Leech Heartbeat

A major goal in neuroscience is to understand how neural circuits coordinate behaviors. The use of invertebrate preparations has been invaluable in this endeavor, as their accessible nervous systems, composed of large, readily identifiable, and stereotyped neurons, allow for a thorough study of the relative timing of activity as well as synaptic connectivity between neurons. The medicinal leech (Hirudo sp.) is an ideal test case for this. The leech has been a very productive preparation for more than 70 years, helping to elucidate the organizing principles that underlie the production and coordination of behavior dating back to experiments on leech swimming in the late 1930s. For nearly 50 years, the leech heartbeat system has been intensely studied using varying approaches and stands, alongside the stomatogastric nervous system, as one of only a few preparations in which enough experimental and theoretical detail has been gathered to allow for a full discernment of the mechanism by which a neural circuit can coordinate stereotyped motor behaviors even when considering the impact of inter-individual variability. The heartbeat central pattern generator is a small network of seven bilateral pairs of interneurons. These interneurons form segment specific patterns of feedforward inhibitory synaptic input to a small set of bilateral motor neurons that innervate a pair of heart tubes. The heartbeat system has allowed for a detailed description of all the relevant parameters for the system, from premotor pattern, patterns of premotor synaptic connections, motor pattern and, finally, heart tube (i.e., constriction) pattern. This description has produced a cellular mechanism for motor control that accommodates inter-individual variability. The principles of coordination described here serve as a roadmap for elucidating how the motor output of neural microcircuits in more complex animals is coordinated.