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

A summary about the characteristics of the axolemma and its functions in the neuron.

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, among which are the axon and the membrane that covers it, the axolemma..

Below we will see more in depth the main characteristics of the axolemma, its most important sections, what kind of substances and structures compose it and what importance it acquires during the transmission of the nerve impulse.

What is the axolemma?

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

The axon in general terms

Before going into more detail about the axolemma, let's take a quick look at the axon, the structure that the axolemma covers. The axon is a cellular prolongation with few branchesThe axon is a cell extension with few branches, at right angles and with a diameter that remains constant along its path. From neuron to neuron the axon can have different diameters and lengths, ranging from 1 to 20 micrometers thick and from 1 millimeter to 1 meter long.

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

The axon originates in the soma, i.e. the body of the neuron, as a triangular structure called the axon cone. It continues with an initial segment that has no myelin sheath, which is a kind of neuronal insulator, very important for the transmission of the nerve impulse. very important for the transmission of the nervous impulse in an efficient and fast way. 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 amyelinated axons.

Description of the axolemma and general characteristics

All cells in the human body are delimited by a cell membrane, and neurons are no exception. As we have already mentioned, axons are covered by axolemmas, and they do not differ much from other cell membranes since they are formed by a double layer of phospholipids bound to different proteins.

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

1. Axon initial segment

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

The initial segment of the axon has a dense layer of finely granular material overlying the plasma membrane. A similar lower layer is found beneath the plasma membrane of myelinated axons in the nodes of Ranvier.

The initial segment acts as a kind of molecule-selective filter that allows the passage of axonally charged, but not dendritic, proteins into the axon.

2. Nodes of Ranvier

The nodes of Ranvier are spaces as small as one micrometer in length that expose the axon membrane to the extracellular fluid.. They are like a kind of interruptions that occur at regular intervals along the myelinated axon.

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, whereas in the peripheral nervous system they may be surrounded by cytoplasmic prolongations of Schwann cells (myelinated fibers) or by myelin from 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 takes place we would be talking about an action potential, being the axolemma very 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 into the extracellular medium exchanging it for potassium.
• Voltage-sensitive sodium channels: determine the reversal of membrane voltage allowing Na+ (sodium) ions to enter, making the inside of the membrane 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 the interior of the axon medium (axoplasm).

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

This myelin is formed by successive cells and, at each boundary between them, there is a kind of ring without myelin that corresponds to a node of Ranvier. It is at the nodes of Ranvier that the flow of ions through the axonal membrane can occur. At the level of the nodes of Ranvier the axolemma presents a high concentration of voltage-dependent soidium channels.

Bibliographic references:

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