Fibromyalgia affects multiple body systems, but the central nervous system is the primary driver. Rather than being a disease of the muscles or joints, fibromyalgia is fundamentally a disorder of how the brain and spinal cord process pain signals, with ripple effects across the stress-response system, the immune system, the digestive tract, and the body’s automatic functions like heart rate and temperature regulation.
The Central Nervous System Is the Core Problem
The defining feature of fibromyalgia is a process called central sensitization, where the brain and spinal cord amplify pain signals far beyond what’s appropriate. Think of it as the volume knob on pain being turned up and stuck there. This leads to two hallmark experiences: feeling intense pain from something that should only be mildly uncomfortable (hyperalgesia), and feeling pain from things that shouldn’t hurt at all, like a light touch or gentle pressure (allodynia). The pain receptive field also expands, meaning more areas of the body become sensitive over time.
This amplification happens at a chemical level. People with fibromyalgia have roughly three times the normal concentration of substance P, a chemical messenger that transmits pain signals, in the fluid surrounding their brain and spinal cord. Levels of glutamate, another excitatory brain chemical involved in pain transmission, are also elevated. Brain imaging confirms these glutamate increases directly in brain tissue. At the same time, the calming side of the equation is weakened. Levels of serotonin, which helps regulate mood and pain, are reduced in both blood and spinal fluid. The brain’s supply of norepinephrine, which normally helps dampen pain, is also lower than expected. So the system is simultaneously getting more “go” signals and fewer “stop” signals.
Neuroinflammation and Immune Involvement
The immune system plays a more active role in fibromyalgia than researchers once thought. A growing body of evidence points to neuroinflammation, a low-grade inflammatory process happening inside the brain and spinal cord rather than in the muscles or joints. The key players are microglia, the brain’s resident immune cells. When these cells become overactivated, they release inflammatory molecules that interact with pain-processing neurons and ratchet up their sensitivity.
PET brain scans have confirmed this directly. Using a specialized tracer that binds to activated microglia, researchers found increased neuroinflammation across widespread brain regions in people with fibromyalgia, including areas responsible for sensory processing, motor control, and cognitive function. In healthy adults, injecting a substance that triggers microglial activation produces the same kind of widespread pain sensitivity seen in fibromyalgia, which supports the idea that these immune cells are actively contributing to the pain state rather than just responding to it.
The process also involves the peripheral nervous system. Pain-signaling nerve fibers release chemical messengers that activate microglia in the spinal cord through specific receptor sites. Those activated microglia then release their own inflammatory substances, which sensitize local nerve connections and can spread pain sensitivity to areas beyond the original trigger point. This creates a self-reinforcing loop between the nervous and immune systems.
The Peripheral Nervous System
Although fibromyalgia is primarily a central nervous system condition, there’s clear evidence that the peripheral nerves are also affected. Studies using skin biopsies show that up to 45 to 50 percent of fibromyalgia patients have small fiber neuropathy, a condition where the tiny nerve endings in the skin are reduced in number or damaged. These small fibers are responsible for sensing pain and temperature, which helps explain why many people with fibromyalgia experience burning sensations, tingling, or unusual sensitivity to heat and cold in their skin.
The Stress Response System
Fibromyalgia appears to disrupt the body’s stress response system, specifically the communication loop between the brain’s hypothalamus, the pituitary gland, and the adrenal glands (the HPA axis). This system controls how your body produces cortisol, the primary stress hormone. A large meta-analysis found that people with fibromyalgia tend to have lower cortisol levels in saliva and urine compared to healthy controls, while simultaneously showing elevated norepinephrine in the blood. This pattern suggests the adrenal glands are underperforming while the sympathetic nervous system, the “fight or flight” branch, is running in overdrive.
This combination may help explain the paradox many people with fibromyalgia experience: feeling simultaneously exhausted and wired. The body is stuck in a state of heightened alertness without the hormonal resources to sustain it. That said, the research on HPA axis dysfunction in fibromyalgia is somewhat inconsistent across studies, and the exact pattern likely varies between individuals.
The Autonomic Nervous System
The autonomic nervous system manages every background process your body runs without your conscious input: blood pressure, heart rate, digestion, sweating, and body temperature. Many people with fibromyalgia experience dysfunction in this system, sometimes called dysautonomia. Common symptoms include a heart rate that doesn’t adjust properly during exercise, episodes of rapid heartbeat, excessive or insufficient sweating, and difficulty regulating body temperature. These symptoms are often some of the most confusing for patients because they seem unrelated to pain, but they trace back to the same nervous system disruption driving the rest of the condition.
The Digestive System and Gut-Brain Axis
Digestive problems are remarkably common in fibromyalgia, and irritable bowel syndrome (IBS) co-occurs so frequently that researchers have investigated whether the two conditions share underlying mechanisms. The evidence increasingly points to the gut-brain axis, a two-way communication network linking the digestive tract and the brain through neural, hormonal, and immune pathways.
Several disruptions converge in the gut. Many fibromyalgia patients have been found to have small intestinal bacterial overgrowth (SIBO), and their gut microbiome composition differs from healthy controls. Specifically, populations of beneficial bacteria like Bifidobacteria and several species that produce short-chain fatty acids are reduced. These fatty acids normally help maintain the integrity of the gut lining. When their production drops, the intestinal barrier can become “leaky,” allowing bacterial toxins to enter the bloodstream and potentially interact with peripheral nerves and the brain, further amplifying pain signaling. The same serotonin deficiency seen in the brain also affects the gut, where serotonin plays a major role in regulating motility and visceral sensitivity.
Muscles and Joints: What’s Actually Happening
Despite the widespread muscle pain that defines fibromyalgia, muscle tissue itself shows no consistent structural damage or inflammation. Biopsies reveal some metabolic changes, including signs of impaired oxygen use and increased byproducts of anaerobic metabolism, suggesting muscles may not be getting adequate oxygen during activity. There’s also evidence of reduced capacity for muscle recovery. But these changes are nonspecific, meaning they can show up in other conditions too, and they aren’t sufficient to explain the level of pain people experience. The pain is real, but the problem originates in how the nervous system interprets signals from the muscles rather than in the muscle tissue itself.
Sleep Architecture Disruption
Fibromyalgia disrupts the structure of sleep at a level that goes beyond simply having trouble falling or staying asleep. People with fibromyalgia are especially prone to a pattern called alpha-delta sleep, where fast-frequency alpha brain waves (normally associated with relaxed wakefulness) intrude into the slow delta waves that define deep sleep. Deep sleep is when the body does its most intensive physical repair and recovery. When alpha waves continuously disrupt this stage, you can spend hours asleep without ever reaching truly restorative rest. Unlike the brief bursts of brain activity that occur naturally during sleep transitions, these alpha intrusions are relatively continuous, essentially keeping parts of the brain in a shallow, wakeful state even during what should be the deepest phase of sleep. Researchers have suggested that this disrupted sleep may itself contribute to the muscle pain and tissue tenderness that characterize the disorder, creating another self-perpetuating cycle.