Axoplasm is the

cytoplasm The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...

within the

axon An axon (from Greek ἄξων ''áxōn'', axis) or nerve fiber (or nerve fibre: see American and British English spelling differences#-re, -er, spelling differences) is a long, slender cellular extensions, projection of a nerve cell, or neuron, ...

of a

neuron A neuron (American English), neurone (British English), or nerve cell, is an membrane potential#Cell excitability, excitable cell (biology), cell that fires electric signals called action potentials across a neural network (biology), neural net ...

(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 (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as Organ (anatomy), organs are to th ...

s and other materials than that found in the neuron's cell body ( soma) or dendrites. In axonal transport (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 paral ...

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 (also known as a nerve impulse or "spike" when in a neuron) is a series of quick changes in voltage across a cell membrane. An action potential occurs when the membrane potential of a specific Cell (biology), cell rapidly ri ...

down an axon. If the axoplasm contains many

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

s that are not electrically conductive, it will slow the travel of the potential because it will cause more ions to flow across the axolemma (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 cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...

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 ...

s, and microtubules. Axoplasm lacks much of the cellular machinery ( ribosomes and nucleus) required to transcribe 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, re ...

. 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 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 vesicles. 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 multidomain enzymes, 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, 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 to reach the

synapse In the nervous system, a synapse is a structure that allows a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or a target effector cell. Synapses can be classified as either chemical or electrical, depending o ...

. 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 neuromuscular junction, muscles or glands. Chemical synapses allow neurons to form biological neural ...

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 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 after many years of learning of the functions and properties of squid giant axons. Axons in general were very difficult to study due to their narrow structure and in close proximity to glial cells. 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. It equals the interior potential minus the exterior potential. This is th ...

. 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 Sheath ( ) is a lipid-rich material that in most vertebrates surrounds the axons of neurons to insulate them and increase the rate at which electrical impulses (called action potentials) pass along the axon. The myelinated axon can be lik ...

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, 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