Axon

by Abdullah Arif · June 14, 2024

Axon . The axon is a long, thin tube that branches at the end into several presynaptic terminal branches. The message that travels through the axon is electrical, all or nothing, and is called an action potential. This originates from a specialized area of the cell body, called the axon cone. The starting cone and the initial segment of the axon occupy a distance equivalent to the diameter of a cell body. From this point the axon begins to be covered by the millennial sheath. This area functions as an activation region, which integrates the numerous signals from other cells and initiates the signal that the neuron sends to its targets or synaptic destinations.

Summary

[ disguise ]

1 Features
2 Classification
3 Function of the Axon
- 1 Transport of organelles and substances
- 2 Nerve impulse conduction
4 Sources

Characteristics

The axon is a long, thin extension of neurons that originates in a specialized region called the axonal eminence or axonal cone, from the soma, or sometimes from a dendrite. The axon is shaped like a cone that tapers toward the periphery. Periodic circular constrictions called nodes of Ranvier are observed on its surface. The cell membrane of the axon is called the axolemma. Axoplasm is the cytoplasm contained within the axon and axonal eminence. It is a viscous fluid within which neurotubules, neurofilaments, mitochondria, granules and vesicles are found, which differ from the cytoplasm soma and proximal dendrites, because they lack rough endoplasmic reticulum, free ribosomes and Golgi apparatus. Axons may or may not be covered by a sheath, called myelin sheath.

In the peripheral nervous system, axons are always covered by Schwann cells, which surround the axon with a multiple layer formed from the membrane of these cells and constitute the myelin sheath. The neurons of the peripheral nervous system that are not surrounded by the myelin sheath are embedded in Schwann cells, forming the bundle of Remak. In the central nervous system, the axons that are myelinated are covered by oligodendrocytes, glia cells, as well as Schwann cells that form the myelin sheath.

Classification

According to the length of the axon, neurons can be classified into two types:

Golgi type I neurons, which have a long axon that

can measure more than one meter

Golgi type II neurons, which have a short, dendrite-like axon that ends near the soma.
Depending on the coverage, the axon can be myelinated or

Amielinco A myelinated axon is covered by a layer of myelin, which is a fatty substance produced by Schwann cells and oligodendrocytes, which are support cells. The myelin that covers the axon has a series of indentations, called nodes of Ranvier, along it. This facilitates the Saltatory nerve impulse. Myelin in turn is covered by the neurilemma, which is a cytoplasmic layer composed of Schwann cells.

Axon Function

The functions of the axon are the transport of organelles and substances, and the conduction of nerve impulses.

Transport of organelles and substances

The transport of organelles, enzymes, macromolecules and metabolites is a function of axoplasm in which microtubules directly intervene. Axoplasmic transport is necessary for the maintenance of the axon and the cells associated with it, and to allow the arrival of regulatory factors that regulate its function to the perikaryon. Transport within the axon can be in two directions:

Anterograde or centrifugal transport: This is what occurs from the neuronal soma to the telodron.
Retrograde or centripetal transport: This is what occurs from the terminal buttons to the neuronal soma.

The transport speed varies between:

Slow flow of 0.5 µm/min, speed at which molecular aggregates move such as the protein subunits that form the cytoskeleton

axonal.

Fast anterograde flow in which organelles move at speeds of about 300 µm/min. The kinesin molecule, attached to a receptor on the membrane of the transported organelle, moves, at the expense of ATP, from the negative end of the microtubule, located in the perikaryon or soma, towards its positive end.
Rapid retrograde flow in which membranous vesicles from the terminal buttons are transported towards the perikaryon or soma at about 200 µm/min. The cytoplasmic dynein molecule (MAP1C) bound to a receptor on the membrane of the transported organelle moves, interacting with tubulin at the expense of ATP, from the positive end of the microtubule, located in the axon terminal or terminal arborization, to its negative end.

Nerve impulse conduction

The axons constitute the nerve fibers, the long efferent branch being the one that transmits the action potential, whether excitatory or inhibitory, through one or more synapses. Axons can also receive inputs through axoaxonal synapses, which are made between two axons, but axon output functions predominately. The conduction of the nerve impulse is the displacement of the action potential generated by changes in the permeability to ions along the axolemma (axon membrane) of the nerve fibers, helped by the support cells that surround the axon like a sheath. In the central nervous system, axons are surrounded by the myelin of oligodendrocytes, while in the peripheral nervous system they can be surrounded, either, by cytoplasmic extensions of Schwann cells (unmyelinated fibers) or by the myelins of Schwann cells. (myelinated nerve fibers of the peripheral nervous system). Nerve impulses are transient waves of reversal of the voltage that exists at the level of the plasma membrane, which begin at the place where the stimulus occurs. Each of these waves corresponds to an action potential. This process is possible thanks to the macromolecules that, as integral proteins, occupy the entire thickness of the axolemma such as:

The sodium-potassium pump, capable of actively transporting sodium to the extracellular medium, exchanging it for potassium.
Voltage-sensitive sodium channels, which determine the inversion of the membrane voltage since when they open and allow the entry of sodium, they cause the inside of the membrane to become positive.
Voltage-sensitive potassium channels, whose activation contributes to the return to the initial polarity, due to the release of potassium ions from the interior of the axoplasm.

In unmyelinated nerve fibers the impulse is conducted as a continuous wave of voltage reversal to the terminal buttons of the axons at a speed that is proportional to the diameter of the axon and varies from one to one hundred meters per second. In myelinated nerve fibers, the axon is covered by a myelin sheath formed by the overlapping or coiling of a series of layers of cell membrane, which acts as an electrical insulator of the axon. Along the axon, myelin is formed by successive cells and at each intercellular boundary there is a ring without myelin that corresponds to the node of Ranvier. In the nodes of Ranvier, the flow of ions through the axonal membrane occurs. The axolemma of the nodes of Ranvier has a high concentration of voltage-sensitive sodium channels. The consequence is a Saltatory conduction of the action potential since the inversion of the voltage induced at the level of a node of Ranvier is continued by rapid passive propagation of the current through the interior of the axon and through the extracellular to the next node where the inversion occurs. of the voltage. The consequence of this structure is that nerve impulse conduction is faster in myelinated axons. The conduction speed of the nerve impulse is proportional to the diameter of the axon and the distance between the [[nodes of Ranvier]] in myelinated axons. The first measurement of nerve impulse speed is attributed to Hermann von Helmholtz , who in 1853 established an average value of 27.25 m/s.