Hello, reader, and welcome to the YNCA’s Basics of Neuroscience Course. This course will help provide a basic understanding of how the brain and nervous system works. Throughout our issues, we will explain every facet of neuroscience, from the level of massive connected systems to the level of discrete cells and molecules. We’ll look at how neuroscience affects what it fundamentally means to be human. We’ll tackle how we feel, move, love, and think.
Neuroscience is one of the fastest growing scientific fields, and neurological disorders affect countless individuals worldwide; despite this, most people don’t learn about neuroscience until they reach college. Our Basics of Neuroscience section provides an introduction to the wonders of the human nervous system. We hope to educate, entertain, and ultimately inspire young minds to enter one of the most important fields of study in the modern world
To fully understand how the brain works, we must consider it on many different scales. From subatomic polarities to complex computational systems of neurons to the many parts of the brain itself, the study of the human mind is an immense field including not only neurobiology, but also mathematics, psychology, engineering, computer science, medicine, chemistry, electrophysics, biology, philosophy, and many other fascinating subjects.
This first lesson will cover the basic structure of neurons, and how the structure of each individual neuron contributes to its function.
The human brain is composed of approximately 80 to 100 billion neurons, which work together to form complex computational systems.
As explained in greater detail in the History of Neuroscience article, Santiago Ramon y Cajal developed a theory, called the Neuron Doctrine, that stated that the brain is composed of many distinct cells. A neuron consists of same basic components as any other cell. Like a muscle cell, or a skin cell, neurons have cell membranes, nuclei, cytoplasm, mitochondria, and the numerous other structures found in any eukaryotic cell. The cell body (also known as the soma) contains the nucleus, and most of the support systems and organelles found in all cells. What differentiates neurons from all these other cells, however, are two large, branching structures protruding from a neuron called the axon and the dendrite. During the process of signal transmission, the dendrite serves as the input end, while the axon is the output end. Dendrites extend in many directions from the soma; the pattern of dendrites protruding from a neuron is known as arborization. The number of dendrites varies throughout neurons, depending on their specific function. The signals sent to neurons are typically received on dendritic spines, specific sites on the dendrite specialized for signal reception.
From the cell body, electrical signals known as action potentials travel through the axon (more on this in the next issue). The cell body transfers signals to the axon at the axon hillock. Many neurons have axons covered by a myelin sheath, a layer of lipids meant to allow faster transmission of these electrical signals (represented in blue on the diagram above). In neurons, myelin allows for propagation of the action potential faster through the axon: they act as a sort of insulation for the electrical signal. Between myelin are Nodes of Ranvier, sites that contain high concentrations of ion channels to propagate the action potential (more on this in the next issue). Axons then branch out (thinning along the way) and end at small bumps called axon terminals.
It is also important to understand how signals are sent between neurons to form the unique communications systems that neurons use. In general, an axon terminal connects with a dendritic spine, forming a synapse. The axon and dendrite are separated by a miniscule gap called the synaptic cleft. This gap allows chemicals called neurotransmitters to cross over from the axon to the dendrite and react with receptors, but more on that in the next issue. The synapse is the main way neurons communicate with each other.
One final thing to keep in mind is that neurons are extremely specialized cells. There are no two identical neurons, as each and every neuron grows and changes to serve a specific purpose. One specialized type of neuron is the Purkinje neuron, which is involved with motor function, and plays a critical role in integrating and transmitting massive amounts of information. These neurons have massive branching dendrites in order to handle such a large amount of information; however, Purkinje cells have only one axon to relay the information to the next cells in the circuit. The structure of every neuron likewise directly correlates to the function they enact.
Aside from neurons, many types of support cells, known as glia, exist throughout the nervous system. Astrocytes, one common type of glial cell named for their star-like shape, provide a wide range of support for neurons, including the uptake of used neurotransmitters. Astrocytes also have end-feet that attach to blood vessels to help create the Blood-Brain Barrier (BBB). The BBB helps filter out dangerous substances from the brain, yet also in some cases makes developing medications difficult as only certain molecules can enter the brain. Also involved in protecting the brain, microglia act as the immune cells of the nervous system. Microglia function to attack and destroy invading pathogens.
Two types of glial cells are involved specifically with creating the myelin sheaths. Oligodendrocytes myelinate CNS neurons, and each cell has many protrusions from its cell body that allow them to myelinate more than one neuron. Schwann cells have an analogous function within the PNS, yet each cell creates only one segment of the myelin sheath.
Much like neurons, glial cells in the nervous system are also structured to match their function. For one, the astrocytic end-feet allow them to filter blood and protect the brain. Furthermore, the differences between oligodendrocytes and Schwann cells exist in order to account for different spatial issues within each main part of the nervous system. Oligodendrocytes have long extensions that allow them to myelinate many neurons, while Schwann cells are only capable of myelinating one neuron. The crowded nature of the CNS has contributed to the development of many mechanisms (such as the use of oligodendrocytes) to save space.
Key Terms (In order of their mention)
Neuroscience - The science of the brain and nervous system
Neuron - The basic functional cell of the nervous system
Central Nervous System (CNS)- The brain and the spinal cord
Peripheral Nervous System (PNS) - All neurons not part of the Central Nervous System
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Jacob Umans is an aspiring physician-scientist in the Stanford University Class of 2020. As a cofounder of the IYNA, he is passionate about science education and hopes to share his excitement about all subfields of neuroscience -- especially glial biology and neuroimmunity -- with students around the world. He hopes to go on to earn an MD/Ph.D. after graduating from Stanford and to use his clinical experience develop a research focused on developing a better understanding of and improved therapies for neurodegenerative diseases. Outside of neuroscience, Jacob is an avid fan of puns, table tennis, and reading.