Neurofibrillary tangles
Neurofibrillary tangles refers to the insoluble twisted fibers found inside the brain's cells. These tangles are made of primarily a protein called tau, it forms part of a structure which is called a microtubule. The microtubule helps in transportation of nutrients and other important substances from one part of the nerve cell to another part. In Alzheimer's disease, this tau protein is abnormal and the microtubule structures collapses. These tangles are formed when tau is misfolded in a very specific way.
In Alzheimer’s disease, the tau takes a C-shape in the core of the tangle with a loose end sticking out randomly. In Pick’s disease, this core takes forms a J-shape instead. There is likely a single molecule responsible for shaping tau into these forms, but (as of the time of writing this) the molecule has not yet been identified by the researchers.