Stress and the Autonomic Nervous System

A near miss in traffic, a difficult exam, a crying child at three in the morning: within seconds the heart races, the palms sweat and the mind narrows to the threat. That response kept our ancestors alive, but when it never fully switches off it reshapes blood pressure, immunity, sleep, mood and pain. This chapter explains the machinery of stress, from the hypothalamus to the adrenal gland and the autonomic nerves, and shows why it matters so much to people living with spinal pain.

Section 1The stress response and the HPA axis

What counts as stress

In everyday speech "stress" means both the problem and how we feel about it. Physiologists separate the two. A stressor is any demand, physical or psychological, that challenges the body's stability: blood loss, cold, infection, a hard workout, a deadline, social rejection. The stress response is the coordinated set of neural, hormonal and behavioral changes that the body mounts to meet that demand. The response itself is not harmful; it is a survival system. Problems arise when it is too strong, lasts too long or is triggered too often.

Two pioneers shaped the field. Walter Cannon, in the early twentieth century, described the fight or flight response driven by the sympathetic nervous system and adrenaline, and coined the term homeostasis for the body's tendency to keep its internal environment stable. Hans Selye, beginning in the 1930s, observed that rats exposed to many different noxious agents developed the same triad of enlarged adrenal glands, shrunken thymus and lymphoid tissue, and stomach ulcers. He proposed the general adaptation syndrome with three stages: alarm (the initial mobilization), resistance (sustained adaptation) and exhaustion (breakdown when demands persist). Later research showed that the "exhaustion" stage is not a simple running out of hormones, but Selye's insight that diverse stressors share a common physiological pathway remains foundational.

Psychological research added a crucial point: the same event can be a strong stressor for one person and a minor one for another, depending on how it is appraised. Stressors that are unpredictable, uncontrollable, novel or socially evaluative (being judged by others) are the most potent triggers of the hormonal stress response. A sense of control, predictability and social support all blunt it.

Two arms working on two time scales

The stress response has a fast arm and a slower arm. The fast arm, the sympathetic adrenal medullary (SAM) system, acts within seconds through nerves and the adrenal medulla. The slower arm, the hypothalamic pituitary adrenal (HPA) axis, acts over minutes to hours through a chain of hormones that ends with cortisol. The two arms are triggered together and reinforce each other.

The fast arm: sympathetic adrenal medullary system

When the brain detects a threat, the amygdala signals the hypothalamus and brainstem. The locus coeruleus in the pons, the brain's main source of norepinephrine, increases arousal and vigilance throughout the forebrain. Descending pathways activate sympathetic preganglionic neurons in the spinal cord. Some of these neurons travel directly to the adrenal medulla, which is in effect a modified sympathetic ganglion: its chromaffin cells release hormones straight into the blood. In humans roughly 80 percent of the medullary output is epinephrine (adrenaline) and about 20 percent norepinephrine (noradrenaline). Within seconds, heart rate and contractility rise, airways dilate, blood is redirected from skin and gut toward skeletal muscle, the liver releases glucose and fat cells release fatty acids, and pupils dilate.

The slow arm: the HPA axis step by step

1. Neurons in the paraventricular nucleus (PVN) of the hypothalamus release corticotropin releasing hormone (CRH), together with arginine vasopressin, into the hypophyseal portal veins, a private blood route to the anterior pituitary.
2. Corticotroph cells of the anterior pituitary respond by secreting adrenocorticotropic hormone (ACTH), cleaved from a larger precursor protein, pro-opiomelanocortin (POMC). The same precursor also yields beta-endorphin, an endogenous opioid, which is one link between stress and pain modulation.
3. ACTH travels in the general circulation to the adrenal cortex, where it stimulates the zona fasciculata to synthesize and release cortisol, the main human glucocorticoid. Cortisol levels rise within about 15 to 30 minutes of a stressor and peak after roughly half an hour.
4. Cortisol acts on almost every tissue through glucocorticoid receptors and the higher affinity mineralocorticoid receptors, which are intracellular receptors that change gene transcription.
5. Cortisol then shuts the system down through negative feedback on the hypothalamus (less CRH), the pituitary (less ACTH) and higher centers, especially the hippocampus, which restrains the HPA axis.

What cortisol does

Cortisol's name comes from the cortex of the adrenal gland, and its classic job is captured by the term glucocorticoid: it raises blood glucose. Its effects are best understood as reallocating resources toward immediate survival.

The anti-inflammatory power of cortisol is why synthetic glucocorticoids such as prednisone are widely used as drugs, and why their long term use reproduces many features of chronic stress. In the rare disease Cushing syndrome, cortisol is persistently excessive, producing central obesity, a rounded face, purple stretch marks, muscle weakness, high blood pressure, diabetes and osteoporosis with vertebral fractures. In Addison disease, adrenal cortex destruction leaves too little cortisol and aldosterone, producing fatigue, weight loss, low blood pressure, salt craving and darkened skin (from high ACTH, whose precursor also yields melanocyte stimulating hormone).

The daily rhythm of cortisol

Cortisol is not secreted at a steady rate. Under control of the body's master clock in the suprachiasmatic nucleus of the hypothalamus, it follows a strong diurnal rhythm. Levels are lowest around midnight, start climbing in the early morning hours and peak shortly after waking. On top of this, most people show a cortisol awakening response: a further sharp rise, commonly around 50 percent above the waking value, during the first 30 to 45 minutes after waking. Levels then fall gradually across the day. Saliva samples collected at several times are often used in research to describe this curve. A flattened diurnal slope (lower morning and higher evening values) has been linked in many studies with chronic stress, depression, poor sleep and worse health outcomes, though individual measurements are noisy.

Other hormones that join the response

Stress also changes the release of many other hormones. Glucagon and growth hormone rise acutely, helping mobilize fuel. Vasopressin (antidiuretic hormone) conserves water. Prolactin often rises. Insulin secretion is suppressed by sympathetic alpha-2 activity, allowing glucose to stay available. Reproductive hormones tend to fall with prolonged stress, which is why intense physical or psychological stress can disrupt menstrual cycles. DHEA (dehydroepiandrosterone), another adrenal steroid released with ACTH, has effects that partly oppose cortisol, and the cortisol to DHEA ratio is sometimes used in research as a marker of stress balance.

Section 2Sympathetic and parasympathetic balance

The autonomic nervous system in outline

The autonomic nervous system (ANS) controls functions we do not usually direct consciously: heart rate, blood vessel diameter, airway size, digestion, sweating, pupil size, bladder emptying and sexual responses. It has three divisions: the sympathetic, the parasympathetic and the enteric nervous system, the large network of neurons in the gut wall that can run digestion largely on its own. Control centers in the hypothalamus, brainstem (especially the nucleus tractus solitarius, which receives visceral sensory input, and the medullary cardiovascular centers) and limbic system integrate signals from the body and from emotional and cognitive brain areas, which is how a thought can make the heart pound.

Unlike the somatic motor system, in which one neuron runs from the spinal cord to skeletal muscle, autonomic output uses a two neuron chain. A preganglionic neuron with its cell body in the brainstem or spinal cord sends a thinly myelinated (type B) axon to an autonomic ganglion. There it synapses on a postganglionic neuron, whose unmyelinated (type C) axon reaches the target organ.

The sympathetic division: thoracolumbar

Sympathetic preganglionic neurons sit in the lateral horn (intermediolateral cell column) of the spinal cord from T1 to L2, which is why the division is called thoracolumbar. Their axons leave through the ventral roots, pass through white rami communicantes and enter the sympathetic chain (paravertebral ganglia), which runs along each side of the vertebral bodies from the upper neck to the coccyx. From there an axon may synapse at its own level, travel up or down the chain to synapse at another level, or pass through without synapsing to reach a prevertebral ganglion (celiac, superior mesenteric, inferior mesenteric) in front of the aorta. Postganglionic fibers rejoin spinal nerves through gray rami communicantes to reach sweat glands, blood vessels and hair follicles throughout the body, or travel along arteries to the viscera. Because the sympathetic chain lies directly beside the vertebral bodies and rib heads, spine practitioners should know it well.

The sympathetic system has short preganglionic and long postganglionic fibers, and each preganglionic neuron branches to many postganglionic neurons. This divergence, plus the hormonal backup of the adrenal medulla, allows sympathetic activation to be widespread, which suits a whole body emergency response. That said, the sympathetic system is also capable of very local, finely graded control, for example adjusting blood flow in one region of skin for temperature regulation.

The parasympathetic division: craniosacral

Parasympathetic preganglionic neurons are found in brainstem nuclei associated with cranial nerves III, VII, IX and X and in the sacral spinal cord at S2 to S4, so the division is called craniosacral. Its ganglia lie in or near the target organs, so it has long preganglionic and short postganglionic fibers, allowing more targeted effects. The vagus nerve (CN X) carries about three quarters of all parasympathetic fibers, supplying the heart, lungs and the gut as far as roughly the left colic flexure. The pelvic splanchnic nerves from S2 to S4 supply the distal colon, bladder and reproductive organs; damage at these levels, as in cauda equina syndrome, affects bladder and bowel control.

A common source of confusion: the vagus nerve is not purely motor. Roughly 80 percent of its fibers are afferent, carrying sensory information from the organs to the brainstem about stretch, chemistry and inflammation. This sensory traffic is how the brain monitors the body's internal state, a process called interoception.

Neurotransmitters and receptors

All autonomic preganglionic neurons release acetylcholine (ACh), which acts on nicotinic receptors (ligand gated ion channels) on the postganglionic neuron. At the target, parasympathetic postganglionic neurons release ACh onto muscarinic receptors (G protein coupled). Most sympathetic postganglionic neurons release norepinephrine (NE) onto adrenergic receptors. The important exception is the sympathetic supply to most eccrine sweat glands, which releases ACh onto muscarinic receptors; that is why anticholinergic drugs reduce sweating. The adrenal medulla is the other special case: preganglionic fibers synapse directly on chromaffin cells, which act as postganglionic neurons that release into blood.

Organ by organ: who does what

Most organs receive dual innervation, and the two divisions usually have opposing effects, like an accelerator and a brake. Some structures, however, receive only sympathetic fibers: most blood vessels, sweat glands, the arrector pili muscles and the adrenal medulla. In those, control works by turning sympathetic tone up or down rather than by an opposing system. In a few places the two divisions cooperate: in male sexual function, parasympathetic activity produces erection while sympathetic activity produces emission ("point and shoot").

Vagal tone and the heart

If all nerve input to the heart is removed, as in a heart transplant, the sinoatrial (SA) node fires at its intrinsic rate of about 90 to 110 beats per minute. A healthy resting heart rate of 60 to 80 is lower because the vagus nerve continuously slows the SA node. This ongoing restraint is called vagal tone, and at rest it dominates over sympathetic input to the heart. Endurance athletes often have resting heart rates in the 40s and 50s, partly from strong vagal tone and partly from larger stroke volume. Vagal effects on the heart are fast, acting within a single beat, while sympathetic effects build over several seconds. This difference in speed matters for heart rate variability, discussed in Section 4.

The baroreflex: autonomic balance in action

The baroreceptor reflex shows how the two divisions work as a single control system. Stretch receptors in the carotid sinus (signaling through CN IX) and aortic arch (through CN X) fire faster when blood pressure rises. Their signals reach the nucleus tractus solitarius, which increases vagal output (slowing the heart) and decreases sympathetic output (relaxing vessels and reducing contractility), so pressure falls. When you stand up, blood pools in the legs, pressure briefly drops, baroreceptor firing falls, and the reflex speeds the heart and constricts vessels within seconds. Failure of this reflex causes orthostatic hypotension, the light headedness of standing too fast, which is common in older adults, in dehydration and with certain medications.

What "balance" really means

Popular health writing often describes the ANS as a seesaw that should sit level, with too much sympathetic activity being bad and more parasympathetic activity always being good. Physiology is more interesting than that. The two divisions are often regulated somewhat independently, can rise together (as during sexual arousal or the "freeze" response), and differ from organ to organ at the same moment. Healthy autonomic function is better described as flexibility: the ability to ramp sympathetic activity up quickly when a challenge appears and then to withdraw it and restore vagal activity quickly when the challenge ends. Problems arise when the system is stuck: sympathetic activity that stays high after the stressor is gone, or a recovery that is slow and incomplete.

Polyvagal theory

Polyvagal theory, proposed by the psychophysiologist Stephen Porges in the 1990s, is widely used in trauma therapy, body work and some chiropractic and yoga settings. It proposes that the vagus has two functionally distinct branches: an evolutionarily older dorsal vagal system from the dorsal motor nucleus, associated with immobilization and shutdown in the face of overwhelming threat, and a newer myelinated ventral vagal system from the nucleus ambiguus, linked to the muscles of the face, voice and middle ear and to calm social engagement. The theory describes a hierarchy of states (social engagement, mobilization, shutdown) and the idea of neuroception, the unconscious detection of safety or danger. Many clinicians find it a helpful language for explaining to patients why feeling safe matters for healing. At the same time, several of its specific anatomical and evolutionary claims, such as the uniqueness of the myelinated vagus to mammals and the role of the dorsal vagus in cardiac shutdown, have been disputed by other autonomic physiologists, and direct experimental tests of the theory as a whole are limited. It is best taught as an influential model with useful clinical metaphors whose specific physiological claims remain debated.

Section 3Allostatic load and chronic stress

From homeostasis to allostasis

Homeostasis describes keeping internal variables near fixed set points. But many set points are not fixed: blood pressure rises when we wake and when we exercise, heart rate climbs before a race, cortisol peaks in the morning. In 1988 the researchers Peter Sterling and Joseph Eyer introduced the term allostasis, meaning "stability through change": the body anticipates demands and adjusts its physiology to meet them. The neuroendocrinologist Bruce McEwen then developed the concept of allostatic load, the cumulative "wear and tear" that results when allostatic systems are activated too often, too long or inefficiently. When that load becomes high enough to cause disease, it is sometimes called allostatic overload.

The idea neatly resolves a paradox: the same mediators that protect us in the short term (cortisol, catecholamines, inflammatory cytokines) damage us when they are chronically elevated. Cortisol raises glucose to fuel the escape from a predator; months of raised cortisol promote insulin resistance and visceral fat. A blood pressure surge supplies the brain during a crisis; years of repeated surges stiffen arteries.

Four patterns of allostatic load

McEwen described four ways the system can go wrong:

1. Repeated hits: frequent stressors, each producing a normal response, but too many of them, for example repeated financial crises or a job with constant pressure.
2. Lack of adaptation: failure to habituate to the same repeated stressor. Most people's cortisol response to public speaking falls after several practice sessions; some people's does not.
3. Prolonged response: failure to shut off the response after the stressor ends, as when rumination keeps blood pressure and cortisol elevated long after an argument.
4. Inadequate response: a blunted response in one system that lets others overreact. For example, too little cortisol fails to restrain inflammation, leaving cytokines unchecked.

Measuring allostatic load

Researchers build an allostatic load index by counting how many biomarkers fall in a high risk range. Primary mediators are the stress chemicals themselves: cortisol, epinephrine and norepinephrine, DHEA-S and inflammatory markers such as interleukin-6 and C reactive protein. Secondary outcomes are the changes they produce: systolic and diastolic blood pressure, waist to hip ratio, glycated hemoglobin (HbA1c), fasting glucose, total and HDL cholesterol, and resting heart rate. Tertiary outcomes are the diseases that follow: cardiovascular disease, diabetes, cognitive decline. In large cohort studies, higher allostatic load scores predict cardiovascular events, cognitive decline and death, and they rise with lower socioeconomic status, discrimination and chronic caregiving. The index varies between studies, which makes comparisons difficult, but the concept has been useful for linking social conditions to biology.

Chronic stress and the brain

The brain is both the organ that decides what is stressful and a target of stress hormones. The hippocampus, rich in glucocorticoid receptors, supports memory and helps shut off the HPA axis. In animal studies, chronic stress causes its neurons to lose dendritic branches and reduces new neuron formation; human imaging studies have linked prolonged stress, depression and Cushing syndrome with smaller hippocampal volume, which can partly recover with treatment. The amygdala, which drives fear and the stress response, tends to show the opposite: dendritic growth and greater reactivity, which in turn drives more HPA activation. The prefrontal cortex, important for planning, flexible thinking and calming the amygdala, loses dendritic complexity under chronic stress in animal models. Together these changes sketch a vicious circle: weaker brakes, stronger accelerator. The encouraging finding is that many of these changes are reversible with stress reduction, physical activity and treatment of depression, an example of neuroplasticity working in both directions.

Chronic stress and the body

The immune story deserves a closer look because it seems contradictory. Acute stress mobilizes immune cells to the skin and lymph nodes, ready for wounds. Cortisol normally suppresses inflammation. Yet chronic stress is associated with more inflammation, not less. The explanation lies in glucocorticoid resistance: after prolonged exposure, immune cells reduce their sensitivity to cortisol, so the hormone's anti-inflammatory brake weakens. Meanwhile, sympathetic signaling continues to promote inflammatory gene expression. The result is chronic low grade inflammation, a common thread in heart disease, diabetes, depression and persistent pain.

Early life adversity

The Adverse Childhood Experiences (ACE) study, a large survey of adults conducted in the 1990s, asked about childhood abuse, neglect and household dysfunction such as parental substance use, mental illness, incarceration and domestic violence. It found a strong dose response relationship: the more categories of adversity a person reported, the higher his or her adult risk of heart disease, lung disease, depression, substance use and early death. People reporting four or more categories had markedly higher risks. Because the study was observational and relied on adult recall, it cannot by itself prove causation, but the pattern has been replicated widely and animal research supports biological pathways: early stress alters HPA axis set points, brain development and inflammatory regulation. ACE scores also predict chronic pain conditions, including back pain and fibromyalgia. The other side of this research is equally important: safe, stable, nurturing relationships buffer these effects, and resilience can be built at any age.

Section 4Heart rate variability

A healthy heart is not a metronome

If you count your pulse for a minute you get a single number, say 64 beats per minute. But the time between individual beats is not fixed at 0.94 seconds. One interval might be 0.98 seconds, the next 0.91, the next 0.95. This beat to beat fluctuation is called heart rate variability (HRV). It might sound like a flaw, yet higher resting HRV generally signals a healthy, adaptable cardiovascular system, and very low HRV is a marker of illness and risk. HRV is measured from the intervals between successive R waves on an electrocardiogram, called RR intervals (or NN intervals, for "normal to normal," when abnormal beats have been removed).

Where the variability comes from

The SA node is constantly being nudged by both autonomic divisions, by the baroreflex, by breathing, by temperature regulation and by hormones. The single biggest source of short term variability at rest is breathing. During inspiration, vagal outflow to the heart is briefly reduced and heart rate speeds up; during expiration, vagal outflow returns and heart rate slows. This rhythmic speeding and slowing is called respiratory sinus arrhythmia (RSA). It is normal and is usually most obvious in children and fit young adults. Because only the fast acting vagus can change heart rate within a single breath, the size of RSA is a useful window on cardiac vagal activity. Slower oscillations, around every 10 seconds, arise mainly from the baroreflex, and still slower ones reflect thermoregulation, hormonal and circadian influences.

Time domain measures

The simplest HRV measures are calculated directly from the list of RR intervals:

• SDNN: the standard deviation of all NN intervals over a recording. It reflects total variability from all sources. For 24 hour recordings, values below about 50 milliseconds are considered markedly reduced, and values above about 100 milliseconds healthy. SDNN depends heavily on recording length, so short and long recordings cannot be compared directly.
• RMSSD: the root mean square of successive differences, calculated by taking the difference between each interval and the next, squaring, averaging and taking the square root. Because it captures fast, beat to beat changes, RMSSD mainly reflects vagal influence, and it is reliable even in short recordings of one to five minutes. It is the measure most consumer devices report as "HRV."
• pNN50: the percentage of successive intervals that differ by more than 50 milliseconds, another vagally dominated index.

Frequency domain measures

Mathematical techniques such as the fast Fourier transform split the RR series into oscillations of different frequencies, like a prism splitting light, and report the power (variance) in each band:

For years the LF/HF ratio was presented as an index of "sympathovagal balance," with a high ratio meaning sympathetic dominance. That interpretation has been strongly challenged. LF power is not a clean measure of sympathetic activity: it is driven largely by the baroreflex and includes a substantial vagal contribution, and it correlates poorly with direct recordings of sympathetic nerve activity. Breathing rate also shifts power between bands; a person who breathes slowly, at 6 breaths per minute, moves respiratory variability into the LF band and raises the ratio even though vagal activity is high. Most experts now advise interpreting LF/HF cautiously, if at all, and favoring vagal indices such as RMSSD and HF power when the question concerns parasympathetic activity.

What raises and lowers HRV

HRV is highly individual, so a person's own trend over time is usually more meaningful than comparison with someone else. Many factors move it:

• Age: HRV falls steadily with age, roughly halving between young adulthood and old age.
• Fitness: regular aerobic exercise raises resting HRV; heavy training without enough recovery can lower it temporarily, which is why athletes use morning HRV to guide training load.
• Sleep: HRV is highest during deep non-REM sleep; short or poor sleep lowers next day values.
• Acute stress: mental stress, pain, anxiety and illness lower HRV, mainly through vagal withdrawal.
• Alcohol lowers HRV for many hours, especially overnight; heavy meals, dehydration and fever do too.
• Breathing: slow breathing at about 6 breaths per minute (about 0.1 Hz) produces the largest oscillations, because breathing and baroreflex rhythms reinforce each other. This is called resonance frequency breathing and is the basis of HRV biofeedback.
• Posture: standing lowers HRV compared with lying down, as vagal activity falls and sympathetic activity rises to hold blood pressure.
• Medications: beta blockers tend to raise HRV; anticholinergic drugs and some antidepressants lower it.

Measuring HRV well

The reference standard is an ECG, from which R waves can be timed to the millisecond. Chest strap heart rate monitors that detect electrical activity are generally accurate at rest. Wrist and finger devices use photoplethysmography (PPG), an optical sensor that detects pulse waves; they estimate HRV reasonably well during sleep and still rest but become unreliable during movement. Good practice is to measure at the same time each day (often just after waking or during sleep), in the same position, after the same routine, and to look at rolling averages over a week rather than single readings. Ectopic beats and artifacts must be removed, since a single missed beat can distort RMSSD dramatically.

What HRV tells us clinically

The clinical importance of HRV was established in heart disease. In a landmark study of survivors of myocardial infarction published in the late 1980s, those with a 24 hour SDNN below 50 milliseconds had several times the mortality of those with SDNN above 100 milliseconds. Low HRV has since been linked with higher risk of death in heart failure, diabetes and the general population. Diabetic autonomic neuropathy, which damages the vagus early, produces very low HRV and a fixed heart rate that barely changes with breathing; the bedside test of heart rate response to deep breathing is part of its standard evaluation. Low HRV is also associated with depression, anxiety disorders, chronic pain and fibromyalgia, although cause and effect are hard to separate.

HRV has become popular among wellness practitioners, including some chiropractors, as a way to track recovery and to demonstrate the effects of treatment. Studies of spinal manipulation and HRV are mostly small, short term and inconsistent; some report brief changes after manipulation, others none, and few compare against credible sham controls. At present the evidence does not show that manipulation produces lasting changes in autonomic function or that HRV changes after treatment translate into better health. HRV is a legitimate research and monitoring tool, but practitioners should avoid presenting a single reading as proof that a treatment has "balanced the nervous system."

Section 5Stress, pain and the body

Two faces of stress in pain

Stress can switch pain off or turn it up. Soldiers and athletes sometimes do not notice serious injuries until the danger or the game is over. This is stress induced analgesia, and it makes evolutionary sense: an animal fleeing a predator should not stop to tend a wound. Yet people with chronic pain almost universally report that stress makes their pain worse. This is stress induced hyperalgesia. Which face appears depends largely on the duration, controllability and meaning of the stressor.

Stress induced analgesia

Acute, intense, especially physical stress activates the brain's own pain control system (described in Chapter 6). The periaqueductal gray (PAG) in the midbrain, driven by the amygdala, hypothalamus and prefrontal cortex, signals the rostral ventromedial medulla, whose descending fibers release serotonin and norepinephrine in the dorsal horn of the spinal cord and inhibit incoming nociceptive signals. Endogenous opioids (beta-endorphin, enkephalins, dynorphin) and endocannabinoids both contribute; in animal studies, some forms of stress analgesia are blocked by naloxone and others by cannabinoid receptor blockers. Cortisol and norepinephrine also have short term analgesic actions.

Stress induced hyperalgesia and chronic pain

When stress is chronic, unpredictable or uncontrollable, the balance tips. Descending pathways that normally inhibit pain can shift toward descending facilitation, increasing dorsal horn excitability. Glial cells in the spinal cord, activated by inflammatory signals and stress hormones, release cytokines that further sensitize pain neurons. Glucocorticoid resistance allows low grade inflammation to persist. Poor sleep, which accompanies stress, independently lowers pain thresholds the next day. Over time these processes contribute to central sensitization, in which the nervous system amplifies signals so that normal touch and movement become painful. Imaging studies show that people with chronic back pain have altered activity and connectivity in regions such as the prefrontal cortex, amygdala and nucleus accumbens, which overlap with stress and emotion circuits; in longitudinal studies, the strength of these emotional learning circuits at the time of an acute episode helped predict who would develop chronic pain.

Muscles, guarding and the sympathetic nervous system

Many people feel stress in their neck, shoulders, jaw and low back. Surface electromyography studies show that mental stress increases activity in muscles such as the upper trapezius, and that people with chronic neck and shoulder pain often relax more slowly after a stressful task. Low level, sustained activation of a small group of motor units, without rest breaks, may contribute to fatigue and pain in some workers, an idea called the Cinderella hypothesis (the same "low threshold" units are always first to work and last to rest). After injury, protective guarding, increased muscle co-contraction and altered movement, can be helpful in the short term, but fear can keep it going long after tissues have healed, stiffening movement and loading tissues unevenly.

The sympathetic system can also interact directly with pain. In most people, sympathetic activity does not activate nociceptors. After nerve injury or in complex regional pain syndrome (CRPS), however, damaged sensory neurons can express adrenergic receptors and become sensitive to norepinephrine, so that stress and cold worsen pain. CRPS also shows striking autonomic signs: changes in skin color, temperature and sweating, swelling and trophic changes in skin, hair and nails of the affected limb.

The fear avoidance cycle

Psychological factors shape the stress and pain relationship as strongly as biological ones. The fear avoidance model describes two paths after a painful episode. If pain is interpreted as manageable, the person stays active, confronts movement and recovers. If pain is interpreted as a sign of serious harm, a cycle begins: pain catastrophizing (rumination, magnification and helplessness about pain) leads to fear of movement and hypervigilance, which lead to avoidance, deconditioning, disability and low mood, which in turn amplify pain. Each element is also a stressor that activates the HPA axis and sympathetic system. Catastrophizing, fear avoidance beliefs, depression and low expectations of recovery are among the strongest predictors of which patients with acute back pain develop chronic disability, often stronger than findings on imaging.

Breaking the cycle

Because stress and pain amplify each other through several linked pathways, the most effective approaches target more than one link. Pain neuroscience education reframes pain as a protective output of the brain rather than a direct measure of tissue damage, reducing threat. Graded activity and graded exposure reintroduce feared movements in manageable steps, showing the nervous system that movement is safe. Regular exercise lowers resting sympathetic activity, raises HRV, improves sleep and mood and activates endogenous analgesia. Cognitive behavioral therapy and newer approaches such as acceptance and commitment therapy and pain reprocessing therapy change appraisal and catastrophizing; trials in chronic back pain show meaningful benefits. Mindfulness, slow breathing, progressive muscle relaxation, yoga and tai chi target arousal and muscle tension directly (Chapter 13 covers them in detail). Sleep is an underrated target: improving sleep improves next day pain. Social connection buffers the HPA response and is one of the strongest predictors of health across the lifespan.

Where manual therapy fits

Some traditional chiropractic models proposed that spinal adjustments improve health largely by normalizing autonomic function, given the close relationship between the spine and the sympathetic chain. Research has documented short lived changes after manipulation in measures such as skin conductance, skin temperature and pressure pain thresholds, which are consistent with brief activation of descending pain modulation and of the sympathetic system. Whether these brief responses translate into lasting autonomic or health benefits has not been shown, and trials of manipulation for autonomic disorders such as hypertension have been negative or inconclusive. A well supported role for manual therapy in stressed, painful patients is more modest but still valuable: short term pain relief that makes movement possible, a reassuring therapeutic relationship, careful explanation that reduces threat, and a bridge to active self management.

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