Dendritic Cytoskeleton Computation Vision of Integration

In previous sections we discussed novel properties of the main components of the neuronal cytoskeleton, in particular ionic wave propagation along actin filaments, and interacting MAPs and C-termini in microtubular tubulins. This is the core of our current hypothesis. The cytoskeletal biopolymers, including actin filaments and microtubules, constitute the backbone, through possible wave propagation via those structures, which in turn interact with membrane components, including ion channels thus rendering novel modulatory effects to synaptic connections. The basic experimental finding that actin filaments support wave propagation [70] suggests the possibility of nonlinear, soliton-type propagation along "one-dimensional" biopolymers [93], provided that ionic condensation is supported [76, 89, 77]. The molecular dynamics mod-

Fig. 8.12. Basic scheme of a neuron showing dendrites, the soma and an axon. Depicted within the dendrites are microtubule networks (MTs interconnected by MAP2), actin filaments and a synapse. Actin bundles and microtubules are interconnected. Actin filaments are also connected to ion channels

eling of microtubules also raises the possibility that these polymeric structures are capable of transmitting electrostatic perturbations both collectively among neighboring carboxy-termini and from these, to adjacent MTs via linking proteins such as MAP2 [95, 1]. Before we begin to describe our hypothesis, let us illustrate the basic relations between the main components of the model. Fig. 8.12 shows a general scheme of the neuron, emphasizing the main components to be discussed later. Some of the basic morphological features include the Tau-MAP interconnected bundles of microtubules (MTs) present in the axon, and the axon hillock, a region rich in ion channels. The dendritic shaft contains microtubule networks (MTN), MAP2-interconnected MTs, and actin filaments and synapses. Microtubules and actin filaments are interconnected, and actin filaments are connected to ion channels.

Fig. 8.13 focuses on the dendritic shaft. The MTs are decorated by C-termini and interconnected by MAP2 (thick line). Connections between MTs and actin filaments are shown as well. Two types of synaptic bindings are depicted. On the upper left side, actin bundles bind to the postsynaptic density (PSD) of a spineless synapse. On the lower right hand, a spiny synapse is shown where the actin bundle enters the spine neck and binds to the PSD, which at the other end, is connected to the MTN. This is further detailed in Fig. 8.13, right, showing the actin bundle linked to the PSD.

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