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This chapter is from the book

My (six) aching knees

Two scientific papers summarize the lack of consensus on this matter. One claims that invertebrate organisms do not experience pain (Nature, September 13, 2001). The other states that invertebrates might suffer the same way as vertebrates (Animal Welfare, February 2001).

Insects and other invertebrates certainly can detect noxious things in their environments. For example, they withdraw from toxic chemicals and hot surfaces, and they attempt to escape mechanical compression. Avoidance or escape behavior could indicate that an insect is in pain. On the other hand, insects also attempt to escape from non-noxious things, such as gentle touch or a sudden change in illumination.

Recent studies on fruit fly larvae show that they detect noxious and non-noxious things in fundamentally different ways. A light touch usually causes larvae to halt and reverse slightly, but larvae touched with a heated probe launch into a sideways roll. Unlike these gymnastically gifted maggots, researchers have discovered mutants, dubbed painless, that respond normally to gentle touch but fail to respond to a heated probe and other harms.

Painless mutants lack a certain type of protein molecule called a nociceptor that detects noxious heat, chemicals, and pressure. In normal fruit fly larvae, detection of these dangers activates the nerves in which the nociceptors reside.

Similar nerves and nociceptors exist in humans. We usually feel pain when they are activated, which suggests that insects can feel pain, too. Because our brains play a significant role in our perception of pain, and insects' brains are much more rudimentary than ours, their experience is probably different.

Our brains do not simply act as "pain-o-meters" that register the signals coming from the nociceptor-containing nerves in our bodies. Brain-imaging studies show that several different areas of our brains process the incoming signals. For example, activation of regions in the limbic system of the brain influences our subjective, emotional experience of pain. Different regions of the brain's cortex—the part of the brain involved in higher thinking—also actively process pain signals.

In humans, several lines of evidence show that pain is much more than a sensory experience. Our expectations about how bad the pain will be influence how bad it feels. When we think that we have control over pain, it increases our pain tolerance. Our emotional state can influence pain perception. Finally, we may still experience pain even after the nerves that carry the pain-related signals from the body are removed.

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