A reflex action such as withdrawing the hand from a hot object is fast because
Correct answer: D. the response is organised in the spinal cord without waiting for the brain
- A. the impulses travel through the cerebrum first
- B. only one neurone is involved in the whole pathway
- C. hormones carry the message directly to the muscle
- D. the response is organised in the spinal cord without waiting for the brain
Explanation
The reflex arc is completed at spinal level, so the muscle contracts before the impulses climbing to the brain arrive and the pain is consciously felt, which is what makes the response protective. The brain is informed afterwards and can modify or suppress some reflexes, but it is not needed for the action itself. At least three neurones are involved, and hormones are far too slow for this.
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About Reflexes and the Reflex Arc
A reflex arc shows how a rapid, automatic response travels from a receptor through a sensory neuron, relay neuron and motor neuron to an effector. The topic covers synapses, the spinal cord's role, and the difference between an involuntary reflex action and a slower voluntary response.
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