Mental Energy Explained: How Neurotransmitters, Motivation & Metabolism Drive Your Brain Power

When the brain is active, it sends messages from one nerve cell (neuron) to another. These messages travel through a combination of electrical and chemical signals. When a message is passed from one neuron to the next, an electrical impulse triggers the release of chemicals into the gap between two neurons. This space is called the synaptic cleft.

One of the main chemicals released to help the message continue over the synaptic cleft is called glutamate.
Glutamate is an excitatory neurotransmitter, meaning it increases the likelihood that the next neuron also fires an electrical signal, allowing the message to carry on.

During mentally demanding tasks, many of these messages are sent rapidly and repeatedly. 

This leads to large amounts of glutamate being released into the synaptic cleft. If this happens continuously over time, glutamate can start to build up in the synaptic cleft.

This happens because the astrocytes, which are specialized support cells responsible for taking up and recycling glutamate, can get overwhelmed and thereby can’t clear the synaptic cleft of glutamate fast enough. 

As a result, glutamate accumulates in the synaptic cleft. (Pellerin, L., & Magistretti, P. J. (1994), (Falkowska, A, et al 2015)

Astrocytes help recycle glutamate back to the neurons. In order to take in glutamate from the synaptic cleft, they bind glutamate together with sodium ions (Na+).

This triggers the astrocyte to take in more glucose and causes a buildup of Na+ inside the cell. 

To restore the balance between sodium and potassium inside and outside the astrocyte, the Na+/K+ pump will help.
This pump actively pumps Na+ out in exchange for K+.

Since glutamate is co-transported with Na+, this process helps move glutamate from the astrocytes back to the neurons.

Running the Na+/K+ pump requires energy. To meet this demand, astrocytes will break down their internal glycogen stores into glucose.

This glucose is then used in a process called glycolysis, which produces energy and lactate. The energy powers the Na+/K+ pump, while the lactate can be passed on to neurons, where it serves as a fuels source.

(Falkowska, A, et al 2015)

The use of glucose or lactate to produce energy essentially involves a process that splits a molecule called ATP. ATP, which is short for adenosine triphosphate, is made up of three phosphate-bound adenosine units. 

When one of these bonds is broken, energy is released, and it’s this energy that powers many of the cell’s functions.

Everything the body does requires energy. And when the brain is active with sending messages from neuron to neuron, this too consumes energy.
Both the astrocytes and the neurons themselves use ATP by breaking it down, releasing adenosine in the process.

When we engage in sustained mental work, this breakdown happens faster than the body can recreate ATP.
As a result adenosine begins to build up in the brain.

The accumulated adenosine binds to A1 receptors on neurons. When that happens, it becomes harder for messages to pass between nerve cells, because the adenosine blocks other signal-carrying molecules from binding and continuing the message across the synaptic cleft.

As this buildup increases, it becomes harder to focus and process information. Eventually, a biological process known as “sleep pressure” starts to rise, which is essentially your brain’s way of telling you it's time to rest. (Reichert, C. F., et al. 2022), (Kok, A 2022).

This acts as a natural brake, slowing the brain's processes down and protecting against overactivation.

The Anterior Cingulate Cortex (ACC) really is a key region involved in mechanisms related to mental energy.
It serves as a central hub for monitoring cognitive effort and task difficulty, regulating performance and attention, and tracking internal energy availability.

The ACC evaluates how energy-demanding a task is by receiving input from astrocytes and monitoring levels of neuromodulators like adenosine, glutamate and dopamine. 

In general, if the ACC detects low energy availability – for example via high levels of adenosine - it may prompt restorative behaviors like taking a break or initiating sleep.

It constantly balances the energy required for a task against current capacity and the perceived outcome.

As adenosine builds up and binds to A1 receptors, it becomes harder for signals to pass across the synaptic cleft. This increases the energy cost of communication between neurons, making tasks feel more cognitively demanding.

Maybe because mentally draining tasks such as focusing and concentrating mainly involves an activation of the frontal part of the brain, when the energy cost outweighs the expected reward, the ACC may reduce top-down control from the frontal regions of the brain. 

This reduction in executive activation can lead to lower motivation and reduced focus. 

At the same time, activity in the Default Mode Network (DMN) may increase, leading to mind-wandering, which will make it harder to stay concentrated. As a result, you’re more likely to shift to easier tasks, as foe example checking your phone or taking a break.

Another key player in this system is dopamine. 

The ACC receives dopaminergic input and uses it to help assess the cost-benefit of mental effort. 

During demanding tasks, dopamine supports the stability of working memory, especially in the prefrontal cortex, by enhancing the likelihood of these messages continuing over the synaptic cleft from one nerve cell to another.

But when energy is low or the predicted reward is not sufficient, dopamine release decreases. This weakens the signaling strength and makes it harder to stay focused.

The involvement og dopamine therefore links motivation to the equation of mental energy.
One must assume, that if one is strongly intrinsically motivated by a given task, more dopamine would be available, strengthening the brains ability to sent these messages.

As energy drains, less demanding activities become more appealing. 

Frontal Midline Theta (FMθ) activity, which is a brain rhythm associated with cognitive control, typically increases during focused work as is amongst else, plays a role in suppressing DMN activity. 

But as the task continues and the brain has less and less energy, FMθ plateaus or drops, allowing DMN activity to rise and focus to decline. (Kok, A 2022), (Westbrook, A.. & Beaver, T. S, 2016), (Wascher, et al, 2013), (Scheeringa, R. 2008)

All this really shows how mental energy is affected by both biology and motivation. 

Mental fatigue is really your brain’s safety brake telling you resources are low. 

I believe we should use that insight.

We should start managing our mental energy, just as one would manage physical energy. After a tough run, one would feel tired at relax on the couch for an extended period of time before doing squats in the gym.

Use small breaks, naps and meditations strategically to optimize your mental energy. 

-Nicolas Lassen, Master in Sport Science

References:

•  Pellerin, L., & Magistretti, P. J. (1994). Glutamate uptake into astrocytes stimulates aerobic glycolysis: A mechanism coupling neuronal activity to glucose utilization. Proceedings of the National Academy of Sciences, 91(22), 10625–10629. https://doi.org/10.1073/pnas.91.22.10625
•  Falkowska, A., Gutowska, I., Goschorska, M., Nowacki, P., Chlubek, D., & Baranowska-Bosiacka, I. (2015). Energy Metabolism of the Brain, Including the Cooperation between Astrocytes and Neurons, Especially in the Context of Glycogen Metabolism. International Journal of Molecular Sciences, 16(11), 25959–25981. https://doi.org/10.3390/ijms161125939
•  Reichert, C. F., et al. (2022). Sleep pressure and mental fatigue: A neurocognitive perspective. Journal of Sleep Research, 31(3), e13597. https://doi.org/10.1111/jsr.13597
•  Kok, A. (2022). Cognitive control, motivation and fatigue: A cognitive neuroscience perspective. Brain and Cognition, 160, 105880. https://doi.org/10.1016/j.bandc.2022.105880
•  Westbrook, A., & Braver, T. S. (2016). Dopamine does double duty in motivating cognitive effort. Neuron, 89(4), 695–710. https://doi.org/10.1016/j.neuron.2015.12.029
•  Wascher, E., Getzmann, S., & Falkenstein, M. (2013). Frontal theta activity reflects distinct aspects of mental fatigue. Biological Psychology, 96(1), 57–65. https://doi.org/10.1016/j.biopsycho.2013.01.001
•  Scheeringa, R., Petersson, K. M., Oostenveld, R., Norris, D. G., Hagoort, P., & Bastiaansen, M. C. M. (2008). Frontal theta EEG activity correlates negatively with the default mode network in resting state. International Journal of Psychophysiology, 67(3), 242–251. https://doi.org/10.1016/j.ijpsycho.2007.05.017