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An especially important cord reflex in some animals is the scratch reflex, which is initiated by itch or tickle sensation. It involves two functions: (1) a position sense thatallows the paw to find the exact point of irritation on the surface of the body, and (2) a to-and-fro scratching movement.
The position sense of the scratch reflex is a highly developed function. If a flea is crawling as far forward as the shoulder of a spinal animal, the hind paw can still find its position, even though 19 muscles in the limb must be contracted simultaneously in a precise pattern to bring the paw to the position of the crawling flea. To make the reflex even more complicated, when the flea crosses the midline, the first paw stops scratching and the opposite paw begins the to-and-fro motion and eventually finds the flea.
The to-and-fro movement, like the stepping move-ments of locomotion, involves reciprocal innervation circuits that cause oscillation.
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