Live imaging studies of spine dynamics reveal that the morphology of spines can be altered by neuronal activity in vitro and experience in vivo ( Matsuzaki et al., 2004 Holtmaat et al., 2006 Roberts et al., 2010). The interesting feature of these spine structures is that they are not static, but change morphology continuously, even throughout adulthood, reflecting the plastic nature of synaptic connections ( Grutzendler et al., 2002 Trachtenberg et al., 2002). Electron microscopy studies have identified roughly three categories of spines based on their morphology thin, filopodia-like protrusions (“thin spines”), short spines without a well-defined spine neck (“stubby spines”) and spines with a large bulbous head (“mushroom spines”) ( Bourne and Harris, 2008). They come in a wide range of sizes and shapes, their lengths varying from 0.2 to 2 µm and volumes from 0.001 to 1 µm 3. Spines consist of three distinct basic compartments: (1) a delta-shaped base at the junction with the dendritic shaft, (2) a constricted neck in the middle, and (3) a bulbous head contacting the axon ( Fig. Spines occur at a density of 1–10 spines per micrometer of dendrite length, and some neurons, such as hippocampal neurons, contain thousands of spines throughout the dendritic arbors ( Sorra and Harris, 2000) ( Fig.
It has also been shown that various memory disorders involve defects in the regulation of the actin cytoskeleton ( Newey et al., 2005).ĭendritic spines are small protrusions that receive input from a single excitatory presynaptic terminal, allowing regulation of synaptic strength on a synapse-by-synapse basis. At synapses, the actin cytoskeleton does not only contribute to overall structure of synapses but also plays important roles in synaptic activities that range from organizing the postsynaptic density ( Sheng and Hoogenraad, 2007) and anchoring postsynaptic receptors ( Renner et al., 2008) to facilitating the trafficking of synaptic cargos ( Schlager and Hoogenraad, 2009) and localizing the translation machinery ( Bramham, 2008). In addition, modulation of actin dynamics drives the morphological changes in dendritic spines that are associated with alteration in synaptic strength ( Matus, 2000 Cingolani and Goda, 2008). During the last decade, numerous studies on postsynaptic signaling pathways demonstrated that the actin cytoskeleton plays a pivotal role in the formation and elimination, motility and stability, and size and shape of dendritic spines ( Halpain, 2000 Luo, 2002 Ethell and Pasquale, 2005 Tada and Sheng, 2006 Schubert and Dotti, 2007). Because the actin cytoskeleton is central to numerous cellular processes involving membrane dynamics such as cell motility and morphogenesis ( Pollard and Borisy, 2003 Carlier and Pantaloni, 2007), it is not surprising that dendritic spine formation and dynamics are determined by the actin cytoskeleton. The primary function of dendritic spines is to compartmentalize local synaptic signaling pathways and restrict the diffusion of postsynaptic molecules ( Nimchinsky et al., 2002 Newpher and Ehlers, 2009). These functional and structural changes at spines and synapses are believed to be the basis of learning and memory in the brain ( Holtmaat and Svoboda, 2009 Kasai et al., 2010). It is now widely believed that information in the brain can be stored by strengthening or weakening existing synapses, as well as appearance or disappearance of dendritic spines, which subsequently leads to the formation or elimination of synapses. Experimental evidence has shown that changes in spine morphology account for functional differences at the synaptic level ( Yuste and Bonhoeffer, 2001 Kasai et al., 2003). Most excitatory synapses in the mammalian brain are formed at tiny dendritic protrusions, named dendritic spines ( Bourne and Harris, 2008).
Precise control of the development and connectivity of synapses is critical for accurate neural network activity and normal brain function. Chemical synapses regulate the electric communication within neural networks and pass information directly from presynaptic axon terminals to postsynaptic dendritic regions. The capacity of neurons to function within neuronal circuits is mediated via specialized cell junctions called synapses. The human brain consists of a hundred billion neurons interconnected into functional neuronal circuits that underlie all our behaviors, thoughts, emotions, dreams, and memories.
0 kommentar(er)
