Load-regulating mechanisms in gait and posture: Comparative aspects

How is load sensed by receptors, and how is this sensory information used t o guide locomotion? Many insights in this domain have evolved from comparat ive studies since it has been realized that basic principles concerning loa d sensing and regulation can be found in a wide variety of animals, both ve rtebrate and invertebrate. Feedback about load is not only derived from spe cific load receptors but also from ogler types of receptors that previously were thought to have other functions. In the central nervous system of man y species, a convergence is found between specific and nonspecific load rec eptors. Furthermore, feedback from load receptors onto central circuits inv olved in the generation of rhythmic locomotor output is commonly found. Dur ing the stance phase, afferent activity from various load detectors can act ivate the extensor part in such circuits, thereby providing reinforcing for ce feedback. At the same time, the flexion is suppressed. The functional ro le of this arrangement is that activity in antigravity muscles is promoted while the onset of the next flexion is delayed as long as the limb is loade d. This type of reinforcing force feedback is present during gait but absen t in the immoble resting animal.