ULTIMATE COMPUTING - Quantum Consciousness Studies

66 From Brain to Cytoskeleton
appear to be evolutionary adaptations necessary for larger and more complex
nervous systems.
Sherrington is a key figure in the history of neuroscience. His concept of
integration by reflex centers illuminated possible modes of information
processing by neural structures. It is now appreciated that information transfer
functions occur at all levels of nervous system organization and include functions
now used in computers such as summation, ramp triggers, analog/digital
conversion, and logic.
4.3.2 Pulse Logic
Electrical signaling in the nervous system was, according to legend,
discovered during a demonstration given by Luigi Galvani to a class in the late
18th century. While Master Galvani was dissecting a frog, an electrostatic
generator (which discharged electric sparks) was being played with by a bored
student. Each time he drew a spark, the frog’s leg (held in a forceps by Galvani)
twitched. Galvani pursued this curious observation and published his treatise on
Animal Electricity in 1791. Galvani felt that animal electricity was the material of
excitation of nerves and of the contraction of muscles. Only after his death did his
followers demonstrate that animals indeed generated their own electrical currents,
rather than merely responding to applied electricity (Freeman, 1972).
The development of the microelectrode and appropriate electronic support
equipment after 1940 revealed discrete electrical nerve impulses to be ubiquitous
throughout the central and peripheral nervous systems. Electrophysiological
techniques afforded an approach to complex problems by recording the activity of
individual cells or small groups of cells. For example, the neural events involved
in perception of touch were studied by recording signals from a neuron that
terminates in the skin and whose function-touch sensation-was unambiguous.
These signals consist of brief electrical pulses about 0.1 volt in amplitude, they
last for about one millisecond, and they move along nerves at a speed up to 120
meters per second. Electrophysiological pulses within “Sherringtonian centers”
had been correlated with appropriate forms of behavior, for example bursts of
firings of neurons in medullary reticular formation were related to breathing.
Neural activity was conceptually limited to actuation of information stored in
specific neural regions. Newer approaches introduced the possibilities of
encoding by dynamic neural firing patterns.
In 1943 McCulloch and Pitts proposed that neurons might be approximately
described by the following assumptions: 1) the output activity of a neuron is an
“all or none” process, 2) a certain number of input synapses must be excited
within a brief period to excite the neuron’s output, and 3) the only significant
delay within the nervous system is synaptic delay. These “McCulloch-Pitts
neurons” connected in networks could, in theory, perform any computation.
Behaviorally significant information (“psychons”) were conceived as action
potential patterns in single neurons much like mass discharge in “centers” had
been previously considered. A finer structure of neural information processing
was perceived.
All or none action potentials which were the central element of pulse logic are
relatively new over the course of evolution compared to graded wave-like events
which occur in more primitive nerve nets. In the olfactory bulb, granule cells have
no axons and do not generate extracellularly detectable action potentials at all.
Thus the nerve impulse is not the only basis for transmission either between
neurons or within neurons. What is the basic substrate of information transfer and
processing within the nervous system McCulloch (1943) considered: