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.
Garcia-Perez
Elizabeth
E
Scuola Internazionale Superiore di Studi Avanzati and Isituto Nazionale di Fisica della Materia, Via Beirut 7, 34014 Trieste, Italy. torre@sissa.it
Zoccolan
Davide
D
Pinato
Giulietta
G
Torre
Vincent
V
D000818:Animals; D004558:Electric Stimulation / Q000379:methods; D066298:In Vitro Techniques; D007865:Leeches / Q000502:physiology*; D009046:Motor Neurons / Q000502:physiology; D009119:Muscle Contraction / Q000502:physiology; D009126:Muscle Relaxation / Q000502:physiology; D011597:Psychomotor Performance / Q000502:physiology*; D014110:Touch / Q000502:physiology*