Axoplasm

Axoplasm is the portion of the cytoplasm of a nerve cell that is localized within the axon. In some neurons, the axon plamsa may comprise more than 99% of the total cytoplasm of the cell. The axoplasm has a different composition of organelles and other elements than those present in the cell body (soma) or dendrites of the neuron. Thus, the axoplasm is composed of diverse organelles and cytoskeletal elements. Axoplasm contains a high concentration of mitochondria, microfilaments and microtubules. In addition, axoplasm contains the presynaptic vesicles of the neurotransmitter that are eventually released into the synaptic cleft. In contrast, axoplasm lacks most of the other cellular elements such as ribosomes and nucleus, elements required for transcription and translation of complex proteins.

Axoplasm is essential for the overall function of neurons for the propagation of the action potential through the axon. Thus, the amount of axoplasm in the axon is also important for the cable-like properties of the axon in the so-called cable theory. In terms of this cable theory, the axoplasmic content determines the resistance of the axon to potential change. The composite cytoskeletal elements of axoplasm, neural filaments, and microtubules enable axonal transport (also known as axoplasmic transport), which ultimately allows neurotransmitters to reach the synapse. Axonal transport involves the transport of substances through the axoplasm to or from the soma.

Axonal transport: Axonal transport occurs either through:

• fast transport
• or
• slow transport.

In fast axoplasmic transport, vesicular contents (such as organelles) of motor proteins are moved along microtubules at a rate of 50-400 mm per day. The electrical resistance of axoplasm, called axoplasmic resistance, is an aspect of the cable properties of a neuron because it affects the speed of movement of an action potential along an axon. When the axoplasm contains many molecules that are not electrically conductive, it slows the movement of the potential because more ions flow through the axolemma (the axon membrane) than through the axoplasm.

Slow axoplasmic transport involves the movement of cytosolic soluble proteins and cytoskeletal elements at a much slower rate of 0.02 to 0.1 mm / day. The exact mechanism of slow axonal transport is unknown. There is evidence that it functions by transient association with fast axonal transport vesicles.

Damage detection and regeneration: The axoplasm contains both the mRNA and ribonuclear protein required for axonal protein synthesis. Axonal protein synthesis has been shown to play an essential role in both neuronal regeneration and localized responses to axonal damage.