When a hot surface is touched, receptors in the hand send a signal to the nervous system. The nervous system then sends a signal to the muscles in the arm to contract and move the hand from the hot surface.

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The nerve pathway followed by a reflex action is called a reflex arc. For example, a simple reflex arc happens if we accidentally touch something hot. Receptor in the skin detects a stimulus (the change in temperature). Sensory neurone sends impulses to relay neurone .

answer: the blood-brain barrier is a layer of specialized membranes and cell processes designed to keep undesirable substances from passing from the bloodstream into the precious tissues of the brain. while the blood-brain barrier does keep most bad things out of the central nervous system, it is not impermeable.

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