Adrenal Medulla: The Nervous System Inside a Gland

The adrenal medulla is the inner core of each adrenal gland, sitting on top of the kidneys, and its primary job is to flood the bloodstream with epinephrine (adrenaline) and smaller amounts of norepinephrine when you face a threat or intense physical demand. Unlike the outer shell of the adrenal gland, which produces steroid hormones, the medulla is essentially a modified piece of the nervous system that got repurposed as a hormone factory. That unusual origin explains a lot about how it works, what can go wrong with it, and why it responds to stress differently than you might expect.

A Piece of the Nervous System Inside a Gland

The cells that make up the adrenal medulla are called chromaffin cells, and they share an origin with the nerve cells of your sympathetic nervous system. During embryonic development, precursor cells migrate from a structure called the neural crest toward the developing adrenal gland. Research in mice has shown that these precursors enter the adrenal tissue around embryonic day 11 and begin differentiating into hormone-producing chromaffin cells within about 24 hours.¹ A 2017 study added a twist to this picture: a large portion of chromaffin cells actually arise not from the neural crest directly, but from Schwann cell precursors, a type of peripheral glial stem cell that hitches a ride along nerves growing toward the adrenal gland, then detaches and transforms into hormone-secreting cells on arrival.²

This developmental backstory matters because it means chromaffin cells are close cousins of the neurons in your sympathetic chain ganglia. They share the same molecular toolkit for making catecholamines, and they respond to the same nerve signals. The key difference is that instead of releasing a chemical messenger across a tiny gap between two nerve cells, chromaffin cells dump their output directly into the blood, acting as a broadcast system rather than a point-to-point wiring network.

How Catecholamines Get Made and Released

Chromaffin cells produce three catecholamines: dopamine, norepinephrine, and epinephrine.³ The pathway starts with the amino acid tyrosine, which gets converted step by step into dopamine, then norepinephrine, and finally epinephrine. That last conversion step requires an enzyme called PNMT, and here is where the adrenal gland’s anatomy becomes crucial: cortisol produced in the surrounding adrenal cortex flows inward through a local blood supply and stimulates chromaffin cells to make more PNMT, boosting the conversion of norepinephrine into epinephrine.⁴ This is why the adrenal medulla is the body’s primary source of circulating epinephrine. Sympathetic nerve endings elsewhere in the body release norepinephrine locally, but they lack the cortisol bath that would let them efficiently produce epinephrine.

Under the microscope, chromaffin cells are arranged in cords and clumps wrapped tightly around blood-filled sinusoids, with a thin space between the cell surface and the blood vessel wall.⁵ The catecholamines themselves are stored in dense-core granules packed inside the cell. When the signal arrives, calcium rushes in, and these granules fuse with the cell membrane and spill their contents into the nearby blood vessel in a process that happens on a millisecond timescale.⁶

More Than Just Adrenaline

Catecholamines are the headline act, but chromaffin granules contain a supporting cast of proteins and peptides that get co-released. These include chromogranin A, chromogranin B, secretogranin II, and neuropeptide Y, all packed together inside the same granules.⁷ These co-secreted molecules help fine-tune the body’s response. The peptides influence blood vessel tone and immune responses, and chromogranin A has become clinically useful as a blood marker for detecting catecholamine-producing tumors.

An elegant study in cats demonstrated that these different molecules take distinct routes through the body after release. Small molecules like catecholamines appeared rapidly and at high concentrations in the venous blood draining the adrenal gland, while the much larger chromogranin A molecule reached roughly equal levels in both blood and lymph, peaking later in lymph than in blood.⁸ The practical upshot is that these different substances reach different tissues at different speeds, creating layered effects rather than one simple on-off signal.

What Happens When the Medulla Fires

The textbook description of the fight-or-flight response captures the broad strokes. Epinephrine and norepinephrine released from the adrenal medulla, along with norepinephrine from sympathetic nerve terminals throughout the body, raise blood pressure and cardiac output, relax the airways, dilate blood vessels in skeletal muscles, and shift metabolism to liberate fuel.⁹ The cardiovascular picture is more detailed than just “blood pressure goes up.” The heart beats faster and harder, veins constrict to push more blood back to the heart, blood flow shifts away from the gut and kidneys and toward the muscles, and the kidneys hold onto sodium.¹⁰

On the metabolic side, epinephrine causes a quick spike in blood sugar. It does this first by stimulating the liver to break down stored glycogen and later by ramping up the production of new glucose from scratch.¹¹ The timing is interesting: in the first hour or so, about 60% of the extra glucose comes from glycogen breakdown, but after that initial burst, the liver shifts to relying mainly on making new glucose.¹² Meanwhile, epinephrine also blocks insulin’s ability to store glucose in muscle tissue and triggers the release of free fatty acids from fat stores, ensuring plenty of fuel stays available in the blood.¹³ This combination of effects explains why people under acute stress sometimes feel shaky and find their blood sugar elevated even if they have not eaten.

How the Brain Controls Medullary Output

The signal to release catecholamines travels from the brain down through the spinal cord to preganglionic sympathetic neurons, which send their axons via the splanchnic nerve directly to chromaffin cells in the medulla. At that junction, the nerve ending releases acetylcholine, which activates receptors on the chromaffin cell surface, triggering depolarization and ultimately the calcium-dependent release of stored catecholamines.¹⁴

The pattern of nerve firing matters as much as its intensity. Experiments in calves showed that delivering the same total number of nerve impulses in short, high-frequency bursts rather than as a steady stream significantly increased epinephrine release.¹⁵ Responses driven by the adrenal medulla were preferentially amplified by this burst pattern, while other sympathetic responses were not.¹⁶ In other words, the nervous system can selectively crank up adrenal medullary output just by changing its firing rhythm, without needing a separate chemical switch.

At especially high firing rates, the type seen during serious stress, a different neurotransmitter called PACAP takes over from acetylcholine as the main driver of catecholamine release. Mice lacking PACAP lost virtually all catecholamine secretion in response to high-frequency splanchnic nerve stimulation, even though low-frequency responses remained intact.¹⁷ This two-transmitter design creates a built-in gear system: acetylcholine handles baseline and moderate activation, while PACAP kicks in for extreme situations.

The Medulla Does Not Habituate Like the Cortisol System

When people face the same stressful situation repeatedly, the cortisol arm of the stress response tends to dial down over time. But the adrenal medulla does not follow suit. In a study that exposed volunteers to the same psychosocial stressor three times, cortisol and ACTH responses dropped significantly across sessions, while epinephrine and norepinephrine levels stayed flat, showing no habituation at all.¹⁸ Even when participants were divided into high and low cortisol responders, the two groups showed no differences in their catecholamine responses. This dissociation suggests that the medullary system operates on a fundamentally different regulatory logic than the cortisol-producing cortex. From a practical standpoint, it means that chronic exposure to the same stressor does not reduce the cardiovascular and metabolic load driven by medullary catecholamines, even as cortisol output fades. The body keeps hitting the accelerator even as the other stress-response arm eases off.

When Chromaffin Cells Become Tumors

Pheochromocytoma is a tumor of chromaffin cells that produces catecholamines in an unregulated fashion.¹⁹ The hallmark symptom triad is severe hypertension, headaches, and sweating, often arriving in sudden episodes.²⁰ One counterintuitive feature is that the severity of blood pressure elevation does not correlate neatly with how much catecholamine the tumor is secreting. Some patients with very high circulating catecholamines have relatively moderate hypertension, likely because prolonged exposure desensitizes the receptors that catecholamines normally activate.²¹ The blood pressure often also loses its normal day-night rhythm, staying high or even rising during sleep.

Diagnosing pheochromocytoma starts with a blood test for plasma free metanephrines, which are breakdown products of catecholamines. This test has high sensitivity and specificity for detecting catecholamine-producing tumors, and measuring the dopamine metabolite methoxytyramine can help catch tumors that produce only modest amounts of the classic catecholamines.²² Once biochemistry confirms the diagnosis, imaging with CT or MRI locates the tumor, and specialized nuclear medicine scans using compounds like MIBG that are taken up preferentially by chromaffin tissue can confirm the tumor’s identity and detect any spread.²³

Neuroblastoma and the Embryonic Connection

The adrenal medulla’s neural crest origin has a darker consequence in children. Neuroblastoma, the most common solid tumor outside the brain in pediatric patients, arises from the same pool of neural crest-derived cells, and roughly half of these tumors originate in the adrenal gland. It accounts for about 10% of pediatric cancer deaths.²⁴ Single-cell analysis has shown that neuroblastoma cells resemble immature neuroblasts or their closest progenitors, suggesting the tumor arises when normal developmental pathways stall or go off track.²⁵ The connection between the medulla’s embryonic precursors and neuroblastoma is an active area of research, with the Schwann cell precursor population now attracting particular attention as a possible cell of origin.

An Evolutionary View

The tight nesting of the medulla inside the cortex is not universal across vertebrates. A comparative study of chromaffin cell distribution across species found that in fish, the catecholamine-producing cells and the steroid-producing cells are often physically separate or only loosely associated. Moving up the vertebrate family tree, these two cell types gradually come into closer contact, reaching their most intimate arrangement in mammals, where the cortex completely envelops the medulla.²⁶ This trend parallels a shift in the balance of catecholamines: the proportion of epinephrine-producing cells increases from fish to mammals, while norepinephrine-producing cells decrease. The explanation ties back to the cortisol story mentioned earlier. As the cortex wraps more tightly around the chromaffin tissue, it delivers higher local concentrations of glucocorticoids, which drive the enzyme that converts norepinephrine to epinephrine. Evolution appears to have selected for a design that maximizes epinephrine production by physically encasing the factory in its own raw-material supplier.

Drugs That Dial Medullary Output Up or Down

Several medications interact with chromaffin cell function in ways that matter clinically. Chromaffin cells carry their own adrenergic receptors, creating a positive feedback loop: catecholamines released by the cell can bind to beta-2 and beta-3 receptors on its own surface and stimulate further release.²⁷ This autocrine loop helps explain why certain beta-agonist drugs can elevate circulating catecholamines beyond what the nerve signal alone would produce.

On the other side, gabapentin, widely used for nerve pain and seizures, has been shown to reduce catecholamine release from chromaffin cells by roughly a quarter when they are stimulated by cholinergic signals. It accomplishes this by reducing the number of granules that fuse with the membrane during stimulation, without affecting the size of each individual release event or the calcium channels that initiate the process.²⁸ This finding may partly explain why some patients report feeling calmer or having reduced physiological stress responses on gabapentin, beyond what would be expected from its primary action on nerve pain circuits. Reserpine, an older antihypertensive drug, takes a different approach: it directly depletes catecholamine stores inside chromaffin granules, which in turn triggers a compensatory increase in nerve-driven stimulation of the cells to ramp up new catecholamine production.²⁹ The tug-of-war between depletion and compensatory synthesis is one reason reserpine’s effects take time to stabilize and why abrupt withdrawal can produce a catecholamine surge.

Living Without a Functional Medulla

Given all these dramatic physiological effects, you might assume that losing adrenal medullary function would be devastating. In practice, it is surprisingly well tolerated. People who have both adrenal glands surgically removed for disease reasons lose their medullary epinephrine source entirely, yet they can live relatively normal lives with cortisol replacement alone. Sympathetic nerve terminals throughout the body still release norepinephrine locally, maintaining most cardiovascular regulation. What these patients do lose is the ability to mount a full hormonal surge during severe stress, heavy exercise, or dangerously low blood sugar. The medulla exists less as an essential minute-to-minute regulator and more as an emergency amplifier, a system that adds a global, blood-borne layer of catecholamine signaling on top of the local sympathetic nervous system when the situation truly demands it.

This functional redundancy also explains why pheochromocytoma removal, once the surgical and anesthetic risks are managed, tends to result in good outcomes. The body adjusts to the absence of tumor-driven catecholamine excess, and the sympathetic nervous system steps in to handle day-to-day cardiovascular regulation without much fuss. Where patients sometimes struggle is in the immediate postoperative period, when the sudden withdrawal of massive catecholamine levels can cause blood pressure to drop sharply, requiring close monitoring and sometimes vasopressor support until the cardiovascular system recalibrates.