Axolema: what are and what characteristics does this part of the neuron have

Neurons are very important cells, mainly because they are the functional unit of our nervous system. Like any other cell, they are made up of different parts, between them the axon and the membrane that covers it, the axolem.

Then we will take a closer look at the main features of the axolem, its most important sections, what kind of substances and structures it is made of and what importance acquires during the transmission of the nerve impulse.

    What is the axolemma?

    axolem it is the part of the cell membrane that surrounds the axon. This part of the neuronal membrane performs several important functions for the nervous system, as it is the cellular part responsible for maintaining membrane potential. It has ion channels through which they can quickly exchange ions between the interior and exterior of the neuron, allowing polarization and depolarization of the neuron’s membrane.

    The axon in general terms

    Before going into more detail on the axolem, we will see a little above what the axon is, in the structure that the axolem covers. The axon is a cell extension with few branches, At right angles and with a diameter that remains constant along its path. Between the neuron and the neuron, the axon can have different diameters and lengths, ranging from 1 to 20 microns in thickness and from 1 millimeter to 1 meter in length.

    In addition to the axolem, 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, as with other types of eukaryotic cells, it has a cytoskeleton, mitochondria, neurotransmitter vesicles, and associated proteins.

    The axon originates in the soma, which 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, that is, a kind of neural insulator very important for the transmission of nerve impulses efficiently and quickly. After this initial first segment comes the main segment, which may or may not have a myelin sheath, which determines the formation of myelin axons or myelin axons.

    Description of the axolem and general characteristics

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

    The peculiarity of the axolem is that it has voltage-dependent ion channels, Fundamental of the transmission of nerve impulses. 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 initial segment of the axon (AIS) and the nodes of Ranvier.

    1. Initial segment of the axon

    The initial segment of the axon is a highly specialized membrane region in close proximity to the neuron soma.

    The initial segment of the axon has a dense layer of finely granular material that covers the plasma membrane. A similar lower layer is found under the plasma membrane of myelinated axons in Ranvier nodules.

    The initial segment acts as a sort of selective filter of molecules which allows axonally charged, although not dendritic, proteins to pass to the axon.

    2. Nodes of Ranvier

    Ranvier knots these are spaces only a micrometer in length that leave the axon membrane exposed to extracellular fluid. They are like a kind of interruption that occurs at regular intervals along the myelinated axon.

      How is the nerve impulse conducted through the axolem?

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

      Nerve impulses are electrical currents that flow through the nervous system, reversing the tension of the membrane of nerve cells. In a very simplified way, each time this process occurs, one would speak of an action potential, the axolemma being very involved. This process could not occur if the axonal membrane did not contain 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 environment by exchanging potassium.
      • Voltage-sensitive sodium channels: they determine the inversion of the membrane voltage allowing the entry of Na + (sodium) ions, which causes the interior of the membrane to become more and more positive.
      • Voltage Sensitive Potassium Channels: Activation of these channels causes the cell to revert to its original polarity, causing K ions (potassium) to emerge from within the axonal medium (axoplasm).

      The nerve impulse is conducted through the myelinated nerve fibers as a continuous wave of voltage reversal to the terminal buttons of the axon. The speed of this process will depend proportionally on the diameter of the axon, varying between 1 and 100 m / s. In myelin nerve fibers, the axon is covered with 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 made up of successive cells, and at each border between them there is a kind of ring without myelin which corresponds to a Ranvier node. It is at the nodes of Ranvier that ionic flow through the axonal membrane can occur. At the level of the nodes of Ranvier the axolema presents a high concentration of tension-dependent soidio channels.

      Bibliographical references:

      • Hamada, MS; Kole, MHP (2015). Adaptations of myelin loss and axonal ion channel associated with neuronal hyperexcitability of gray matter. Journal of Neuroscience 35 (18): p. 7272 – 7286. PMC 4420788. PMID 25948275. doi: 10.1523 / JNEUROSCI.4747-14.2015.
      • Moreno Benavides, C. (2017). chap.3: Ultrastructure of the axon ”In Bru Benavides, C; Velasquez-Torres, A; Amador-Muñoz, D; López-Guzmán, S., ed. The peripheral nerve: structure and function. Colombia: University of the Rosary, School of Texts of Medicine and Health Sciences.
      • Kole, M .; Stuart, GJ (2012). Signal processing in the initial segment of the axon. Neuron (review) 73 (2): 235-247.
      • Triarhou, LC (2014). Axons emanating from dendrites: phylogenetic repercussions with Cajal nuances. Frontiers in neuroanatomy. 8: 133. doi: 10.3389 / fnana.2014.00133. PMC 4235383. PMID 25477788.
      • Yau, KW (1976). Receptive fields, geometry and conduction block of sensory neurons in the leech central nervous system. The Journal of Physiology. 263 (3): 513–38. doi: 10.1113 / jphysiol.1976.sp011643. PMC 1307715. PMID 1018277.
      • Squire, Larry (2013). Fundamental Neuroscience (4th ed.). Amsterdam: Elsevier / Academic Press. pages 61 to 65. ISBN 978-0-12-385-870-2.

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