ACTH; Corticoids; HPA axis; Stress; Stressor; Stress hormones
‘Stress’ is the word which is known to everyone. We often use the term in our daily lives. Stress is not a situation or a condition during an adverse condition, as it is generally assumed. In fact, it is a way by which the body overcomes a demanding or undesirable situation. Whenever we are in some unfavourable condition (whether it is physical or mental) our body tries to maintain the homeostasis (internal milieu) and protect itself from such events adopting some ‘changes’. Stress is a series of events our body follows to cope with such situations. Selye used the term ‘stress’ to represent the effects of anything that seriously threatens homeostasis of the body . Both external and internal factors affect the homeostasis of the body.
Though stress is commonly associated with disagreeable states and experiences, but it is not always the case. Stress is simply a response to physical and emotional demands. Whenever we feel stressed this usually means that the demands of the situation exceed all our available resources. Generally when we talk about the stress in our lives we are talking about ‘stressors’.
Any factor or event that causes stress is called a stressor. Stressors can be of many types: physical or physiological changes in the body, changes in the environment, life events or behaviours. Even an unreal (imaginary) situation can act as a stressor and could be the reason of stress. In fact in most of the cases it is just an imagination which is the cause of a stress. Hence, it is very important how we perceive an event or a situation.
Depending on the nature, duration and impact on the body, stress can be categorized into different categories:
Following a stressful event the body acts at different levels to cope with the stressor. This is achieved through two major changes in the body: 1. change in pattern/amount of the release of the energy and 2. change in the distribution of energy. Several events occur to bring these changes. All such events are collectively called stress or the stress response. Stress is a multidimensional phenomenon which involves both nervous and endocrine system.
The first step in stress response is the perception of the threat (stressor). Whenever there is some stressor - real or imagined, it acts at the level of brain. In the brain, it is the hypothalamus which perceives the stressor. When the hypothalamus encounters a threat it performs some specific functions: 1. activates autonomic nervous system (ANS) 2. Stimulates Hypothalamic Pituitary Adrenal (HPA) axis by releasing Corticotrophin Releasing Hormone (CRH) and 3. Secrets arginine vasopressin (Antidiuretic Hormone ADH). Autonomic nervous system consists of sympathetic (arousal) and parasympathetic (relaxed) nervous system. The ANS regulates visceral activities like circulation, digestion, respiration, temperature regulation and some vital organs.
The sympathetic system accounts for the flight-or-flight response. In response to a stressor catecholamines: epinephrine (adrenaline) and norepinephrine (nor adrenaline) are released at various neural synapses. The release of these catecholamines causes several changes like increase in the heart rate and force of myocardial contraction vasodilatation of arteries throughout working muscles and vasoconstriction of arteries to nonworking muscles; dilation of pupil and bronchi and reduction of digestive activities in the body. All these changes are required to prepare the body for fight-or-flight response. The effects of these hormones - epinephrine and nor epinephrine last for few seconds. The functions of parasympathetic nervous system are opposite to that of sympathetic nervous system and help in energy conservation and relaxation.
CRH acts at the anterior pituitary gland an endocrine gland located in the brain. Pituitary gland is also called ‘master gland’, as it controls the secretion of other endocrine glands in the body. On stimulation by CRH, anterior pituitary secretes Adrenocorticotropin Hormone (ACTH). According to Scantamburlo et al., arginine vasopressin modulates the effect of CRH on ACTH secretion .
ACTH released from anterior pituitary gland in response to CRH stimulates adrenal glands located on the kidneys. There are two parts of adrenal - the outer part called cortex and the inner part known as medulla.
ACTH stimulates adrenal cortex to release corticoids (glucocorticoids and mineralocorticoids). The major function of glucocorticoids is to release energy, which is required to cope with the ill effects of stressor. The energy is released by conversion of glycogen into glucose (glycogenolysis) and also by breakdown of fats into fatty acids and glycerol (lipolysis). In addition to this corticoids have several other functions such as: increased urea production, appetite suppression, suppression of immune system, exacerbation of gastric irritation, associated feeling of depression and loss of control. These are the symptoms generally seen in a person under stress. Mineralocorticoid (aldosterone) promotes Na+ retention and elimination of K+. It increases blood pressure by increasing blood volume. The medulla part of the adrenal gland secretes epinephrine and norepinephrine. The functions of these hormones are the same as that of those secreted from nerve endings of sympathetic nervous system. These hormones secreted by adrenal medulla, reinforce the functions of sympathetic nervous system. The release of these hormones from adrenal medulla acts as a backup system to ensure the most efficient means of physical survival. The effects brought out by epinephrine and norepinephrine from the sympathetic nervous system may be termed as immediate effects and the effects brought out by those of adrenal medulla are intermediate effects.
The basic function of vasopressin or ADH synthesised by hypothalamus and released by posterior pituitary is to regulate fluid loss through urinary tract. This is achieved by reabsorption of water. In addition, ADH also has a prominent role on regulation of blood pressure during stress when the homeostasis of the body is disturbed in addition to release of energy second major change occurring during stress is distribution of energy to a particular organ that needs it most. This is achieved by increasing blood pressure. This occurs either through enhanced cardiac output or through constriction of blood vessel.
In addition to HPA axis some other hormones such as Growth Hormone (GH) and thyroid hormones also play significant role in stress. Growth hormone is a peptide hormone, released from anterior pituitary gland. GH is a stress hormone that raises the concentration of glucose and free fatty acids . It has been observed that, in human beings psychological stimuli increase the concentration of thyroid hormones [4,5]. Thyroid releases thyroxin and triiodothyronine. These hormones also have some significant function in stress . The main function of thyroid hormones is to increase overall metabolic rate or Basal Metabolic Rate (BMR). Thyroxin also increases heart rate and also the sensitivity of some tissues to catecholamines.
Though, serotonin and melatonin are not considered as stress hormones yet they are associated with mood. A decrease in the levels of these hormones is thought to be related to depression.
Citation: Sharma DK (2018) Physiology of Stress and its Management. J Med Stud Res 1: 001.
Copyright: © 2018 Dushyant Kumar Sharma, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.