Neural control of locomotion

Locomotion is a particularly richly studied but frustrating aspect of biology. Much is known about individual components of the process, but there is no general hypothesis for how it works. Vertebrate locomotion is distinguished from invertebrate locomotion by the conjunction of two features. The skeletal movements are complex but they are usually regular, repeating sequences (see Hildebrand page 766 this issue), and most of the organisms are massive enough that the amount of energy and force invested in moving the whole animal constitutes a significant percentage of its metabolic economy and is a significant determinant of the details of its body plan (see Biewener page 776 this issue). At the phenomenological level, we have good descriptions of the movements and even the mechanics involved in walking, swimming, and flying. At reductionistic and comparative levels, we have catalogued and differentiated the properties of skeletons (see Gordon page 784 this issue), muscles (see Weeks page 791 this issue), and sensory and motor neurons. We even know that the critical

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