Adaptive reaction

An adaptive reaction is an appropriate reaction to a stimulus in the environment. It is also known as an adaptive response. An example is when the iris of the eye reacts to the brightness level in the environment by automatically expanding and contracting. Humans and animals have many automatic adaptive reactions, such as the “fight-or-flight” response when exposed to a stressful or frightening situation or salivation when one smells food. These reactions are controlled by the autonomic nervous system.

A great deal of research has been conducted on adaptive responses to stress. These responses involve multiple systems and can greatly affect the body, causing serious and sometimes chronic health problems. Post-traumatic stress disorder (PTSD), for example, has been found to affect many more people than previously realized. Understanding these responses is essential to developing effective treatments.

Background

In the mid-nineteenth century, French experimental psychologist Claude Bernard introduced his theory about how body systems function when faced with stress. Bernard believed in the idea of the inner world, where the body systems try to maintain balance by keeping the internal systems constant. He believed every system in the body was involved by constantly adjusting, like a person standing on a skateboard for the first time and having to shift weight very slightly but constantly to find balance. Bernard’s theory was revolutionary at the time.

Around the start of the twentieth century, American physiologist Walter B. Cannon built on Bernard’s research on the inner world or inner environment. Cannon was interested in how the body adjusts to maintain the constant internal environment. The many experiments he conducted over several decades of research led him to conclude that adrenaline or epinephrine is crucial to maintaining the inner world’s balance. Cannon also introduced three ideas that have become widely accepted: homeostasis, fight-or-flight responses, and the sympathoadrenal system.

Homeostasis is a word that Cannon created to describe the stability of Bernard’s inner world. Cannon theorized that the brain is responsible for the coordination of the body systems. The brain works to maintain targets, such as a core temperature of 98.6 F, optimum blood glucose levels, and so on. Anything that threatens homeostasis, whether an external threat or an internal infection, triggers the internal nervous and hormone systems to act. These responses are directly related to the brain seeking to return the body to homeostasis. Cannon stated that all stressful events cause the brain to react in the same way by increasing the release of adrenaline. Later research indicated he was wrong on this aspect of the adaptive reaction to stress, however.

Cannon also coined the phrase “fight or flight.” He stated that both physical and psychological emergencies would trigger the release of adrenaline into the bloodstream and viewed both the fight and the flight reaction as the same. Later research showed that his dependence on adrenaline as a single variable limited Cannon’s awareness of other response patterns.

Cannon also theorized the existence of a sympathoadrenal system. He believed the sympathetic nervous system and adrenal gland worked together to attain and maintain homeostasis. The methods he used in his experiments, though, prevented him from appreciating the function of the sympathetic nerves in maintaining homeostasis.

Overview

The body’s responses are controlled by the autonomic nervous system. This comprises the sympathetic and parasympathetic nervous systems. The sympathetic nervous system prepares the body for fight or flight in the face of danger or stress. The parasympathetic nervous system returns the body to a balanced, calm state by inhibiting the stress response. While Cannon referred to this inner balance as homeostasis, the modern term is allostasis, or adaptive change to maintain balance.

The autonomic nervous system automatically regulates many body processes, including breathing rate, heart rate, digestion, the production of body fluids such as sweat, and blood pressure. Some organs are controlled by one system or the other, however, others are affected in opposing ways by the nervous systems.

The sympathetic nervous system is composed of nerves that link the internal organs with the brain. When activated, they prepare the body for fight or flight. They increase heart rate and blood flow to the muscles while decreasing blood flow to low-priority systems such as the skin. The parasympathetic nerve fibers are the cranial and lumbar spinal nerves. When stimulated, they reduce the heartbeat and increase the secretion of digestive fluids.

The fight-or-flight stress response of the sympathetic nervous system is triggered by the release of acetylcholine from the preganglionic sympathetic nerves. This triggers the release of hormones, including the catecholamines adrenaline and norepinephrine from the adrenal glands. The heart rate and breathing increase, blood vessels constrict, and muscles tighten. Catecholamines are readily available at neuroreceptor sites, preparing the body for spontaneous reactions such as running from or lashing out at the threat.

These adaptive reactions are appropriate when an individual faces a danger, such as an oncoming car or an aggressive person. After the danger has passed, the body counteracts the aroused state and returns the body to homeostasis. However, internal stresses can trigger the same responses, which may keep an individual in a near-constant fight-or-flight state. This places stress on the body that can have lasting effects. Among the health problems that are associated with chronic stress are gastrointestinal problems such as ulcerative colitis, heart disease, high blood pressure, and mental health problems such as depression.

Individuals with Post-Traumatic Stress Disorder (PTSD) have been found to have high levels of stress hormones even when not in danger. Researchers believe that parts of the brain, such as the hippocampus, that are involved in emotional processing have been affected. In people with PTSD, the hippocampus, which is responsible for emotions and memory, appears to be smaller. This reduction of the hippocampus might prevent the individual from processing flashbacks and nightmares. The stress caused by flashbacks and nightmares may continue to remain at high levels rather than gradually reduce. Researchers have found that helping patients with PTSD properly process memories helps the flashbacks to disappear over time.

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