[From Bruce Abbott (950518.1145 EST)]
The May-June 1995 issue of _American Scientist_ contains several articles of
potential interest to PCTers, including the one I'll describe here, entitled
"Dynamic Networks of Neurons," authored by Simmers, Meyrand, and Moulins.
The topic of the reported research is a "central pattern generator" that
produces rhythmic, coordinated ingestion and food-processing movements in
the foregut of the lobster. The pyloric network in the somatogastric
ganglion consists of 14 neurons, most of which are motor neurons, which are
interconnected both via inhibitory synapses _and_ a bidirectional
_electrical_ pathway. A sine-wave-like pattern of ion channel
openings/closings on the cell membranes of some of the motor neurons
produces a rhythmic pattern of polarization/depolarization in these cells
that contributes to the production of a regular pattern of impulses to
produce a specific sequence of muscle contractions. Normally these outputs
generate activities in three separate areas of the foregut for grinding and
filtering the food. However, input from "pyloric suppressor neurons"
produces a functional change in the way the pyloric cells interact; as a
result the three independent activities cease and are replaced by a
coordinated action which produces swallowing and motion of the food through
the gut. The pyloric system has been shown to operate normally after
dissection (when placed in a physiological saline solution without any
sensory connections that might supply feedback; thus the system is
open-loop. (This makes sense given the "mission" of the system; however, it
may be that the switch from grinding/filtering to swallowing/moving is
closed-loop, a possibility not touched on in the article.)
What I think is of interest to PCTers in the article is not that the system
is an open-loop one, but the way in which its pattern of output can be
changed, effectively reorganizing the circuit. The pattern is not only
influenced by the suppressor-neuron activity, but also is sensitive to the
levels of various neurotransmitters, which can alter such characteristics as
the threshold for initiating an action potential and whether there is just a
single spike or a sustained depolarization of the neuron the produces a
high-frequency succession of impulses. The authors note that understanding
of neuron functioning is changing:
Until rather recently, most neurobiologists considered neurons as
logical threshold units that sum incoming signals and then linearly
transform the analog input into digital output, or pulses. We now
know, however, that neurons are not simply "all-or-nothing" devices
that can only be on or off. Instead, they can possess additional
bioelectrical properties that have far-reaching consequences for their
computational ability and function.
The authors do seem to buy into the notion that coordinated behavior such as
walking is produced by "rhythm-generating circuits" (controlling output
rather than input), but despite that defect the article is worth reading for
the information it provides about how pre-organized systems of neurons can
effectively become temporarily reorganized to perform alternate functions,
and for its tutorial on current understanding of neuronal function.
Regards,
Bruce