Axon | Definition, Structure, Types, Functions


Axons typically carry signals away from the cell body. An axon, or nerve fiber, is a long slender projection of a nerve cell, or neuron, that conducts electrical impulses away form the neuron’s cell body or soma. Axons are in effect the primary transmission lines of the nervous system, and as bundles they help make up nerves.

Structure of Axon
Structure of Axon

Structure of Axon

Individual axons are microscopic in diameter – typically abut one micrometer across – but may extend to macroscopic lengths. The longest axons in the human body, for example, are those of the siatic nerve, which run from the base of the spine to the big toe of each foot. These single cell fibre may extend a meter or even longer. In firaffes have single axons running along the whole length of their necks, several meters in length. Mostly the neurons have only one axon, but this axon may and usually will undergo extensive branching, enabling, communication with man target cells. The part of the axon where it emerges from the soma is called the ‘axon hillock’.

The axon terminal a specialized structure at the end of the axon that is used to release neurotransmitter and communicate with target neurons. Each axon is filled with the cytoplasm called axoplasm called axoplasm called axoplasm i.e. continuous with the cytoplasm of cell body. The axon includes numerous mitochondria, rough endoplasmic reticulum and neurofibrils but lacks Nissl’s granules, Golgi apparatus and nucleus. The covering of axon is called axolemma.

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Functions Of Axon

Axons are important in the transmission of information between neurons via the cable system. They produce side branches known as axon collaterals in order to be able to convey messages to multiple neurons at the same time.

Smaller extensions known as axon terminal branches, or nerve terminals, are formed when these branches split into smaller extensions. Each terminal contains a synapse, which is where neurotransmitters send their messages and where neurotransmitters receive their messages.

Simply defined, axons enable nerve cells to communicate with one another by transmitting electrical and chemical signals to other neuron, gland, and muscle cells through an internal communication process.

Types of Axon

Bundles of axons and dendrites are called nerves. they are sensory if they consist of dendrites only, motor if they consist of axons only and mixed if they consist of both. There are two types of Axons on the basis of presence and absense of the myelin sheath.

1. Nivelinated or Medullated / Myelinated

In vertebrates, the axons of many neurons are sheathed in myelin. which is formed by many satellite cells called Schwann cells. Schwann cells secrets a fatty insulating layer called myelin sheath all around the axon. Along myelinated nerve fibers, gaps in the sheath known as nodes of Ranvier which occurs at regular intervals, ft enabling the nerve fiber to conduct the electrical impulse at far greater speed. Actually, the impulse jumps from one node to another.

Myelinated axons are a type of neuron or nerve cell that is surrounded by a fatty layer known as the myelin sheath. The myelin sheath, like the rubber coating of an electrical cord, insulates and shields the axon of the nerve cell while also conducting the electrical impulse along the nerve. Nerves are made up of a network of neurons, which are specialized cells that transfer electrical signals from the brain and spinal cord to the muscles or from the body’s various tissues back to the brain and spinal cord. Myelinated axons carry these impulses from nerve cell to nerve cell through this channel, ensuring that the message arrives to its destination as rapidly as possible.

2. Non-Myelinated or Non-Medullated

The axon or the nerve fiber lacks of myelin sheath. The Schwann cells are present around the axon but without nodes of Ranvier. The cells do not secrete the myelin sheath. Almost all the invertebrates have non-myelinated nerve fiber. But in vertebrates these nerves are only present in autonomic nervous system.

Schwann cells and oligodendrocytes can also interact with axons, but they do not form myelin sheaths around them. In this electron micrograph, we see a Schwann cell connected to multiple tiny axons, but the axons are not myelinated. Unlike in myelinated nerves, in unmyelinated nerves, Schwann cells are able to interact with many axons.

Note: The demyelination of axons causes the multitude of neurological symptoms found in the disease Multiple Sclerosis.

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Axon Vs Dendrites

Difference between Axons and Dendrites fere are different internal structural characteristics between axons and dendrites. Axons typically almost sever contain ribosomes, except some in the initial segment, Dendrites contain granular endoplasmic reticulin or ribosomes, with diminishing amounts with distance from the cell body. Although the canonical view of the neuron attributes dedicated functions to its various anatomical components, dendrites and axons very often act contrary to their so-called main function.

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