Practice question · Multiple choice
A withdrawal reflex has your hand off the hot pan before you consciously feel the heat. Why is the circuit wired through the spinal cord rather than up to the brain and back?
Hints
- Compare the length of the two possible routes, and remember what is happening to the skin while the signal travels.
- You do feel the heat. Ask when, relative to the withdrawal.
Show the answer
A. Because a trip to the brain and back costs time the tissue cannot spare.
Why
Everything here is distance: hand to cord to muscle is about a metre and a half, and routing through the brain adds most of the body. Roughly 20 ms against 250 ms is the difference between a reddened palm and a burn. You do feel it, a moment later, the sensory neuron branches, and the loop finishes before the brain is told.
Practise Central Nervous System
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More questions on Central Nervous System
- Order the events of a withdrawal reflex, from stimulus to the moment you feel it.
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