Part 2 of a 3-part series on anxiety, depression, and PTSD
In Part 1 of this series, I walked through anxiety as a nervous system state: a stress response that spreads through the body, and one that can loop when anticipation never resolves into behavior change. Here’s where that leads.
The dominant model for understanding depression in the research is the vulnerability-stress model, sometimes called the diathesis-stress model. It holds that depression develops when a biologically vulnerable brain meets sufficient stress. Genetic, developmental, and biological vulnerabilities predispose a person, but it typically takes an environmental stressor to actually trigger the disorder. Depression, in this framing, is what happens when that vulnerability and that stress load converge.
That convergence is where the relationship between depression and brain energy actually plays out, specifically on the stress side of the equation: what “sufficient stress” actually looks like at the level of brain resources, and why chronic anxiety and pain are two of the most common ways a person accumulates that load.
Depression can be understood as a condition with strong roots in how the brain uses its resources: a state in which the brain, over time, has less brain energy left to sustain thought, mood, and self-regulation. Chronic anxiety and pain are two of the most common ways that capacity gets drained. That’s the mechanism this post is built on. It isn’t the whole vulnerability-stress picture, but it’s the piece that has to be understood before the rest makes sense.
Anxiety and Pain as Energy Drains
Sustaining a thought, regulating a mood, and inhibiting an impulse all draw on the same limited pool of mental resources. Anxiety and chronic pain are two of the most common processes that quietly consume that pool.
An anxious brain runs its stress response repeatedly, often without full resolution. Chronic pain does something similar: it occupies attention, keeps the nervous system in a heightened state, and requires ongoing effort just to function around it. This isn’t only a subjective impression. Depression, anxiety, and chronic pain overlap in the same large-scale brain networks, sharing changes in the very regions that manage attention, self-reference, and emotional regulation.[1]
Neither anxiety nor pain has to be constant or severe to have this effect. Run either one long enough in the background, and the brain has less left over for everything else it’s supposed to be doing, including holding a stable mood and a clear train of thought.
This is the mechanism I want to establish first: depression, in many cases, isn’t a separate problem that shows up out of nowhere. It’s what can happen when the brain’s capacity has been steadily drawn down by processes like these for long enough.
What “Runs Out of Fuel” Actually Points To
I want to be careful here, because it’s easy to overstate the biology, and I don’t want the argument to rest on a claim I can’t stand behind.
What the research does support is that anxiety, chronic pain, and depression share overlapping disturbances in brain networks and in the systems that regulate stress and neurotransmission, and that these disturbances travel together rather than staying neatly separated.[2][3] Chronic stress and its effects on the limbic system, the region responsible for memory and emotional regulation, are a recurring theme across all three conditions.[4]
The honest, defensible version of “runs out of fuel” is this: the systems that carry mood, motivation, and clear thinking are not limitless, and when they’re taxed continuously by stress and pain, their output suffers. Metabolism and thought process are not separate systems. That’s the point worth making, not a precise claim about receptors or blood vessels, but the larger truth that how the brain is resourced and how it thinks are the same conversation.
Another Piece of the Picture: Reward Signaling
The energy-drain mechanism is the foundation, but it isn’t the only piece. A second layer worth understanding is that depression often involves a breakdown in the brain’s reward system itself, not just a shortage of the resources that run it.
The research describes this in terms of blunted dopamine signaling in the brain’s reward circuitry.[7] When that circuitry isn’t responding normally, things that would normally register as pleasurable or motivating don’t land the same way. That’s the clinical picture of anhedonia: loss of interest in things once enjoyed, and one of the most common and underappreciated symptoms of depression.[5][6]
Here’s the part I think matters most, and the part that’s easy to get wrong. This is not fundamentally a willpower problem. It’s a messaging problem. Think of dopamine signaling as the brain’s way of keeping a running tally: an acknowledgment that something good was done, a sense of completion, of satisfaction, of “that counted.” In a person with blunted reward signaling, the good deed still happens, the person still does the constructive thing, but the body doesn’t register the acknowledgment. The counter doesn’t tick over. The signal that’s supposed to say “you can rest now, that mattered” never arrives.
That’s why the same lifestyle and behavior advice works beautifully for one person and seems to do almost nothing for another. Some people register “I did something good for my body” as genuinely rewarding, and that registration reinforces the behavior. Others don’t get that signal, regardless of whether the underlying behavior was constructive, so they keep seeking, and seeking, looking for a reward response that isn’t landing the way it should. The difference isn’t character. It’s whether the acknowledgment is being delivered.
This is also, I think, the piece that explains something patients describe often but rarely have language for. Someone comes in for chiropractic care around lower back pain and afterward reports feeling noticeably better in ways that go beyond the back itself. Part of that is straightforward: less pain means less of the nervous system’s bandwidth is being consumed by pain signaling and the effort of working around it.
Some of it may also involve better efficiency in related physiology, breathing mechanics, for instance, that frees up more of that same bandwidth. When a person isn’t as encumbered by pain and the distraction that comes with it, there’s simply more capacity available. That can feel like more energy, even though nothing was done to directly treat “energy.”
The Default Mode Network: A Preview
There’s a piece of brain-imaging research worth mentioning briefly here, because it’s where this series is headed next. The brain has a default mode network, the circuitry active during self-referential thought, memory, and mental simulation. In anxiety and depression, this network’s ability to switch on and off appropriately breaks down, and that disruption is part of why rumination and repetitive self-focused thought take hold and won’t let go.[8]
It’s an encouraging finding, because attention is trainable, and the control that’s lost here can be rebuilt. I’ll go deeper into this, along with what QEEG and other functional testing can show us about it, in a future post focused specifically on how testing in this space has modernized.
Where the Leverage Actually Is
Understanding depression this way changes where the useful intervention points are. If the brain’s capacity is being drawn down by anxiety and pain, and the reward system’s acknowledgment isn’t landing, then the most actionable place to work isn’t at the level of mood. It’s at the level of behavior, attention, and lifestyle, the patterns that are either draining capacity further or helping the brain rebuild it.
This is the reveal I want this post to land: the makeup of depression, understood honestly, actually gives you more avenues to adapt and rebalance, not fewer. Thought process, focus and attention, introspection, these aren’t soft add-ons to “real” treatment. They’re the levers, and they’re levers you can learn to operate intentionally. I’ve written previously about Emotional Freedom Technique work as one approach we use for this, and I’ll go deeper into treatment, including Cognitive Behavioral Therapy, in future posts.
Where This Connects Back to Anxiety
This is why I think of anxiety and depression as points on the same continuum rather than separate conditions. An anxious brain running its stress response on a loop is expensive. Over time, that expense can draw down the very capacity, for attention, for regulation, for experiencing reward, that a healthy mood system depends on. Anxiety, in that sense, can be the process that leads toward depression, and depression can be what’s left when that process has run long enough.
In the next post in this series, I’ll turn to PTSD, how a specific kind of disruption to brain connectivity, rather than a resource shortage or a reward-signaling issue, produces a different but related set of symptoms. Anxiety, depression, and PTSD are the launch point for a longer series that will go deeper into both testing (QEEG, default mode network findings) and treatment (CBT, EFT, and more) in the posts ahead.
If this is resonating, if what’s described here sounds like your own experience with depression, that’s worth a conversation. If you’d like to read more on the EFT work referenced above, see our earlier post on EFT and emotional resilience, or reach out directly to talk through what a different starting point could look like for you.
Recovery, resilience, and clarity, they all have a biology.
At Well Rooted Health, mindfulness and recovery are understood through the lens of nervous system function. How you regulate stress, process experience, and restore capacity is connected to how your body is working at a foundational level. Every new patient relationship begins with a comprehensive physical exam, in Westfield NJ, New York City, or virtually.
Request an Appointment: https://wellrootedhealth.clientsecure.me
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References
- Brandl F, Weise B, Mulej Bratec S, et al. Common and specific large-scale brain changes in major depressive disorder, anxiety disorders, and chronic pain: a transdiagnostic multimodal meta-analysis of structural and functional MRI studies. Neuropsychopharmacology. 2022;47(5):1071-1080.
- Lee SH, Kim YK. Chronic pain and major depression: shared psychological and neurobiological mechanism. Adv Exp Med Biol. 2026.
- Ma M, Zhang Y, Tao K, Lu Z. Neurochemical crossroads: exploring the neurotransmitter network in chronic pain and depression comorbidity. Front Mol Neurosci. 2025.
- Haidary M, Arif S, Hossaini D, et al. Pain-insomnia-depression syndrome: triangular relationships, pathobiological correlations, current treatment modalities, and future direction. Pain Ther. 2024;13(4).
- Pizzagalli DA. Toward a better understanding of the mechanisms and pathophysiology of anhedonia: are we ready for translation? Am J Psychiatry. 2022;179(7):458-469.
- Wang S, Leri F, Rizvi SJ. Anhedonia as a central factor in depression: neural mechanisms revealed from preclinical to clinical evidence. Prog Neuropsychopharmacol Biol Psychiatry. 2021;110:110289.
- Szczypiński JJ, Gola M. Dopamine dysregulation hypothesis: the common basis for motivational anhedonia in major depressive disorder and schizophrenia? Rev Neurosci. 2018;29(7):727-744.
- Bertocci MA, Afriyie-Agyemang Y, Rozovsky R, et al. Altered patterns of central executive, default mode and salience network activity and connectivity are associated with current and future depression risk in two independent young adult samples. Mol Psychiatry. 2023;28(3).