Walter Cannon first described the role of the sympathetic nervous system in 1915. Stress can be defined as any perturbation from homeostasis, and although perceived as harmful, the causes and response to stress are instrumental in maintaining health.

The stress response involves interactions between the central and peripheral nervous systems and other body systems. Although much is known about the peripheral response, research investigating the role of the brain, as well as its interactions with peripheral systems, has made great strides in recent years, especially with the advent of new technologies allowing greater manipulation of the genes and circuits involved.

Stress. Image Credit: Rawpixel.com/Shutterstock.com

What is the effect of stress on the body?

In response to a stressor, the body activates coordinated and dynamic processes to restore homeostasis.  This 'stress response' leads to the release of noradrenaline from synapses and adrenaline from the adrenal medulla. As well as homeostatic disturbance, purely psychological processes can determine the magnitude of the stress response. These trigger the following physical reactions:

• Increased blood  pressure
• More blood flow to active muscles and less blood flow to organs not needed for rapid motor activity
• Increased rate of blood coagulation
• Increased rates of metabolism
• Increased muscle strength
• Increased mental activity
• Increased mobilization of glucose
• These factors all improve a person’s ability to respond to a potentially hazardous or challenging situation.

The main types of stress

There are several types of stress:

1. Acute stress: an immediate reaction to a perceived threat, challenge, or scare. This produces a cascade of changes in the nervous, cardiovascular, endocrine, and immune systems. Two features make the stress response adaptive. First, energy stores are mobilized by stress hormones for immediate use. Second, the energy distribution is altered; energy is diverted to the skeletal muscles and the brain.
2. Chronic stress: this occurs when the acute stress response becomes maladaptive. The elevated basal levels of stress hormones associated with chronic stress are associated with damage to the cardiovascular system, including increases in blood pressure, vascular hypertrophy, and damaged arteries and plaque formation. It may also suppress immunity which is associated with a slower rate of wound repair, surgery recovery, poorer antibody responses to vaccination, and increased vulnerability to viral infections. Chronic stress can lead to atrophy of the brain mass and decrease its weight

Stressors are environmental factors that trigger the stress response. Examples include noises, aggressive behavior, and threatening or risky situations. Feelings of stress tend to increase in tandem with the number of stressors.

Physiological vs physical stressors: common causes

The stress response is mediated by the hypothalamus-pituitary-adrenal (HPA) axis, whose main effector is glucocorticoids (GCs). GC release affects systemic changes in the body due to the ubiquitous expression of the GC receptor – resulting in the cardiovascular, metabolic, cognitive, and immunologic effects experienced in response to stress. Stress can be divided into two major categories: physical stress and psychological stress.

Psychological and physical stressors engage different neuronal and cell activities, that cause distinct pathways to be activated in the brain. Stimuli that overwhelm the organism and result in disturbances of physiological status, e.g., infection are considered as physical stressors. However, psychological stressors are perceived in anticipation e.g., negative environmental stimuli, and internal dissatisfaction such as failure to satisfy eudemonia (authenticity, fulfillment, self-actualization, and purpose) and hedonia (pleasure, enjoyment, and satisfaction).

Physical stressors and their common causes

Physical stresses of processed by the brainstem on hypothalamic regions for, first time, and they usually result in a systemic reaction. Therefore, the first phase of the stress response in response to physical stimuli, results in a physiological adaptation, resulting in short-lasting responses such as increased vigilance, alertness, and assessment of the situation to facilitate a strategic decision to be made in response. This is called the sympathetic adrenomedullary system, or SAM.

The secondary phase, however, involves the HPA, which is a delayed effect compared to the synaptic mechanisms that activate the SAM. The HPA results in an amplified and protracted secretary response which is long-lasting. These can be internal or external. Internal stressors include:

• Malnutrition and/or dehydration
• Sexual frustration
• Illness or injury
• Substance use (alcohol, nicotine, caffeine)
• Illness or injury
• Obesity/ Anorexia
• Impaired immune system
• Muscular and aerobic fatigue
• Sleep deprivation
• External stressors relate to the environment and include:
• Heat, cold, wetness, and dust
• Ionizing radiation
• Infectious agents
• High altitude
• Poor visibility including bright lights, darkness, and haze
• Noxious odors such as fumes, poisons, and chemicals
• Loud noise
• Physical assault

Therefore, when an external stressor is perceived through sensory mechanisms, such as touch as a result of inflammation, pain, etc. by the brainstem, neuropsychiatry is activated did generate a rapid autonomic nervous system and HPA axis response.

Psychological stressors and their common causes

Psychological stresses impact both physical and cognitive stress responses. As a result, parts of the limbic circuits, which include the prefrontal cortex (PFC), amygdala, hippocampus (HIPPO), PVN, ventral tegmental area (VTA), and nucleus accumbens (NAc), are activated. Example of physiological stressors that impact cognitive process include:

• Lack of, or excess information
• sensory overload or deprivation
• Difficult decision-making
• Organizational dynamics and changes
• Working beyond the skill level
• Time pressure or waiting
• Ambiguity, uncertainty, and unpredictability
• Previous failures

Psychological stresses also include emotional triggers; common causes of emotional psychological stresses include:

• New environments
• Loneliness and isolation
• Experiencing prejudice/exposure to bigotry
• Bereavement
• Resentment, anger, frustration, and guilt
• Boredom
• Conflict
• Spiritual confrontation
• Interpersonal conflicts
• Homesickness
• Victimization
• Exposure to violence

Internal stresses may exacerbate the response to external stresses as a result of overlapping processing.

The cumulative science linking stress to negative health outcomes is vast. Individuals’ responses to stress can vary – from maladaptive responses to resilience. These differences can be due to sex, genetic predisposition, personality, and mindset. Stress may also produce beneficial effects such as the preservation of cells/species homeostasis. The type, timing, and severity of the applied stimulus evoke a compensatory response known as stress responses. These range from alterations in homeostasis to life-threatening effects and death.

References:

• Yaribeygi H, Panahi Y, Sahraei H, Johnston TP, Sahebkar A. The impact of stress on body function: A review. EXCLI J. 2017;16:1057-1072. Published 2017 Jul 21. doi:10.17179/excli2017-480
• www.mind.org.uk. (n.d.). Causes of stress. [online] Available at: https://www.mind.org.uk/information-support/types-of-mental-health-problems/stress/causes-of-stress [Accessed 22 Jun. 2021].

Further Reading

• All Stress Content
• Why can Uncertainty Cause Stress?
• Stress and Alzheimer’s Disease

Written by

Hidaya Aliouche

Hidaya is a science communications enthusiast who has recently graduated and is embarking on a career in the science and medical copywriting. She has a B.Sc. in Biochemistry from The University of Manchester. She is passionate about writing and is particularly interested in microbiology, immunology, and biochemistry.