ULTIMATE COMPUTING - Quantum Consciousness Studies

Cytoskeleton/Cytocomputer 99
possess “ATPase activity,” that is they .contract conformationally due to
hydrolysis of ATP. Dynein arms contract in waves and mediate sliding between
the adjacent outer tubules to drive ciliary and flagellar motility. Dynein arms
attached to MT in neuronal axons act collectively to pass material in “bucket
brigade” axoplasmic transport. These and other MT “bending sidearms” will be
discussed in Section 5.5.2.
Other MAPs are bridges between parallel MT, filaments, organelles and the
“microtrabecular lattice” and appear to integrate microtubules with the rest of the
cytoplasm (Figure 5.6). MAPs from mammalian brain neurons have been
characterized into three main groups. These are high molecular weight (greater
than 100 kilodalton) MAP 1 and MAP 2, and a number of closely related 56–62
kilodalton proteins designated as “tau.” Studies in rats show an increase in
number and an altered distribution of the various forms of tubulin, high molecular
weight MAPs, and tau which correlate with rat brain development and learning.
Both MAP 2 and tau are heat stable, stimulate MT polymerization (lower Cc) and
share binding domains on intact MT; they are heterogeneous protein families
whose functions are unknown. MAP 2 is concentrated in cell bodies and dendrites
of neurons, found in a ratio of MAP 2 to tubulin of one to twelve, occurs in only
small amounts in axons and is absent in glia. Conversely, brain tau is largely
confined to axons. Both MAP 2 and tau are phosphorylated by cyclic AMP
dependent protein kinases, proteins which amplify effects of calcium ions.
Intracellular levels of calcium ions, in turn, are regulated by diverse extracellular
signals including neurotransmitters, hormones and electrical factors to regulate
cell shape, motility, secretory processes and other functions. Thus these MAPs
can mediate diverse inputs into the cytoskeleton. Each MAP attached to an MT
may function like a “synapse” in a parallel, laterally interconnected network.