A neurofilament is an element of the cytoskeleton that is specific to neurons. Neurofilaments are similar to cytoskeletal elements in other cells, but they are made up of a different set of proteins. They are especially numerous in the axons, i.e., long extensions of neurons that generally transmit nerve impulses away from the cell body toward other cells. Several neurological disorders are directly linked to overproduction of neurofilaments.
A cell maintains its shape and conducts the transport of certain cellular components through the cytoplasm by use of its cytoskeleton. Cytoskeletal elements hold the organelles in place and even allow movement in some cells because they are a structural component of cilia and flagella. The cytoskeleton is built of microtubules of about 23 nm, microfilaments of approximately 6 nm, and intermediate filaments of about 10 nm. Intermediate filaments in skin and hair are typically made mostly of keratin while those that are part of the structure of the nuclear membrane in each cell are made of lamin.
A neurofilament is an intermediate filament that is made up of at least two of the three different types of specialized protein subunits. These three types are called Neurofilament light (NF-L), Neurofilament medium (NF-M) and Neurofilament heavy (NF-H); each neurofilament consists of NF-L and either NF-M or NF-H. Other proteins in certain neurofilaments such as Nestin, are found in developing neurons, and vimentin is found in some neurons of the retina as well as developing neurons.
Both dendrites and axons depend heavily on intermediate filaments to form their framework. The surface of a sidearm extension from one end — the C terminus — of each neurofilament is polarized so that neurofilaments within the same dendrite or axon repel each other. This creates a space between them that acts as a passageway for nerve signal conduction and gives the axon added strength. The number of neurofilaments then determines the width of the signal pathway and thus the speed of signal transmission.
Alzheimer’s disease, Lou Gehrig’s disease, and giant axonal neuropathy are all disorders that involve an overabundance of neurofilaments. The axons in a neuron with excessive neurofilaments are crowded and unable to transmit nerve signals efficiently. Another neurological disorder linked to a neurofilament dysfunction is Charcot-Marie-Tooth disease which causes axons to deteriorate and nerve cells in some muscle tissue to die. While studying this disease, scientists recently discovered a mutation in the gene that encodes the protein NF-L that may also be involved in other neuropathies.