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. It is found in most animals. Its main job is to carry electrical signals, called action potentials, away from the nerve cell body. These signals go to other neurons, muscles, and glands. This helps the body send messages quickly.

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 and often branch out to connect with many other cells. In some neurons, there are special branches at the ends of axons called telodendria. The swollen tips are known as axon terminals. These tips connect with other neurons to form important junctions called synapses.

Axons are covered by a special membrane called an axolemma. The inside of the axon is filled with a substance called axoplasm. In the human brain, bundles of axons form important pathways, like the corpus callosum. These pathways help 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 small, 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 talk to 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, so the signal stays strong all the way.

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.

Development and growth

The axon grows to reach its target during the development of the nervous system. Neurons 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. 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 by their size and how fast they send signals. Thicker axons send signals faster. In 1941, researchers Erlanger and Gasser studied this and made 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 fibers that carry signals to the brain (afferents) and 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, scientists found two groups of Aa fibers that carry signals from the body to the brain. They made 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. Sometimes, small parts of cells that carry messages can be affected.

If an axon gets crushed, the far end of it can break down. This is called degeneration, and it happens quickly after the injury.

Damage to the covering of axons can cause symptoms in some diseases. Problems with this covering can also lead to other health issues.

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

Terminology

Some dictionaries call a "nerve fiber" any part of a nerve cell, which includes both axons and dendrites. But in medical use, "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. Swiss scientist Rüdolf Albert von Kölliker and German scientist Robert Remak studied the beginning of the axon. In 1896, Kölliker gave the axon its name. Louis-Antoine Ranvier described small gaps on axons, called the nodes of Ranvier. Spanish anatomist Santiago Ramón y Cajal explained that axons help neurons send information.

Later, Joseph Erlanger and Herbert Gasser found a way to group nerve fibers. Then, Alan Hodgkin and Andrew Huxley studied signals in a squid's axon. By 1952, they explained how signals work inside cells, leading to the Hodgkin–Huxley model. They won the Nobel Prize in 1963. Their work helped scientists learn more about ion channels.

Other animals

Scientists study axons in invertebrates. 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 its jet propulsion system. Some pelagic Penaeid shrimps have very fast conduction speeds.

Additional images

Recordings in the hippocampus from different cell types and axons