Axon

An axon (from Ancient Greek: ἄξων, romanized: áxōn, lit. 'axis'; also called a nerve fiber or fibre) is a long, thin part of a nerve cell or neuron found in most animals. Its main job is to carry electrical signals, called action potentials, away from the nerve cell body to other neurons, muscles, and glands. This helps the body send messages quickly and efficiently.

Axons are different from another part of the neuron called a dendrite. Dendrites receive signals, while axons send them out. Axons can be very long, much longer than the cell body itself, and they often branch out to connect with many other cells. In some neurons, there are special branches at the ends of axons called telodendria, and the swollen tips are known as axon terminals. These tips connect with other neurons to form important junctions called synapses, where signals pass from one cell to another.

Axons are covered by a special membrane called an axolemma, and the inside of the axon is filled with a substance called axoplasm. Most axons can branch many times, creating thousands of connections. In the human brain, bundles of axons form important pathways, like the corpus callosum, which helps different parts of the brain communicate.

Anatomy

Axons are long parts of nerve cells that help send messages throughout the body. They can be very tiny, like one millimeter, or as long as a meter in some animals. In humans, the longest axons connect the spinal cord to the toes. Axons can be thin, about one micrometer wide, or thick like a pencil lead in some sea animals.

There are two main types of axons: those with a fatty coating called myelin and those without. Myelin helps speed up the messages and is made by special supporting cells. The spaces between myelin sections are called nodes of Ranvier, where the message jumps from one node to the next, making the signal travel faster.

At the end of axons are tiny branches called telodendria, which connect to other cells to pass on the message. These endings store special chemicals called neurotransmitters that help cells communicate with each other.

Action potentials

Main article: Action potential

Further information: Neural coding and Active zone

Most axons send signals as action potentials. These are special waves of electricity that move quickly along the axon, starting from the cell body and ending where the axon connects with other cells. Every action potential looks the same size and shape, so the signal stays strong all the way along the axon.

When an action potential reaches the end of the axon, it helps send chemicals called neurotransmitters to the next cell. These chemicals travel across a tiny space and connect with the next cell, telling it what to do next. This whole process happens very fast, in less than a thousandth of a second!

Development and growth

The development of the axon to its target is one of the major stages in the development of the nervous system. Studies on neurons show that they first create several similar parts, but only one becomes the axon. If an unfinished axon is cut, the longest part can become the new axon.

Growing axons move using a special tip called the growth cone, which helps them explore their surroundings. These axons need sticky surfaces to grow on, provided by special molecules. Even if axons are damaged, they can regrow if the main cell part stays healthy.

Classification

Further information: Nerve conduction velocity

The axons of neurons in the human peripheral nervous system can be grouped based on their size and how fast they send signals. Scientists found that thicker axons send signals faster. In 1941, two researchers named Erlanger and Gasser studied this and created the first way to group axons.

Axons are sorted into two main systems. The first system, made by Erlanger and Gasser, uses letters A, B, and C. These groups include both the fibers that carry signals to the brain (afferents) and the fibers that carry signals from the brain (efferents). Group A is divided into alpha, beta, gamma, and delta fibers – Aα, Aβ, Aγ, and Aδ. The motor neurons for these fibers are called lower motor neurons, such as alpha motor neuron, beta motor neuron, and gamma motor neuron.

Later, other scientists found two groups of Aa fibers that carry signals from the body to the brain. They created a system called the Lloyd classification, which uses Roman numerals: Type Ia, Type Ib, Type II, Type III, and Type IV.

Motor

Lower motor neurons have two kinds of fibers:

Sensory

Different parts of the body that sense things are connected by different types of nerve fibers. For example, proprioceptors are connected by type Ia, Ib, and II fibers, mechanoreceptors by type II and III fibers, and nociceptors and thermoreceptors by type III and IV fibers.

Autonomic

The autonomic nervous system has two kinds of peripheral fibers:

Clinical significance

Main articles: Nerve injury, Peripheral neuropathy, and Demyelinating disease

When nerves in the body get hurt, it can cause problems. There are different levels of harm, from mild to very serious. One mild type of brain injury can affect the tiny parts of cells that carry messages.

When an axon gets crushed, a process happens where the far end of the axon breaks down. This is called degeneration, and it happens quickly after the injury.

Damage to the covering of axons can cause many symptoms seen in certain diseases. Problems with how this covering forms can also lead to other health issues.

Serious head injuries can damage many axons at once, which can cause serious long-term effects. Scientists study these injuries to understand how to help people recover better. Special tools can sometimes help guide the growth of axons to fix damaged nerves.

Terminology

Some dictionaries describe a "nerve fiber" as any part of a nerve cell, which includes both axons and dendrites. But in medical use, the term "nerve fiber" usually means just the axon.

History

Many important scientists helped us learn about axons. German anatomist Otto Friedrich Karl Deiters was the first to discover the axon and tell it apart from other parts of nerve cells. Swiss scientist Rüdolf Albert von Kölliker and German scientist Robert Remak were the first to study the beginning part of the axon. In 1896, Kölliker gave the axon its name. Louis-Antoine Ranvier was the first to describe small gaps on axons, now called the nodes of Ranvier. Spanish anatomist Santiago Ramón y Cajal explained that axons help neurons send information out.

Later, Joseph Erlanger and Herbert Gasser created a way to group nerve fibers based on how fast they send signals and other features, including myelination. Then, Alan Hodgkin and Andrew Huxley used a large axon from a squid to study how these signals work. By 1952, they fully explained how tiny parts inside cells create these signals, leading to the Hodgkin–Huxley model. They won the Nobel Prize in 1963 for this important work. Their ideas were later applied to animals, and scientists learned more about the tiny gates in cells that help send these signals, called ion channels.

Other animals

Scientists have studied the axons in invertebrates a lot. The longfin inshore squid is often used in experiments because it has the longest known axon. The giant squid has the biggest axon, which can be up to 1 mm wide and helps control its jet propulsion system. Some pelagic Penaeid shrimps have the fastest conduction speed ever recorded, reaching up to 210 meters per second.

Additional images

Recordings in the hippocampus from different cell types and axons