Response To Stress

Every environment can expose livestock to some type of stressor. These stressors can be thermal, physical, and social. How the animal is able to cope with these potential stressors depends on the species abilities, for instance, swine can not sweat. Similarly, past experiences are important for coping in situations that may involve confrontations between individuals. When livestock are exposed to a stressor, the body automatically initiates a stress response. Stress responses are characterized by a behavioral response as well as a physiological response. The first behavioral response is simply for the animal to remove itself from the stressor, to move away. However, often this is not possible, such as when livestock are exposed to heat stress or to a pen mate that is aggressive. If unable to escape the stressor, a chronic physiologic stress response is maintained. Physiologically, the stress response is characterized by activation of the sympatho-adrenal axis (SA) and the hypothalamic-pituitary-adrenal axis (HPA) (see Fig. 1). The SA axis is activated immediately upon being exposed to a stressor and is the body's short-term response. This response is characterized by activation of the sympathetic nervous system, which stimulates the adrenal medulla to secrete epinephrine (also called adrenaline) into the circulation. Epinephrine is responsible for increasing blood flow and pressure in the body by increasing heart rate and vasoconstriction. This increase in blood flow allows the body to deliver more oxygen and energy (glucose) to muscle and brain tissue, thereby allowing the animal to react to the stressor (often termed the fight-or-flight response.[1] Activation of the HPA axis is used to maintain a physiologic status that enables the animal to continue to respond to the stressor. This axis is characterized by the release of corticotropic-releasing hormone (CRH) from the hypothalamus, which then causes the release of adrenocorticotropic hormone (ACTH) from the anterior pituitary gland. ACTH is then released into the circulation to cause the release of glucocorticoids from the adrenal cortex. Glucocorticoids increase the availability of energy to the muscles and brain by facilitating glucose availability in the circulation. Thus, the stress response[2,3] is critical for livestock to cope with stressful situations.

However, several products of the stress response can cause the immune system to be suppressed. Glucocorti-coids, CRH, and epinephrine have all been shown to alter immune function and typically are considered immuno-suppressive. For example, glucocorticoids are known to suppress the function of lymphocytes, macrophages, natural killer cells, and neutrophils. Some of these immune cells, for instance, lymphocytes and macrophages, have receptors for glucocorticoids and/or cat-echolamines (i.e., epinephrine), which allow these compounds to have direct effects on the immune system. Other parameters of the immune system are altered indirectly when the stress hormones cause lymphocytes and macrophages to have an altered expression of cytokine release. A more in-depth review of the stress response on immune function can be found under ''Immune System: Stress Effects,'' and in several reviews.[4-6]

Fig. 1 The hypothalamic pituitary adrenal (HPA) axis and the sympatho adrenal (SA) axis are activated in response to a stressor. CRH, corticotropic releasing hormone; ACTH, adrenocortico tropic hormone. CRH causes the release of ACTH, which causes the release of glucocorticoids from the cortex of the adrenal gland (the HPA response). Sympathetic activation of the autonomic nervous system causes the release of epinephrine from the medulla of the adrenal gland (the SA response). (View this art in color at www.dekker.com.)

Fig. 1 The hypothalamic pituitary adrenal (HPA) axis and the sympatho adrenal (SA) axis are activated in response to a stressor. CRH, corticotropic releasing hormone; ACTH, adrenocortico tropic hormone. CRH causes the release of ACTH, which causes the release of glucocorticoids from the cortex of the adrenal gland (the HPA response). Sympathetic activation of the autonomic nervous system causes the release of epinephrine from the medulla of the adrenal gland (the SA response). (View this art in color at www.dekker.com.)

How To Bolster Your Immune System

How To Bolster Your Immune System

All Natural Immune Boosters Proven To Fight Infection, Disease And More. Discover A Natural, Safe Effective Way To Boost Your Immune System Using Ingredients From Your Kitchen Cupboard. The only common sense, no holds barred guide to hit the market today no gimmicks, no pills, just old fashioned common sense remedies to cure colds, influenza, viral infections and more.

Get My Free Audio Book


Post a comment