ULTIMATE COMPUTING - Quantum Consciousness Studies

76 From Brain to Cytoskeleton
an important mechanism of short term memory, and a link between the
cytoskeleton and synaptic level neural networks. Another link is axoplasmic
transport which maintains and supplies the form and functions of dendritic spines
and all neuronal synapses and structures.
4.3.6 Axoplasmic Transport
Synaptic membrane proteins including ion channels and receptors,
cytoskeletal protein structures which expel neurotransmitter vesicles, organelles
including mitochondria, and enzymes required for the synthesis and metabolism
of transmitters are manufactured only in the cell bodies of neurons where
biochemical machinery exists for protein synthesis and assembly (Golgi apparatus
and ribosomes). These materials or their precursors are then moved through the
axon (or dendrite) to the nerve terminal by a cytoskeletal mechanism similar to a
conveyer belt or bucket brigade. Time lapse photography of neurons in cell
culture show mitochondria (large organelles which produce chemical energy in
the form of ATP) floating down axons like barges on a river. Recent technology
such as video enhanced contrast microscopy (Allen, 1987) has shown vesicles
zipping along microtubules on the surfaces of axoplasm extruded from squid
neurons. Transmitters are synthesized throughout the entire neuron as the
enzymes which catalyze transmitter formation move along the cytoskeletal
apparatus from cell body to terminal. The highest enzymatic activity and
transmitter concentrations are reached in the terminal boutons. Thus the plasticity
of a synapse, its efficacy, readiness and threshold which appear to regulate
learning and memory in neural nets over time all depend on axoplasmic transport.
Several separate and independent axoplasmic transport processes have been
identified by following the movement of various tracers. The fastest move at a
rate of 400 millimeters per day (about 500 nanometers per second), the slowest
barely one millimeter per day. The mechanical parts of the system are
microtubules and contractile proteins attached to specific sites on microtubule
walls. These contractile proteins (dynein or kinesin) utilize chemical energy in the
form of ATP hydrolysis to contract in orchestrated sequences of bucket brigade
activity. What is not understood is the mechanism by which microtubules
orchestrate the cooperative sequential activities of the attached contractile
proteins. The main stream of axoplasmic transport can be stopped by drugs such
as colchicine, which depolymerizes microtubules. Axoplasmic transport generally
flows from the cell body toward the tips of fibers. In motor nerves, axoplasmic
transport flows in the same direction as the impulse traffic; in primary sensory
neurons it flows in the opposite direction to the sensory impulses. In dendrites, the
main flow is also from the cell body to the periphery. These are all examples of
anterograde axoplasmic flow. There is also simultaneous transport in the opposite
direction toward the cell body called retrograde axoplasmic flow which
apparently brings feedback information to the cell machinery to regulate the
production of transmitter enzymes and other materials. It might also return worn
or broken down cell constituents to be recycled. These trophic feedback
mechanisms create dynamic neurons capable of changing shape and function as
an adaptation to ongoing experience without excessive loss of old information.
Synapses, dendritic spines, dendritic branch patterns and membrane proteins are
continually changing, yet the memories they contain are somehow maintained by
the ever present and ever-changing cytoskeleton.