[From Bruce Abbott (960118.0910 EST)]
John Anderson (960118.0745) --
It seems to me that finding the various components of control systems in real
nervous systems is tricky business, even in very simple ones. For example,
consider the gill withdrawal circuit of the marine snail Aplysia, . . .
John, I looked into this (briefly) about four or five months ago. I wonder
if there is something missing from the diagram, as there is no indication of
muscle spindles or other sensors of muscle length/force. As currently
reported, the diagram's muscle-contraction element is a pure S-R mechanism,
lacking feedback about the state of the muscle.
There IS feedback in the larger system comprising the mantle
touch-receptors, conducting neurons, motor neurons, and gill withdrawal
muscle. The diagram should look something like this:
ref (touch)
>
+--------->[comp]--------------------+
> >
> v
[touch recept] [muscle]
^ |
+----[contact]---[mantle position]<--+
^
>
disturbance
In this system what is being controlled is the perception of contact with
the mantle. The actual system probably responds more readily to sudden
changes than to gradual ones; this effect would be mediated at the
receptor/sensory neuron level. I haven't represented this explicitly in the
diagram. If the touch reference is at some relatively low value, light
stimuluation would not produce gill-withdrawal. Stronger stimuluation would
produce an error which would then produce muscle contraction; muscle
contraction would withdraw the mantle, pulling the mantle/gill/siphon system
out of contact with the source of touch-stimulation, reducing the error to
zero. If the muscle reacts strongly, so that it contracts rapidly and
fully, but relaxes only slowly, the system as diagrammed would oscillate
rather than stabilizing at some mantle position that produces light touch
stimulation; this oscillation appears to characterize the behavior of the
actual system if the reopening of the mantle puts the mantle back into
contact with the source of stimulation. What pulls the mantle open? Is it
merely the relaxation of the gill-withdrawal muscles? What produces the
gradual re-opening, as opposed to the sudden withdrawal? Again, I'm not
sure that the diagrams usualy given for aplysia are complete enough for a
full control-system analysis.
The habituation-sensitization mechanisms seem to affect the mantle-contact
control system by altering the output gain: the strength of gill-muscle
contraction under a given rate of sensory-neural stimulation. This appears
to be accomplished at the axonal-axonal junction at the sensory neuron
terminals.
How's that for starters?
Regards,
Bruce