ULTIMATE COMPUTING - Quantum Consciousness Studies

Models of Cytoskeletal Computing 159
(1965) also proposed that receptor cilia operated “by virtue of transmitting
mechanical signals to the base of the cilium.”
Biologist Jelle Atema (1973) of the Wood’s Hole Oceanographic Institute,
whose work had focused on acoustical perception at the cellular level, also linked
microtubules and sensory transduction. Noting common cilia-like structure in
sensory receptor organelles among a wide variety of organisms, Atema proposed
that sensory cilia conveyed environmental information to the rest of the cell. He
suggested that transduction occurred by propagated conformational changes in the
microtubule subunits which constituted these cilia. He argued that microtubules
were active functional units in reception and transduction of sensory information.
Atema reviewed conformational changes in MT tubulin subunit dimers
observed by a variety of authors and suggested that these occurred under
physiological conditions. For example, oscillations of sperm flagella are
apparently not controlled by their cell membrane but rather are direct properties of
their microtubules in the presence of ATP (Lindemann and Rikmensboel, 1972).
Atema concluded that a sequence of subunit conformational changes is likely to
occur in microtubules. In motor cilia and flagella, the distortion would originate at
the base of the structure and be propagated distally, resulting in wavelike or
whiplike motions which propel the organism. In sensory cilia, the distortion
apparently originates near the distal end of the ciliary MT and propagates in either
direction or at least proximally towards the cell body. There the signal may
propagate via either an excitable membrane or through the anchoring basal body
and cytoskeleton. Because mechanical deformation and/or local chemical
distortion are sufficient stimuli to start a propagating signal in sensory cilia,
Atema’s microtubule theory assumed that distortion of tubulin conformation by
any number of sources was sufficient to propagate a conformational wave. Thus
light energy, chemical bond energy, and mechanical forces could be transduced
by sensory cilia.
Atema’s view of MT information processing was an “all or none”
propagation by allosteric conformational changes along tubulin protofilaments.
His was the first theory to look beyond the global behavior of cilia to consider
conformational effects in tubulin components. Subsequent theories became more
elaborate to consider localized analog functions, switching, and collective
neighbor interactions among MT subunits.
8.2.2 MT Mechano-lonic Transducers/Moran and Varela
Biological sensory systems ranging from human inner ears to honey bee
hairplate receptors use sensory cilia to transduce mechanical deformation into
cognitive information like position in space (“proprioception”). Harvard
biologists Moran and Varela (1971) studied the proprioceptive transduction of
mechanical deformation in the legs of cockroaches. There, tactile spines contain
mechanoreceptor structures called “campaniform sensilla” which consist of a
single bipolar neuron from whose dendritic tip extends a modified cilium
containing 350 to 1000 microtubules. Moran and Varela determined that this
parallel bundle of MT was directly involved in the transduction of mechanical
deformation to the neuron.
All neurons are mechanical receptors, being stretch sensitive to cause ionic
currents. In the cockroach campaniform sensilla, the cilium is the only bridge and
transduction does not occur when MT are incapacitated with tubulin binding
drugs like colchicine and vinblastine (Moran and Varela, 1971). MT are thus
directly involved in proprioceptive determination of the system’s spatial relation
to its environment. Moran and Varela saw two possible roles for MT in their
mechanoreceptor function: as passive translation rods, or as generators of