Dynamics and Reproducibility of a Moderately Complex Sensory-Motor Response in the Medicinal Leech

Local bending, a motor response caused by mechanical
stimulation of the leech skin, has been shown to be remarkably
reproducible, in its initial phase, despite the highly variable firing of
motoneurons sustaining it. In this work, the reproducibility of local
bending was further analyzed by monitoring it over a longer period of
time and by using more intact preparations, in which muscle activation
in an entire body segment was studied. Our experiments showed
that local bending is a moderately complex motor response, composed
of a sequence of four different phases, which were consistently
identified in all leeches. During each phase, longitudinal and circular
muscles in specific areas of the body segment acted synergistically,
being co-activated or co-inhibited depending on their position relative
to the stimulation site. Onset and duration of the first phase were
reproducible across different trials and different animals as a result of
the massive co-activation of excitatory motoneurons sustaining it. The
other phases were produced by the inhibition of excitatory and
activation of inhibitory motoneurons, and also by the intrinsic relaxation
dynamics of leech muscles. As a consequence, their duration and
relative timing was variable across different preparations, whereas
their order of appearance was conserved. These results suggest that,
during local bending, the leech neuromuscular system 1) operates a
reduction of its available degrees of freedom, by simultaneously
recruiting groups of otherwise antagonistic muscles and large populations
of motoneurons; and 2) ensures reliability and effectiveness of
this escape reflex, by guaranteeing the reproducibility of its crucial
initial phase.