The cytoskeleton is the cytoplasmic structure that supports the axon. It consists of three types of protein filaments working together: microtubules, neurofilaments, and microfilaments (actin filaments). These components maintain the axon’s shape, provide mechanical strength, and serve as tracks for transporting materials along the axon’s length.
Three Filaments That Form the Axonal Cytoskeleton
Each of the three filament types differs in size, composition, and what it contributes to the axon.
Microtubules are the largest, with a diameter of about 25 nanometers. They are hollow cylinders built from repeating units of a protein called tubulin. Of the three filament types, microtubules contribute the most to axonal stiffness. Experiments disrupting microtubules produced axons with the lowest resistance to deformation, confirming their role as the primary source of structural rigidity. Beyond physical support, microtubules act as railway tracks for intracellular transport, which is covered in more detail below.
Neurofilaments are mid-sized, roughly 10 nanometers in diameter. They are a neuron-specific version of the intermediate filaments found in other cell types. Neurofilaments provide mechanical strength and stability, and they are the main factor determining how thick an axon grows. They accomplish this through arm-like extensions that project outward from a central core and space neighboring neurofilaments at regular intervals across the axon’s width. Adding more neurofilaments, or chemically modifying those arms through a process called phosphorylation, increases the axon’s overall caliber.
Microfilaments (actin filaments) are the thinnest, only about 6 nanometers across. They are made of two twisted strands of actin, one of the most abundant proteins in neurons. Actin is primarily responsible for changes in cell shape and is found throughout the neuron, with especially high concentrations in the axon and dendrites.
How Microtubules Are Organized in the Axon
Microtubules have an inherent directionality, with a “plus end” and a “minus end.” In the axon, they are arranged in a nearly uniform orientation: plus ends pointing away from the cell body. This consistent alignment is what allows efficient one-way transport of cargo toward the axon tip or back toward the cell body. It also distinguishes the axon from dendrites, where microtubules point in mixed directions. The neuron actively maintains this uniform polarity, because it is constantly at risk of being disrupted.
A protein called tau plays a key role in keeping axonal microtubules intact. Tau was originally discovered as a factor essential for microtubule assembly in the lab. It binds to microtubules and influences their stability and dynamics. When tau is depleted from neurons, the total mass of microtubules in the axon decreases. This relationship becomes clinically important in diseases where tau malfunctions, as described later.
The Actin-Spectrin Lattice Under the Membrane
In addition to the three main filaments running through the axon’s interior, super-resolution microscopy revealed a previously hidden structure in 2013. Just beneath the axon’s outer membrane sits a repeating scaffold made of short actin filaments arranged in ring-like structures that wrap around the circumference of the axon. These rings are connected by rod-shaped spectrin proteins spaced about 190 nanometers apart, like rungs on a ladder. This periodic skeleton is present in mature axons across every neuronal type examined so far and likely helps the axon maintain its tubular shape and resist mechanical stress at the membrane level.
The Cytoskeleton as a Transport Highway
The axon can extend enormous distances relative to the cell body. A motor neuron reaching from your spinal cord to your foot, for example, might stretch over a meter. The cell body cannot simply diffuse proteins and organelles across that distance. Instead, motor proteins walk along microtubules to deliver cargo.
Kinesin motors carry cargo in the anterograde direction (away from the cell body, toward the plus end of microtubules), while dynein motors carry cargo in the retrograde direction (back toward the cell body). Both move at roughly similar speeds, around 0.85 micrometers per second. This transport system is entirely dependent on the structural integrity of the microtubule tracks.
What Happens When the Cytoskeleton Breaks Down
Because the cytoskeleton supports both the axon’s structure and its transport system, damage to these filaments has serious consequences. Disruption of axonal transport is an early event in several major neurodegenerative diseases, including Alzheimer’s, Parkinson’s, Huntington’s, and ALS.
In Alzheimer’s disease, swollen, distorted axons packed with stalled cargo and motor proteins appear in early stages of the disease, even before the hallmark amyloid plaques and tau tangles have fully formed. The connection to tau is direct: disease-linked mutations or excessive phosphorylation of tau reduce its ability to bind microtubules, leading to microtubule instability, impaired transport, and eventually neuronal death. This mechanism underlies an entire class of neurodegenerative conditions called tauopathies.
In ALS, postmortem tissue from patients shows axonal swellings filled with accumulated neurofilaments, a clear sign that transport has failed. Mutations in genes encoding kinesin, dynein, and related motor proteins have been identified as direct causes of familial ALS, reinforcing that axonal transport disruption is not just a symptom but a driving mechanism of the disease. Early structural changes to neurofilaments, such as alterations to their spacing arms, may serve as a trigger for further cascading damage to the axon.