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I. Pain and Distress

Definitions

Pain is registered by the brain as an experience. What that experience is can be conveyed with words by a human, as well as by facial expression and sounds that the human brain interprets as reflecting pain and suffering. For animals, we have to guess, as they cannot tell us.

Distress is an even more complicated judgment call than pain, for humans trying to assess what animals feel.

Biology of Pain

Pain is a response to damaging or potentially damaging contact with the outside world. Pain is a unique, individual experience. Its perception can change with age, health, and experience. That is to say, the cognitive centers of the brain can dampen or sharpen perception of a painful stimulation.

Neuroscientists define pain as nociception, the transmission of signals along certain types of nerve fibers to the spinal cord and up into the brain. The tissues of the body contain receptors for painful stimuli, called nociceptors. These are nerve endings, and they respond to crush, temperature, or chemical stimulation that threatens or actually causes tissue damage1.

Nociceptor nerve endings feed into nerves that carry a signal to the spinal cord, which produces a withdrawal reflex causing muscles to contract. This is the local response to pain, and it takes place with or without the perception of pain, as human experiments have shown.

The nociceptive signal also ascends to the brain. Here is where the signal is interpreted as painful.

The Physiology of Pain

Recording electrical signals through the skin, scientists find that people say they feel pain when activity takes place in the smallest myelinated nerves, called the A delta fibers, and the unmyelinated or C nerve fibers (myelin is the fatty sheath covering those nerves that transmit signals most rapidly-some pain signals arrive quickly, and others more slowly.)

These same nerve fibers appear to be present in all vertebrates. Research on anesthetized mammals indicates that these same types of nerve fibers are activated primarily by stimuli of noxious intensity for people. Similarities between humans and animals also appear in the pathways within the spinal cord and brain involved in pain perception2.

But the brain is key to sensing pain. Unless certain regions of the brain are there to process the sensory information arriving from peripheral nerves, pain as people perceive it may not be experienced by animals in the same way. The information has to be processed in the cerebral cortex for pain to be experienced.

Distress and stress are even less objective entities than is pain. These are terms to describe the emotional components that accompany pain, and emotions are a cognitive process residing within the brain, as compared to the simple electrical signals brought into the central nervous system by the A and C nerve fibers. Higher cognitive processes beyond basic reflexes and vegetative functions (regulating heart rate and respiration) are carried out by the brain structures that have been layered over the primitive core of the brainstem during evolution of reptiles, birds, and mammals from simpler creatures.

Fear and anxiety are subsets of distress. Anxiety has been defined as an emotional state of alertness or arousal prompted by an unknown threat in the environment, while fear is the emotional state prompted by a known danger3.

Fear and anxiety are emotions, entirely within the brain initially, but can fan out to affect the entire body adversely.

Biology of Distress

The part of the brain that registers emotion is called the limbic system. It encircles the primitive core of the brain, but lies below the more recently evolved cortex, the brain's covering that handles complex thinking, that is, cognition.

In evolutionary terms, the limbic system first appears in reptiles. Covering the limbic structures are the complex networks of nerve cells called the cerebral cortex, the dominant feature of the mammalian brain. Regions of the cortex govern reasoning and anticipation, and they modulate the emotional signals generated in the limbic structures. Conscious control, at least in people, can govern the emotional content of the limbic system's activity. Stress and distress are created by thinking about pain, anticipating it.

People have a highly developed cerebral cortex (note they are not the only animals with extensive cortex-other primates, dolphins and whales, and elephants, are comparative examples). Especially developed are the human frontal lobes. Other mammals also have appreciable frontal lobes and assuredly are capable of suffering stress and distress4. Even rats readily develop stress ulcers in their digestive systems when placed in situations contrived so they cannot escape repeated pain.

The immune system has its own separate response to continued pain, and an immune response can be an objective indicator that the individual is experiencing chronic pain, or stress, or both5. The immune response puts out hormones and other molecular signals that change the functioning of other parts of the body. On a long term basis, persistent activation of the immune system produces damage to the body's organs, and this activation can result from the continued perception of pain and accompanying stress.

Stress and distress do not have to come from pain but can be created by emotions alone. Unfortunately, gauging this kind of distress in animals is especially difficult. Objective detection is needed for what is by definition a subjective experience. Distress in laboratory animals that is not due to pain is still an unmet challenge as standard methods for its measurement do not exist. In fact, experts have been unable to agree on a set definition of distress to be a basis for setting regulations for the care of laboratory animals6.

Does Brain Equal Pain?

Scientists debate how much brain is required to register pain. Do fish feel pain? Do ants?

Many biologists think that animals without a limbic system do not suffer. Their response to potentially damaging contacts is to draw back reflexively. But they do not feel pain, some researchers have argued.

This view is being challenged, however. Some investigations suggest that fish, for example, might experience fear, if not pain 7.

The USDA guideline on pain in animals states that if one has reason to believe that a stimulus would be painful to humans, it should also be regarded as painful to animals8.

NEXT: Recognition and assessment of pain and distress

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Readings and Resources on the Biology of Pain and Distress

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