Fibril, micro-fibre or micro-fibril is a fine fiber, such as a myofibril or neurofibril. Neurofilaments are about 10 nanometers in diameter.
Cytoplasmic fibrils are observed on the protoplasmic cylinders found in most spirochetal species, although no function of the cytoplasmic fibrils has been ascribed.
Polysaccharides, the union of several linked monosaccharides, sometimes serve as a structural compound. Cellulose forms cable-like strings, known as fibrils in the tough walls that enclose plant cells. While cellulose is a compilation of glucose monomers, they form unbranched, long strands instead of coils like starch or glycogen. These are arranged in parallel lines which form on top of each other in an intricate layer through hydrogen bonding.
Insect flight muscle is said to be fibrillar, in that it contracts in response to being stretched by antagonistic muscle, so as to allow very rapid (up to 1000 Hz) contraction.