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Structures
Neuron Structures
What is a neuron?
Neurons, or nerve cells, are a type of cell found within our central nervous system (or our brain and spinal cord) and peripheral nervous system (or the neurons outside of the brain/spinal cord that extends to muscle and organ tissue). It's amazing to think that these tiny cells are so key to our experience as humans. By communicating through electrical and chemical signaling, their interactions lead to processes like vision, pain, taste, language, emotion, thoughts, etc.
Key neuron structures
There are 4 different types of neurons (described below). Although the structures pictured here are common to neurons, they are actually not a part of all types of neurons. For example, not all neurons have myelin sheath.
Soma
cell body containing organelles
where dendrites/axons branch off
(see below)
Axon
long, thin structure that sends information to other neurons
Dendrites
receives information from other neurons
Synapses
small space between two neurons where information passes (see below)
Myelin sheath
fatty substance surrounding the axon that allows for quick sending of information
Nodes of ranvier
gaps between myelin sheath
Parts of the Soma
Nucleolus
Director of Ribosome Production - responsible for transcribing ribosomal RNA to make ribosome subunits; these subunits get sent out to the rest of the cell and become complete ribosomes that are important for protein synthesis
Factory walls and roof - the cell's outer layer of fats and proteins acts as a barrier against substances outside of the cell (shown as avocado in the picture)
Cytosol
Cell Membrane
Nucleus
Factory's CEO - contains genetic material that provides information about cell development and maintenance; this boss oversees the factory's production and sends out orders, so the other organelles can get their jobs done properly
Think of the soma as a factory, where the organelles are workers that make products (i.e., proteins and enzymes) that get distributed to dendrites and axons for further use. Here's an introduction to some of the factory's key parts:
Inside workspace of the factory - a substance filling the space within the cell membrane where organelles are situated to produce their functions (shown as rice)
Mitochondria
Powerhouse - our factory needs power to produce necessary cellular activities, so it's up to these guys to make sure enough power is supplied
Endoplasmic Reticulum
Producers - these workers divide into two separate teams: the Smooth ER team's job is to make lipids, and the Rough ER team's job is to make proteins via ribosomes. These proteins eventually become neurotransmitters.
Golgi Apparatus
Packagers - these workers are responsible for packaging the Rough ER team's protein products. Neurotransmitters get put into vesicles and are distributed down the axon.
Synapses
Individual neurons don’t actually physically touch. Instead, their communication occurs through a small space between the end of one neuron sending a signal and another neuron receiving this information.
The neuron sending information (i.e., presynaptic neuron) goes through a process called an “action potential,” which sparks a release of little chemical messengers called “neurotransmitters” (pictured here as the moving ponzu pearls).
If all goes well, these messengers bind to specialized spots in the receiving neuron (i.e., postsynaptic neuron).
Click the play button!
You can think of this process like emails between two people. With the right email addresses, good internet connection, and appropriately configured firewall, important information can travel between two people.
But if one of these things is off, communication breaks down. When communication breaks down in the brain, humans can experience different physical signs or symptoms, ranging from depression to tremors, depending on where the miscommunication is happening.
Types of Neurons
We have an estimated 86.1 billion neurons in our brain, with ~8% of those in our prefrontal cortex. With that many neurons in such a small space, these cells must get on each other's nerves! 😬
Neurons are found outside of our brain too. Neurons in the brain and spinal cord make up our central nervous system, whereas neurons leading out to our limbs and organs make up our peripheral nervous system.
There are 4 kinds of neurons commonly taught in classes, but research is actually ongoing to determine what other types exist and their functions. Neurons in the spinal cord are more established and understood than those in the brain. The 4 better-understood neurons are defined by their shape and tend to vary in amount/functions.
types of neurons
Unipolar
These cells have one axon extending from the soma and are the most common cell in invertebrates (e.g., insects). In vertebrates, like humans, unipolar neurons are mostly substituted by:
Bipolar
These cells are called "bipolar" because they have two extensions: one axon and one dendrite. They are found in places like our retinas and our nose, so are important for sensory functions like vision and smell.
Pseudounipolar
These cells actually start out as bipolar neurons during development, but the axon and dendrite fuse into just one axon that extends from two sides of the soma. This soma is commonly found in regions of the spinal cord called "dorsal root ganglion." The axon extends between this soma in the CNS to limbs/organs in the PNS. These neurons are very important for sensory processing (e.g., touch, pain).
Mutlipolar
Most neurons in the CNS are multipolar. They have one axon and multiple dendrites extending from the soma. Because they have so many dendrites, these neurons are great for integrating a lot of information.
References
[1] Bear, Connors, & Paradisio (2015). In Neuroscience: Exploring the Brain (4th edition)
[2] Banich & Compton (2018). In Cognitive Neuroscience (4th edition)
[3] Hagan, C. E., Bolon, B., & Keene, C. D. (2012). Nervous system. In Comparative Anatomy and Histology (pp. 339-394).
[4] Hendry, S., & Hsiao, S. (2014). The somatosensory system. In Fundamental Neuroscience (Fourth Edition) (pp. 531-551).
[5] Patestas, M. A., & Gartner, L. P. (2016). A textbook of neuroanatomy. John Wiley & Sons.
[6] Azevedo, F. A., Carvalho, L. R., Grinberg, L. T., Farfel, J. M., Ferretti, R. E., Leite, R. E., ... & Herculano‐Houzel, S. (2009). Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled‐up primate brain. Journal of Comparative Neurology, 513(5), 532-541.
[7] Gabi, M., Neves, K., Masseron, C., Ribeiro, P. F., Ventura-Antunes, L., Torres, L., ... & Herculano-Houzel, S. (2016). No relative expansion of the number of prefrontal neurons in primate and human evolution. Proceedings of the National Academy of Sciences, 113(34), 9617-9622.
[8] Rolls, M. M., & Jegla, T. J. (2015). Neuronal polarity: an evolutionary perspective. Journal of Experimental Biology, 218(4), 572-580.
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