ULTIMATE COMPUTING - Quantum Consciousness Studies

Cytoskeleton/Cytocomputer 123
retraction of the cell process with subsequent formation of one or more growth
cones from the cell body. These observations together with ultrastructural
evaluations of treated cells, show that MT are necessary for the growth of
neurites, while actin filaments are essential to growth cone protrusion, a
phenomenon similar to amoeboid motion. A complex interplay of dynamic
activities of actin and MT are required for neurite sprouting and growth cone
extension. A composite view of growth cone activities is that, under direction of
the MT cytoskeleton, actin assembly generates protruding lamellipodia and
filopodia. Upon contact with an appropriate external substrate, filopodia adhere
and actin bundles form from the filopodium tip into the growth cone. Myosin
colocalizes with actin and the actin-myosin interaction produces a “muscle-like”
tension which provides an anchorage for the growth cone to the filopodium
adherence site. Lamellipodia, sheet-like ruffles, dart out among the finger-like
filopodia, particularly near points where decisions about branching are required.
Growth cone activities related to embryological differentiation are at the very
ground floor of intelligence. Evidence suggests that expression of tubulin, MAPS,
and other cytoskeletal elements are also important for determination of cell form
and function. Barra and colleagues (1974) have shown changes in alpha and beta
tubulin during brain development. The relative amount of alpha and beta tubulin
in rat brain peaks at about the 14th day after birth and then declines to adult levels
as a result of reduction in the rate of synthesis. The pattern of alpha and beta
tubulin genetic variants (isozymes) undergoes marked changes during brain
development with an increase in the variety of tubulin. For example, 3–4 different
types of beta tubulin are observed in embryonic mouse brain compared to 13
types in adult mice. Modification of alpha tubulin isotypes begins in the
embryonic brain whereas the beta tubulin modification begins to occur after birth
and coincides with a period of extensive outgrowth of processes and
synaptogenesis in the developing brain.
MAPs are also implicated in development and expression (Barra et al., 1974).
In the case of rat brain, two tau polypeptides are present in the first few days of
birth. By 35 days the adult pattern has emerged in which four major tau
polypeptides are apparent. The tau proteins of maturity are more adept at
promoting MT assembly than are the tau proteins of immaturity. MAP 1 can be
resolved into three groups of MAP 1A, 1B, and 1C. In chicken brain
development, MAP IA is initially present at low levels and increases during late
embryonic development and post hatching. Similarly, MT from 10 day old rat
brains are depleted of MAP 1A in comparison to adult brains. MAP 2 is restricted
to dendrites and cell bodies in adult brain and is particularly concentrated in
dendritic tips, Purkinje cell dendrites and all neuronal cell bodies, but absent from
axons. Localization of MAP 2 to the dendritic cytoskeleton begins at the earliest
times of appearance of dendritic outgrowths. In the adult brain, MAP 2A and
MAP 2B show up at different times and brain regions and are localized in
neurofilament rich axons.
All these changes are due to localized cytoskeletal mechanisms in addition to
genetic expression. The symphony of alterations in tubulin and MAP isozymes
has the score written in DNA, but its performance requires collective cooperative
interactions among the conductor and orchestra. For example, “tyrosination” of
beta tubulin occurs due to signaling and conditions present in the cytoplasm.
Modifications are involved in the control of the outgrowth of axonal and dendritic
processes, transport of constituents to the tips of the growing processes, cell
migration and division, and establishment and maintenance of synapses and the
adult form of the neuron. The temporal relationships between changes in neuronal
MT proteins and differentiation is essential to the final product: the brain/mind.