Small steps and larger strides in understanding the neural bases of crawling in the medicinal leech

The nervous system of the medicinal leech (Hirudo species) has often provided elegant solutions to long-standing questions in the field of locomotor control. Such outcomes stem from its easily accessible and relatively large neurons, metameric organization, identified aminergic neuromodulation, and known role of action-selection neurons descending from the brain. Neural circuits underlying leech swimming were first revealed in the 1970s, but it has only been in the past two decades that crawling, the focus of this chapter, has shared the spotlight. In this review, we will discuss the functional architecture of crawling, its differences compared to swimming, and the inherent plasticity that enables the leech to recover its locomotion after injury. We will also underscore the full utility of the leech model while looking toward the future, focusing on the technological advances and expanded tool kits that will ensure that the leech persists as a valuable preparation for generations to come.