Neurons have elaborate cytoskeletal structures.

(A, B) Neurons are specialized eukaryotic cells that extend long processes to form connections in the nervous system. Like other eukaryotic cells, neurons have a cytoskeleton that consists of three main polymers: microtubules (green), intermediate filaments (purple) and actin filaments (red). Microtubules emanate from the axon, and actin-filament networks form sheet-like structures and filopodial protrusions at the leading edge. Scale bar, 20 μm. (C) The neuronal axon is a long membrane-bounded extension, in which neurofilaments (a class of intermediate filament in neurons) form a structural matrix that embeds microtubules, which transport materials from the cell body to the axon terminals at the synapse. (D) The growth cone contains dendritic actin filament networks and parallel actin filament filopodia. (E) Microtubules consist of 13 protofilaments of tubulin dimers arranged in a hollow tube. (F) Neurofilaments have flexible polymer arms that repel neighboring neurofilaments and determine the radius of the axon. (G) Actin filaments are arranged into networks. These networks can have many architectures, including the branched structures depicted here, which are formed by the Arp2/3 complex (blue). The diameters of microtubules, intermediate filaments, and actin filaments are within a factor of three of each other; the diagrams in E, F, and G are drawn approximately to scale. However, the relative flexibilities of these polymers differ markedly, as indicated by their persistence lengths: from least to most flexible, microtubules (5,000 μm), actin filaments (13.5 μm) and intermediate filaments (0.5 μm).