ULTIMATE COMPUTING - Quantum Consciousness Studies

68 From Brain to Cytoskeleton
meticulous detail with which neural connections are formed, and
initially supported the concept of the nervous system as a pulse logic
device, superseding the older concept of the brain as a switchboard
of reflex centers. Adrian’s discovery that neural pulse coding was
limited to a frequency/intensity coupling shifted emphasis to
connectionism per se. Because the electric signals the brain uses to
communicate among cells were seen as stereotyped, or nearly
identical, they were viewed as symbols which do not themselves
resemble the external world they represent. The consensus of
opinion regarding brain functions shifted to a concept in which the
shape of a neuron and its fiber origins and destinations determine
mental representation as part of a neural network. The meaning of
stereotyped signals was thought to be derived from specific
connections of neurons.
The high degree of precision with which nerve cells are connected to each
other and to different tissues in the periphery became emphasized in the
connectionist concept. Orderliness of connections formed during development
became viewed as essential for integrating mechanisms and representation of
information in some way. The nervous system appeared to be constructed as if
each neuron had built into it an awareness of its proper place in the system. The
question of mental representation refocused on the embryological development
during which synaptic connections were formed. During development, the neuron
grows towards its target, ignores some cells, selects others, and makes permanent
contact-not just anywhere on a cell but with a specified part of it. Further, neurons
behave as if they were aware when they have received an appropriate synaptic
connection. When they lose their synapses they respond in various ways. For
example, neurons or muscle fibers disconnected from their neuronal contacts may
die, but first develop “super-sensitivity” to their chemical neurotransmitter by
means of an abundance of new synaptic membrane receptor proteins. Cell death
or dysfunction induced by denervation occurs due to a loss of morphologicial
“trophism,” a neural function which conveys structural and functional material
and information by cytoskeletal axoplasmic transport. Atrophy, dystrophy and
spasticity of muscles and limbs which occur after strokes and other nervous
system insults are examples of the loss of normal trophism. Microtubules and
other cytoskeletal proteins responsible for trophism and axoplasmic transport also
allow growth and extension of neuronal axon growth cones, dendrites and
dendritic spines and thus play a key role in neural connections. Super-sensitivity,
spasticity, atrophy and dystrophy are examples of “synaptic plasticity,” changes
in connections or connection strength among neurons which are relevant to brain
and bodily function.
Association of learning with ongoing alteration of synaptic function was
considered by several late 19th century writers and was popularized due to the
Pavlovian and behaviorist influences of conditioned responses. Pavlov (1928)
proposed that conditioned reflexes are established by forming new connections
between cortical neurons that receive a conditioned stimulus (one accompanied
by a reward or punishment) and those that receive an unconditioned stimulus.
Once a new pathway was established the unconditioned stimulus would acquire
the same power of evoking the response that only the conditioned stimulus
originally possessed. Pavlov’s idea of a new connection became fused with
Donald Hebb’s (1949) concept of plastic changes in synaptic efficacy to correlate
with learning. Because it was believed that new fibers and therefore new synaptic
connections, could not grow in adults, long term facilitation of anatomically
preformed, initially nonfunctional connections became the likely alternative. This