The Parasympathetic Nervous System (PNS)
I. Parasympathetic 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, feeling excited, and running.
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II. Activites Regulated by the Parasympathetic Nervous System: digestion, blood pressure,...
|Parasympathetic Nervous System (PNS) |
|fuel storage (increases insulin activity)|
|resistance to infection|
|rest and recuperation|
|circulation to nonvital organs, (skin,extremities...)|
|endorphins, the "feel good" hormones|
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Figure 2. Parasympathetic Dominance
Under Normal Circumstances, the Parasympathetic Nervous System promotes the ability to recover, digest, play, and create. An individual who is exposed to states of PNS dominance has an increased risk for Symptoms and Illnesses listed below.
The symptoms and illnesses associated with PNS dominance are those of faint or freeze and include:
depression, weight gain, dizziness and light headedness, low blood pressure, low heart rate, fatigue, diarrhea...
Decreases clarity of thought as a part of a numbing process to avoid pain and overwhelming emotions.
Symptoms: foggy thinking, confusion, slowed thinking, difficulty organizing thoughts
Illness: depression, numbness
Decreases blood pressure in times of safety, and decreases it even further to save energy when survival is at stake.
Symptoms: low blood pressure; dizziness - light headedness
Decreases heart rate, which is associated with the ability to feel calm, to bond, and to be in relatinship. In times of grave threat, heart rate is decreased further to save energy through a state of immobility.
Symptoms: slow heart rate
Illness slow arrhythmias; sudden death; ventricular fibrillation
Increases fuel storage (sugar, fats...) in the body, which serves as a reserve for times of scarcity. The PNS promotes the secretion of insulin and makes tissues sensitive to insulin to promote the storage of glucose in particular.
Symptoms: low blood sugar; low cholesterol;high insulin; weight gain
Illness: obesity; hypoglycemia?
Increases oxygen circulation to non-vital organs in times of safety. Provides fuel and removes wastes from the skin, the extremities, the digestive tract, reproductive organs....; In times of grave threat, shuts down metabolism to save energy until the threat is gone. Some of these activities are similar to SNS processes and may be difficult to differentiate.
Symptoms: difficulty thinking; increased or decreased intestinal activity (diarrhea; slowed gastric emptying)
Illness: peripheral neuropathy?
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Figure 3. Parasympathetic Dominance: Freeze Dominance of the Parasympathetic nervous system suppressing Sympathetic Arousal.
The freeze state represents the most extreme defense strategy at our disposal, and hence is only used as a last resort: "feigned death". Although incompletely understood, the freeze state is known to occur when an animal in fight or flight perceives that its current state sympathetic arousal will not work. When this happens, unconscious processes in the brain suppress the SNS response with the very powerful freeze state, which is associated with severe downregulation in a last attempt to survive until the danger is past. This freeze or immobility state may resemble a form of hibernation or near death and is associated with trauma See below for more.
A prototype of the freeze state is Chronic Fatigue:
This is a state in which fatigue is predominant yet an underlying activation of sympathetic arousal often precedes diagnosis (history of being highly industrious ("workaholic")) for many years, and with diagnosis have poor sleep, hypervigilance, little to no weight gain despite increased appetite, agitated depression,... Some individuals may experience fatigue at the "faint" end of the spectrum (faint) without the signs of sympathetic arousal.
Other possible associations include:
Bronchoconstriction is the hallmark of asthma and represents a freeze state (dorsal vagal nerve: Steve Porges).
Flattened affect, internal focus, lack of social interaction are hallmarks of the freeze state (mediated by the dorsal vagal nerve: Steve Porges). Sympathetic arousal may be seen in the bright intelligence that lies behind the difficulty in communication, as well as high motor activity and energy.
Type 2 diabetes:
Insulin resistance is a normal part of sympathetic arousal and is aimed at maximizing fuel availability rather than increasing fuel storage. An increase in fuel storage, generally seen as weight gain, may be a "side effect" of increased glucose and insulin, or could represent a sign of increased parasympathetic activity. There may be two types of type 2 diabetes, one associated with greater sympathetic arousal (type 1.5) and another dominanted by weight gain / obesity, and depression.
Alzheimer's is a disease of "forgetting", that is also associated with agitation and rapid mood fluctuations, among other symptoms... Alzheimer's may represent a prolonged and extreme form of the freeze state, which is associated with dissociation from the memory of past events too overwhelming to tolerate or integrate.
V. The Parasympathetic Nervous System: Background
The Parasympathetic Nervous System: Rest and Restoration, Faint and Freeze
The PNS consists of 4 cranial nerves originating in the brainstem, including the vagus (see Figure 2-3), which is the principal nerve involved in effecting parasympathetic activity to most areas of the body (Ganong, 2001). The vagus nerve conducts information between the posterior hypothalamus and brain stem of the CNS, and vital organs and glands (Oslen, 2002). Higher brain functions of the cortex also influence vagal function. PNS responses are generally highly specific, as supported by the direct innervation of target organs (Cannon, 1923/1915). The PNS is inhibitory, and promotes energy conservation.
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Functions of the Parasympathetic Nervous System
The primary defense strategies supported by the PNS involve energy conservation. According to Schore (2001), PNS activation promotes passive coping strategies such as withdrawal or disengagement, dissociation, and the immobility response (Schore, 1994). Examples of passive coping styles include physical or emotional withdrawal. Emotions commonly associated with PNS function have a negative valence, such as shame, disgust, hopelessness, and despair (Schore, 1994).
The Dorsal Vagal Complex
The unmyelinated part of the vagal nerve (the dorsal vagal complex or DVC) influences organs below the diaphragm. The myelinated and more rapid portion of the vagal nerve (the ventral vagal complex or VVC) mediates parasympathetic activity above the diaphragm, including motor movement of many aspects of communication, such as facial gesture and sound (Porges, 2001). The DVC has been introduced as the primary agent associated with the immobility / freeze state, and this response is generally activated only in the face of extreme threat (Schore, 1994). The more common functions of the DVC are activities of digestion, rest, and restoration (Porges, 2001).
Parasympathetic Activity is Inhibitory
The evolutionarily ancient DVC is functional at birth but other aspects of the PNS are immature at this time (Schore, 1994). The sympathetic system, on the other hand, is more mature at birth, and this evolutionary strategy enables the organism to be shaped by its interaction with the environment, which is mediated by mobilization behavior of the SNS (Schore, 1994). Initial sympathetic activity also fosters early attachment and bonding (Schore, 1994). The gradual maturation of the parasympathetic system is associated with the capacity to inhibit sympathetic activity, and results in a reduction in mobilization and baseline levels of arousal. If parasympathetic functions come online too early in life, they inhibit the degree to which a developing organism can learn from and adapt to its unique environment because they prevent important exploratory functions associated with mobilization (Schore, 1994).
Moderate Arousal and Social Engagement.
Parasympathetic activity is associated with inhibition of the intrinsic pacemaker of the heart, and results in a lower resting heart rate. A mature PNS is also associated with the capacity to live and to engage with the environment from a state of moderate arousal. This is a state of energy conservation that fosters the capacity to engage with others (Schore, 1994) through VVC-mediated activities such as communication (Porges, 2001).
Capacity to Self-Soothe.
In mammals, early phases of mobilization do not require sympathetic catecholamine activity. As a consequence of the increased functionality and diversity of parasympathetic activity, the organism can usually rapidly self-soothe (Porges, 2001) and can also be comforted by supportive others.
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Rest and Restoration
When the organism feels safe, as opposed to when it is in an energy-conservation mode of self-protection or defense, rest becomes possible. "Without external challenges, [the DVC]... optimizes the function of the internal viscera" (Porges, 2001, p. 17). The PNS, which initially consists primarily of the DVC, stimulates the secretion of digestive enzymes in the presence as well as in the anticipation of food (Braunwald et al., 2001). It furthers digestion by stimulating intestinal motility and peristalsis to move food through the digestive tract, and relaxes intestinal sphincters (Ganong, 2001). In most tissues, the activities of energy storage and digestion are facilitated by the presence of insulin, which is also stimulated by the PNS (Ganong, 2001).
Growth and Reproduction, Attachment and Integration.
"Often, it is during the parasympathetic state, including daydreams,...[and] doodling, that things start to "make sense", and when we have a clear insight... about seemingly disparate thoughts and experience (Olsen, 2002, p. 51).
During periods of low threat, stored energy is available as the substrate for less essential activities such as social engagement (Porges, 2001), growth, and reproduction (Ganong, 2001). During periods of rest and PNS activity, the opportunity for the integration of information also becomes possible, in part because of low levels of norepinephrine, which interferes with certain types of memory storage (Scaer, 2001, p. 15).
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The "faint" response is a defense strategy that is mediated by parasympathetic arousal. Little is available concerning this response in the trauma literature and differences between the faint and freeze response are incompletely understood.
The faint response is a condition of parasympathetic arousal elicited in the face of emotional stress or shock, which results in lowered heart rate and blood pressure. The decrease in blood supply to the brain can lead to loss of consciousness such as fainting. In order for faint to occur, levels of sympathetic arousal must be moderate or low. The faint response is fairly common, can be elicited by environmental triggers such as the sight of blood, and is usually transient.
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The Freeze Response
A Survival Response to Extreme Threat
Like fight/flight, the freeze response is an important strategy of defense and is evoked in the face of perceived threat. The immobility/freeze response is the least understood of the survival responses (Scaer, 2001) and it is elicited as a last resort. Sympathetic arousal always precedes the freeze state and becomes a part of this state (Levine, 1997; Scaer, 2001) due to circulating chemical (catecholamines such as epinephrine) activity.
The freeze state represents a response to extreme threat that cannot effectively be addressed through the use of mobilization strategies such as fight or flight (Scaer, 2001). Hypoxia (low oxygen) is one example of a life-threatening situation that requires a state of extreme energy conservation. In this situation, sympathetic arousal initially facilitates delivery of available oxygen to the most vital organ: the brain. In this circumstance, however, energy conservation is also activated in an effort to optimize chances of survival by minimizing the use of oxygen by less vital organs (Scaer, 2001) such as the intestines and stomach. When the freeze response is elicited, the energy-conserving strategies mediated by the PNS "truncate" (Scaer, 2001) and dominate over the existing state of high sympathetic arousal, leaving the organism in a state of high arousal of both the SNS and the PNS. The resulting condition is associated with characteristics of both energy-conservation and mobilization.
Activities Fostered by the Freeze State: decreased blood pressure, numbness...
The freeze state is associated with the shunting of blood from muscles towards the organs in the body core (Scaer, 2001). Scaer (2001) states that during the freeze state, the racing heart slows to a crawl, blood pressure drops precipitously, tense muscles collapse and become still as a result of the assumption of an "apparent enforced vegetative state" (Scaer, 2001). During freeze, the mind becomes numb and dissociated, at least in part due to high levels of endorphins, and memory access and storage are impaired. Amnesia may be expected for at least some of the events occurring during a freeze state (Scaer, 2001).
Examples in which Freeze is Elicited
The freeze response is perceived to have adaptive value for promoting survival. This state is associated with a variety of characteristics and situations including 1) energy conservation, such as exhibited by reptiles and mammals during underwater dives; 2) an immobility state that serves to conserve energy until a predator loses interest, in which case the state of ongoing high sympathetic arousal permits instantaneous escape at full sympathetic arousal; 3) apparent immobility to fool a predator into thinking the animal is dead, which requires convincing evidence for a death-like state, as seen in the opossum (Scaer, 2001a), and 4) high endorphin states associated with the numbing of pain and a lack of fear in the face of imminent death, sometimes described as a form of "grace" that spares an organism from experiencing the pain when becoming another's prey (Scaer, 2001). These states all form a continuum of the freeze response.
The freeze response plays an important role in the promotion of symptoms following trauma, such as PTSD, and also appears to foster chronic illness states such as chronic fatigue, depression, fainting, and may also affect risk for narcolepsy and Alzheimer's, among others.
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Cannon, W. (1923/1915). Bodily changes in hunger, fear, pain and rage. New York: D. Appleton and Co.
Levine, P. (1997). Waking the tiger. Berkeley: North Atlantic Books.
Porges, S. W. (2001). The polyvagal theory: phylogenetic substrates of a social nervous system. International Journal of Psychophysiology.
Scaer, R. C. (2001). The body bears the burden: trauma, dissociation, and disease. New York: Haworth Medical.
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