The cerebrum is the largest part of the brain and the one most responsible for who we are. It covers the top of the brain and helps us think, plan, remember, move, and make sense of the world.

It is divided into two halves, right and left, that are constantly talking to each other through a powerful connection called the corpus callosum. Each side controls the opposite side of the body.

We used to think one side of the brain was “creative” and the other “logical,” but today we know the brain works as a team. Still, there are some preferences: the left side is more involved in language, while the right side helps with spatial awareness and understanding our body in space.

In short, the cerebrum is the brain’s command center—helping us think clearly, make decisions, and interact with the world around us.

The cerebral cortex, the outer surface or gray matter of the brain, is divided into four sections known as lobes, which perform different tasks, but work together.  Each lobe has specified tasks like vision, speech, decision-making, and more (unfortunately, the brain does not look like this beautiful graphic, with vibrant colors and clear markings between the lobes!): 

🧠 The Brain Stem

🔗 Connects the brain to the spinal cord

🌬️ Helps control breathing

❤️ Helps control heart rate

🌡️ Helps keep the body’s temperature steady

✨ Important:

👍 These functions work automatically, even when we sleep

The limbic system is part of our primitive brain region, spanning both hemispheres and containing parts of the frontal, parietal, and temporal lobes.  It is responsible for basic emotions (fear, pleasure, and anger) and drives (hunger, sex, dominance, and care of offspring).  Structures within the limbic system include the hippocampus, a seahorse-shaped structure that is key to learning and memory consolidation, and the amygdala, an almond-shaped structure, which is involved in memory, attention, and emotional processes.

Our brains are comprised of brain cells, also called “nerve cells” or “neurons.” It is estimated there are 100 billion neurons in the human brain.  Each of these neurons connects with other neurons in an intricate network.  A neuron consists of the cell body (soma), nucleus, dendrites (which receive electrical signals), an axon (through which electrical signals travel), and axon terminals (which transmit the electrical signal from one neuron to another). The axon is wrapped with a myelin sheath that helps speed the transmission of electrical impulses as they travel toward the axon terminals.

The brain also contains a type of cell called a glial cell.  It has been thought of as the most abundant type of cell in the nervous system with a 10:1 ratio of glial cells to neurons (100 billion neurons and 1 trillion glial cells).  However, compelling evidence from a 2009 study by neurophysiologist Suzana Herculano-Houzel and her colleagues has refuted that number and suggested it is closer to a 1:1 ratio (von Bartheld, Bahney, Herculano-Houzel, 2016).  These researchers invented a new, highly efficient way to count cells and found a much lower glial cell/neuron ratio than before believed.  Additionally, Scientific American researched professional literature dating back to the 1950s and could not find a single published study supporting the 10:1 glia to neuron ratio (Jabr, 2012). 

Glial cells come in many varieties, including oligodendrocytes, astrocytes, ependymal cells, Schwann cells, microglia, and satellite cells.  Glial cells are responsible for providing support to neurons, such as insulating axons and helping to amplify the electrical impulses traveling through them.  The glial cells do this by wrapping themselves around the axon and producing a myelin sheath to help speed transmission of the messages.  In the diagram of a neuron (above), you can see the neuron body, the axon (where the signal travels) and the thick covering surrounding part of the neuron, which are the glial cells (shown as green, though they are whitish in appearance, thus being called “white matter.”) The glial cells act like a tunnel, and communication can only take place outside of the glial cell area of the axon. 

You have probably heard the brain referred to as “grey matter.”  But what does that really mean?  Grey matter is the darker tissue of the brain (pinkish-grey), consisting mainly of nerve cell bodies (neurons) and branching dendrites.  White matter is made of dense bundles of axons connecting different parts of grey matter to each other (the axons get their whitish appearance as they are covered in myelin, which is white).  If we added up the length of all the axons interconnecting the neurons in just the forebrain of a young adult, it would easily extend to the moon and back!   

Here is an easy way to remember the difference between grey matter and white matter: 

grey matter = cell bodies

white matter = networks

If you would like an immersive, 3-D experience with the ability to look at the brain and move it around to see if from different perspectives, try this activity on BrainFacts.org to review what we have just learned about the various parts of the brain and where each is located.  

Communication within our vast network of brain cells is accomplished by brain chemicals called neurotransmitters and neuromodulators. A neurotransmitter is a brain chemical that is responsible for the transmission of electrical signals (messages) from one neuron to another. Scientists have managed to identify over 100 neurotransmitters in the human brain, but evidence suggests we have significantly more than this number (wiseGEEK, n.d.).  Scientists also believe that only about 10 of these brain chemicals do 99% of the work of the brain (King, 2013). 

Neurotransmitters include glutamate, dopamine, acetylcholine, noradrenalin, serotonin, and endorphins. These neurotransmitters are what tell the body how to respond--such as heartbeat, digestion, and breathing.  They also affect mood, sleep, concentration, and weight. Stress, diet, toxins, drugs, alcohol, and caffeine can affect the amount of neurotransmitters released.  

The process of communication in the brain involves electrical signals that travel from neuron to neuron.  When an electrical signal travels from one neuron to another, an electrical impulse (action potential) is generated.  The axon, a long, slender, tube-like projection of a nerve cell, conducts nerve impulses away from the cell body.  At the end of the axon, there are branch-like extensions called axon terminals. When an action potential nears the axon terminal, an impulse is generated, resulting in the release of neurotransmitters into a synaptic gap—a narrow gap, or synapse, between neurons.  There are roughly between 1,000 and 10,000 synapses terminating on each neuron.  Estimates of the numbers of these synapses in the mature human brain range between about 60 trillion to about 240 trillion. 

After being released into the synaptic gap, the neurotransmitter is then taken up by receptors on a neighboring dendrite.  Dendrites are tree-like structures in neurons that extend away from the cell body to receive messages from other neurons at synapses.  The group of dendrites on a neuron is called a dendritic tree.  Each branch of the tree receives many synaptic inputs (sent from axon terminals on other neurons) that travel across the synaptic gap.  The signal then continues through that neuron to the next.  The neurotransmitter is then reabsorbed into the originating axon terminal.  This is called "reuptake."  Some neurotransmitters are lost and do not get reabsorbed. 

Below is a depiction of the communication cycle and actual images of impulses between two neurons (different colors).  Also shown is a close-up of what it looks like under a microscope (Segev, 2015).

Modern medical technology has given us a new frontier—what happens inside our brain.  Using scanning techniques, scientists can detect which areas of the brain are stimulated under certain conditions or during specific activities and determine which parts of the brain are responsible for certain functions.  Activities that can be measured include those related to thoughts, sensations, movements, pathologies (like a tumor), libido, mood disorders, choices, regrets, motivation, and even racism.  

There are many methods used today to measure brain activity:

• EEG (electroencephalogram): A scan that measures the activity of electrochemical pulses within the brain.  The signals have wave-like patterns ranging from alpha (when we are relaxing or sleeping) to gamma (when we are actively thinking).  When the activity loses its rhythm, a brain seizure occurs.  EEGs can detect changes in electrical activity in the brain on a millisecond level, and have high resolution.

• fMRI (functional magnetic resonance imaging): A scan that measures brain activity by detecting changes associated with blood flow and the movement of water molecules in brain tissues.

• PET (positron emission tomography): A scan that uses a radioactive substance (tracer) to look for disease or injury in the brain. 

• CT (computed tomography): A scan that builds a picture of the brain based on X-rays.  It is useful for revealing gross features of the brain.  

• MEG (magneto encephalography): A scan that measures magnetic fields produced by electrical activity in the brain.  Used in clinical settings and research, it can help locate a pathology or assist in determining the function of parts of the brain. 

• NIRS (near infrared spectroscopy): An optical technique that measures blood oxygenation in the brain.  It shines light through the skull to see how much the remerging light is attenuated (lost), which can determine blood oxygenation (Demitri 2010).

Here are some real examples of what can be revealed through brain scans:

We’ve had a look at what makes up our remarkable brains.  We’ve learned about basic structures and functions of major parts of the brain and how our brain cells communicate with each other (we will have more on this in another lesson).  We have learned how modern scanning technology enables us to learn even more about how our brains work, and how pathologies are detected.   Knowing this should give us a greater appreciation for the complexity of this amazing control center in our heads, and motivate us to care for our brains the very best we can.

Bhandari, Tamara. (October 25, 2018). Mind’s quality control center found in long-ignored brain area. Science Daily. [Online article]. Washington University School of Medicine. Retrieved from: https://www.sciencedaily.com/releases/2018/10/181025142018.htm

Demitri, Michael M.D. Types of Brain Imaging Techniques. 2010.  Retrieved from: http://psychcentral.com/lib/types-of-brain-imaging-techniques/

Gazerani P., The Neuroplastic Brain: Current Breakthroughs and Emerging Frontiers, 2025 — an evidence-based overview of how the brain reorganizes itself through neural connections as part of learning, memory, and adaptation.

King, P. (Apr. 2, 2013). How many types of neurotransmitters are there in a human brain? Quora [Website blog]. Retrieved from: https://www.quora.com/How-many-types-of-neurotransmitters-are-there-in-a-human-brain

Jabr, F. (June 13, 2012).  Know Your Neurons: What Is the Ratio of Glia to Neurons in the Brain?

Scientific American.  [Website].  Retrieved from: https://blogs.scientificamerican.com/brainwaves/know-your-neurons-what-is-the-ratio-of-glia-to-neurons-in-the-brain/

Phillips, Helen. Introduction: The Human Brain. September 4, 2006. Retrieved from: http://www.newscientist.com/article/dn9969-introduction-the-human-brain.html?full=true&print=true#.VZQa7vlVhBc

Segev, Ivan. "Synapses, Neurons and Brains." Coursera. 2015.

Wikipedia. (n.d.). History of neuroscience. Retrieved from: https://en.wikipedia.org/wiki/History_of_neuroscience

von Bartheld, C.S., Bahney, J, Herculano-Houzel, S.  (2016).  The search for true numbers of neurons and glial cells in the human brain: A review of 150 years of cell counting. Journal of Comparative Neurology, 524(18): 3865-3895.  doi: 10.1002/cne.24040.  Retrieved from: https://www.google.com/search?client=safari&rls=en&q=journal+comp+neurol&ie=UTF-8&oe=UTF-8 

Wikipedia. (n.d.). History of neuroscience. Retrieved from: https://en.wikipedia.org/wiki/History_of_neuroscience

wiseGEEK. (n.d.). How many neurotransmitters are there? [Website]. Retrieved from: https://www.wisegeek.com/how-many-neurotransmitters-are-there.htm