Fig. A dendritic spine (S) as seen with freeze-fracture preparation. The plane of fracture exposed the cytoplasmic half of the membrane of the dendrite (D) and of the spine protruding from it. Apposed to the head of the spine is the extracellular half of the fractured membrane of the presynaptic bouton (B). (Rat, hippocampal area CA1; adapted from Harris & Landis, Neuroscience 19(3):857-872, 1986 [PDF].)