Our brains use a sophisticated communication and command-and-control system that monitors and manages interactions between roughly 100 billion neurons, each with 5,000-10,000 synaptic connections, for as many as 500 trillion synapses in adults. Neurons and associated glial cells like astrocytes monitor the nervous system, process information, and orchestrate adaptive responses in the service of homeostasis (Breedlove & Watson, 2023).
This post provides an executive summary of neuron structure and function, which is ideal for neurofeedback technicians and undergraduates. We will cover Neuron Functions, Neuron Structure, and Three Neuron Designs. For readers interested in the "fine print," we have added Appendices on Cell Body Organelles and Rough ER Proteins.
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The human brain operates through a highly sophisticated communication system involving approximately 100 billion neurons, each forming thousands of synaptic connections. These connections facilitate an immense network of interactions that enable complex cognitive functions, sensory processing, and motor control. Neurons, along with glial cells such as astrocytes, play a pivotal role in maintaining and regulating the nervous system, ensuring homeostasis through their intricate and adaptive responses. Understanding the structure and function of neurons is essential for neurofeedback technicians and students of neuroscience, providing a foundation for exploring how the brain processes information and adapts to its environment. This overview delves into the fundamental aspects of neuron functions, structures, and designs, offering a comprehensive guide to these vital components of the nervous system.
The Brain Research Through Advancing Innovative Neurotechnologies (BRAIN) Initiative
Silleti and colleagues (2023) identified over 3,000 distinct cell types in examining the brains of three human subjects. The lungs, in comparison, may contain 100 cell types. Another BRAIN initiative is studying the connectome using technologies like diffusion MRI tractography. Jim Stanis, Ryan Cabeen, and Arthur Toga created the video below.
Neuron Functions
We can divide neurons into sensory neurons, motor neurons, and interneurons. Sensory neurons are specialized for sensory intake from internal and external environments. They are called afferent because they transmit sensory information towards the central nervous system (brain, retina, and spinal cord). T
Motor neurons convey commands to glands, muscles, and other neurons. They are called efferent because they convey information towards the periphery. Motor neuron graphic © Designua/Shutterstock.com.
Interneurons provide the integration required for decisions, learning and memory, perception, planning, and movement. They are predominantly multipolar, have short processes, analyze incoming information, and distribute their analysis with other neurons in their network. Interneurons are entirely confined to the central nervous system, account for many of its neurons, and comprise most of the brain (Breedlove & Watson, 2023).
Local interneurons analyze small amounts of information provided by neighboring neurons. Relay interneurons connect networks of local interneurons from separate regions to enable diverse functions like perception, learning, and memory, and executive functions like planning (Carlson & Birkett, 2021).
Neuron Structure
Although neurons have over 200 different designs to perform specialized jobs in the nervous system, they generally have five structures: a cell body or soma, dendrites, an axon and axon hillock, and terminal buttons.
Cell Body
The cell body or soma contains the machinery for the neuron’s life processes. It comprises part of a neuron's input zone, receiving and integrating EPSPs and IPSPs, small graded positive and negative changes in membrane potential generated by axons and dendrites. Cell body graphic © Andrii Muzyka/Shutterstock.com.
The cell body of a typical neuron is 20 μm in diameter, and its spherical nucleus, which contains chromosomes comprised of DNA, is 5-10 μm across. Since this requires ribosomes, the cell body is the only location where neurons manufacture proteins (like enzymes, receptors, and ion channels) and peptides (neurotransmitters NT like oxytocin). Check out the Khan Academy YouTube video, Anatomy of a Neuron.
Dendrites
Dendrites are branched structures that extend from the neuron's cell body. They are the primary component of a neuron's input zone. Their extensive surface area maximizes the number of synapses and information a neuron can receive. Dendrites receive messages from other neurons via axodendritic synapses (junctions between axons and dendrites) and send messages to other neurons through dendrodendritic synapses (junctions between the dendrites of two neurons). Dendrites receive thousands of synaptic contacts and have specialized proteins called receptors for neurotransmitters (NTs) released into the synaptic cleft (Bear, Connors, & Paradiso, 2020). Once dendrites receive a signal, they convert it into an electrical impulse transmitted to the cell body.
The graphic below depicts dendrites with raised areas called dendritic spines. Dendrites construct and disassemble spines as part of neuroplasticity. Spines allow dendrites to pack more receptors into a limited space.
The brain may use spiny synapses to provide stability and silent synapses on filopodia for flexibility. Filopodia are thin, elongated, and highly dynamic protrusions extending from developing neurons' growth cones or cell bodies. Stimulation in adulthood may cause them to express glutamate (AMPA) receptors and remodel their membranes to resemble spines (Vardalaki et al., 2022).
Holly Barker (2022) writing for The Scientist, explained:
The ability of the brain to use different synapses 'solves the plasticity versus flexibility dilemma,' says Harnett. If all the brain’s synapses are flexible, then you can’t preserve old information. But if they’re all stable, then it is difficult to learn new things, he says. Instead, the brain employs both: spiny synapses for stability and filopodia for flexibility.
But instead of distinct categories, Harnett’s group are beginning to think about dendritic projections as existing on a continuum, from filopodia on one end to mature spines at the other. 'It is a spectrum of maturity, strength, and plasticity,' says study author Dimitra Vardalaki, a PhD candidate in Harnett’s lab.
Graded positive and negative changes in a neuron's apical dendrite produce the electroencephalogram (EEG). The scalp EEG is the voltage difference between two recording sites recorded over time. The EEG is primarily generated by large pyramidal neurons in layers 3 and 5 of the 2-4.5-mm-thick cortical gray matter. The movie below is a BioTrace+/NeXus-32 display of the raw EEG with voltage shown as μV peak to peak © John S. Anderson.
The image of a pyramidal neuron was created using Golgi silver chrome © Jose Luis Calvo/Shutterstock.com. Note that the apical dendrite arising from the top of the cell body and basilar dendrites feature an extensive network of spines.
Axon
An axon is a cylindrical structure only found in neurons specialized for the distribution of information within the central and peripheral nervous systems. Axons range from 1 to 25 µm in diameter and 0.1 mm to more than a meter in length. Some human neurons lack an axon entirely (Purves, 2018).
Over 90% of neurons are interneurons whose axons and dendrites are very short and do not extend beyond their cell cluster. Axons usually branch repeatedly. Each branch is called an axon collateral. Axon graphic © Designua/ Shutterstock.com.
An axon hillock is a cell body swelling where the axon begins. Neurons integrate EPSPs and IPSPs at the axon hillock and initiate all-or-none action potentials when its membrane becomes ~ 15 mV less negative than at rest. Graphic by M.alijar3i from the Wikipedia article Axon Hillock.
Axon Terminal
The axon branches out at its end to form axon terminals. Axon terminals contain vesicles that store NTs for release when an action potential arrives. Their presynaptic membrane may have reuptake transporters that return NTs from the synapse or extracellular space for repackaging. They make synaptic contacts with other neurons or effector cells. When an action potential reaches the axon terminals, it triggers the release of NTs stored in synaptic vesicles. The NTs cross the synaptic cleft and bind to receptors on the post-synaptic cell, transmitting the signal.
Three Neuron Designs
Despite differences in their structure, all neurons are characterized by input, integration, conduction, and output zones. Neurons can be categorized into three principal types: multipolar, bipolar, and unipolar.
Multipolar neurons possess multiple dendrites and a solitary axon. Examples of multipolar neurons include motor neurons and interneurons.
Bipolar neurons have one dendrite and one axon. They are typically involved in sensory systems like sight, smell, or hearing. For instance, bipolar cells in the retina receive input from photoreceptor cells and transmit this information to the ganglion cells, which then carry the visual signal to the brain.
Unipolar neurons display a unique formation where a single extension emerges from the cell body and bifurcates into two directions. In unipolar neurons, the integration zone is uniquely situated at the base of the dendritic branches, not within the cell body as is typically seen. Unipolar neurons are typically found in the peripheral nervous system, transmitting touch and pain signals from the body to the central nervous system.
Appendix A: Cell Body Organelles
The cell body contains the nucleus, endoplasmic reticulum (rough and smooth), Golgi apparatus, mitochondria, lysosomes, peroxisomes, neurofilaments, and microtubules.
Nucleus
The nucleus is a neuron's control center. It contains the neuron's genetic material (DNA), which dictates the cell's function and structure. The nucleus regulates gene expression and controls protein synthesis.
Endoplasmic Reticulum
The endoplasmic reticulum is a network of tubules, including the rough and smooth ER. The rough ER is studded with ribosomes, which are the sites of protein synthesis. Nissl bodies are rough ER with rosettes of free ribosomes and are the sites of protein synthesis. ER graphic © TimeLineArtist/Shutterstock.com.
The rough ER's protein synthesis function is critical for various processes within neurons, from intracellular signaling and structural maintenance to inter-neuronal communication.
The smooth ER synthesizes lipids, detoxifies molecules, and traffics proteins. The smooth ER synthesizes phospholipids and cholesterol, essential cellular and organelle membrane components. Smooth ER enzymes detoxify endogenous and exogenous substances, such as drugs and toxins, by making them more soluble and easier to excrete from the body.
Golgi Apparatus
The Golgi apparatus is responsible for packaging proteins and lipids into vesicles for transport to their respective destinations within the cell or for export out of the cell. The Golgi apparatus is also crucial in forming synaptic vesicles containing neurotransmitters in neurons. Golgi apparatus graphic © Gunita Reine/Dreamstime.com.
Local Protein Synthesis
Neurons, with their intricate compartmentalized structure, rely heavily on localized protein synthesis to sustain and adapt their complex functions. The presence of the necessary protein synthesis machinery, including ribosomes and mRNAs, within dendrites and axons underscores the capability of neurons to produce proteins locally. This local production is crucial for maintaining synaptic function and facilitating plasticity, which is essential for processes such as learning and memory (Biever et al., 2019; Dastidar & Nair, 2022; Holt et al., 2019; Kim & Jung, 2015).
Advanced imaging techniques have provided direct visualization of protein synthesis within neuronal processes. For example, studies utilizing protein synthesis reporters have demonstrated that growth factors like BDNF can trigger local protein synthesis in dendrites. Additionally, ribosome profiling has revealed active translation occurring in both dendrites and axons, with monosomes playing a significant role in this process. These findings highlight the dynamic nature of local protein synthesis within neurons (Aakalu et al., 2001).
The functional importance of local translation is evident in its role in synaptic plasticity (Dastidar & Nair, 2002; Steward & Schuman, 2003). This mechanism allows neurons to quickly respond to synaptic activity and environmental cues by producing proteins on-site, which in turn modifies synaptic strength and structure. Such rapid and localized response is pivotal for the adaptability and functionality of neuronal networks. Local protein production graphic from Dastidar and Nair (2022) in Frontiers in Molecular Neuroscience.
Furthermore, the translation of specific mRNAs in axons and dendrites is tightly regulated by extracellular signals such as guidance cues, growth factors, and synaptic activity. This regulation ensures that proteins are synthesized in response to specific stimuli, supporting processes like axon guidance and synaptic modification. The ability to synthesize and degrade proteins in a compartmentalized manner allows neurons to precisely control the local proteome, which is vital for maintaining synaptic function and plasticity (Biever et al., 2019).
The spatial and temporal dynamics of local protein synthesis are finely tuned, with neurons employing various strategies to ensure that proteins are synthesized at the appropriate time and location. For instance, translational "hot spots" near synapses are areas where protein synthesis is consistently active, indicating the presence of spatially regulated protein production (Aakalu et al., 2001).
Recent studies have also highlighted the significant role of monosomes in local protein synthesis (Biever et al., 2019). Unlike polysomes, which are typically associated with bulk protein synthesis, monosomes are often responsible for translating synaptic mRNAs. This adaptation may be due to the limited space within synaptic compartments, necessitating a more compact and efficient translation machinery.
In conclusion, the evidence robustly supports the concept that neurons manufacture proteins locally. This local synthesis is essential for the dynamic regulation of the neuronal proteome, enabling neurons to adapt to environmental changes and maintain their complex functions.
Mitochondria
Known as the cell's "powerhouses," mitochondria produce adenosine triphosphate (ATP) through cellular respiration. Most of a neuron's ATP is used to power its sodium-potassium transporters to restore resting membrane potentials. The mitochondrion graphic © Shadow_cluster/ Dreamstime.com.
Lysosomes
Lysosomes are the cell's waste disposal system. They break down waste materials and cellular debris into simple compounds, which are transferred back into the cytoplasm as new cell-building materials. In lysosomal storage diseases like Tay-Sachs, lysosomes cannot break down and eliminate wastes. Lysosome graphic © Designua/Shutterstock.com.
Peroxisomes
Peroxisomes are involved in lipid metabolism and the breakdown of reactive oxygen species, thereby protecting the cell from oxidative damage. Peroxisome graphic © snapgalleria/iStock by Getty Images.
Neurofilaments
Neurofilaments provide structural support and shape to the neuron. Chemical synapse graphic © rob9000/Shutterstock.com.
Microtubules are involved in transport within the neuron. Motor proteins "walk" bidirectionally along the axon's length, carrying transport vesicles filled with crucial molecules.
This transport system is essential in neurons due to their elongated shape. The microtubule graphic © Kateryna Kon/Dreamstime.com.
Appendix B: Rough ER Proteins
The rough ER synthesizes membrane, secreted, vesicular, organelle, cytoskeletal, enzyme, and chaperone proteins. Membrane proteins are embedded in the cell membrane and play a variety of roles, including serving as ion channels, transporters, and receptors. Secreted proteins are synthesized, processed, and then packaged into vesicles as neurotransmitters (NTs) for transport out of the cell. Vesicular proteins are integral parts of the vesicles that store and release neurotransmitters. These proteins are involved in the trafficking, docking, and fusion of vesicles with the cell membrane and neurotransmitter uptake into the vesicles. Organelle proteins contribute to the structure and function of various organelles within the neuron. For instance, proteins synthesized by the rough ER may become integral parts of the Golgi apparatus, mitochondria, or lysosomes, each having specific functions. Cytoskeletal proteins are essential components of the neuron's cytoskeleton, which maintains the cell's shape, anchors organelles, and is involved in intracellular transport and cell division. The rough ER also synthesizes various enzymes involved in multiple biochemical pathways within the neuron. For example, some enzymes ensure that proteins are correctly folded and functional.
Chaperone proteins help to fold and assemble newly synthesized polypeptides and proteins in the ER lumen. They also play a crucial role in breaking down misfolded proteins.
Conclusion
Neurons are the fundamental units of the nervous system, equipped with specialized structures and functions that enable them to process and transmit information efficiently. The intricate architecture of neurons, including the cell body, dendrites, axon, and synaptic terminals, supports their role in communication within the brain and throughout the body. The differentiation into sensory neurons, motor neurons, and interneurons highlights the diversity of functions that neurons perform, from sensory perception to motor coordination and cognitive processing. The local synthesis of proteins within neurons further underscores their ability to adapt rapidly to environmental changes and maintain synaptic plasticity, which is crucial for learning and memory. By understanding these basic yet complex aspects of neuron biology, we gain deeper insights into the workings of the brain and the potential for therapeutic interventions in neurological disorders.
Quiz
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Glossary
action potentials: brief, rapid, and large changes in membrane potential during which the potential reverses.
afferent: neurons or pathways that carry information from sensory receptors to the central nervous system.
axodendritic synapses: synapses between one neuron's axon and another's dendrite.
axon collateral: a branch off the main axon of a neuron. Collaterals can propagate action potentials and help transmit signals to multiple targets, enhancing communication within the neural network.
axon hillock: a specialized region of the cell body (or soma) of a neuron that connects to the axon; it is the last site in the soma where membrane potentials propagated from synaptic inputs are summated before being transmitted to the axon.
axon terminal: the end of an axon or axon collateral, which forms a synapse with a neuron or an effector cell.
BDNF (Brain-Derived Neurotrophic Factor): a protein that belongs to the neurotrophin family of growth factors, which are essential for the development, maintenance, and function of neurons in the central and peripheral nervous systems. BDNF plays a crucial role in promoting the survival of existing neurons, encouraging the growth and differentiation of new neurons and synapses, and supporting synaptic plasticity, which is vital for learning and memory.
bipolar neurons: neurons that have one axon and one dendrite.
cell body: the part of a neuron containing the nucleus, most of the cytoplasm, and the organelles.
chaperone proteins: proteins that assist in the folding of other proteins.
central nervous system: the brain, retina, and spinal cord.
conduction zone: the region (axon) where information can be transmitted over long distances.
cytoskeletal proteins: proteins that contribute to a cell's shape, support, and movement capabilities.
dendrite: A branch-like projection of neurons that carries information toward the cell body.
dendritic spines: small protrusions from a dendrite that typically receive input from a single synapse of an axon.
dendrodendritic synapses: synapses between dendrites of different neurons.
efferent: neurons or pathways that carry information away from the central nervous system towards the periphery.
electroencephalogram (EEG): a noninvasive test used to record electrical patterns in the brain.
endoplasmic reticulum (ER): a network of tubular membranes in the cell that works in the synthesis and transport of proteins (rough ER) and lipids (smooth ER).
enzyme: a type of protein that acts as a catalyst in biological systems to accelerate specific chemical reactions without being consumed. Enzymes are essential in metabolism, DNA replication, protein synthesis, and signal transduction.
filopodia: thin, elongated, and highly dynamic protrusions extending from the growth cone or the cell body of developing neurons. They serve a crucial function during neural development as they explore the surrounding environment, guide neuronal growth and pathfinding, and initiate the formation of synapses. Filopodia can form dendritic spines, the primary sites for excitatory synapses in the brain.
Golgi apparatus is an organelle that modifies, sorts, and packages proteins and lipids to transport to targeted destinations.
input zone: the region comprised of dendrites and the cell body where neurons collect information from other cells.
integration zone: the site where a neuron "decides" to initiate an action potential. This is the axon hillock for multipolar and bipolar neurons.
interneurons: CNS neurons that process information locally and relay information between other neurons.
local interneurons: interneurons that form short axon connections within the same region to analyze small pieces of information.
lysosomes: organelles that contain digestive enzymes to break down waste materials and cellular debris.
membrane proteins: proteins embedded within, or associated with the cell membrane, serving various functions such as transport, signal transduction, and cell recognition.
messenger RNA (mRNA): a type of RNA that carries genetic information from DNA to the ribosome, where it serves as a template for protein synthesis. The sequence of nucleotides in mRNA is transcribed from a gene and dictates the order in which amino acids are added to a growing polypeptide chain, effectively determining the primary structure of the resulting protein.
microtubules: tiny, hollow cylinders made up of protein that provides structural support and are involved in cell division and transportation of substances within the cell.
mitochondria: organelles that are the sites of aerobic respiration within the cell and are responsible for producing most of the cell's supply of adenosine triphosphate (ATP), used as a source of chemical energy.
monosomes: single ribosomes that are engaged in translating mRNA into protein. Unlike polysomes (polyribosomes), which are clusters of multiple ribosomes translating a single mRNA simultaneously, monosomes function independently. Monosome translation is particularly significant in confined cellular spaces, such as synaptic compartments in neurons, where efficient use of limited space is crucial for local protein synthesis.
motor neurons: neurons that carry information from the central nervous system to muscles and glands to trigger action.
multipolar neurons: neurons that have one axon and two or more dendrites.
neurofilaments: intermediate filament proteins that provide structural support for neurons and their synapses.
neurotransmitter (NT): chemical messengers that neurons use to communicate.
nucleus: the central and most important part of a cell, containing genetic material.
organelle proteins: proteins that contribute to the structure and function of cell organelles.
output zone: the region where a neuron transfers information to other cells (e.g., axon terminal).
peripheral nervous system (PNS): the nervous system outside the brain and spinal cord. It consists of all the nerves that carry signals between the central nervous system (CNS) and the rest of the body. It's divided into the sensory (afferent) division, which transmits signals from the body to the CNS, and the motor (efferent) division, which sends signals from the CNS to muscles, glands, and other tissues.
peroxisomes: small organelles that contain enzymes involved in oxidation reactions.
proteome: the proteome refers to the entire set of proteins expressed by a cell, tissue, or organism at a specific time under defined conditions. It encompasses not only the full complement of proteins but also their modifications, interactions, and functional states. The proteome is dynamic and can change in response to various physiological and pathological stimuli, reflecting the cellular environment and function at any given moment. Understanding the proteome is crucial for comprehending cellular processes, disease mechanisms, and the development of targeted medical therapies.
pyramidal neuron: a type of neuron in the cerebral cortex and hippocampus, named for its pyramid-like cell body.
relay interneurons: interneurons that form long axon connections that allow communication between neurons in different brain regions.
rough endoplasmic reticulum (rough ER): a part of the endoplasmic reticulum studded with ribosomes and involved in protein synthesis.
rough endoplasmic reticulum enzymes: enzymes located in the rough ER that assist in protein synthesis and modification.
secreted proteins: proteins released from a cell into the extracellular space, often to send signals to other cells.
sensory neurons: neurons that carry information from sensory receptors to the central nervous system.
silent synapses: synapses on filopodia missing glutamate (AMPA) receptors, which contribute flexibility.
smooth endoplasmic reticulum (smooth ER): a part of the endoplasmic reticulum that lacks ribosomes and is involved in lipid synthesis and detoxification.
spiny synapses: synapses on dendritic spines, which contribute stability.
synaptic vesicles: small sacs in the axon terminal that store NTs.
Tay-Sachs disease: a rare, inherited neurodegenerative disorder characterized by a deficiency of an enzyme called beta-hexosaminidase A. This deficiency results in the accumulation of GM2 gangliosides, a type of lipid, in neurons, leading to progressive damage and death of these cells.
unipolar neurons: neurons with one process extending from the cell body that branches into two, acting as the dendrite and axon.
vesicular proteins: proteins that are components of, or associated with, the vesicles that transport various molecules within or outside the cell.
References
Aakalu, G., Smith, W., Nguyen, N., Jiang, C., & Schuman, E. (2001). Dynamic visualization of local protein synthesis in hippocampal neurons. Neuron, 30, 489-502. https://doi.org/10.1016/S0896-6273(01)00295-1 Barker, H. (2022). Silent synapses may provide plasticity in adulthood. The Scientist.com. Bear, M. F., Connors, B. W., & Paradiso, M. A. (2020). Neuroscience: Exploring the brain (4th ed.). Jones & Bartlett Learning. Biever, A., Donlin-Asp, P., & Schuman, E. (2019). Local translation in neuronal processes. Current Opinion in Neurobiology, 57, 141-148. https://doi.org/10.1016/j.conb.2019.02.008 Breedlove, S. M., & Watson, N. V. (2023). Behavioral neuroscience (10th ed.). Sinauer Associates, Inc. Carlson, N. R., & Birkett, M. A. (2021). Physiology of behavior (13th ed.). Pearson.
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Steward, O., & Schuman, E. (2003). Compartmentalized synthesis and degradation of proteins in neurons. Neuron, 40, 347-359. https://doi.org/10.1016/S0896-6273(03)00635-4 Vardalaki, D., Chung, K., & Harnett, M. T. (2022). Filopodia are a structural substrate for silent synapses in adult neocortex. Nature, 612(7939), 323–327. https://doi.org/10.1038/s41586-022-05483-6
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