1. Why Does the Brain Use So Much of the Body's Energy?
You might not feel like your brain is working hard during a normal day. However, while the adult brain weighs only about 2% of your body weight, it uses about 20% of the oxygen and energy your body needs. [1] [2] If you compare energy use per unit of weight, the brain uses about 10 times more energy than the average body tissue (calculated as 20 divided by 2). [1]
Is this energy used for difficult thinking tasks? Actually, no. It is thought that the brain’s energy consumption does not change much between times when you are focusing hard and times when you are zoning out. [1]
Most of this energy is used for "communication." Estimates suggest that 60 to 80% of energy use goes to the process of neurons sending signals to each other. The extra energy used for a specific task in front of you is estimated to be only about 0.5 to 1% of the total. This estimate comes from a secondary introduction citing a 2006 explanatory paper by Raichle. [1] [9]
Think of it like a shop that keeps the lights on and the refrigerator running even when no customers are there. But for the brain, that "electricity" is used for the work of connecting cells to each other. From these numbers alone, we cannot say that thinking does not tire the brain. Other organs, like the heart and kidneys, also use a lot of energy. The brain is not the only big eater. [1]
2. Which Brain Parts Control Breathing, Feeling, Moving, and Thinking?
Even while you sleep, your breathing does not stop. This is not because you are thinking "I should breathe." It is thought that the lower parts of the brain keep adjusting this automatically. [5]
Looking from the bottom up, the medulla oblongata is in the lower part of the brainstem. It has centers that regulate breathing, heart function, the width of blood vessels, and swallowing. The brainstem, which includes the medulla, also has the reticular formation, which helps keep the cerebrum awake. [5] The NINDS also describes the hindbrain (brainstem and cerebellum) as a part involved in maintaining life functions like breathing and heart rate. [6]
Above this is the thalamus. It relays information from most senses (except smell) to the cerebral cortex. Just below it, the hypothalamus is a high-level center for regulating body temperature, blood pressure, heart rate, eating, and drinking. [5]
The cerebellum is toward the back and lower part of the brain. It regulates movement based on information from the senses and the cerebral cortex. It supports actions like walking while keeping balance, speaking smoothly, and doing fine tasks. It is also involved in learning through practice, such as playing piano or tennis. [5] [6]
The largest part, the cerebrum, is considered the source of conscious thoughts and actions. It allows you to hold memories, plan, imagine, and think. [6] These parts work together. For example, sensory news goes through the thalamus to the cortex, and the cerebellum uses that information to adjust movement. Consciousness is maintained by the work of the cerebral cortex and the reticular formation in the brainstem. [5]
3. Has the Number of 100 Billion Neurons Really Been Measured?
You may have heard that the brain has 100 billion neurons. However, according to MIT explanatory materials (which are secondary sources, not research papers), there are no peer-reviewed papers supporting this number. It was estimated from other measurements. [4]
In 2009, Azevedo’s research team studied the brains of four adult men. They dissolved brain tissue and counted the cell nuclei. The result was an average of about 86.1 billion neurons and about 84.6 billion non-neuronal cells, which are almost equal in number. The common belief that "non-neuronal cells are 10 times the number of neurons" did not fit these actual measurements. [3] However, this study only looked at 4 people, and there was a variation of about 8.1 billion. So, remembering "exactly 86 billion" is not accurate. [3]
The breakdown by part was also surprising. The cerebral cortex makes up about 82% of the brain's mass but contains only about 19% of the neurons. On the other hand, the cerebellum is only about 10% of the mass but holds about 80% of the neurons. [3] 80% of 86.1 billion is about 69 billion cells, and about 16 billion are in the cerebral cortex (these are calculations). Per unit of weight, the cerebellum has over 30 times as many neurons compared to the cerebral cortex. [3]
You might think these cells are used for walking, speaking, and smoothing out fine movements. However, we should be careful about linking the number of cells directly to their function one-to-one. [3]
4. Does the Brain Predict What Will Happen Next?
Have you ever stepped down a dark staircase and felt a jolt when the last step was missing? Your foot had already moved out, expecting the floor to be there. [7]
A theory called "predictive coding" explains this "mismatch." The idea is that the brain predicts the outside world based on past experiences. It then uses the mismatch (prediction error) between what it predicted and what actually came in through the senses to update its predictions. [7]
Materials from a research group at the National Institute for Physiological Sciences also explain that the brain is not passively reacting to stimuli from sensory organs. Instead, it compares predictions from an internal model with actual sensory signals. It updates the internal model based on the mismatch to actively create perception. [8]
However, this is a framework being studied as a hypothesis or theory. It is not a confirmed fact about how the brain works. How this is realized within neural circuits is still not understood. [8] The staircase example is just a situation where this idea is easy to explain. [8]
5. How Can You Notice Different Brain Jobs in Your Own Body?
You can feel a little bit of the brain’s division of labor in your own body. First, try counting 10 breaths. Do not force yourself to breathe deeply; just keep your normal breathing. Even though you are not consciously giving orders, your breathing continues. This is explained as the work of the medulla and other parts. [5]
Second, in a safe, flat place, walk slowly and say the order of your foot movements out loud, one step at a time. Usually, you do not think about this, but your feet move smoothly. The cerebellum is said to be involved here. [5]
These are not experiments but games to notice the brain working where you usually do not pay attention. It is okay if you do not notice it clearly. Many things that happen without us thinking are likely handled by the brain. Before you take a step, try to predict where your foot will land. If it lands in a different place than you thought, how does it feel? That moment of surprise is a small example of prediction and updating. [7]