Axoplasm is the

cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. The ...

within the

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 po ...

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. N ...

(nerve cell). For some neuronal types this can be more than 99% of the total cytoplasm. Axoplasm has a different composition of

organelle In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' the ...

s and other materials than that found in the neuron's

cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...

body (

soma Soma may refer to: Businesses and brands * SOMA (architects), a New York–based firm of architects * Soma (company), a company that designs eco-friendly water filtration systems * SOMA Fabrications, a builder of bicycle frames and other bicycle ...

) or dendrites. In

axonal transport Axonal transport, also called axoplasmic transport or axoplasmic flow, is a cellular process responsible for movement of mitochondria, lipids, synaptic vesicles, proteins, and other organelles to and from a neuron's cell body, through the cytopla ...

(also known as axoplasmic transport) materials are carried through the axoplasm to or from the soma. The

electrical resistance The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual parallels ...

of the axoplasm, called axoplasmic resistance, is one aspect of a neuron's cable properties, because it affects the rate of travel of 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, ...

down an axon. If the axoplasm contains many

molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...

s that are not

electrically conductive Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allow ...

, it will slow the travel of the potential because it will cause more

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

s to flow across the

axolemma In neuroscience, the axolemma (, and 'axo-' from axon) is the cell membrane of an axon, the branch of a neuron through which signals (action potentials) are transmitted. The axolemma is a three-layered, bilipid membrane. Under standard electron m ...

(the axon's membrane) than through the axoplasm.

Structure

Axoplasm is composed of various organelles and cytoskeletal elements. The axoplasm contains a high concentration of elongated

mitochondria A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...

microfilament Microfilaments, also called actin filaments, are protein filaments in the cytoplasm of eukaryotic cells that form part of the cytoskeleton. They are primarily composed of polymers of actin, but are modified by and interact with numerous other pr ...

s, and

microtubules Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nm and have an inner diameter between 11 an ...

. Axoplasm lacks much of the cellular machinery (

ribosomes Ribosomes ( ) are macromolecular machines, found within all cells, that perform biological protein synthesis (mRNA translation). Ribosomes link amino acids together in the order specified by the codons of messenger RNA (mRNA) molecules to f ...

and

nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucle ...

) required to

transcribe Transcription refers to the process of converting sounds (voice, music etc.) into letters or musical notes, or producing a copy of something in another medium, including: Genetics * Transcription (biology), the copying of DNA into RNA, the fir ...

and translate complex

proteins Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

. As a result, most enzymes and large proteins are transported from the soma through the axoplasm. Axonal transport occurs either by fast or slow transport. Fast transport involves vesicular contents (like organelles) being moved along microtubules by

motor proteins Motor proteins are a class of molecular motors that can move along the cytoplasm of cells. They convert chemical energy into mechanical work by the hydrolysis of ATP. Flagellar rotation, however, is powered by a proton pump. Cellular functions ...

at a rate of 50–400mm per day. Slow axoplasmic transport involves the movement of cytosolic soluble proteins and cytoskeletal elements at a much slower rate of 0.02-0.1mm/d. The precise mechanism of slow axonal transport remains unknown but recent studies have proposed that it may function by means of transient association with the fast axonal transport

vesicle Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane * Synaptic vesicle ; In human embryology * Vesicle (embryology), bulge-like features o ...

s. Though axonal transport is responsible for most organelles and complex proteins present in the axoplasm, recent studies have shown that some translation does occur in axoplasm. This axoplasmic translation is possible due to the presence of localized translationally silent

mRNA In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of Protein biosynthesis, synthesizing a protein. mRNA is ...

and ribonuclear

protein complexes A protein complex or multiprotein complex is a group of two or more associated polypeptide chains. Protein complexes are distinct from multienzyme complexes, in which multiple catalytic domains are found in a single polypeptide chain. Protein c ...

Function

Signal transduction

Axoplasm is integral to the overall function of neurons in propagating action potential through the axon. The amount of axoplasm in the axon is important to the cable like properties of the axon in cable theory. In regards to

cable theory Classical cable theory uses mathematical models to calculate the electric current (and accompanying voltage) along passive neurites, particularly the dendrites that receive synaptic inputs at different sites and times. Estimates are made by model ...

, the axoplasmic content determines the resistance of the axon to a potential change. The composing cytoskeletal elements of axoplasm, neural filaments, and microtubules provide the framework for axonal transport which allows for

neurotransmitters A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a synapse. The cell receiving the signal, any main body part or target cell, may be another neuron, but could also be a gland or muscle cell. Neur ...

to reach the

synapse In the nervous system, a synapse is a structure that permits a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or to the target effector cell. Synapses are essential to the transmission of nervous impulses from ...

. Furthermore, axoplasm contains the pre-synaptic vesicles of neurotransmitter which are eventually released into the

synaptic cleft Chemical synapses are biological junctions through which neurons' signals can be sent to each other and to non-neuronal cells such as those in muscles or glands. Chemical synapses allow neurons to form circuits within the central nervous syste ...

Damage detection and regeneration

Axoplasm contains both the mRNA and ribonuclearprotein required for axonal protein synthesis. Axonal protein synthesis has been shown to be integral in both

neural regeneration Neuroregeneration refers to the regrowth or repair of nervous tissues, cells or cell products. Such mechanisms may include generation of new neurons, glia, axons, myelin, or synapses. Neuroregeneration differs between the peripheral nervous syst ...

and in localized responses to axon damage. When an axon is damaged, both axonal translation and retrograde axonal transport are required to propagate a signal to the soma that the cell is damaged.

History

Axoplasm was not a main focus for neurological research until many years of learning of the functions and properties of

squid giant axon The squid giant axon is the very large (up to 1.5 mm in diameter; typically around 0.5 mm) axon that controls part of the water jet propulsion system in squid. It was first described by L. W. Williams in 1909, but this discovery was fo ...

s. Axons in general were very difficult to study due to their narrow structure and in close proximity to

glial cells Glia, also called glial cells (gliocytes) or neuroglia, are non-neuronal cells in the central nervous system (brain and spinal cord) and the peripheral nervous system that do not produce electrical impulses. They maintain homeostasis, form mye ...

. To solve this problem squid axons were used as an animal model due to the relatively vast sized axons compared to humans or other mammals. These axons were mainly studied to understand action potential, and axoplasm was soon understood to be important in

membrane potential Membrane potential (also transmembrane potential or membrane voltage) is the difference in electric potential between the interior and the exterior of a biological cell. That is, there is a difference in the energy required for electric charges ...

. The axoplasm was at first just thought to be very similar to cytoplasm, but axoplasm plays an important role in transference of nutrients and electrical potential that is generated by neurons. It actually proves quite difficult to isolate axons from the

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 ...

that surrounds it, so the squid giant axon is the focus for many studies that touch on axoplasm. As more knowledge formed from studying the signalling that occurs in neurons, transfer of nutrients and materials became an important topic to research. The mechanisms for the proliferation and sustained electrical potentials were affected by the fast axonal transport system. The fast axonal transport system uses the axoplasm for movement, and contains many non-conductive molecules that change the rate of these electrical potentials across the axon, but the opposite influence does not occur. The fast axonal transport system is able to function without an axolemma, implying that the electrical potential does not influence the transport of materials through the axon. This understanding of the relationship of axoplasm regarding transport and electrical potential is critical in the understanding of the overall brain functions. With this knowledge, axoplasm has become a model for studying varying cell signaling and functions for the research of neurological diseases like

Alzheimer's Alzheimer's disease (AD) is a neurodegenerative disease that usually starts slowly and progressively worsens. It is the cause of 60–70% of cases of dementia. The most common early symptom is difficulty in remembering recent events. As t ...

, and Huntington's. Fast axonal transport is a crucial mechanism when examining these diseases and determining how a lack of materials and nutrients can influence the progression of neurological disorders.

References

{{Authority control Neurohistology