These studies employed either electrical stimulation, which produces LTP in a selective pathway, or chemical LTP, which is likely to activate most learn more if not all of the synapses. In general, these studies did not reveal massive changes in spine head volume, although changes in postsynaptic density and changes in the proportion of thin to mushroom spines were noted (Medvedev et al., 2010). In all, these studies demonstrate that populations

of spines can shift to having larger spine heads following a tetanic stimulation of an afferent pathway, and it is possible that large changes in spine volume take place in a small subset of spines, although this is not seen in the averaged data. Assuming that spine volume does change after a specific intense stimulation, it is still not clear what are the relations between spine AG-014699 manufacturer shape, size

and density and ambient network activity: do spine shapes vary in a dynamic fashion as a function of ambient activity, such that an increase in activity results in an increase in spine size or density and, conversely, a decrease in activity results in elongation of spines and a collapse of their heads. Alternatively, if spines model ‘memory’ irrespective of ambient activity, then once a spine is formed following a specific ‘pairing’ it should

persistent irrespective of ongoing activity. These two conditions assume opposite demands on the spines, to constantly change their morphology or be stable and store a ‘memory’. This issue is difficult to address directly, but some of the following studies are relevant to this issue. One of the factors that contribute to the difficulty in generalizing some rules that govern the behavior of spines is the different preparations, ages and imaging conditions used. Obviously, when one images remote dendrites of young cortical neurons in vivo, where spine density is rather low, Dimethyl sulfoxide one cannot expect to generalize a priori to mature, highly spiny neurons recorded in an acute slice or in a cultured slice. The heterogeneity is built into the spine, and any attempt to produce a ‘rule’ has to consider different conditions, ages and preparations. The following provides some illustrations of this complexity. The role of ambient activity in formation and maturation of dendritic spines can be learned from the order of events that take place during spine formation and maturation.