LOCOMOTION IS ONE OF THE MOST FUNDAMENTAL of animal behaviors and is common to all members of the animal kingdom. As one might expect of such an essential behavior, the neural mechanisms responsible for the basic alternating rhythmicity that underlies locomotion are highly conserved throughout the animal kingdom, from invertebrates to vertebrates, and from the early vertebrates to primates. However, while the basic locomotor-generating circuits have been conserved, the evolution of limbs, and then of ever more complex patterns of behavior, has resulted in the development of progressively more complex spinal and supraspinal circuits (Figure 33–1).
The locomotor system. Multiple regions of the central nervous system interact to initiate and regulate locomotion. Locomotor networks in the spinal cord—the central pattern generators (CPGs)—generate the precise timing and patterning of locomotion. Proprioceptive sensory feedback modulates the activity of the locomotor CPG. The initiation of locomotion is mediated by neurons in the mesencephalic locomotor region (MLR) that project to neurons in the medial reticular formation (MRF) in the lower brain stem, which in turn project to the spinal cord. Descending fibers from the vestibular nuclei, pontomedullary reticular formation, and the red nucleus (brain stem nuclei) maintain equilibrium and modulate the ongoing locomotor activity. Cortical activity from the posterior parietal cortex (not illustrated) and the motor cortex is involved in the planning and execution of visually guided locomotion, while the basal ganglia (not illustrated) and cerebellum are important for the selection and coordination of locomotor activity.
Scientists have been intrigued with the neural mechanisms of locomotion since the beginning of the 20th century, when pioneering work by Charles Sherrington and Thomas Graham Brown showed that the isolated spinal cord of the cat is able to generate the basic aspects of locomotor activity and subsequently that this capacity was intrinsic to the spinal cord. Throughout the 20th century, major advances were made in detailing both the rhythm- and pattern-producing capacities of the spinal cord, leading ultimately to the groundbreaking concept of a central pattern generator for locomotion in the spinal cord. This single concept, more than any other, has driven research into the mechanisms underlying locomotor control since the 1970s, allowing a detailed electrophysiological examination of the neuronal mechanisms involved in the control of locomotion that is not possible for most other motor acts.
Most research throughout the 20th century on the spinal mechanisms mediating locomotion was performed on the cat, which remains an important model for studying many aspects of locomotor control. However, the complexity of the spinal circuits in mammals led to the search for simpler preparations that would allow a better understanding of the synaptic connectivity and neuronal properties responsible for the generation of locomotion. This search led to the development of the lamprey and the tadpole models (Box 33–1; Figures ...