The gemmula dendritica (also called as the dendritic spine) is a small membranous protrusion from a neuron’s dendrite that typically receives input from a single axon at the synapse. Dendritic spines serve as a storage site for synaptic strength and help transmit electrical signals to the neuron’s cell body. In addition, they may also serve to increase the number of possible contacts between neurons. The dendrites of a single neuron can contain hundreds of thousands of spines.
The Spanish neuroscientist and pathologist Santiago Ramón y Cajal discovered the gemmula dendritica in 1888 after using Golgi’s stain on the cerebellum of birds. In his monograph published in 1888, he notes the presence of tiny protrusions on the surfaces of Purkinje cell (a type of neuron in the cerebellum) dendrites. To quote him, “The surface of the Purkinje cells dendrites appear ruffled with thorns or short spines, which in the terminal dendrites look like light protrusions. Early on we thought that these eminences were the result of a tumultuous precipitation of the silver; but the constancy of their existence and its presence even in preparations where the staining appears with great delicacy in the remaining elements, incline us to consider them as a normal disposition.” On the left is a photomicrograph of Santiago Cajal’s original preparation of a dendrite from a pyramidal neuron with the dendritic spines clearly visible and on the right is a labelled photomicrograph of a neuron taken by a confocal microscope: