The Brain Has an Energetic Operating Range: What Mitochondria Can Teach Us About Stress, Resilience, and Recovery

Why can the same person think clearly, exercise well, tolerate stress, and remain motivated on one day, yet struggle with concentration, effort, or emotional regulation on another?

The conventional explanation often focuses on individual neurotransmitters, hormones, or psychological states. A major new review in Nature Reviews Neuroscience offers a broader perspective: the ability of neural circuits to function depends partly on the energetic capacity available to support them.

In Brain mitochondria as key drivers of cognition and behaviour, Sandi and colleagues argue that mitochondria are much more than cellular power plants. They help establish the physiological conditions that determine whether neural circuits are ready to operate, how strongly they can be recruited, how long their activity can be sustained, and how well they adapt to changing demands.

This perspective fits remarkably well with two broader ideas: the Energy Constraint Principle and the emerging concept of bioenergetic impedance.

Together, they suggest a different way of thinking about fatigue, stress vulnerability, cognitive performance, and recovery: not simply as failures of individual organs or molecules, but as consequences of how much useful energetic throughput a biological system can generate, distribute, and sustain.

The brain does not run at maximum capacity all the time

One of the most interesting ideas raised by Sandi and colleagues is that neural circuits may normally operate below their maximum computational capacity.

There is a good biological reason for this. Information processing is expensive.

Neurons must maintain ion gradients, propagate electrical signals, recycle synaptic vesicles, synthesize neurotransmitters, control calcium, repair cellular structures, and continuously maintain their enormous axonal and dendritic networks. All of these processes require energy.

The review therefore asks whether neural computation is constrained by bioenergetic budgets and whether mitochondrial capacity helps establish an energetic operating range for individual circuits.

This distinction is important.

A neural circuit can physically exist without being equally recruitable under every physiological condition. Anatomical wiring tells us what a network could potentially do.

Energetic capacity helps determine what it can actually do at that moment and for how long.

Two ways mitochondria support the brain

Sandi and colleagues organize mitochondrial function into two complementary modes.

The first is baseline support. Over longer periods, mitochondrial capacity, distribution, quality control, redox state, and interactions with surrounding glial cells help maintain synapses, axons, dendrites, and the physiological environment required for circuit stability.

This baseline state influences what the authors call circuit readiness—how easily a circuit can be recruited when it is needed.

The second is activity-evoked support. When a circuit becomes active, mitochondria must respond locally. They increase ATP production, handle calcium, generate controlled redox signals, and reorganize themselves to support synaptic transmission and plasticity.

The distinction helps explain why mitochondrial function can influence both relatively stable characteristics, such as vulnerability or motivational capacity, and rapidly changing states, such as learning, stress responses, or effort.

The mitochondria supporting a circuit therefore do not simply determine whether neurons live or die. They influence how much functional demand the circuit can accommodate.

This resembles the Energy Constraint Principle

The Energy Constraint Principle starts from a simple biological reality:

energy available for physiological work is finite, while competing demands can be very large.

Energy must continually be allocated among different functions.

Some expenditure represents vital cost: maintaining membrane potentials, circulation, respiration, temperature, and basic cellular integrity.

Some represents stress cost: the additional energetic expenditure required to detect, respond to, and adapt to challenges.

And some supports growth, maintenance, and repair—GMR: protein turnover, antioxidant defense, mitochondrial quality control, tissue repair, immune resolution, structural remodeling, and rebuilding future capacity.

These demands compete within an available energetic budget.

The Sandi review provides an unusually clear neural-circuit counterpart to this concept.

A brain circuit must first pay the energetic cost of maintaining itself. Additional energy is then required when it is recruited. If stress, inflammation, metabolic disturbance, or another challenge raises ongoing energetic expenditure, the amount of capacity available for additional computation may become smaller.

This means that the energetic operating range of a circuit may reflect not merely its maximum mitochondrial capacity, but how much capacity remains after other biological demands have already been accommodated.

Reserve matters as much as output

Consider two people who can both generate sufficient ATP while sitting quietly.

Their apparent resting function might look similar.

But one may possess substantial reserve capacity, whereas the other may already be operating close to the upper limit of sustainable throughput.

The difference becomes apparent only when demand rises.

Exercise, infection, sleep deprivation, psychological stress, fasting, heat, cognitive workload, or tissue repair can all increase energetic requirements.

The first person increases energetic throughput, accommodates the challenge, and subsequently returns toward baseline.

The second may initially compensate as well—but with less reserve. Additional demand can therefore produce disproportionate fatigue, impaired concentration, reduced exercise tolerance, slower recovery, or other forms of functional prioritization.

The relevant question is no longer simply:

“Can this system produce energy?”

It becomes:

“How much additional demand can this system accommodate while still maintaining stability, repair, and recovery?”

That is the practical meaning of an energetic operating range.

Bioenergetic impedance explains why the operating range can narrow

The Energy Constraint Principle establishes the existence of energetic limits.

Bioenergetic impedance asks what restricts usable energetic throughput inside the biological system.

Energy metabolism is not one reaction occurring inside an isolated mitochondrion.

For oxidative metabolism to proceed efficiently, oxygen must reach tissues. Nutrients must be digested, transported, and delivered. Substrates must enter cells and mitochondria. Carbon must move through metabolic pathways. NADH and FADH₂ must transfer electrons through the respiratory chain. Redox systems must recycle. ATP must be generated and used. Metabolic products must be transported or cleared.

Constraints can therefore appear at many levels.

Reduced perfusion, hypoxia, persistent inflammation, substrate overload, impaired mitochondrial respiratory throughput, excessive redox pressure, inadequate transport capacity, disrupted metabolic coupling, or insufficient recovery can all make biological energy transformation more difficult.

This is what the bioenergetic impedance framework attempts to capture.

Impedance does not necessarily mean complete mitochondrial failure. A system may still produce ATP while requiring progressively greater compensation to do so.

That distinction matters.

A stressed system may increase glycolysis, alter substrate selection, redistribute nutrients, activate stress hormones, change immune activity, or modify mitochondrial dynamics. These adaptations can preserve immediate function.

But compensation has a cost.

If the underlying constraint resolves, the system can return toward baseline and perhaps become more resilient.

If it does not resolve, compensation itself can become part of the new physiological state.

The operating range gradually narrows.

The brain is also a multicellular energetic network

Another strength of the Sandi review is that it moves beyond the neuron.

Neurons depend heavily on surrounding glial cells.

Astrocytes provide metabolic substrates such as lactate and contribute importantly to redox buffering. Oligodendrocytes support axonal metabolism as well as myelination. Microglial mitochondrial metabolism influences inflammatory signaling and synaptic remodeling.

Brain energetics is therefore a network property.

A neuron can possess structurally intact mitochondria yet still operate poorly if substrate delivery, redox buffering, inflammatory conditions, or glial metabolic support become unfavorable.

This is highly compatible with the impedance concept because energetic flow depends upon the conductance of the entire pathway, not merely the final ATP-producing machinery.

Stress can temporarily raise impedance

Acute stress is not necessarily harmful.

Sandi and colleagues describe evidence that acute stress can rapidly alter mitochondrial respiration, morphology, gene expression, and redox metabolism. Some of these changes return toward baseline after the stressor disappears.

That looks much more like adaptive impedance cycling than mitochondrial disease.

Demand rises.

Mitochondrial and metabolic systems reorganize.

Resources are temporarily redistributed.

The challenge passes.

Recovery restores the previous operating range.

Repeated challenges with complete recovery may even expand future capacity.

The problem emerges when the cycle remains incomplete.

When stress cost remains high, something else may have to give

If a substantial portion of the energetic budget remains committed to stress adaptation, the organism faces increasingly difficult allocation decisions.

Vital functions cannot simply stop.

Stress-response machinery may remain necessary because the perceived or biological challenge has not been resolved.

The adjustable component may therefore be GMR and discretionary function.

Maintenance can be postponed.

Repair can slow.

Physical activity may decline.

Motivation may decrease.

Complex cognition may become more difficult to sustain.

The organism may still be successfully surviving. But it is surviving within a progressively smaller energetic operating range.

This provides a useful bridge to energetic-allocation models of stress: what appears clinically as loss of motivation, exercise intolerance, poor concentration, reduced resilience, or delayed repair may sometimes reflect prioritization under energetic constraint, rather than failure of a single pathway.

Sandi and colleagues provide supporting neural-circuit biology for this possibility. In the nucleus accumbens, for example, mitochondrial respiratory capacity, redox state, and quality-control pathways are associated with the ability of circuits to support effort, motivation, and anxiety-related behavior; experimental manipulation of mitochondrial function can alter these outcomes.

What does this mean clinically?

The immediate clinical implication is not that every patient with fatigue, anxiety, or cognitive difficulty has “mitochondrial dysfunction.”

That would replace one oversimplification with another.

A more useful translation is to ask whether the individual's energetic operating range has become constrained and, if so, why.

This requires looking beyond individual abnormal laboratory values.

A person may have apparently adequate nutrient concentrations yet poor delivery or utilization. They may have adequate calories but excessive substrate pressure. They may produce sufficient resting energy but possess little reserve for exercise or immune challenge. They may successfully mount a stress response but recover unusually slowly afterward.

The clinical pattern therefore matters as much as a single measurement.

Assessment can begin with three questions:

1. What is increasing energetic demand? Persistent inflammation, infection, sleep disruption, pain, psychological stress, environmental exposures, metabolic overload, or excessive exercise may continually raise energetic pressure.

   
   

2. What may be limiting throughput? Oxygen delivery, perfusion, substrate handling, mitochondrial oxidation, redox recycling, protein and micronutrient availability, endocrine regulation, and tissue integrity all influence the ability to convert available resources into useful work.

   
   

3. Is recovery occurring? The trajectory following exertion, illness, poor sleep, or psychological stress may reveal more than the resting state itself. How quickly does energy return? Does cognitive function normalize? Does exercise tolerance improve with rest? Does inflammation resolve? Can the person repeatedly respond to challenges without progressively losing reserve?

This shifts clinical attention from simply correcting isolated numbers toward understanding the respond–adapt–recover cycle.

Recovery may be one of our most useful clinical signals

Modern medicine is very good at measuring resting states.

We measure glucose while a patient is sitting still. We measure inflammatory markers at a single time point. We assess blood pressure after several minutes of rest.

But biological resilience is inherently dynamic.

The more informative question may be what happens after a challenge.

How rapidly does heart rate normalize after exercise?

How long does fatigue persist?

How well is a meal handled metabolically?

How quickly does sleep loss resolve?

Does an infection produce a temporary disturbance followed by restoration, or does the patient remain physiologically altered for weeks?

From an impedance perspective, successful recovery means that the temporary constraints created by adaptation have been cleared sufficiently for the previous operating range to be restored.

Incomplete recovery means that part of the energetic cost of the previous challenge remains.

Repeated often enough, yesterday's adaptation becomes today's baseline.

From “mitochondrial dysfunction” to energetic capacity

The emerging science described by Sandi and colleagues encourages a more nuanced vocabulary.

Mitochondria can change because the system is adapting.

They can relocate toward sites of demand.

They can undergo fission or fusion.

They can alter substrate use.

They can change ROS production.

They can increase or decrease respiratory activity depending on context.

Calling every change “mitochondrial dysfunction” misses the biological purpose of these responses.

A better question is whether mitochondrial and metabolic adaptation is successfully preserving an adequate operating range.

If yes, stress is followed by recovery and adaptive capacity is maintained.

If no, energetic throughput becomes increasingly constrained, reserve falls, and biological priorities may have to be reorganized.

A different way to understand resilience

The Sandi review provides a powerful bridge between mitochondrial biology and behaviour.

Its central message can be extended beyond neuroscience.

Human function depends not simply on how much energy exists in the body, but on how readily energy can move through biological systems and be converted into useful work when demand appears.

The Energy Constraint Principle tells us that the energetic budget is finite.

Bioenergetic impedance describes the constraints that determine how much of that budget can actually flow through the system.

The energetic operating range describes the function that remains possible.

And recovery determines whether that range is restored before the next challenge arrives.

From this perspective, resilience is not merely the ability to withstand stress.

Resilience is the ability to increase energetic throughput when required, allocate it appropriately, resolve the resulting physiological disturbance, and recover sufficient reserve for the next challenge.

That may ultimately prove to be one of the most clinically useful lessons emerging from mitochondrial neuroscience.

Sandi, C., Lobo, M. K., Hollis, F., Hirabayashi, Y., de Juan-Sanz, J., & Bolaños, J. P. (2026). Brain mitochondria as key drivers of cognition and behaviour. Nature Reviews Neuroscience. https://doi.org/10.1038/s41583-026-01061-1