How do itches come about, and why does it feel good to scratch them? —B. Ericsson, Sweden

Mark A. W. Andrews, associate professor of physiology at the Lake Erie College of Osteopathic Medicine, replies:

An itch, also known as pruritus, arises from the irritation of nerve cells associated with the skin. Pruritus serves as an important sensory and self-protective mechanism—as do many other skin sensations—by alerting us to harmful external agents. Constant itching, however, can become unbearable if the underlying condition is not treated.

Pruritus is a dominant symptom of many skin diseases and also occurs in some ailments that affect the entire body. An itching sensation results from the stimulation of pruriceptors—itch-sensing nerve endings—by certain mediators. These stimulating agents include chemicals for immune response (such as histamines) and pain relief (such as opioids); neuropeptides, which include endorphins and other pain-regulating messengers released within the brain; the neurotransmitters acetylcholine and serotonin, which relay impulses between nerve cells; and prostaglandins—lipids that, among other functions, create the sensation of pain in spinal nerve cells. Stimulation by any of these agents is typically related to inflammation, dryness or other damage to the skin, mucous membranes or conjunctiva of the eye.

Itching generally involves activation of the pruriceptors of specialized nerve cells called C-fibers. These C-fibers are identical to those associated with the sensation of pain, but they are functionally distinct and only convey the itch sensation. When stimulated on the skin, the C-fibers carry signals along the nerve to the spinal cord and on to the brain.

Scratching and rubbing interfere with the sensations arising from pruriceptors by stimulating various pain and touch receptors in the same areas. Like many sensory systems in the body, activation of one signal, in this case that of the pain and touch receptors, causes “surround inhibition” of another signal, that coming from the pruriceptor. This lack of pruriceptor firing “turns off” the itch sensation for a short period. Although it is helpful in relieving an itch, scratching offers only temporary relief and may cause the skin to become further irritated and possibly even to tear.

Despite approximately a century of research, no single effective antipruritic treatment exists. Several topical and orally administered agents can help suppress itching, however. These include lotions and creams (such as calamine and hydrocortisone), antihistamines, opioid antagonists (such as naltrexone, a drug used to treat narcotic or alcohol dependence), aspirin and ultraviolet light therapy.

How did the sun wind up in the middle of the solar system? —A. Sommers, East Brunswick, N.J.

Michael A. Jura, an astrophysicist at the University of California, Los Angeles, explains:

The best model of our solar system's history states that the planets formed from a spinning disk of dust encircling the sun, leaving a collection of bodies with the sun at its center.

According to this model, the solar system formed from the collapse and flattening of an interstellar cloud. The cloud may initially have measured as much as a light-year across—10 million times wider than the sun. As the cloud compacted and cooled, its own gravity overpowered any forces acting to stabilize the system, causing it to further contract dramatically.

Before this collapse, the original cloud probably began with a fixed mass and a slight, random rotation relative to some central axis. Most of the cloud's mass helped to form the sun, but some of it flattened out and remained as a disk encircling that newly created star. Observations from elsewhere in the galaxy indicate that this disk most likely gave birth to Earth and the other planets, leaving them in naturally heliocentric orbits.

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