Readings for this Lesson

Two Families of Brain Cells

Like every organ in the body, the brain and nervous system are made up of cells. Brain cells can be classified into two rough categories: Neurons and Glia. Neurons are the basic building block of the nervous system, the business end. They can generate a series of electrical impulses that allow them to send information throughout the brain. Neurons talk to each other by sending a receiving little packets of chemicals called neurotransmitters at specialized connections between them called synapses. One major goal of neuroscience is to understand how groups of neurons interact, using these electrical and chemical signals, to generate behavior. The other category of cell in the nervous system are called Glia. There are many types of glia and they are responsible for a variety of functions. Glia are traditionally thought of as the supporting cast of the nervous system, and have important roles such as regulating the chemical environment in the brain, enhancing electrical signaling of neurons and removing debris left by dead neurons. That being said, there is increasing evidence that glia may also play important roles in brain signaling, just like neurons. In this exercise we will give you a brief introduction to the structure of neurons and tell you a little about the different types of glia. In a separate exercise we will learn about different classes of neurons and how to classify and identify them.

The Structure of a Neuron

The brain of mammals contains anywhere between 100 million to 100 billion neruons, depending on the species. Humans are at the top of this range. 100 billion neurons are A LOT of neurons. For example, our galaxy, the Milky Way contains an estimated 300 billion stars, so take two of your friends and together you all have as many neurons as stars in our galaxy. In many ways, neurons are just like regular cells, they have a cell body, also known as a soma, which contains a nucleus and various organelles (if you are unfamiliar with the basic parts of a cell check out this article and this resource for a brief primer). A neuron's soma ranges in size between 5 and 50 micrometers (0.0002 to 0.002 inches). Tiny. The nucleus contains the cell's DNA and is surrounded by structures known as the endoplasmic reticulum and Golgi apparatus, which are responsible for synthesizing and delivering newly made proteins to their location within the cell. Surrounding the neuron is a cell membrane made up of a fatty substance called a phospholipid. This fatty membrane keeps the inside of the cell separate from the outside of the cell (remember, fats and water don't mix) and is critical for allowing neurons to generate electrical impulses.

Neurons, however, have key differences from other cells in the body, like say skin cells or white blood cells. Neurons are polarized cells, meaning that one end of the cell is different from the other. This polarized structure allows the neurons to grow branchy extensions called processes. These processes can either be axons or dendrites. Axons are long outgrowths that allow neurons to send electrical signals throughout the nervous system. They can be up to a meter long (for example the distance between your brain and your lower spinal cord) and often branch into various directions to send signals to different places. The part of the axon closest to the cell body is called the axon hillock, or axon initial segment. This is where nerve impulses are first generated. At the other end of the axon is an axon terminal which the neuron uses to form one half of the synapse when communicating with other cells. The axon terminal contains little packets of neurotransmitter contained in little spherical bits of membrane known as synaptic vesicles. When we learn about synaptic transmission you will understand how the axon terminal uses these neurotransmitters to talk to other cells. Axons are like electrical cables, and most have an insulating sheath made up of a substance called myelin, which is provided by surrounding glia. This allows electrical impulses to travel faster. Dendrites are also branchy extensions coming out of the cell soma. Dendrites are thicker and branchier than axons, and not as long. Unlike axons, they contain no myelin. Dendrites allow neurons to receive chemical messages from axon terminals belonging to other neurons. One dendrite can get inputs from thousands of other neurons, and likewise, one axon can contact thousands of cells. This tangle of neurons sending and receiving messages, is known as a neural network or neural circuit. One good rule of thumb to remember is that electrical signals in dendrites travel TOWARD the soma, while signals in axons travel AWAY from the soma toward the axon terminal.

Some fun facts about neurons

Did you know that the main axon of a giraffe sensory neuron (from toe to neck) is about 15 feet? Or that neurons conduct nerve impulses anywhere between 1.2 - 250 miles per hour? How about the fact that the neuronal membrane is only 5 nanometers thick (0.0000002 inches)? Betcha' didn't know octopuses have 300 million neurons (compared to the 100 billion humans have)? You can explore these interesting facts and figures about neurons (and the nervous system in general) in this fun page.

Glia

The name glia comes from the Latin meaning 'glue'. That's because it was thought initially that glia are the glue that holds the brain together. But in reality there is much more to glia. In fact your brain has about 8 times more glia than neurons. That would be about 800 billion glia! The most common type of glia in the brain are called astrocytes. They have the job of controlling the chemical composition of the fluid surrounding the neurons, controlling the growth of neurons, regulating synaptic communication between neurons and altering blood flow in regions of the brain that are highly active. A second type of glial cells, are ones responsible for creating the myelin sheath which surrounds axons. In the brain, glia called oligodendrocytes play this role, wrapping their cell membrane around portions of a neuron's axon, in order to electrically insulate it from the outside. This allows axons to transmit nerve impulses much faster and much more efficiently. Oligodendrocytes do not completely cover the whole axon with myelin; there are small gaps along the axon that remain unmyelinated. These gaps are called nodes of Ranvier and are important for propagating electrical signals in the axons. In the nerves outside of the brain and spinal cord, glia called Schwann cells have the job of myelinating nerves. Many neurodegenerative diseases, such as multiple sclerosis involve damage to the myelin sheaths. The last type of glia are microglia. Microglia are responsible for clearing damage after injury and can be thought of as the immune system of the brain.

Image shows diversity of glial cell types found in the nervous system.

Are Glia the Genius Cells?
New research, however is beginning to indicate that glia are not just supporting cells, but that they also play an active role in brain signaling and in brain development. For example, astrocytes surround synapses and are also activated by neurotransmitters and can even release their own gliotransmitters that affect how neurons function. Likewise, astrocytes are thought to be critical for the formation of new synapses in the developing brain and maybe even during learning. Read the following interview with Dr. Ben Barres, a neuroscientist that has been pioneering important work aimed at determining the role of glia in the development of synapses in the immature brain. As you read this article keep in mind the following question: how do these new findings about glia challenge the view that neurons are the main functional unit of the nervous system?

Click image to download interview as a .pdf

Aside from allowing new synapses to form, glia have been thought to actually process neural information using unique forms of communication that are different than what you would find in neurons.

Interestingly, when they examined Einstein's brain after he died to see if

he had more neurons than the average person, what they found instead was that

he had way more glia than most people.

Does this mean that having all these extra glia make you smarter? Are glia the genius cells? Probably not, it might just be coincidence. But it is definitely worth rethinking the role glia play in brain signaling and thought processing. The following video contains an interview with Dr. Doug Fields, one of the pioneers in understanding how glia communicate:

Together these last two pieces suggest an evolving and changing view of glia. When you learn about the Neuron Doctrine in the next lesson, think about what you learned about glia and whether its time to revise this view to include the function of glia in brain signaling.

Squishy Brains!

One last thing to think about is that with all those billions of neurons and glia taking up every empty space in your brain, and based on the fact that neuronal and glial cell membranes, as well as myelin are all made of fatty substances, this means that your brain is mostly composed of fat, and maybe some protein. Many times when brain dissections are made as part of a school class, brains have been treated with fixative to preserve the tissues. What this does is that it makes the brain sort of hard and rubbery, it gives it a grayish appearance. Thus when you hear talk about "gray matter" and "white matter", this isn't exactly accurate. What people call gray matter refers to parts of the brain that have very little myelin, such as regions containing neuronal somata and dendrites. In fixed brains this appears as gray(ish). In fresh brains these regions are more pink. White matter refers to parts that have lots of myelinated axons, and these always are kind of white. So by looking at a fixed brain one really fails to appreciate how squishy real brain really are and to get a sense of their real colors. Check out this video to get a good sense of the real texture and color of an unfixed, fresh human brain:

Proceed to the quiz in the next page, to test your knowledge about neurons and glia!