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Scratch reflex

QUESTION : How does scratching cure itching sensation?

K. R. Sankaran, Chennai

ANSWER : Very sensitive, rapidly adapting, mechanoreceptive free nerve endings that elicit only the tickle and itch sensation are found almost exclusively in the superficial layers of the skin. This sensation is transmitted by very small type - C nerve fibres. These nerve fibres are unmyelinated fibers with a diameter of 0.5 to 2 microns. These fibres transmit impulses as slowly as 0.5 m/sec, unlike the type - A fibres, which transmit impulses at velocities as great as 120 m/sec.

The process of scratching can relieve itch. This scratch reflex is the important spinal cord reflex found in some animals including man. This reflex is initiated by the itch and tickle sensation. It involves two different functions (1) a position sense that allows the hand to find the exact point of irritation on the surface of the body and (2) a to and fro scratching movement. This scratching removes the irritant and the itch is relieved. Sometimes the scratch is strong enough to elicit pain. The pain signals are believed to suppress the itch signals in the cord by the process of lateral inhibition. This lateral inhibition is by the transmission of sensory signals by the anterolateral system, in contrast to the dorsal column system. The anterolateral system is a cruder type of transmission system than the dorsal column system.

S. Palaniappan, Pudukkottai, T. N.

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