The parasympathetic branch generates the variability between adjacent heartbeats. The sympathetic branch regulates slower heart rate changes across several beats. The hormones angiotensin, epinephrine, and vasopressin modulate heart rate over periods from seconds to hours (Karemaker, 2020).
Brindle and colleagues (2014) meta-analyses concerning acute psychological stress mechanisms found equal roles for beta-adrenergic activation and vagal withdrawal: "Cardiovascular reactivity to acute psychological stress would appear to reflect both beta-adrenergic activation and vagal withdrawal to a largely equal extent" (p. 964).
This unit challenges the simplistic beliefs that we respond to acute stressors using only sympathetic fight-or-flight or parasympathetic vagal withdrawal responses. These branches do not operate independently. The autonomic nervous system integrates sympathetic and parasympathetic responses for more nuanced control. This is the accentuated antagonism model's premise. The background activity of one branch moderates brief changes in the other (Uijtdehagge & Thayer, 2000).
Stephen Porges (2011) expanded our appreciation of the parasympathetic branch's versatility. We respond to everyday stressors with vagal withdrawal. In extreme cases, we may use immobilization, feigning death, passive avoidance, or shutdown. When we perceive that we are safe, we may socially engage, supported by the release of the hormone oxytocin, and practice self-regulation skills.
BCIA Blueprint Coverage
This unit addresses I. HRV Anatomy and Physiology: C. ANS Anatomy and Physiology.
Professionals
completing this module will be able to discuss:
A. The anatomy and physiology of the three autonomic branches
B. The distribution and functions of the vagus nerve
This unit covers the Autonomic Nervous System (ANS), Sympathetic Division, Parasympathetic Division, The Relationship Between the Sympathetic and Parasympathetic Branches, Porges' Polyvagal Theory, and the Enteric Division.
Please click on the podcast icon below to hear a full-length lecture.
Autonomic Nervous System (ANS)
The central nervous system (CNS) includes the brain, spinal cord, and retina.
The peripheral nervous system consists of the somatic nervous system and
the three branches of the autonomic nervous system (Breedlove & Watson, 2023).
Caption: This image illustrates the three-quarter right anterior-lateral view of the male torso, with the nervous system highlighted. The central nervous system CNS (brain and spinal cord) is colored blue, and the peripheral nervous system PNS (major peripheral nerves) is colored yellow. Shown are the brain inside the cranium, the spinal cord inside the vertebral column, and the spinal nerves exiting the intervertebral foramen.
The somatic nervous system controls the contraction of skeletal muscles
and transmits somatosensory information to the CNS. The autonomic nervous
system regulates cardiac and smooth muscle, and glands, transmits sensory
information to the CNS, and innervates muscle spindles.
The autonomic nervous system is divided into three main systems: sympathetic, parasympathetic, and enteric. Check out the YouTube video The Autonomic Nervous System.
In the ANS, preganglionic neurons communicate with postganglionic neurons. Preganglionic axons originate in the CNS, the brainstem, or the spinal cord. These axons travel from the CNS to nerve cell body clusters in the PNS. They release acetylcholine (ACh) to communicate with postganglionic neurons.
Postganglionic axons originate in the autonomic ganglia and project to target organs like the heart, lungs, and digestive system. They release ACh (PNS) or norepinephrine (SNS) to adjust organ activity.
The term peripheral biofeedback is misleading because the ANS originates in the CNS. The ANS is central and peripheral, and regulated by networks that span these divisions.
Sympathetic Division
The sympathetic nervous system (SNS) readies us for action and regulates activities that expend stored energy.
In concert with the endocrine system, the SNS responds to threats to our safety through mobilization, fight-or-flight, and active avoidance. The SNS responds more slowly (> 5 seconds) and for a more extended period than the more rapid (< 1 second) parasympathetic vagus system (Nunan et al., 2010). Porges (2011) theorized that the SNS inhibits the unmyelinated vagus (dorsal vagal complex) to mobilize us for action.
SNS cell bodies are found in the gray matter of the thoracic (from T1) and lumbar (to L2) spinal cord segments. The sympathetic division is thoracolumbar.
SNS preganglionic neurons, which originate in the CNS, exit the spinal
cord via the ventral root. Most of these axons synapse with the sympathetic chain's autonomic ganglia
(collection of neurons), which parallel the spinal cord on each side.
Preganglionic neurons branch extensively and can communicate with sympathetic ganglia in complex ways:
1. Divergence – one preganglionic neuron synapses with multiple ganglia.
2. Convergence – many preganglionic neurons from different spinal levels synapse with one ganglion cell.
Both divergence and convergence produce mass activation, allowing integrated sympathetic action (e.g., increased blood pressure, heart rate, and respiration rate) during emergencies but not at rest. The sympathetic branch does not exhibit this degree of integration during resting conditions (Lehrer & Gevirtz, 2021).
SNS preganglionic axons also directly innervate the adrenal medulla
(the central portion of the adrenal gland). When stimulated, the adrenal medulla releases epinephrine and norepinephrine, which reinforces the sympathetic activation of visceral organs. The release of epinephrine and norepinephrine increases muscle blood flow and converts stored nutrients into glucose to power skeletal muscle contraction. Postganglionic neurons exit the sympathetic chain and project axons to target organs, like the heart, lungs, and sweat glands.
Adrenergic receptors produce changes through G-proteins. The following table is adapted from Fox and Rompolski (2022).
Organs With Sympathetic Innervation Only
The adrenal medulla, arrector pili muscles of the skin, cutaneous sweat glands, and most blood vessels receive sympathetic innervation exclusively (Fox & Rompolski, 2022).
Preganglionic and Postganglionic Neurotransmitters Are Different
Heart rate variability (HRV) consists of the beat-to-beat changes in HR, including changes in the time intervals between consecutive heartbeats. The SNS does not appear to contribute significantly to the low-frequency (LF; 0.04-0.15 Hz) component of HRV under resting conditions, as was previously believed.
Stephen Porges has emphasized that most stressors do not require the sympathetic branch's intensive energy expenditure during fight-or-flight. He argues that when we perceive daily stressors as threatening, this causes vagal withdrawal instead of sympathetic activation.
In vagal withdrawal, we disengage parasympathetic control of our viscera (e.g. large internal organs) and reduce HRV. We shift from a more calm, socially engaged state. We are now ready for mobilization, either fight or flight (mediated by the sympathetic nervous system) or immobilization.
Vagal withdrawal increases power in the very-low-frequency (VLF) band (≤ 0.04 Hz) and lowers it in the high-frequency (HF) band (0.15 – 0.40 Hz).
However, mild stressors can also trigger graded SNS responses.
We may observe finger cooling and sweating during effortful slow-paced breathing. Brindle and colleagues (2014) found equal roles for beta-adrenergic activation and vagal withdrawal.
Cardiovascular reactivity to acute psychological stress would appear to reflect both beta-adrenergic activation and vagal withdrawal to a largely equal extent" (p. 964).
The parasympathetic division regulates activities that increase the
body’s energy reserves, including salivation, gastric (stomach) and
intestinal motility, gastric juice secretion, and increased blood flow to
the gastrointestinal system (rest and digest). When we discuss Porges' polyvagal theory, we will learn that this system is also involved in self-regulation, social engagement, and passive responses to threats.
Heart rate variability biofeedback uses slow-paced breathing and rhythmic skeletal muscle contraction to restore healthy parasympathetic activity.
Approximately 75% of the parasympathetic fibers in the human body are contained within the vagus nerve (cranial nerve X). The remaining parasympathetic fibers are found in the cranial nerves III (oculomotor), VII (facial), and IX (glossopharyngeal), as well as in the pelvic splanchnic nerves, which originate from the sacral segments (S2-S4) of the spinal cord. The parasympathetic division is craniosacral.
Unlike the sympathetic division, parasympathetic ganglia are located near
their target organs. This arrangement means that preganglionic axons are
relatively long, postganglionic axons are relatively short, and
PNS changes can be selective.
Preganglionic neurons travel with the oculomotor, facial, glossopharyngeal, and vagus cranial nerves. Preganglionic neurons that exit the vagus (X) nerve at the medulla synapse with terminal ganglia within the heart, lungs, esophagus, stomach, pancreas, liver, and intestines. Both PNS
preganglionic and postganglionic axons release acetylcholine (Fox & Rompolski, 2022).
The vagus nerve's sensory branch detects inflammation/infection via tissue necrosis factor (TNF) and interleukin-1 (IL-1).
The vagus nerve, through its sympathetic components, is involved in anti-inflammatory pathways. It can stimulate the splenic sympathetic nerve, which plays a role in inhibiting the release of pro-inflammatory cytokines like TNFa, thus contributing to the body's anti-inflammatory responses (Bonaz, Sinniger, & Pellissier, 2017, 2018).
The motor branch of the vagus signals descending neurons to release norepinephrine, which prompts spleen immune cells to release acetylcholine to macrophages to dampen inflammation (Schwartz, 2015). Resonance frequency breathing may influence the vagal cholinergic cytokine control system (Gevirtz, 2013; Tracey, 2007).
Sympathetic fibers are consistently found within the human vagus nerve, as evidenced by the presence of tyrosine hydroxylase (TH)-positive fibers, which are indicative of sympathetic activity. These fibers are distributed throughout the cervical and thoracic regions of the vagus nerve. Human vagus nerves contain 3.97%-5.47% sympathetic nerve fibers (Kawagishi et al., 2008; Ruigrok et al., 2023; Seki et al., 2014).
The distribution and quantity of these sympathetic fibers can vary significantly between individuals and even between the left and right vagus nerves of the same individual
(Ruigrok et al., 2023; Wallace et al., 2022).
The sympathetic fibers within the vagus nerve may contribute to the physiological effects observed with vagal nerve stimulation (VNS), such as modulation of autonomic balance and potential therapeutic benefits in conditions like depression and chronic heart failure (Ruigrok et al., 2023; Seki et al., 2014).
The Relationship Between the Sympathetic and Parasympathetic Branches
Berntson, Cacioppo, and Quigley (1993) challenge the concept of a
continuum ranging from SNS to PNS dominance. They
argue that the two autonomic branches do not only act antagonistically
(reciprocally). They also exert complementary, cooperative, and independent actions.
This adrenergic and cholinergic effects table was adapted from Fox and Rompolski (2022). Adrenergic receptors are alpha (α) or beta (β), and cholinergic receptors are muscarinic (M).
SNS and PNS actions are complementary when they
produce similar changes in the target organ. Saliva production serves as
an example. PNS activation produces watery saliva, and
SNS activation constricts salivary gland blood vessels
producing thick saliva.
Cooperative Actions
SNS and PNS actions are cooperative when their
different effects result in a single action. Sexual function provides an
example. PNS activation produces erection and vaginal
secretions, while SNS activation produces ejaculation and orgasm.
Exclusive Sympathetic Control
The SNS exclusively innervates several organs. They are controlled by increasing or decreasing the firing of SNS postganglionic fibers: adrenal medulla, arrector pili muscle, sweat glands, and most blood vessels.
The Relationship Between the SNS and PNS is Dynamic
There is a dynamic relationship between sympathetic nervous system (SNS) and parasympathetic nervous system (PNS) influences in a healthy heart. The synergistic relationship between these autonomic branches is complex: sometimes reciprocal, additive, or subtractive (Gevirtz, Schwartz, & Lehrer, 2016).
PNS control predominates at rest, resulting in an
average heart rate of 75 beats per minute (bpm) that is significantly slower than the SA node's intrinsic rate,
which decreases with age, from an average 107 bpm at 20 years to 90 bpm at 50 years (Opthof, 2000).
Parasympathetic nerves exert their effects more
rapidly (< 1 second) than sympathetic nerves (> 5 seconds) (Ecksberg & Eckberg, 1982; Nunan et al., 2010; Shaffer, McCraty, & Zerr, 2014; Tortora & Derrickson, 2017).
While the SNS can suppress PNS activity, it can also increase PNS reactivity (Gellhorn, 1957). Parasympathetic rebound may occur following high stress levels, resulting in increased nighttime gastric activity (Nada et al., 2001) and asthma symptoms (Ballard, 1999).
The relationship between the PNS and SNS branches is complex (both linear and nonlinear) and should not be described as a “zero sum” system (Ginsberg, 2017). Increased PNS activity may be associated with a decrease, increase, or no change in SNS activity.
For example, immediately following aerobic exercise, heart rate recovery involves PNS reactivation while SNS activity remains elevated (Billman, 2017). Likewise, teaching clients to breathe slowly when they experience high levels of SNS activity can engage both branches and increase respiratory sinus arrhythmia (RSA) (Ginsberg, 2017).
We must reject the simplistic view that increased sympathetic activity is unhealthy. This is false when you swim laps in the pool or are startled by an unexpected sound. Adaptive SNS activation in response to an increased physical workload or sudden threat is desirable.
A depressed SNS response could signal physical depletion and compromised ability to cope. In fact, 24-hour HRV recordings of patients diagnosed with medical and psychological disorders show low SNS and normal PNS activity.
Reduced very-low-frequency (VLF) power is strongly associated with future health crises like sudden cardiac death (Arai et al., 2009; McCraty, 2013).
The degree and duration of SNS activation should be appropriate to the current challenge, and recovery should be rapid.
According to Porges' (2011) polyvagal theory, the autonomic nervous system must be considered a “system,” with the vagal nerve containing specialized subsystems that regulate competing adaptive responses.
Caption: Stephen Porges
“Polyvagal” means that there are two vagal systems, the unmyelinated vagus (dorsal motor nucleus) and myelinated vagus (nucleus ambiguus).
Porges' theory proposes competing roles for the unmyelinated fibers in the vagus, which originate in the dorsal motor complex, and newer myelinated nerves which originate in the nucleus ambiguus. Although the polyvagal theory is controversial (Grossman & Taylor, 2007), it highlights diverse PNS adaptive functions.
Click on the Read More button to learn about myelinated vagus, sympathetic branch, and unmyelinated vagus roles in polyvagal theory.
The Myelinated Vagus
Porges theorizes that the evolution of the autonomic nervous system was central to developing emotional experience and affective processes involved in social behavior. We are not limited to fight, flight, or freezing behavioral responses as human beings.
In concert with the endocrine system, the SNS responds to threats to our safety through mobilization, including fight-or-flight and active avoidance. The SNS responds more slowly and for a more extended period (i.e., more than a few seconds) than the parasympathetic vagus system. The SNS responds more slowly and for a more extended period (i.e., more
than a few seconds) than the parasympathetic vagus system.
The SNS inhibits the unmyelinated vagus to mobilize us for action instead of fainting or freezing. In contrast, the
parasympathetic myelinated vagus rapidly adjusts cardiac output, promotes social engagement (the tend-and-befriend response), and enables self-regulation. These three changes promote biofeedback training.
When our nervous system perceives safety, we activate the myelinated vagus to conserve and rebuild energy stores (rest and digest), socially bond with others (tend and befriend), and engage in executive functions like self-regulation and planning.
When our nervous system perceives danger, we activate the sympathetic and the endocrine systems' SAM pathway and HPA axis, inhibiting the unmyelinated vagus for fight or flight or active avoidance.
When our nervous system perceives that our life is threatened and that fight, flight, or active avoidance will not succeed, like a mouse in the jaws of a cat, we activate the unmyelinated vagus. This results in passive avoidance through dissociation,
fainting, feigning death, immobilization, and shutdown. Dani S@unclebelang on fiverr.com created the Polyvagal Theory graphic from the author's design.
Enteric Division
The enteric division is the largest part of the autonomic nervous system (Rao & Gershon, 2016). The ENS comprises a vast network of 200-600 million neurons and four times more glial cells throughout the gastrointestinal tract (Boesmans et al., 2013; Popowycz et al., 2022).
Most CNS neurotransmitters are also present in the ENS. This similarity underscores the ENS's complexity and its role as an independent integrative center (Belkind- Gerson, Graeme-Cook, & Winter, 2006).
The ENS controls peristalsis and enzyme secretion in the GI tract to maintain fluid and nutrient balance (Breedlove & Watson, 2023). While the ENS locally regulates intestinal functions, SNS and PNS efferents can override its activity during intense emotions like anger and fear (Fox & Rompolski, 2022). During physical activity and stress, SNS firing inhibits intestinal motility. During rest, PNS firing promotes it by exciting enteric preganglionic neurons (Khazan, 2013).
ENS and the central nervous system communicate bidirectionally via the vagus nerve (Carabotti et al., 2015).The gut-brain axis integrates neuronal, endocrine, and immune mechanisms. Enteric glial cells modulate neuronal activity, inflammation, and gut barrier function (Gulbransen & Sharkey, 2012).
The gut microbiome may contribute to the onset and progression of serious mental illnesses (SMIs). Gut microbiota imbalances may contribute to systemic inflammation and neuroinflammation (Nguyen et al., 2021). The ENS may play an integral role in Alzheimer's disease, Parkinson's disease, and autism spectrum disorder (Fung et al., 2017).
The gut contains more than 90% of the body’s serotonin (Khazan, 2013) and about 50% of its dopamine. Check out the Khan Academy YouTube video Control of the GI Tract.
A popular author wrote that since excessive sympathetic nervous system activity causes many diseases, people should practice exercises that strengthen the protective parasympathetic nervous system. What mistake did this author make?
The idea of a bad sympathetic nervous system and good parasympathetic nervous system is one of the most common misconceptions about the ANS. The body depends on both systems to function. Both systems are "on" all the time, but their relative activation depends on the immediate demands for adjustment and our response to these challenges. Health and optimal performance depend on autonomic balance in which each branch of the autonomic nervous system is activated to the degree and for the required time to meet current demands. There are no "good" and "bad" systems. The popular author should remember that parasympathetic overactivity contributes to diseases like asthma, hypotension, and irritable bowel syndrome (IBS).
Glossary
accentuated antagonism: the parasympathetic nervous system's ability to directly oppose sympathetic action, such as slowing the heart by 20 or 30 beats.
adrenal medulla: the inner region of the adrenal gland that produces the hormones epinephrine and norepinephrine.
autonomic nervous system: the subdivision of the peripheral nervous system that includes enteric, parasympathetic, and sympathetic divisions.
beta-adrenergic: relating to receptors stimulated by epinephrine and norepinephrine that mediate heart rate acceleration, vasodilation, and metabolic effects.
central nervous system: the division of the nervous system that includes the brain, spinal cord, and retina.
ganglia: a collection of neuronal cell bodies outside of the CNS.
hypothalamus: the forebrain structure located below the thalamus that dynamically maintains homeostasis through its control of the autonomic nervous system, endocrine system, survival behaviors, and interconnections with the immune system.
homeostat: a device that maintains homeostasis. For example, the hypothalamus.
mass activation: the simultaneous stimulation of adjacent ganglia (cell bodies) in the sympathetic chain allows the sympathetic nervous system to produce many coordinated changes simultaneously. For example, increased heart rate, respiration rate, and sweat gland activity.
medulla: the brainstem structure that regulates blood pressure, defecation, heart rate, respiration, and vomiting. The medulla influences the autonomic nervous system and distributes signals between the brain and spinal cord.
myelinated vagus: the phylogenetically newer ventral vagal complex that rapidly adjusts cardiac output and promotes social engagement.
parasympathetic division: the autonomic nervous system subdivision that regulates activities that increase the body’s energy reserves, including salivation, gastric (stomach) and intestinal motility, gastric juice secretion, and increased blood flow to the gastrointestinal system.
parasympathetic withdrawal: daily stressors can inhibit the myelinated vagus suppressing parasympathetic activity.
peripheral nervous system: nervous system subdivision that includes autonomic and somatic branches.
polyvagal theory: theory that the unmyelinated vagus (dorsal vagus complex) and newer myelinated vagus (ventral vagal complex) mediate competing adaptive responses.
somatic nervous system: peripheral nervous system subdivision that receives external sensory and somatosensory information and controls skeletal muscle contraction.
sympathetic division: autonomic nervous system branch that regulates activities that expend stored energy, such as when we are excited.
sympathetic preganglionic neurons: the neurons that originate in the CNS, leave the spinal cord via the ventral root, and mainly synapse with sympathetic chain ganglia.
unmyelinated vagus:
the phylogenetically older dorsal vagus complex that responds to threats through immobilization, feigning death, passive avoidance, and shutdown.
vagal withdrawal: the inhibition of the myelinated vagus, often by daily stressors.
vagus nerve: the tenth cranial nerve, which supplies parasympathetic nervous system innervation for the heart.
very low frequency (VLF): the ECG frequency range of 0.003-.04 Hz that may represent
temperature regulation, plasma renin fluctuations, endothelial, and physical activity influences, and possible intrinsic cardiac nervous system, PNS, and
SNS contributions.
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Assignment
Now that you have completed this module, consider how you might better explain the relationship between the sympathetic and parasympathetic divisions of the autonomic nervous system to your clients. Consider how negative emotion interferes with HRV biofeedback training and why emotional self-regulation can be an important training component for some individuals.
Billman, G. E. (2017). Personal communication to J. P. Ginsberg regarding the LF/HF ratio.
Bonaz, B., Sinniger, V., & Pellissier, S. (2016). Anti-inflammatory properties of the vagus nerve: Potential therapeutic implications of vagus nerve stimulation. The Journal of Physiology, 594. https://doi.org/10.1113/JP271539
Bonaz, B., Sinniger, V., & Pellissier, S. (2017). The vagus nerve in the neuro-immune axis: Implications in the pathology of the gastrointestinal tract. Frontiers in Immunology, 8. https://doi.org/10.3389/fimmu.2017.01452
Boron, W. F., & Boulpaep, E. L. (2017). Medical physiology (3rd ed.). Saunders.
Breedlove, S., M., & Watson, N. V. (2020). Behavioral neuroscience (9th ed.). Sinauer Associates, Inc.
Brindle, R. C., Ginty, A. T., Phillips, A. C., & Carroll, D. (2014). A tale of two mechanisms: A meta-analytic approach toward understanding the autonomic basis of cardiovascular reactivity to acute psychological stress. Psychophysiology, 51(10), 964–976. https://doi.org/10.1111/psyp.12248
Eckberg, D. L., & Eckberg, M. J. (1982). Human sinus node responses to repetitive, ramped carotid baroreceptor
stimuli. American Journal of Physiology, 242 (Heart and Circulatory Physiology, 11), H638–H644. https://doi.org/10.1152/ajpheart.1982.242.4.h638
Fox, S. I., & Rompolski, K. (2022). Human physiology (16th ed.). McGraw-Hill.
Gevirtz, R. N., Lehrer, P. M., & Schwartz, M. S. (2016). Cardiorespiratory biofeedback. In M. S. Schwartz & F. Andrasik (Eds.). Biofeedback: A practitioner’s guide (4th ed.). The Guilford Press.
Ginsberg, J. P. (2017). Personal communication regarding autonomic balance.
Grossman, P., & Taylor, E. W. (2007). Toward understanding respiratory sinus arrhythmia: Relations to cardiac vagal tone, evolution and biobehavioral functions. Biol. Psychol. 74, 263–285. https://doi.org/10.1016/j.biopsycho.2005.11.014
Karemaker,
J. M. (2020). Interpretation of heart rate variability: The art of looking through a keyhole. Front. Neurosci. https://doi.org/10.3389/fnins.2020.609570
Kawagishi, K., Fukushima, N., Yokouchi, K., Sumitomo, N., Kakegawa, A., & Moriizumi, T. (2008). Tyrosine hydroxylase-immunoreactive fibers in the human vagus nerve. Journal of Clinical Neuroscience, 15, 1023-1026. https://doi.org/10.1016/j.jocn.2007.08.032
Khazan, I. Z. (2013). The clinical handbook of biofeedback: A step-by-step guide for training and practice with mindfulness. John Wiley & Sons, Ltd.
Lehrer, P. M., & Gevirtz, R. (2021). BCIA HRV Biofeedback didactic workshop. Association for Applied Psychophysiology and Biofeedback.
McCraty, R. (2013). Personal communication concerning autonomic balance.
Porges, S. W. (2011). The polyvagal theory: Neurophysiological foundations of emotions, attachment, communication, and self-regulation. W. W. W. Norton & Company.
Ruigrok, T., Mantel, S., Orlandini, L., De Knegt, C., Vincent, A., & Spoor, J. (2023). Sympathetic components in left and right human cervical vagus nerve: Implications for vagus nerve stimulation. Frontiers in Neuroanatomy, 17. https://doi.org/10.3389/fnana.2023.1205660
Schwartz, S. (2015). Viva vagus: Wandering nerve could lead to range of therapies. Science News, 188(11), 18.
Seki, A., Green, H., Lee, T., Hong, L., Tan, J., Vinters, H., Chen, P., & Fishbein, M. (2014). Sympathetic nerve fibers in human cervical and thoracic vagus nerves. Heart rhythm, 11(8), 1411-1417. https://doi.org/10.1016/j.hrthm.2014.04.032
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Wallace, C., Glueck, E., Kuhnert, S., Brauer, P., Lemmons, C., Kilmer, M., & Barry, A. (2022). The origin of sympathetic postganglionic fibers in the cervical region of the vagus nerve. The FASEB Journal, 36. https://doi.org/10.1096/fasebj.2022.36.s1.r3076
The term peripheral biofeedback is misleading because the ANS originates in the CNS. The ANS is central and peripheral, and regulated by networks that span these divisions.
