Neural Economics: Understanding the Brain’s Energy Budget

Post by Rachel Sharp

How much of your body’s energy does your brain use?

The average adult brain makes up ~2% of our total body weight. And yet, brain processes to maintain proper functioning account for as much as 25% of the body’s energy use. On top of that, when under mental stress, the brain’s energy supply can increase by as much as 12%. The disproportional use of bodily energy sources by the brain is unique to humans and primates - the central nervous system (the brain and spinal cord) in other vertebrate species uses only about 2-8% of the body’s energy.

Previously, it was thought that this difference in energy use was because as primate brains developed more advanced cognitive and social abilities, the amount of energy required by the brain also increased. More recently, we’ve learned that the amount of energy a human neuron uses is similar to the amount of energy used by a mouse neuron. This finding suggests that the actual reason human brains use such a large amount of energy is because the density of neurons in our brains is much higher than in other species. So, the question then becomes: 

What are our neurons doing with all that energy?

Scientists have identified four main activities that neurons use energy for: synaptic transmission, generating action potentials, maintaining resting potentials, and housekeeping.

Synaptic transmission is the process through which neurons communicate with each other via the release of signaling molecules called neurotransmitters. Neurotransmitters, such as dopamine, serotonin, or endorphins, are released from the axon terminal of a neuron, and then bind to receptors of a nearby neuron, causing a variety of responses in both cells. Synaptic transmission is constantly occurring in millions of neurons throughout the brain, and this process takes a lot of energy. When neurotransmitters are released from axon terminals, they have to be packaged into small bubbles called vesicles and pumped from the interior of the neuron to the extracellular space. The receiving neuron must engage various processes as well, like adjusting the amount of time neurotransmitter-receiving channels are open and the number of active receptors available to bind to the neurotransmitters, which also uses energy. Together, the processes required for synaptic transmission, which occurs throughout the brain both at rest and in heightened states, account for about 45% of the brain’s total energy use.

Action potential generation can occur as a result of synaptic transmission. Let’s consider three neurons: A, B, and C. Neuron A communicates with neuron B through synaptic transmission. If the signal received by Neuron B is strong enough, it will generate an action potential to communicate that message to Neuron C. An action potential is a rapid electrical signal transmitted along the whole length of a neuron. There are two main sources of energy used in the process of generating action potentials: initiating an action potential and then maintaining the electric current that allows the action potential to travel all the way down a neuron’s axon. These processes are estimated to account for 25-30% of the brain’s total energy use.

Maintaining resting potentials is an ongoing process that neurons are always engaged in (outside of an active action potential). Resting potential refers to the balance of electric charge between the interior and exterior of a cell. For neurons, this is actually an imbalance, as the inside of a neuron typically has a charge that’s 60-90 millivolts lower than the outside of the neuron. This electrical imbalance allows the neuron to maintain a “baseline” state distinct from its “activated” state of action potential generation. An action potential occurs because of positively charged particles entering the neuron and increasing the internal electrical charge. The processes involved in maintaining resting potential, mainly pumping positively charged particles out of the cell and negatively charged particles into the cell, ensure that the neuron doesn’t simply increase in internal charge over time. The maintenance of resting potentials across neurons is estimated to account for 20-25% of total brain energy use. 

Housekeeping refers to necessary processes between neurons that don’t involve signaling or communication. Currently, the way these processes use energy is not well understood, but estimates for the use of energy by cellular structure modeling proteins, protein creation, and vesicle transport have been investigated. These largely unmeasured processes are thought to account for roughly 20% of the brain’s total energy use. 

How does the brain maintain its energy sources?

Sleep is the most important component of energy maintenance by the brain, particularly non-REM sleep, when brain activity, breathing, and heart rate all slow down and muscles relax. While awake, the processes above increase the brain’s consumption of energy from its energy stores, depleting them over time. During non-REM sleep, these processes slow down, energy use lessens, and energy conservation processes increase, so that the brain can replete and maintain energy storage. This is vital because the brain does not store much of its own energy. In fact, most of the energy the brain uses is supplied through the blood from the rest of the body. 

Overall, while research about neuronal energy use is still underway, we know that it’s a complicated and vital process: non-optimal energy use and storage in the brain has been linked to several disorders such as Alzheimer’s and Parkinson’s disease. Understanding the brain’s energy consumption not only highlights its complex functionality, but also shows the importance of maintaining our cognitive health through proper rest, ensuring that our most energy-demanding organ can continue to perform at its best.

References +

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