Obviously, the spine is a modifiable compartment whose neck length can be controlled by afferent activity and which can regulate the spread of the [Ca2+]i rise evoked at the synapse and perhaps prevent further spreading into the parent dendrite (Korkotian & Segal, 2007); it can also control the access of synaptic molecules into the sphere of the spine head. One category of molecule which is delivered into and out of the synapse in relation

to activity is the ionotrophic AMPA-subtype glutamate receptor. LTP is assumed GSI-IX order to involve the addition of glutamate receptors into the postsynaptic density, and LTD results from the removal of AMPA receptors from the spine head. Recently it has been suggested (Korkotian & Segal, 2007; Ashby et al., 2006) that the spine neck is a barrier to the diffusion of glutamate receptors into the synapse. Whether this barrier is determined by the calcium signal delivered to the dendrite or by the diffusion of receptor molecules is less Z-VAD-FMK price critical; the outcome is that spine neck restricts access of glutamate receptors to the synapse. Consequently, synapses on the parent dendritic shaft should produce larger synaptic currents than those in the spine head, and the length of the spine will determine synapse efficacy. In addition to the influx of calcium through NMDA-gated channels, voltage-gated calcium channels and GluR1-gated,

GluR2-lacking channels, the spines are endowed with calcium stores of the ryanodine type, which are activated by influx of calcium or by direct activation of the ryanodine receptors (e.g. by caffeine). These stores have been linked oxyclozanide recently to the spine apparatus, en enigmatic structure in the spine neck, via synaptopodin, a molecule found to be in close association with the spine apparatus (Vlachos et al., 2009). Synaptopodin and the spine apparatus have been found primarily in large, mature spines. Thus, it is likely that synaptopodin regulates the levels of ambient [Ca2+]i, which is raised transiently by influx of calcium ions. It is likely that large spines, where a larger influx of calcium is expected, need the

stores in order to regulate excess amount of [Ca2+]i. Whether synaptopodin contributes to the stability of the spine is not entirely clear, as time-lapse imaging of synaptopodin and spines show that neither entity is stable over time (Vlachos et al., 2009). Regardless of their plastic properties, spines have been shown to constitute an independent physical compartment in which [Ca2+]i can rise to high levels, independent of the parent dendrite, suggesting that the spine protects the parent neuron from uncontrolled rises in [Ca2+]i, which may otherwise activate apoptotic processes leading to cell death (Schonfeld-Dado et al., 2009). In spiny neurons, shaft synapses are more likely to be harmful to the parent neuron than spine synapses.