, accurate responses are the direct benefit of carrying messages as electrical impulses.
6. Inside a neuroneA neurone is the basic unit of the nervous system, made of a cell body and extensions: dendrites receive the message and pass it towards the cell body, while the axon carries it away to the next neurone or effector. Many fibres are wrapped in an insulating myelin sheath that makes conduction faster and more reliable. Messages travel one way only through a neurone — in at the dendrites, out along the axon — keeping the whole pathway orderly and free of confusion.

4Diagrams Diagrams

5Worked example Example

6Common mistakes Mistakes

7Quick check Quick check

8Recap Recap

• Stimulus → receptor → sensory neurone → CNS → motor neurone → effector
• Neurones use electrical signals — extremely fast
• Brain and spinal cord are the processing centres
• A neurone is one cell; a nerve is a bundle of many neurones
• Sensory neurones carry messages in; motor neurones carry commands out