Dendritic Spines: What Are They? What Are They For?

machinery is also identified in spines, within specialized subdomains (88, 102). In addition, polyribosomes and protein translational machinery are often anchored at the base of spines (80). Thus increasing evidence suggests that individual spines represent partially autonomous compartments, having their own regulated membrane-trafficking events that shuttle components into and out of the spine membrane. Together, these specialized molecular assemblies determine spine shape and, most importantly, enable the postsynaptic neuron to respond biochemically to glutamate or other transmembrane signals (44, 100, 127). Proteomic approaches have begun to unravel the molecular composition of isolated PSD fractions and glutamate receptor complexes and have uncovered a remarkable degree of complexity (48, 83, 116, 124). Although it appears that all spines share several core components, at least some of the apparent molecular complexity probably reflects the fact that individual spine synapses are heterogeneous, exhibiting variable levels of important signaling molecules that can finetune individual synaptic responses. Despite advances in our understanding of the molecular composition of spines, their function is not entirely understood. For example, why do some synapses occur on spines and others (“shaft synapses”) directly on the dendrite? One widely accepted explanation is that spines provide biochemical compartmentalization (76). The narrow neck of the spine creates a spatially isolated compartment where biochemical signals can rise and fall without spreading to neighboring synapses along the parent dendrite, thus allowing the isolation and/or amplification of incoming signals. Similarly, a spine compartment may help confine membrane trafficking to a localized region. Such restriction of molecular signals to one spine may contribute to the phenomenon of “input specificity,” allowing a given set of nerve terminals to induce changes only at those synapses that are specific to their postsynaptic contacts and not at other synapses on the same neuron that are driven by different axons (64).