The Sympathetic Nervous System (SNS)
I. Sympathetic Nervous System Activity Supports Homeostasis and Balance
Figure 1. Maintaining Homeostasis Across Activities.
A healthy nervous system maintains homeostasis by balancing input from both branches of the ANS during activites ranging from relaxing, digesting and sleeping, to waking, walking, feeling excited, and running.
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II. Activites Regulated by the Sympathetic Nervous System: blood pressure, heart rate...
|Sympathetic Nervous System|
|fuel availability (sugar, fats...) |
|oxygen circulation to vital organs|
|blood clotting (minimizes loss of blood if wounded)|
|pupil size and peripheral vision (improves vision)|
|fuel storage (decreases insulin activity to store glucose, for example)|
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Figure 2. Sympathetic Dominance. Dominance of the Sympathetic nervous system.
Under Normal Circumstances, the Sympathetic Nervous System promotes the ability to be active and the defense mechanism of fight or flight. It affects activites in red. An individual who is exposed to states of SNS dominance has an increased risk for Symptoms and Illnesses listed below, which have long been associated with stress.
The symptoms and illnesses associated with SNS dominance are those of fight/flight, and include:
hypertension, hypercholesterolemia, fast arrhythmias, heart disease, type 1 diabetes, sjogren's; anxiety, panic attacks, hypervigilance, poor sleep,...
Increases vigilance and arousal to quickly notice and respond to danger.
Symptoms: hypervigilance; startling easily;nervousness;anxiety;fear;
Illness: chronic or severe anxiety; panic attacks
Increases blood pressure to get blood to the brain and vital organs:
Symptoms: ? white coat high blood pressure;
Illness: hypertension, strokes from prolonged high blood pressure, heart disease; heart attacks
Increases heart rate to circulate fuel and oxygen to vital organs for activity and defense:
Symptoms: fast heart rate
Illness fast arrhythmias (?atrial fibrillation; PSVT?)
Increases fuel availability (sugar, fats...) to the brain, muscles and other organs who need it during exercise and defense. Because insulin promotes food storage, it is inhibited during sns activity to maximize fuel availability.
Symptoms: high blood sugar; high cholesterol;low insulin;
Illness type 1 diabetes, with increased risk for high blood pressure, high cholesterol, heart disease,...; hypercholesterolemia
Increases adrenalineto facilitate changes in blood pressure, heart rate etc
Symptoms: shakiness; palpitations; ?butterflies; difficulty concentrating
Illnesschronic anxiety, panic, hypertension, and others listed here
Increases oxygen circulation to vital organs to provide fuel for activity and defense while decreasing circulation to non-vital organs such as skin, the extremities,...
Symptoms: cold hands and feet;headaches?
Increases blood clotting ,which minimizes blood loss if wounded during defense such as fight/flight
Symptoms: strokes; clotting disorders
Increases pupil size and peripheral vision to maximize awareness of sources of potential danger
Symptoms: blurry vision when trying to focus on narrow vision (reading...)
Illnessprolonged visual changes requiring corrective lenses?
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IV. The Sympathetic Nervous System: Background
The Sympathetic Nervous System: Active Coping and Mobilization
Due to its participation in generating the well-known fight/flight response, the SNS is the more familiar and the more studied of the two branches of the autonomic nervous system (ANS). Contrary to its name, the fight/flight response, coined by the physiologist Walter Cannon (Ganong, 2001), represents a broad range of mobilization activities.
The sympathetic nervous system (SNS) is geared towards mobilization. It is associated with active coping strategies, such as the energized activity utilized to complete a project, fight a foe, or flee a predator.
Events that stimulate SNS involvement include exercise (Ganong, 2001), the experiencing of emotions such as positive states of interest, joy (Lipsitt, 1976, as cited in Schore, 1994) and excitement (Ganong, 2001), as well as terror, rage, and elation, and exposure to moderate changes in temperature (Sapolsky, 1998).
A stronger activation of SNS arousal arises following experiences that are emotionally distressing (Goldstein, 2000) such as the perception of threat. In the face of danger, the SNS facilitates changes in the body that promote survival. These changes include elevations in heart rate to facilitate the rapid circulation of oxygen and glucose to muscles that need fuel to flee or fight, pupil dilatation for better vision, water retention to minimize loss in fluid volume, and increases in the clotting of blood that minimize blood loss in the event of wounding.
Examples of threats that may induce the fight/flight or stress response include those arising within the internal environment, such as in the case of low blood sugar, which represents a global threat to the organism sugar (glucose) is the only food used by the brain (Ganong, 2001). Threat may also be perceived in the external environment, such as occurs when someone comes face to face with a bear, is in an accident, or is threatened by a hurricane. Excitement (an internal stimulus) or a long jog (an external stimulus) may stimulate the same level of sympathetic arousal that is mediated in the face of threat (Braunwald et al., 2001). The context in which threat occurs, the perception of the degree of danger, and the ability to successfully escape from a situation all affect the degree of SNS response.
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Responding to Threat
In the case of potentially life-threatening events, the organism is geared to respond rapidly, since fast responses improve chances of survival (Porges, 2001). The hypothalamus promotes this function, raising a body-wide alarm response within minutes, followed at a slightly slower pace by an even broader "stress response". The stress response, described by Hans Selye (1978) is elicited through stimulation of the hypothalamic-pituitary-adrenocortical (HPA) axis, which results in secretion of the cortisol. The adrenal cortex facilitates conversion of norepinephrine to epinephrine (Porges, 2001). Together, the effects of cortisol, epinephrine, and norepinephrine engage system-wide defense mechanisms, fostering mobilization through processes such as secretion and distribution of glucose and stored energy, in part through inhibition of insulin secretion and promotion of insulin resistance (Meaney et al., 1996).
While the epinephrine and norepinephrine responses are rapid and fairly short lived, the cortisol effect lasts for hours and helps the body repair damage and reduce inflammation (Sapolsky, 1998). Cortisol also helps shut down or contain SNS activation and helps the body return to baseline (Sapolsky, 1998).
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Characteristics of the Fight/Flight Response
The global fight/flight response has a number of very specific effects aimed at maximizing survival, mediated by circulating epinephrine and cortisol (Braunwald et al., 2001). These effects include a state of heightened alertness, increased energy with which to meet a potentially difficult situation, and augmented muscle strength (Ganong, 2001). In preparation for battle, chemicals are released into the blood to facilitate clotting, and blood vessels in the skin are constricted to prevent heavy blood loss in the event of wounding (Ganong, 2001). Similarly, blood pressure and heart rate increase and the kidneys retain water, all in support of tissue perfusion and the maintenance of fluid volume in the event of sweating or blood loss (Ganong, 2001). In addition, the spleen deposits red blood cells into the blood stream in order to increase oxygen delivery to muscles (Juhan, 1998), and pupils dilate to let more light into the eyes in order to increase visual acuity (Ganong, 2001).
Specific Example 1: Insulin and Digestion
In support of all the increases in metabolic demand, epinephrine and cortisol increase the production and release of glucose, and inhibit insulin activity and secretion (Braunwald et al., 2001). During fight/flight, digestion and other activities such as energy storage are also inhibited (Ganong, 2001). In effect, during fight/flight "we stop digesting the food in our stomachs and intestines and begin digesting ourselves", as stored glucose, proteins and fats are utilized for energy (Juhan, 1998).
Specific Example 2: The Immune System
An additional feature of the acute phase of the fight/flight response is an increase in immune system activity (Sapolsky, 1998; Scaer, 2001). Rises in norepinephrine and epinephrine in this acute phase are associated with increases in the organism's front line of defense, expressed through augmentation of natural killer cell activity (Scaer, 2001). As acute stress becomes chronic, however, elevations in cortisol levels impair the immune response and inhibit natural killer cell activity, rendering the organism more susceptible to infection (Scaer, 2001).
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V. Summary of Sympathetic Nervous System Activity
In summary, the SNS fosters the release and production of epinephrine and cortisol, as well as glucose and other metabolites in support of mobilization (Porges, 2001; Schore, 1994). The level of SNS response occurs on a continuum, ranging from the local release of norepinephrine, to slightly broader responses associated with a decrease in vagal tone (PNS), to the global response of fight/flight associated with epinephrine secretion, to the stress response and cortisol secretion associated with activation of the HPA axis. SNS activity also promotes attachment behavior between infant and caregiver through dopamine release that fosters engagement, interaction, and exploratory behavior (Schore, 1994). In health, mobilization fosters the body's ability to utilize glucose and is associated with a decreased need for insulin in mobilized tissues, such as skeletal muscles in limbs, which require insulin at rest (Braunwald et al., 2001).
Examples representing variation in energy and mobilization responses include a change from sitting to standing, which elicits local norepinephrine release (Braunwald et al., 2001), and mild exercise such as a saunter through a park, which can stimulate a decrease in vagal tone, allowing heart rate to increase without SNS activation. Increasing the level of activity to a jog may then stimulate a more global response in order to meet the greater energy demand.
Finally, if a jogger suddenly comes face to face with a ferocious bear or a purse-snatcher, the HPA axis may be recruited, resulting in the combining of many SNS responses. Activation of the global response and cortisol secretion are costly, both on an energetic level due to the degree to which glucose and metabolites must be mobilized, and on a behavioral level due to the effects of epinephrine and norepinephrine, which increase irritability and emotional reactivity, and require time reabsorption (Ganong, 2001). Because of the prolonged reabsorption phase following fight/flight, a high degree of sympathetic activation reduces the ability to self-soothe and self-regulate (Porges, 2001).
The order in which the organism responds to threats proceeds from the most energy efficient and energy conserving states associated with the relational interactions, to more primitive energy conserving states consisting of immobility and freeze states (PNS). These responses occur in a blended fashion and are not necessarily distinctly separate phases (Porges, 2001).
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