The implication of subcortical motor centers in voluntary human activities

Resumen

The main objective of the present dissertation was to analyse the subcortical implications in the preparation and execution of complex voluntary movements. Three different tasks were selected on purpose. They all are everyday activities which although functionally related have differential characteristics. The first task was the sit-to-stand manoeuvre. Simple ballistic movements are executed faster in reaction time task paradigms when the imperative signal is accompanied by a startling auditory stimulus (SAS). We examined whether this effect also occurs in complex movements such as the sit-to-stand manoeuvre, taking into account both anticipatory postural adjustments and prime movers. Reaction time was significantly shortened when SAS was applied at an interval of 0 ms with respect to the imperative signal. The onset latency of EMG bursts recorded from tibialis anterior, lumbar paraspinal, quadriceps and biceps femoris muscles reduced proportionally to the shortening of take-off. However, these effects were not observed if SAS was delivered 150 ms after the imperative signal, when the manoeuvre had already started. Our results suggest that stimuli acting on subcortical motor structures speed-up but do not otherwise interfere with the execution of the motor programs. The second task was gait initiation followed by gait-pattern. Human gait involves a repetitive leg motor pattern that emerges after gait initiation. While the automatic maintenance of the gait-pattern may be under the control of subcortical motor centres, gait initiation requires the voluntary launching of a different motor program. We sought to examine how the two motor programmes respond to an experimental manipulation of the timing of gait initiation. Subjects were instructed to start walking as soon as possible at the perception of an imperative signal that, in some interspersed trials, was accompanied by a SAS. We recorded the gait phases and the EMG activity of four muscles from the leg that initiates gait. In trials with SAS, latency of all gait initiation-related events showed a significant shortening and the bursts of EMG activity had higher amplitude and shorter duration than in trials without SAS. The events related to gait-pattern were also advanced but otherwise unchanged. The fact that all the effects of SAS were limited to gait initiation suggests that startle selectively can affect the neural structures involved in gait initiation. Additionally, the proportional advancement of the gait-pattern sequence to the end of gait initiation supports the view that gait initiation may actually trigger the inputs necessary for generating the gait-pattern sequence. The third task was obstacle avoidance during walking. We hypothesized that inducing a StartReact effect may be beneficial in obstacle avoidance under time pressure, when subjects have to perform fast gait adjustments. Subjects walked on a treadmill and obstacles were released in specific moments of the step cycle. On average the EMG onset latency in the biceps femoris shortened by 20 % while amplitude increased by 50 %, in trials in which an auditory startle accompanied obstacle avoidance. The presentation of a startle increased the probability of using a long step strategy, enlarged stride length modifications and resulted in higher success rates, to avoid the obstacle. We also examined the effects of the startle in a condition in which the obstacle was not present in comparison to a condition in which the obstacle was visibly present but it did not fall. In the latter condition, the obstacle avoidance reaction occurred with a similar latency but smaller amplitude as in trials in which the obstacle was actually released. Our results suggest that the motor programmes used for obstacle avoidance are likely stored at subcortical structures. The release of these motor programmes by a SAS may combine intersensory facilitation and the StartReact effect.