"

1 The Neuron

Casey Henley

Resources

Neurons are the basic units of the brain. Their main function is to send electrical signals over short and long distances in the body, and they are electrically and chemically excitable. The function of the neuron is dependent on the structure of the neuron. The typical neuron consists of the dendrites, cell body, axon (including the axon hillock), and presynaptic terminal.

A diagram of a typical neuron, showing its dendrites, cell body, axon hillock, axon, and presynaptic terminal. Link to detailed alternative text in caption.
Figure 1.1. Complete neuron structure. A typical neuron consists of dendrites that receive inputs, a cell body containing the nucleus, an axon hillock where action potentials initiate, a myelinated axon that conducts signals, and presynaptic terminals that release neurotransmitters to communicate with other cells. ‘Neuron’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

The structure of the neuron affects how it functions

Dendrites

Dendrites, shown here in green, are processes that branch out in a tree-like fashion from the cell body. They are the main target for incoming signals received from other cells. The number of inputs a neuron receives depends on the complexity of the dendritic branching. Dendrites may also have small protrusions along the branches known as spines. Spines, illustrated in the inset box, are the sites of some synaptic contacts. Spines increase the surface area of the dendritic arbor, which may be an important factor in receiving communication.

A diagram of a neuron with its dendrites highlighted in green. An inset box magnifies a section of a dendrite, showing small, mushroom-shaped protrusions called "Dendritic Spines" extending from its surface.
Figure 1.2. Dendritic structure and spines. Dendrites branch from the cell body in a tree-like pattern to receive synaptic inputs. Dendritic spines are small protrusions that increase surface area and serve as sites of synaptic contact with other neurons. ‘Dendrites’ by Casey Henley (CC-BY-NC-SA).

Cell Body

The cell body, shown here in green and also known as the soma, contains the nucleus and cellular organelles, including endoplasmic reticulum, Golgi apparatus, mitochondria, ribosomes, and secretory vesicles. The nucleus houses the DNA of the cell, which is the template for all proteins synthesized in the cell. The organelles, illustrated in the inset box, in the soma are responsible for cellular mechanisms like protein synthesis, packaging of molecules, and cellular respiration.

A diagram of a typical neuron, showing its dendrites, cell body, axon hillock, axon, and presynaptic terminal. Link to detailed alternative text in caption.
Figure 1.3. Neuronal cell body organelles. The soma contains the nucleus and organelles essential for cellular function, including mitochondria for energy production, endoplasmic reticulum for protein synthesis, and Golgi apparatus for protein processing and packaging. ‘Neuron Cell Body’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Axon

The axon, highlighted in green, is usually a long, single process that begins at the axon hillock and extends out from the cell body. The axon hillock is located where the cell body transitions into the axon. Axons can branch in order to communicate with more than one target cell.

the basic structure of two connected neurons, showing the cell body, dendrites, myelinated axon with Nodes of Ranvier, and synaptic connections between them. Link to detailed alternative text in caption.
Figure 1.4. Axon structure and axon hillock. The axon is a single projection that begins at the axon hillock, the region where the cell body transitions into the axon. The axon hillock is the typical site of action potential initiation. ‘Axon’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Action Potential

The axon transmits an electrical signal, called an action potential, from the axon hillock to the presynaptic terminal where the electrical signal will result in a release of chemical neurotransmitters to communicate with the next cell. The action potential is a very brief change in the electrical potential, which is the difference in charge between the inside and outside of the cell. During the action potential, the electrical potential across the membrane moves from a negative value to a positive value and back.

 


Animation 1.1. The action potential is a rapid change in membrane potential, moving from a resting value of –65 mV to positive and then back to rest. This electrical signal begins at the axon hillock, travels down the axon, and triggers neurotransmitter release at the presynaptic terminal. ‘Action Potential Propagation’ by Casey L. Henley (CC-BY-NC-SA). View static image of animation. View alternative text.

Myelin

Many axons are also covered by a myelin sheath, a fatty substance that wraps around portions of the axon and increases action potential speed. There are breaks between the myelin segments called Nodes of Ranvier, and this uncovered region of the membrane regenerates the action potential as it propagates down the axon in a process called saltatory conduction. There is a high concentration of voltage-gated ion channels, which are necessary for the action potential to occur, in the Nodes of Ranvier.

Illustration of saltatory conduction in myelinated neurons, showing how action potentials jump between Nodes of Ranvier along the axon from one neuron to another. Link to detailed alternative text in caption.
Figure 1.5. Myelin wraps around and insulates the axon. The spaces between the myelin sheath, where the axon is uncovered, are call the Nodes of Ranvier. ‘Myelin’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Axon Length

The length of an axon is variable depending on the location of the neuron and its function. The axon of a sensory neuron in your big toe needs to travel from your foot up to your spinal cord, whereas an interneuron in your spinal cord may only be a few hundred micrometers in length.

Anatomical diagram illustrating the variation in axon lengths, comparing short interneuron axons within the spinal cord to long sensory and motor neuron axons that extend from the spinal cord to peripheral body parts like the foot. Link to detailed alternative text in caption.
Figure 1.6. Axon length varies dramatically depending on neuron function and location. Short interneurons within the spinal cord may have axons only hundreds of micrometers long, while sensory or motor neurons connecting the spinal cord to peripheral targets like the foot require axons that can be over a meter in length. ‘Axon Length’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Axon Diameter

Axon diameter is also variable and can be used to differentiate different types of neurons. The diameter affects the speed at which the action potential will propagate. The larger the diameter, the faster the signal can travel. Additionally, larger diameter axons tend to have thicker myelin.

An illustration that demonstrates how axon diameter and myelin thickness affect action potential propagation speed, showing that larger, more heavily myelinated axons conduct signals faster than smaller, unmyelinated ones. Link to detailed alternative text in caption.
Figure 1.7. Axon diameter and myelination affect conduction speed. Large diameter axons with thick myelin sheaths (left) conduct action potentials fastest, while small diameter unmyelinated axons (right) have the slowest conduction speeds. The amount of myelination typically correlates with axon diameter. ‘Axon Diameter’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Presynaptic Terminal

The axon terminates at the presynaptic terminal or terminal bouton. The terminal of the presynaptic cell forms a synapse with another neuron or cell, known as the postsynaptic cell. When the action potential reaches the presynaptic terminal, the neuron releases neurotransmitters into the synapse. The neurotransmitters act on the postsynaptic cell. Therefore, neuronal communication requires both an electrical signal (the action potential) and a chemical signal (the neurotransmitter). Most commonly, presynaptic terminals contact dendrites, but terminals can also communicate with cell bodies or even axons. Neurons can also synapse on non-neuronal cells such as muscle cells or glands.

An illustration that demonstrates synaptic communication between a presynaptic and postsynaptic neuron, with the synaptic connections highlighted in green to show where neurotransmitter transmission occurs. Link to detailed alternative text in caption.
Figure 1.8. Synaptic contact formation. The presynaptic terminal forms synaptic contacts (green) with the postsynaptic cell, enabling neuronal communication through neurotransmitter release. ‘Presynaptic Terminal’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

The terms presynaptic and postsynaptic are in reference to which neuron is releasing neurotransmitters and which is receiving them. Presynaptic cells release neurotransmitters into the synapse and those neurotransmitters act on the postsynaptic cell.

A diagram showing synaptic communication between two neurons, highlighting how the presynaptic cell releases neurotransmitters that act upon the postsynaptic cell to transmit information. Link to detailed alternative text in caption.
Figure 1.9. Synaptic communication between neurons. The presynaptic cell (left) releases neurotransmitters from its terminal (green) that act upon the postsynaptic cell (right). The terms presynaptic and postsynaptic refer to which neuron is releasing versus receiving the chemical signal. ‘Postsynaptic Cell’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Variations in Structure

Although these typical structural components can be seen in all neurons, the overall structure can vary drastically depending on the location and function of the neuron. Some neurons, called unipolar, have only one branch from the cell body, and the dendrites and axon terminals project from it. Others, called bipolar, have one axonal branch and one dendritic branch. Multipolar neurons can have many processes branching from the cell body. Additionally, each of the projections can take many forms, with different branching characteristics. The common features of cell body, dendrites, and axon, though, are common among all neurons.

A comparison of different neuron types showing structural diversity including multipolar, bipolar, and unipolar neurons with varying dendritic branching patterns while maintaining common components of dendrites, cell body, and axon. Link to detailed alternative text in caption.
Figure 1.10. Structural diversity of neurons. Although neurons vary dramatically in their overall structure depending on location and function, all share the common components of dendrites (brown), cell body (black), and axon (blue). Examples shown include multipolar neurons with extensive branching, bipolar neurons with two main processes, and neurons with varying degrees of dendritic complexity. ‘Neuron Types‘ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Conclusion

The neuron’s structure is essential to its function, from receiving inputs to transmitting signals. Understanding these basics lays the groundwork for exploring the complex networks that drive behavior and cognition.

Key Takeaways

  • Neurons are the fundamental units of the brain, designed to transmit information via electrical and chemical signals
  • The main structures of a neuron—dendrites, soma, axon, and presynaptic terminal—each play a critical role in communication
  • Overall structure of the cell can vary depending on location and function of the neuron

Test Yourself!

Try the quiz more than once to get different questions!

  1. From memory, draw a neuron and identify the following structures: dendrites, soma, axon hillock, axon, myelin, nodes of Ranvier, presynaptic terminal.
  2. Describe functions of each neuronal structure depicted in your illustration.
  3. Predict what would happen to neuron function if myelin was destroyed

Video Lecture

 

License

Icon for the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License

The Neuron Copyright © 2021 by Casey Henley is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

Share This Book