A central pattern generator underlies crawling in the medicinal leech.

Crawling in the medicinal leech has previously been thought to require sensory feedback because the intact behavior is strongly modulated by sensory feedback and because semi-intact preparations will only crawl if they can move freely. Here we show that an isolated leech nerve cord can produce a crawling motor pattern similar to the one seen in semi-intact preparations, which consists of an anterior-to-posterior wave of alternating excitatory circular and longitudinal motor neuron bursts in each segment. The isolated cord also reproduces the patterns of activity seen in semi-intact preparations for several other kinds of cells: the dorsal inhibitor cell 1, the ventral excitor cell 4, and the annulus erector motor neuron. Because this correspondence is so strong, there must be a central pattern generator in the isolated cord that can produce the basic motor pattern for crawling without sensory feedback. A quantitative analysis of the isolated motor pattern, however, reveals that isolated and semi-intact preparations have longer periods than the intact behavior and that there are deficiencies in the timing of motor neuron bursts in the isolated pattern. These results suggest that sensory feedback modulates the isolated central pattern generator to help produce the normal motor pattern.

Development of neuronal circuits and behaviors in the medicinal leech.

We are studying the neuronal mechanisms responsible for establishing circuitry underlying the local bending response in the medicinal leech. Local bending replaces an embryonic behavior, circumferential indentation, during the time of initial chemical synaptogenesis in leech embryos. We found that the electrical connections among the motor neurons are established first, about 5% of embryonic time (almost 2 full days) before chemical connections form. The inhibitory connections from muscle inhibitors to muscle excitors are, we hypothesize, responsible for the emergence of local bending. We have also found that the central processes of the excitors--but not the inhibitors--have much longer central processes when their peripheral processes are kept from contacting their target muscles. This system should allow us to test ideas about how individual neurons find their appropriate targets to form functional neuronal circuits.