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potentials). Analog signaling is also seen in synaptic transmission, which is
graded, and there is tonic release of transmitter.
36
Recordings using patch electrodes (electrodes that form seals on the
surface of the membrane rather than being inserted through it, as with
sharp electrodes) in C. elegans show that the membrane resistance is also
high; this means that in neurons as small as those in C. elegans, the cells are
essentially iso-potential, and action potentials may not be needed.
37
Analog signaling and synaptic transmission fit well with such properties.
It is perhaps more surprising that large nematodes function well without
having developed action potentials for long distance signaling. This may
be a factor that contributes to the upper limit in the size of nematodes.
Pairwise recordings from motor neurons showed the neuron
neuron
synapses. Besides the input to inhibitory motor neurons from their
opposite excitors (DE to VI, and VE to DI) mediating reciprocal inhibition
as mentioned above, there are synapses from inhibitors to the same side
excitors (VI to VE, and DI to DE;
Figure 6.2
B). Taken together with another
property of VI and DI motor neurons, namely that they generate slow
oscillations of the membrane potential when they are depolarized,
36,38
this
led to a hypothesis of how the alternating dorsal and ventral contractions
that characterize A. suum (and indeed nematode) locomotory behavior are
generated (
Figure 6.2
A,B). Once a dorsal excitor is depolarized (by input
from interneurons) it will depolarize VI and cause it to oscillate. The
inhibitory synapse from VI to VE will make VE oscillate out of phase with
VI, and the excitatory synapse from VE to DI will make DI respond with
oscillations in antiphase to VI; similarly the inhibitory synapse from DI to
DE will make DI and DE oscillate in antiphase. Peaks of DE oscillations
should produce dorsal contractions and ventral relaxations, and peaks in
VE oscillations should produce ventral contractions and dorsal relaxa-
tions, and the oscillations will produce alternating dorsal and ventral
contractions, which interchange with the characteristic frequency of the
e
=
FIGURE 6.2
Model for generation of locomotory waves, and dissections used for
electrophysiological testing of model. A. Predictions of alternation of contractions and
relaxations in dorsal muscle (DM) and ventral muscle (VM), and activity in dorsal excitatory
(DE) motor neurons, dorsal inhibitory motor neurons (DI), ventral excitatory motor neurons
(VE), and ventral inhibitory motor neurons (VI). B. Synaptic connectivity between motor
neurons and neuromuscular synapses. Open triangles, excitatory synapses; filled triangles,
inhibitory synapses. The sole input to inhibitory motor neurons is from the opposite exci-
tors. Excitors receive excitatory input from interneurons (IN). C. Fully dissected preparation
for recording from ventrodorsal commissures. A short length of worm is removed (dotted
lines) and slit open longitudinally and pinned flat. Intracellular microelectrode recordings
are made from commissures. D. Semi-intact preparation for correlating electrical activity in
commissures of motor neurons with body waveform propagation. A small region of the
body wall is slit open and pinned flat to allow recording from commissures. The rest of the
body is free to generate locomotory movements.
