As we continue our tour of the fascinating academic discipline of the neurosciences, we will take a temporary stop to see the Hall of Fame of stress, the sympathetic nervous system.
As students, and more so, as humans, we have all experienced stress in our lives. As with anything, we conjure definitions to put a verbal stamp on the sensations we experience. A typical definition of stress utilizes a psychosocial approach: “physical, mental, or emotional strain or tension” or “a condition or feeling experienced when a person perceives that demands exceed the personal and social resources the individual is able to mobilize (1).” Speaking from a biopsychological perspective, stress is explained as any environmental or physical pressure that elicits a response from an organism (2). There are several levels of stress: acute, chronic, or traumatic. When we discuss acute stress, we are referencing the sympathetic stress response or “fight or flight response.”
To review, the human nervous system has two major subdivisions: the central, comprised of the brain and spinal cord, and peripheral, consisting of sensory organs and outer nervous, nervous systems. The sympathetic nervous system (SYS) is a section of the autonomic nervous system, which is a subdivision of the peripheral nervous system. The SYS is activated during times of physiological distress, i.e. when the fight or flight response is initiated, and it has some telltale symbols of its activation, which we will get into later.
The stress response can be explained by Hans Selye’s General Adaptation Syndrome (GAS). It is called the “general” adaptation syndrome because the phases of the GAS are universal regardless of the stressor. The three stages are alarm, resistance, and exhaustion.
When the body is alarmed, it perceives itself to be under attack by some environmental threat. Thus, the stress response begins in the central nervous system, namely in the limbic system. Before the brain’s visual centers can even assess the threat, the amygdala, a limbic brain structure that processes emotion, sends a signal to the hypothalamus. The hypothalamus, a limbic brain structure that controls biological drives, acts as the command center of the whole fight or flight operation. As the master gland, the hypothalamus sends neural signals through the autonomic nervous system to the adrenal glands, which secrete the stress steroids: epinephrine (adrenaline), norepinephrine (noradrenaline), and glucocorticoids (cortisol). As these stress hormones flood the bloodstream and supply the body’s muscles with energy, numerous actions are physiologically taking place, which result in the recognizable symptoms of stress arousal: the pupils dilate, salivation decreases, skin perspires, respiration increases, the heartbeat accelerates, digestion is inhibited, etc.
The second phase of the GAS, resistance, increases the level of cortisol in order to support the SYS. During the resistance phase, the body has two options: if the threat has passed, return to its pre-arousal homeostatic state or if the threat persists, the body reeves up its output of stress hormones until the stressor is dealt with.
In exhaustion, the GAS’ third phase, if the stress response has been activated for an extended period of time, the body’s resources are depleted, the immune system is exhausted, and the alarm symptoms can kick unprovoked.
There are two alternative stress theories: The General Inhibition to Stress Syndrome, where the body directs stress inward, possibly resulting in learned helplessness, passivity, and depression, and the Tend and Befriend Model, where evolutionarily speaking, women, because they would care for the young, could not flee without the young offspring. Both theories are intriguing descriptions of the very well known sensation of stress.
In my next article, we will backtrack from the peripheral nervous system and explore the county seat of cognition: the central nervous system.
References and Footnotes
(4) Featured Image Credit: http://ainsworthinstitute.com/patient-information/anatomy/sympathetic-nervous-system/