Oxidative Stress Series Part 5: The Mind-Body Connection

Oxidative Stress Series Part 5: The Mind-Body Connection

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When you comment at the end of difficult day that you’re “stressed out,” did you ever stop to think what that really means?

Psychological and emotional anxiety results in physiological responses. Sometimes, those physiological responses increase oxidative stress in cells. Oxidative stress, as discussed in previous posts in this series, can take a toll on various organs and functions, including hearing, where it is often manifest as intractable tinnitus (ringing in the ears) attendant with a hearing loss.

Thanks to our hunter-gatherer ancestors living by their wits several tens of thousands of years ago, humans, along with other animals, developed a survival reflex—a physiological stress response—often referred to as a “fight-or-flight” response. Fight-or-flight is an acute stress reaction controlled by a primitive part of the brain called the hypothalamus, located deep within the convoluted cerebral cortex. Loud sounds, both sudden and continuous, can trigger stress reactions (called audiogenic stress), as can other situations that cause anxiety. When we use the word “stress” to describe psychological and emotional anxiety, what we’re really referring to is the physiological strain that results from the way our bodies physically react to threatening situations.

The hypothalamus is part of a structure in the nervous system called the limbic system. The limbic system controls autonomic body functions vital for survival and continuance of the species: heart rate, blood pressure, respiration, libido and sexual response, hunger, thirst, fat storage, and metabolic rate, among myriad others. These functions are regulated by the release of hormones into the blood stream by the endocrine system. The hypothalamus exerts primary control over endocrine function, but it also responds to signals being returned to it by the release of hormones from elsewhere in the body. The loop connecting the brain to glandular hormones is the neuroendocrine system; it is the mind-body connection.

Research has identified two possible neuroendocrine response systems related to psychological stress [1]. Both response systems are mediated by the small adrenal glands which sit atop each kidney and take their cues either directly or indirectly from the hypothalamus1. The adrenal glands have two distinct layers—the medulla and the cortex—and each layer secretes specific chemicals. The inner layer, or medulla, of the adrenal gland is responsible for releasing the catecholamines epinephrine (adrenaline) and norepinephrine into the bloodstream. The adrenal medulla is activated by sympathetic neurons directly from the hypothalamus. This is the sympathetic adrenomedullary (SAM) system. Activation of the SAM system is immediate.

In contrast, the adrenal cortex secretes about 30 different hormones called glucocorticoids. Activation of the adrenal cortex by the hypothalamus takes place indirectly; i.e., the hypothalamus stimulates the pituitary gland to release hormones into the bloodstream which in turn trigger the release of glucocorticoids from the adrenal cortex. This is called the hypothalamic pituitary adrenocortical (HPA) system. Activation is much slower than the SAM system and its effects persist much longer.

Acute stressful events, like that of a singular frightening nature (perhaps a loud, sudden sound), involves the SAM system and results in the “fight or flight” response with the release of epinephrine and norepinephrine. Instantaneous demand for fuel and oxygen is made in anticipation of a physical response that has not yet happened. In order to quickly deliver the extra fuel and oxygen to the muscles and organs that will be needed when the physical response takes place, heart rate, respiratory rate, respiratory volume, and blood pressure are increased ahead of the exertion. When the hunter becomes the prey, survival depends on sharpened sensory awareness and being able to move quickly. These chemicals enter the bloodstream quickly and are rapidly removed. Only small amounts are needed.

About 30 minutes after the onset of a stressful event, the HPA system has released peak levels of the primary glucocorticoid cortisol into the bloodstream. Cortisol attempts to make the body resistant to another acute stress reaction by keeping it ready to access extra fuel, oxygen, and other resources more easily. It does this by inhibiting the immune response, maintaining open airways, and storing fuel (converting glucose and storing it as fatty acids in adipose tissue).

Chronic stress and anxiety keeps the HPA system activated and cortisol (often called the “stress hormone”) is maintained at higher-than-normal levels in the bloodstream. This suppresses the immune system, inhibits insulin to promote glucose uptake, and promotes storage of fat instead. A person may feel “on-edge” all the time because heart rate and blood pressure remain elevated, as if primed for another acute episode. The metabolic changes and weakened immune response can mean a buildup of oxidative agents in the cells that contribute to accelerated aging of the skin, degeneration of connective tissues, and damage to organs, including the heart, the pancreas, the kidneys, and the cochlea.

The best way to deactivate the HPA system, reduce the level of cortisol in the bloodstream, and reverse the effects of oxidative stress is to remove one’s self from the environment that is triggering the stress response and initiate a period of recovery2. It is vitally important to take vacation time, relax, and intersperse work with periods of play. This is also the reason that taking quiet breaks away from noise is beneficial to both hearing and overall well-being.

Because oxidative stress contributes to cochlear damage, it’s easy to see why exposure to loud noise can be so devastating. The physical exposure of the ear to the noise is one factor; anxiety and fatigue from noise exposure is another factor. Noise is a sensory stimulus known to trigger a fight-or-flight response in humans and other animals3. Continuous exposure to loud noise keeps our stress triggers primed and “on-edge.” As the increased oxidative deterioration takes its toll on the hardest-working structures, like the heart and the rest of the cardiovascular system, failure of these systems can occur, resulting in heart attack and stroke.

Seen from this perspective, wearing hearing protection and taking quiet, restful breaks away from noisy and demanding environments may not only preserve your hearing, but it may also help you avoid other effects of chronic stress. It might contribute to slower aging and lower blood pressure. It might even save your life.

 

 

 

Footnotes
[1] Lundberg, U. (2005). Stress hormones in health and illness: The roles of work and gender. Psychoneuroendocrinology, 30(10), 1017 – 1021.

[2] Sluiter, J.K., de Croon, E.M., and Frings-Dresen, >H.W. (2003). Need for recovery from work related fatigue and its role in the development and prediction of subjective health complaints. Occupational and Environmental Medicine, 60(Suppl 1), i62 – i70.

[3] Burow, A., Day, H.E.W., and Campeau, S. (2005). A detailed characterization of loud noise stress: Intensity analysis of hypothalamo-pituitary-adrenocortical axis and brain activation. Brain Research, 1062(1-2), 63 – 73.

Blog Author:  Robert Ghent  AuD.  
Blog Catagories:  Stress  Oxidative Stress  Hearing Health  

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