No abstract

It had long been known that the decapitated cock can cross a yard. During the last century an automatic mechanism controlling stepping movements has also been found in other vertebrates. The system controlling locomotion has many features similar to these systems controlling other natural movements: respiration (28), micturition (98), scratching (154), mastication (33), etc. Today we know that there are spinal automatisms for each limb generating its stepping movements. Activity of these automatisms depends essentially on the afferent inflow from the moving limbs. There also is interaction of the limbs during locomotion that promotes their coordination. The existence of two descending systems with different functions in the control of locomotion (Fig. 1) also can be considered as an established fact. Activity of a number of neurons involved in the control of locomotion has been studied directly during locomotion in decorticate, thalamic, and mesencephalic cats. To explain the experimental data at hand, several hypotheses of organization of the spinal automatism of stepping have been forwarded: a chain-reflex hypothesis, a hypothesis of two reciprocal half-centers, and a ring hypothesis (Fig. 2). Although general features of the system controlling locomotion are more or less clear, many questions are not yet answered. It is unknown what relative contributions to motoneuronal activity are made by proprioceptive reflexes versus influences from the automatism of stepping. Furthermore the structure of the spinal stepping automatism is not known. It is not clear if the spinal stepping automatisms of the forelimbs are as potent as those of the hindlimbs. The descending system responsible for activation of the spinal automatism of stepping has not yet been identified in direct experiments. The inputs and outputs of the subthalamic and midbrain "locomotor" regions have not been found, and we know almost nothing about intrinsic interaction of neurons in these regions. The role of inhibitory thalamic influences is scarcely known. Finally, we have no data concerning the influence of either cortical (42, 186) or visual mechanisms in locomotor control.

No abstract

A deviation from the dorsal-side-up body posture in quadrupeds activates the mechanisms for postural corrections. Operation of these mechanisms was studied in the rabbit maintaining balance on a platform periodically tilted in the frontal plane. First, we characterized the kinematics and electromyographic (EMG) patterns of postural responses to tilts. It was found that a reaction to tilt includes an extension of the limbs on the side moving down and flexion on the opposite side. These limb movements are primarily due to a modulation of the activity of extensor muscles. Second, it was found that rabbits can effectively maintain the dorsal-side-up body posture when complex postural stimuli are applied, i.e., asynchronous tilts of the platforms supporting the anterior and posterior parts of the body. These data suggest that the nervous mechanisms controlling positions of these parts of the body can operate independently of each other. Third, we found that normally the somatosensory input plays a predominant role for the generation of postural responses. However, when the postural response appears insufficient to maintain balance, the vestibular input contributes considerably to activation of postural mechanisms. We also found that an asymmetry in the tonic vestibular input, caused by galvanic stimulation of the labyrinths, can affect the stabilized body orientation while the magnitude of postural responses to tilts remains unchanged. Fourth, we found that the mechanisms for postural corrections respond only to tilts that exceed a certain (threshold) value.

No abstract