Neurofibrillary tangles:

neurofibrillary tangles are represented by the small red lines within the neuron.

accumulations of tau protein that are seen in neurodegenerative diseases like Alzheimer's disease. The exact role of neurofibrillary tangles in disease is not known, and although they appear as disease progresses, it is unclear if they are causing damage or are simply a response to it.

Watch this 2-Minute Neuroscience video to learn more about Alzheimer's disease.