Reflex Actions and the Reflex Arc
FoundationA reflex is a fast, automatic response that protects the body — no conscious thought needed. The pathway is the reflex arc: stimulus → receptor → sensory neurone → spinal cord → motor neurone → effector. Skipping the brain's slow decision-making is what makes it fast.
- Reflex = fast, automatic, protective
- The arc relays at the spinal cord without the brain
- The pain signal arrives after the response
§Text Passage
1A reflex is the body's fast, inborn reaction, and its purpose is protection before harm is done. When a finger touches a hot pan, heat receptors in the skin detect the high temperature at once and the signal races along a sensory neurone into the spinal cord. Such responses are present from birth, need no learning, and finish before you are even aware of them; blinking, sneezing and the knee-jerk are familiar examples.
2A complete reflex arc has five links: a receptor picks up the stimulus; a sensory neurone carries the signal into the spinal cord; a relay neurone inside hands it straight to a motor neurone; the motor neurone delivers it to an effector; and finally the muscle contracts, jerking the hand away. Because the signal never detours through the brain for analysis, the whole sequence finishes in well under a second — the reason reflexes are so much faster than ordinary responses.
3Only after the hand has pulled away does the pain signal reach the brain. This act-first-feel-later design greatly shortens the time the skin stays on the heat, so the burn is less severe. Note carefully that although a reflex bypasses the brain it still passes through the spinal cord, which is part of the central nervous system, so the claim that 'a reflex skips the CNS' is wrong. Damage to any link of the arc makes the protection disappear.
4Reflexes, however, are fast but crude and handle basic protection only; inspecting the injury and deciding what to do next remain the brain's work. Reflexes also come in two kinds: pulling away and blinking are inborn reflexes completed by lower centres such as the spinal cord, while conditioned reflexes are built by learning — Pavlov's dogs salivated at a bell after repeated pairing. Both types must pass through the central nervous system.
1Summary 重點摘要
A reflex is a fast, automatic response that protects the body — no conscious thought needed. The pathway is the reflex arc: stimulus → receptor → sensory neurone → spinal cord → motor neurone → effector. Skipping the brain's slow decision-making is what makes it fast.
2Vocabulary cards Vocabulary
An inborn, automatic, fast protective response — e.g. pulling your hand away, blinking.
The message pathway of a reflex — the brain isn't needed for the decision.
The processing centre for reflexes — messages don't have to reach the brain first.
A learned reflex, e.g. salivating at a bell (Pavlov).
A reflex present without learning — pulling away, blinking, knee-jerk.
Passes the signal from the sensory neurone straight to the motor neurone inside the spinal cord.
The inborn reflex where a tap below the knee makes the lower leg swing — a classic demonstration.
Reaches the brain only after the response — reflexes act before you feel.
3Step by step Steps
- StimulusA reflex is the body's fast, inborn response whose purpose is protection: when a finger touches a hot pan, heat receptors in the skin detect the high temperature instantly and the reflex arc is triggered at once. Responses like these are present from birth and need no learning; the stronger the stimulus, the more vigorous the response. Blinking, sneezing and the knee-jerk are all similar examples, completed before you are consciously aware, minimising the time the body stays exposed to danger.
- InThe receptor is in fact the ending of a sensory neurone, converting stimuli such as heat or touch into electrical signals. The sensory neurone then carries the signal into the spinal cord — the inward leg of the reflex arc — and the message travels only towards the centre, never through the brain first. Skipping that leg of the journey is exactly why a reflex completes its protective action within a fraction of a second, far faster than any brain-led response; this is the key design of the arc.
- RelayOnce the signal enters the spinal cord, a relay neurone inside hands it straight to a motor neurone, with no waiting for the brain to analyse and decide. Missing out the detour through the brain dramatically shortens the reaction time — the key to reflexes protecting the body. If any part of the arc is damaged, for instance by a spinal injury, the reflex cannot be completed and its protection is lost at once: all five links are interlocked and indispensable.
- OutThe motor neurone carries the signal from the spinal cord to the effector, such as the muscles of the arm. Note carefully: a reflex bypasses the brain but still passes through the spinal cord, which is part of the central nervous system — so the claim that 'a reflex skips the CNS' is wrong. Effectors can be muscles or glands; for example, salivary glands secrete saliva when stimulated, another everyday example of a reflex response.
- ResponseOn receiving the signal the muscle contracts at once and the hand jerks away from the hot pan; only afterwards does the pain signal reach the brain. This act-first-feel-later design greatly shortens the time the skin stays on the heat, so the burn is less severe. Reflexes are fast but crude, handling basic protection only; fine judgement — inspecting the injury and deciding what to do next — remains the brain's job.
- Inborn vs conditionedPulling away, blinking, the knee-jerk and sneezing are inborn reflexes: present from birth, needing no learning, and relayed by lower centres such as the spinal cord. Conditioned reflexes are built by learning: Pavlov's dogs salivated at a bell after repeated pairing, and people grasp a bicycle's balance only through repeated practice. Both types pass through the central nervous system, but conditioned reflexes also involve the brain's learning and memory, making them the higher-level response.
4Diagrams Diagrams
5Worked example Example
Why do you pull your hand off a hot pan before you 'decide' to?
- Waiting for the brain to analyse would take longer
- The spinal cord relays directly — much faster
- Only afterwards does the brain receive the pain signal
- So a reflex happens 'before you feel it'
- Answer: Because the reflex arc completes its relay in the spinal cord, without waiting for the brain to analyse and decide, the hand pulls away before you are consciously aware; pulling away is an inborn reflex — a protective instinct — and pain is only the afterwards notification. In your answer, follow the chain 'spinal relay → faster than via the brain → act before feeling → protection', and support it with examples such as pulling away and blinking. The whole arc takes only a fraction of a second — far faster than responses routed through the brain.
6Common mistakes Mistakes
7Quick check Quick check
Q1Where does the relay happen in the reflex arc?
Q2List the 5 parts of the reflex arc.
Q3Why must reflexes be fast?
Q4What is the key difference between a reflex and a voluntary action such as writing?
Q5What is the role of the relay neurone in the reflex arc?
8Recap Recap
- Reflex = fast, automatic, protective
- The arc relays at the spinal cord without the brain
- The pain signal arrives after the response
- Arc: receptor → sensory neurone → spinal cord → motor neurone → effector
- Inborn reflexes need no learning; conditioned ones are trained