Excitable neurofibrils and the problem of identifying the structure of central excitatory synapses in the nineteenth century.

Neurofibrils, identified after staining with Cajal's reduced silver nitrate, for example, were thought by many senior histologists in the nineteenth and early-twentieth centuries to conduct action potentials. There was no basis for this popular idea, although it was the impetus for intense study of the "neurofibrillar network" within neurons by Golgi, Cajal, Freud, and many others. Here, I trace the way in which this "excitable neurofibrillary" hypothesis led to major problems in the attempt by histologists to identify the central excitatory synapse, postulated by Sherrington on functional grounds and eventually described by Berkley.