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Fig. 6. The anatomical and physiological characterization of an individual
depressor motor neuron (Dep MN) that, when viewed from above, has branches
that cover the whole neuropil. (Ai) This neuron was recorded from its main
neurite, the thick branch in the posterior-lateral quadrant that leads
directly to the axon. This depressor motor neuron has the 4th largest
extracellular spike of the 12 depressor motor neurons in this experiment. The
response properties of this neuron are summarized above the histogram of
extracellular spike amplitude. The neuron shows a resistance response (R) to
movements imposed on the CBCO strand and receives monosynaptic (E) and
polysynaptic (P) excitatory input. (Aii) This neuron is represented from three
different viewpoints. Based on previous work describing the anatomy of the
CBCO sensory neurons, it may receive monosynaptic input from CBCO sensory
neurons in the region indicated by the gray oval
(El Manira et al., 1991). In
particular, the lateralmost branch, which is also the most ventral, is a
likely area of contact. The direction indicated by the arrows is dorsal (d).
(B) This neuron depolarizes during the release (R) phase of a sinusoidal
movement (mvt) imposed on the CBCO strand. The resting membrane potential,
indicated by the dotted line, is –69 mV. The data shown are averages of
eight cycles triggered by a timing pulse that was phase locked to the movement
trace. (C) A ramp-and-hold stimulus reveals that this neuron is phaso-tonic.
It strongly depolarizes during the release phase of ramp movements (Ci; mvt)
and also shows a small, slowly decaying depolarization. Note that the ramp
movements may appear to be instantaneous (perfectly vertical) at the time
scale in Ci but are in fact ramps (Cii). The data shown are averages of eight
cycles. (D) Stimulation of the CBCO nerve reveals that this neuron receives
mono- and polysynaptic excitatory inputs, which were distinguished based on
the delay of the peaks of the compound EPSPs from the stimulus artifact marked
with an asterisk (5 ms and 22.7 ms, respectively).
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