Introduction

The nervous system consists of neurons and neuroglial cells. Neurons are the excitable cells that can generate an action potential when stimulated by an environmental stimulus or other neurons. Neuroglial cells are responsible for providing support to the neurons.

Neurons consist of a cell body that gives rise to cellular processes. These cellular processes are the cytoplasmic projections of neurons that can conduct nerve impulses towards or away from the cell body. The two types of cellular processes are dendrites and axons.

Axons are the cytoplasmic projections of neurons that carry nerve impulses away from the cell body. Most of the neurons have only one axon. These nerve fibres are encapsulated by the myelin sheath that provides electrical insulation from the influences of extracellular fluid. All the spinal and cranial nerves are the bundles of axons with cell bodies in the CNS.

In this article, we will discuss the structure of axons, their embryologic development, axons in different types of neurons, and physiological processes associated with them.

Structure

Axons are the fine cylindrical processes that originate from the cell body of neurons. The length and diameter of axons vary among the different types of neurons. They are usually very long processes having a length in centimetres. For example, axons of motor neurons that innervate muscles in the foot have a length of about 100 cm. These axons travel in the Sciatic nerve, the largest nerve in the body.

The plasma membrane of axons is called axolemma. The cytoplasm present in the axons is called axoplasm. The important structural features of axons are described below.

Axoplasm

In most of the neurons, axoplasm contributes to around 99% of the total cytoplasm of the cell. It has different organelles and cytoskeletal structures as compared to the cytoplasm in the cell body or perikaryon. The axoplasm contains abundant mitochondria and cytoskeletal components like microtubules and microfilaments. It lacks ribosomes and thus cannot make proteins. Neurotransmitter vesicles that were formed in the cell body are also abundantly present in the axoplasm.

Axon Hillock

Axons originate from a pyramid-shaped region of the neuronal cell body known as axon hillock. The segment of the axon just beyond the axon hillock is called the initial segment of the axon. It is the part where all the excitatory and inhibitory signals are algebraically computed and a decision is made whether to propagate the nerve impulse or not. The plasma membrane of the initial segment contains various ion channels that are needed to generate an action potential.

Terminal Arborization and Collaterals

Axons do not undergo branching as seen in the case of dendrites. Their diameter remains constant throughout the length of the axon. However, the distal end of the axon shows terminal arborization. It branches into less thick nerve terminals that take part in synapse formation.

Axons of some motor neurons and interneurons have small branches called the collaterals. These collaterals form synapses with the surrounding neurons and influence their activity.

In both cases, each branch of an axon ends in a terminal dilation known as a terminal bouton. This dilation takes part in synapse formation with other neurons or non-nerve cells and transmits the nerve impulse.

Myelin Sheath

Myelin sheath is a layer of lipids surrounding axons. It is formed by the Schwann cells in the peripheral nervous system and the oligodendrocytes in the central nervous system. The entire length is not covered by the myelin sheath as some segments remain uncovered. These are called nodes of Ranvier.

Nerve impulse can occur only at these nodes. So, in the case of axons, nerve impulse jumps from one node to the other. These jumping impulses greatly increase the conduction speed of nerve fibres.   

Read more about Myelinated and Non-myelinated Axons

Embryologic Development

Neurons are derived from the neuroepithelial cells present in the neural tube. These cells first differentiate into apolar neuroblasts. Different types of neurons are derived from these neuroblast cells.

These cells move to their final location after differentiation from the neuroepithelium. These cells undergo neuronal polarization during the migration process. The cells usually have to travel a long distance before reaching their final destination.

As a result of neuronal polarization, the neurons form a leading process and a trailing process. These processes later become axon or dendrite. It has been found that the axon and dendrite polarity of neurons is based on the apical and basal polarity of their progenitor cells.

During this process, migrating neurons make multiple neurites. However, only one of these neurites becomes an axon. The choice of neurite is based on the polarity of neurons. It was been found that if an axon is cut before its complete development, the polarity can be reversed and another neurite can become an axon.

Certain intracellular and extracellular signalling mechanisms control the development and growth of axons.

Axonal Growth

Axonal growth is an important phenomenon during the development of the nervous system. Once the polarity is established, a cone type structure is formed at the end of the axon called a growth cone. The axon moves through the surrounding environment increasing its length via this growth cone.

Growth cones of axons contain a broad sheet-like structure having multiple protrusions called filopodia. The axons explore the extracellular environment and adhere to certain surfaces during the growth process via these protrusions.

Types of Neurons

This classification of neurons is based on the number of processes arsing from their cell body. The structure of axons in these types of neurons is discussed below.

• Multipolar Neurons: They are characterized by multiple dendrites and a single axon. The axon is long and is covered with the myelin sheath. Most of the neurons in our body belong to this category
• Bipolar Neurons: As evident from the name, these neurons have two cellular processes. One of these is axon while the other is a dendritic process. Axon is a long process with minimum or no branching at all. Bipolar neurons are present in the sensory epithelial tissues like the retina, olfactory epithelium, and in the inner ear. The axons of these neurons serve to carry the sensory signals to the neurons present in the brain.
• Unipolar Neuron: These cells have a single axon that divides into two processes soon after leaving the perikaryon. Both these processes are covered with the myelin sheath. The central process carries the nerve impulses away from the cell body and is regarded as a true axon. These neurons are also called pseudounipolar neurons.  
• Anaxonic Neurons: As the name indicates, these cells lack any axon. Because of this reason, these cells are unable to generate an action potential and transmit nerve impulses. They are responsible for regulating the neuronal activity of the surrounding neurons.

Myelination of Axons

Myelin sheath is an important component of axons. Recall that it provides electrical insulation to the neurons and serves to increase the speed of conduction. It is produced by the neuroglial cells in the CNS as well as the PNS.

Myelin sheath is consists of multiple layers of plasma membranes that are wrapped around each other. This is the reason why it is rich in phospholipids and proteins.

The myelination of axons involves the following steps.

• The axonal fiber is wrapped by the glial cell. In the case of the Schwann cells, one cell can make only one segment of myelin. On the other hand, one oligodendrocyte in the CNS can make myelin sheath on 40 to 50 different axons.
• Once an axon is incorporated into the glial cell, the cell starts rotating. It lays multiple layers of plasma membrane on the axon separated by the cytoplasm of the glial cells.
• In the next step, the cytoplasm between the membrane layers begins to condense.
• Eventually, multiple layers of plasma membranes are left, surrounding the axon in the form of a whorl. These membranes form the myelin sheath.

Recall that axons have myelin in the form of segments instead of having a continuous sheath. Multiple myelin segments can be made at the same time by the glial cells. It is especially true in the case of oligodendrocytes.

Physiological Processes

Two important physiologic processes associated with the axons are the generation and conduction of action potential and the release of neurotransmitters. A brief detail of these processes is mentioned below.

Generation and Conduction of Action Potential

Nerve impulse passes into the axon from the cell body of neurons. However, an action potential can also be initiated in the axon itself when it is stimulated by a synapse. In this case, the action potential is generated in an axon by the following process.

• A stimulus causes the opening of voltage-gated sodium channels. The sodium ions rush into the axon from the surrounding fluid.
• In the meantime, potassium channels are closed. The outflux of potassium ions from the axon is stopped.
• Both these processes result in the built-up of positive charges inside the axon.
• The axon becomes depolarized and an action potential is generated.

The action potential thus generated is carried to the axon terminal in a jumping pattern. The electric currents are set up inside the axon that travels towards the axon terminal causing depolarization. Depolarization can only occur at the nodes as they are devoid of the myelin sheath. So, nerve impulse jumps from node to node and is conducted in a jumping fashion.

Release of Neurotransmitters

Recall that the terminal branches of the axon synapse with the other neurons or non-nerve cells. They form the presynaptic terminal at these synapses. When a nerve impulse reaches the synapse, the depolarization causes the opening of voltage-gated calcium channels.

Calcium ions diffuse into the axon via these channels. These ions bind to certain release sites and open them. The neurotransmitter vesicles fuse with the axolemma at these release sites.

As a result, the neurotransmitters are dumped into the synaptic cleft. They diffuse through the cleft, bind to the receptors on the post0synaptic cell, and initiate an action potential in it.

Axonal Transport

The transport of molecules in the axons is important for their normal function as they cannot make proteins such as neurotransmitters. The transport of small and large molecules in the axon is always bidirectional.

• Organelles and macromolecules that are made in the cell body move in an anterograde direction towards the synaptic terminal of axons. Kinesin protein is responsible for the anterograde flow of the axoplasm.
• Substances that are taken by the axon from the periphery or the synapse are carried towards the cell body in a retrograde direction. The protein responsible for this type of transport is called dynein.

Summary

• Axons are the efferent nerve fibers that are responsible for carrying nerve impulses away from the cell body of neurons.
• Most of the neurons have a single large axon having constant diameter along its length. Axons from multiple neurons make the nerve bundles.
• Axoplasm contains abundant mitochondria and cytoskeletal components but lacks synthetic organelles like ribosomes. It cannot make proteins and other macromolecules and is dependent on the cell body for support.
• Axon hillock is followed by the initial segment of the axon where the inhibitory and excitatory signals are analyzed.
• Myelin sheath is an insulation cover that surrounds the axons and increases their conduction velocity.
• The development of axons begins as a result of the polarization of neurons during their migration to the final location. The polarization is dependent on the polarity of the progenitor cells. The axons grow in the surrounding environment via growth cones.
• Based on the number of axons, neurons are divided into different types such as multipolar, unipolar, bipolar, and anaxonic neurons.
• Myelination of neurons is carried out by the neuroglial cells in the nervous system.
• An action potential can be generated in the axon as a result of synaptic stimulation. Besides, nerve impulse can be passed to axon from the cell body of a neuron.
• The nerve impulse is conducted by axon in the form of depolarization waves that occur at the nodes of Ranvier.
• Molecules move inside the axon in two directions:
  • Anterograde direction i.e. from the cell body to the synaptic terminal
  • Retrograde direction i.e. from synaptic terminal to the cell body

Frequently Asked Questions

What are axons?

Axons are the cytoplasmic projections that originate from the axon hillock of the cell body and connect a neuron to other neurons or non-neuronal cells present in the periphery.

What is the function of axons?

Axons are efferent fibres that transmit nerve impulses away from the cell body of neurons. They carry information from a neuron to other neurons or various other cells present in the body such as muscle fibres.

Do axons regenerate?

Axons present in the central nervous system are not capable of regeneration. However, axons present in the peripheral nervous system can regenerate themselves. 

How long do axons take to heal?

Axons do not heal rapidly to restore neuronal functions. Rather it requires around 3 to 4 years for complete regeneration of axons.

References

1. Debanne D, Campanac E, Bialowas A, Carlier E, Alcaraz G (April 2011). “Axon physiology” (PDF). Physiological Reviews. 91 (2): 555–602. doi:10.1152/physrev.00048.2009. PMID 21527732. S2CID 13916255.
2. Nelson AD, Jenkins PM (2017). “Axonal Membranes and Their Domains: Assembly and Function of the Axon Initial Segment and Node of Ranvier”. Frontiers in Cellular Neuroscience. 11: 136. doi:10.3389/fncel.2017.00136. PMC 5422562. PMID 28536506.
3. Squire, Larry (2013). Fundamental neuroscience (4th ed.). Amsterdam: Elsevier/Academic Press. pp. 61–65. ISBN 978-0-12-385-870-2. Yau, K. W. (1976). “Receptive fields, geometry and conduction block of sensory neurones in the central nervous system of the leech”. The Journal of Physiology. 263 (3): 513–38. doi:10.1113/jphysiol.1976.sp011643. PMC 1307715. PMID 1018277.
4. Carlson, Neil R. (2013). Physiology of Behavior (11th ed.). Boston: Pearson. ISBN 978-0-205-23939-9.
5. Squire, Larry (2013). Fundamental neuroscience (4th ed.). Amsterdam: Elsevier/Academic Press. pp. 61–65. ISBN 978-0-12-385-870-2.