neuroscience Neuroscience is the scientific study of the nervous system (the brain, spinal cord, and peripheral nervous system), its functions and disorders. It is a multidisciplinary science that combines physiology, anatomy, molecular biology, developme ...

, the axolemma (, and 'axo-' from axon) is the

cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...

of an

axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action p ...

, the branch of a

neuron A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. ...

through which signals (

action potentials An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells ...

) are transmitted. The axolemma is a three-layered, bilipid membrane. Under standard electron microscope preparations, the structure is approximately 8 nanometers thick. Axolemma_Histology,_OpenStax_College

Composition

The skeletal framework of this structure is formed by a spectrum of hexagonal or pentagonal arrangement on the inside of the cell membrane, as well as

actin Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of ov ...

connected to the transmembrane. The metric cellular matrix is bound by

transmembrane protein A transmembrane protein (TP) is a type of integral membrane protein that spans the entirety of the cell membrane. Many transmembrane proteins function as gateways to permit the transport of specific substances across the membrane. They frequent ...

s, including the β1-integrin, to the cytoskeleton via the membrane skeleton. The axolemma is a phospholipid bilayer membrane, and charged ions/particles cannot directly pass through it. Instead,

s, such as specialized energy dependent ion pumps (the sodium/potassium pump), and ion channels ( ligand-gated channels, mechanically gated channels, voltage-gated channels, and leakage channels) that sit within the axolemma are required to assist these charged ions/particles across the membrane, and to generate transmembrane potentials that will generate an

action potential An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...

Function

The primary responsibility of cell membranes, including those surrounding the axon, is to regulate what goes into the cell and what goes out of the cell. The axolemma plays an important role in the nervous system, specifically the sensation, integration, and response pathways within the nervous system. Communication between neurons within the nervous system relies on excitable membranes, especially the axolemma. The axolemma is responsible for relaying signals between the

and it's

Schwann Cells Schwann cells or neurolemmocytes (named after German physiologist Theodor Schwann) are the principal glia of the peripheral nervous system (PNS). Glial cells function to support neurons and in the PNS, also include satellite cells, olfactory en ...

. These signals control the proliferative and

myelin Myelin is a lipid-rich material that surrounds nerve cell axons (the nervous system's "wires") to insulate them and increase the rate at which electrical impulses (called action potentials) are passed along the axon. The myelinated axon can be ...

-producing functions of the Schwann Cells, and also partly play a role in the regulation of the size of the axon.

The Axolemmas Role in the Generation of Action Potentials

The variations in electrical state of the axolemma is referred to as the membrane potential – a potential being the distribution of charge between the inside and outside of the cell, which is measured in millivolts (mV). The transmembrane proteins keep the concentration of ions inside the cell and the concentration of ions outside the cell relatively balanced, with a net neutral charge, but if a difference in charge occurs right at the surface of the axolemma, either internally or externally, electrical signals, such as

s, can be generated. When the cell, or axon, is at rest, the concentration of

sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...

(Na⁺) outside of the cell is greater than the concentration of Na⁺ inside of the cell, and the concentration of

potassium Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosph ...

(K⁺) inside of the cell is greater than the concentration of K⁺ outside of the cell. This difference in charge is referred to as the

resting membrane potential A relatively static membrane potential which is usually referred to as the ground value for trans-membrane voltage. The relatively static membrane potential of quiescent cells is called the resting membrane potential (or resting voltage), as oppo ...

– which is measured at -70mV. The opening of channels within the axolemma, allows for Na⁺ to flow down its concentration gradient, and into the cell. Na⁺ is a positively charge ion, so the influx on Na⁺ causes the membrane potential to move toward zero. This is referred to as

depolarization In biology, depolarization or hypopolarization is a change within a cell, during which the cell undergoes a shift in electric charge distribution, resulting in less negative charge inside the cell compared to the outside. Depolarization is ess ...

. However, the concentration gradient of Na⁺ is strong enough to allow Na⁺ to flow into the cell until the membrane potential to reach +30mV. The membrane potential reaching +30 mV, and the concentration of Na⁺ being do high, causes other voltage-gated channels, that are specific to K⁺ to open. K⁺ then flows down its concentration gradient and out of the cell. Since the positively charged K⁺ is leaving the cell, the membrane potential goes back down toward its resting membrane potential. The movement of the membrane voltage back toward -70 mV is referred to as repolarization. However, repolarization overshoots the resting membrane potential, because the K⁺ channels experience a delay when closing, which causes a period of hyperpolarization. This change in charge, voltage, and membrane potential generates an electrical signal referred to as an action potential. Action potentials are used for communication between neurons within nervous tissue. 1221 Action Potential

References

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