Axolemma: what is it and what characteristics does this part of the neuron have?

Neurons are very important cells, basically because they are the functional unit of our nervous system. Like any other cell, they consist of different parts, including the axon and the membrane that covers it, the axolemma.

Next we will see in more depth the main characteristics of the axolemma, its most important sections, what type of substances and structures that compose it and what importance it acquires during the transmission of the impulse highly strung.

• Related article: "What are the parts of the neuron?"

What is the axolemma?

the axolemma is the part of the cell membrane that surrounds the axon. This part of the neuronal membrane fulfills diverse and important functions for the nervous system, since it is the cellular part in charge of maintaining the membrane potential. It has ion channels through which ions can be rapidly exchanged between the neuronal interior and exterior, allowing the polarization and depolarization of the membrane of the neuron.

The axon in general terms

Before going into more detail about the axolemma, let's see a little above what the axon is, the structure that the axolemma covers.

The axon is a cellular extension with few branches., at a right angle and with a diameter that remains constant throughout its path. From neuron to neuron, the axon can have different diameters and lengths, ranging from 1 to 20 micrometers in thickness and from 1 millimeter to 1 meter in length.

In addition to the axolemma, which is the structure that covers and protects the axon, it has other structures. The cytoplasmic medium of the axon is called the axoplasm. and, like other types of eukaryotic cells, it has a cytoskeleton, mitochondria, vesicles with neurotransmitters, and associated proteins.

The axon originates from the soma, that is, the body of the neuron, as a triangular structure called an axon cone. It continues with an initial segment that does not have a myelin sheath, which is a kind of neuronal insulator. very important for the transmission of the nerve impulse efficiently and quickly. After this first initial segment comes the main segment, which may or may not have a myelin sheath, which determines the formation of myelinated axons or unmyelinated axons.

Description of the axolemma and general characteristics

All cells in the human body are bounded by a cell membrane, and neurons are no exception. As we have already commented, the axons are covered by the axolemes, and they do not differ too much from the rest. of cell membranes since they are formed by a double layer of phospholipids linked to different proteins.

The particularity of the axolemma is that it has voltage-gated ion channels., fundamental for the transmission of the nerve impulse. Three types of ion channels can be found in this structure: sodium (Na), potassium (K) and calcium (Ca). The axolemma can be divided into two main sections: the axon initial segment (AIS) and the nodes of Ranvier.

1. Initial segment of the axon

The initial segment of the axon is a highly specialized region of the membrane in the immediate vicinity of the soma of the neuron.

The initial segment of the axon has a dense layer of finely granular material that lines the plasma membrane. A similar lower layer is found below the plasma membrane of myelinated axons in Ranvier's nodes.

The initial segment acts as a kind of selective filter for molecules that allows proteins with axonal load, although not dendritic, to pass into the axon.

2. Ranvier nodes

Ranvier's nodes gaps only 1 micrometer in length that expose the axon membrane to extracellular fluid. They are like a kind of interruptions that occur at regular intervals along the length of the myelinated axon.

• You may be interested in: "Ranvier's nodules: what they are and how they serve neurons"

How is the nerve impulse conducted thanks to the axolemma?

In the central nervous system, axons are surrounded by myelin from oligodendrocytes or myelinated nerve fibers, while in the peripheral nervous system they may be surrounded by cytoplasmic processes of Schwann cells (unmyelinated fibers) or by the myelin of the Schwann cells themselves (myelinated nerve fibers of the PNS)

nerve impulses are electrical currents that travel through the nervous system, reversing the voltage of the nerve cell membrane. In a very simplified way, every time this process occurs we would be talking about an action potential, with the axolemma being highly involved. This process could not occur if the axon membrane did not have certain types of macromolecules in its composition, such as integral proteins. Among these structures we can find some such as the following:

• Sodium-potassium pump: actively transports sodium to the extracellular medium, exchanging it for potassium.
• Voltage-sensitive sodium channels: determine the inversion of the membrane voltage allowing the entry of Na+ (sodium) ions, causing the interior of the membrane to become increasingly positive.
• Voltage-sensitive potassium channels: Activation of these channels causes the cell to return to the initial polarity, causing K (potassium) ions to exit from within the axonal milieu (axoplasm).

The nerve impulse is conducted through the unmyelinated nerve fibers as a continuous wave of voltage reversal to the terminal buttons of the axon. The speed of this process will depend proportionally to the diameter of the axon, varying between 1 and 100 m/s.. In myelinated nerve fibers, the axon is covered by a myelin sheath, which is made up of the apposition of a series of layers of cell membrane, which acts as a kind of electrical insulator of the axon.

This myelin is made up of successive cells and, at each limit between them, there is a kind of ring without myelin that corresponds to a Ranvier node. It is at the nodes of Ranvier that ion flow across the axonal membrane can occur. At the level of the Ranvier nodes, the axolemma presents a high concentration of voltage-gated sodium channels.

Bibliographic references:

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