Neurophysiology of locomotor automatism

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.

Neural control of locomotion; Part 1: The central pattern generator from cats to humans

“…Forelimb movements may induce hindlimb stepping in forward gait. Grillner and Zangger (1984) claimed that interlimb coordination during hindlimb walking deteriorated fol lowing deafferentation in the 'mesencephalic' cat [17], This is a decerebrated cat (obtained after intercollicular transection at the level of the brain stem) in which complete quadrupedal stepping can be evoked by elec trical stimulation of a specific brainstem site below the transection (the mesencephalic locomotor region (MLR), [19]). Depending on the strength of the stimu lus, different gait patterns could be produced (walking» trotting, galloping).…”

“…Reticulospinal, rubrospinal and vestibulospinal path ways are capable of influencing locomotor related neu ral circuits in the spinal cord [110]. Both amplitude modulation of EM G activity in different phases of the step cycle and shifts in timing of rhythm are seen as a result of stimulation of the descending tracts in the decerebrated cat [19,107] (for review see [13]). …”

Lamina VII neurons are rhythmically active during locomotor-like activity in the neonatal rat spinal cord

“…Other regions of the brainstem are also capable of generating locomotion (for reviews see L7 Shik et al 1976;Grillner 1975 Noga et al 1991).…”

Timing factors in the coordination of speech movements

“…For most functional multiarticulate tasks, the nervous system apparently establishes consistent relations among movements and muscle activity of multiple structures to reduce the control complexity (Bernstein, 1967;Gel'fand et al, 197 1;Turvey, 1977). Such diverse observations as constant phase relations during locomotion, consistent patterns of activation of functional synergists during postural stabilization, and invariant relations between elbow and shoulder joints demonstrate task-dependent simplifications apparently facilitating coordination (Grillner, 1975;Shik and Orlovsky, 1976;Nashner, 1977;Soechting and Lacquaniti, 1981;Lacquaniti and Soechting, 1982). In contrast to such basic motor actions, the specific manner in which speech production relies on such simplifying strategies to facilitate coordination is not well understood.…”