Influence of emotional stimuli on lower limb cutaneous reflexes during human gait.

Previous research has shown that cutaneous reflexes are modulated when walking with a threat to stability. It is unclear if this reflex modulation is purely related to the context of the imposed threat or if emotional changes associated with the threat exert an independent influence on reflex excitability. This study investigated the influence of emotional stimuli on lower limb cutaneous reflexes during treadmill walking. Twenty-eight healthy young adults walked at a self-selected pace while viewing pictures that manipulated emotional arousal and valence (confirmed with electrodermal and self-report measures). Throughout each trial, cutaneous reflexes were evoked by electrically stimulating the sural nerve at heel contact, mid-stance, or toe off. Surface electromyography of the ipsilateral soleus (SOL), medial gastrocnemius (MG), tibialis anterior (TA), biceps femoris (BF), and vastus lateralis (VL) was recorded to assess reflexes. Highly arousing pictures, independent of valence, significantly facilitated TA, and trended toward facilitating SOL and BF reflexes during mid-stance. Unpleasant pictures, independent of arousal, significantly reduced reflex amplitudes in BF during mid-stance and TA during toe off. While changes in background muscle activity and step cadence were observed, they did not correlate with reflex changes. This study provides the first evidence that emotional stimuli exert an independent influence on cutaneous reflex excitability during gait. As cutaneous reflexes contribute to stability during gait, these findings support the notion that emotional state influences important sensorimotor processes underlying balance control.

Hindlimb unloading affects cortical motor maps and decreases corticospinal excitability.

A chronic reduction in neuromuscular activity through prolonged body immobilization of humans results in muscle atrophy and weakness as well as motor tasks performance impairment, which is correlated to a change in corticospinal excitability. In rats, hindlimb unloading (HU) is commonly used to mimic the effects of confinement to bed in patients. Several studies have reported changes in the representation of the somatosensory cortex in rodents submitted to HU or sensorimotor restriction by casting: remapping and enlargement of receptive fields, changes in the response of layer 4 neurons to peripheral stimulation. However, we have no data about motor cortical maps in rats submitted to a period of motor restriction during adulthood. Therefore, the objectives of the present study were twofold: to determine, in control rats and in rats submitted to a 14-day period of HU, the size and organization of hindlimb representation in the M1 cortex and to evaluate the overall excitability of M1 cortex by determining the stimulation thresholds. HU led to a dramatic decrease in the hindlimb representation on the M1 cortex (-61%, p<0.01). In addition, current thresholds for eliciting a movement were increased. The toes were less strongly affected by HU than other joint. Our main conclusion is that HU dramatically affects the organization and functioning of cortical motor maps and decreases corticospinal excitability.

The time course of autonomic parameters and muscle tension during recovery following a moderate cognitive stressor: dependency on trait anxiety level.

It has proved difficult to demonstrate the differences in physiological response in individuals with contrasting trait anxiety levels. The purpose of this study was to examine tonic autonomic activity and muscle tension in male subjects with different levels of trait anxiety during and especially after a stressful situation. Twenty-eight low anxiety (LA) and thirty-one high anxiety (HA) individuals performed a video-recorded Stroop color-word interference test. Heart rate, skin conductance level, finger temperature, electromyographic activity of gastrocnemius and trapezius muscles were recorded; state anxiety and valence of thought content were assessed. Physiological activity increased during the stressful situation, whatever the group. However, prolonged gastrocnemius muscle tension and prolonged decrease in finger temperature were found in HA subjects after the stressful situation, but not in LA subjects. These results suggest that physiological recovery following a moderate cognitive stressor could depend on trait anxiety level.

Chapter 16--spinal plasticity in the recovery of locomotion.

Locomotion is a very robust motor pattern which can be optimized after different types of lesions to the central and/or peripheral nervous system. This implies that several plastic mechanisms are at play to re-express locomotion after such lesions. Here, we review some of the key observations that helped identify some of these plastic mechanisms. At the core of this plasticity is the existence of a spinal central pattern generator (CPG) which is responsible for hindlimb locomotion as observed after a complete spinal cord section. However, normally, the CPG pattern is adapted by sensory inputs to take the environment into account and by supraspinal inputs in the context of goal-directed locomotion. We therefore also review some of the sensory and supraspinal mechanisms involved in the recovery of locomotion after partial spinal injury. We particularly stress a recent development using a dual spinal lesion paradigm in which a first partial spinal lesion is made which is then followed, some weeks later, by a complete spinalization. The results show that the spinal cord below the spinalization has been changed by the initial partial lesion suggesting that, in the recovery of locomotion after partial spinal lesion, plastic mechanisms within the spinal cord itself are very important.

Determinants of locomotor recovery after spinal injury in the cat.

After a spinalization at the most caudal thoracic spinal segment, the cat can recover locomotion of the hindlimbs when they are placed on a moving treadmill. This chapter summarizes some of the determinants of such a dramatic recovery of motor function. Fundamental to this recovery is undoubtedly the genetically based spinal locomotor generator, which provides an essential rhythmicity to spinal motoneurons and hence the musculature. Other factors are also important, however. Sensory feedback is essential for the correct expression of spinal locomotion because spinal cats, devoid of cutaneous feedback from the hindfeet, are incapable of plantar foot placement. The neurochemical environment also adapts to spinalization, i.e., the loss of all modulation by descending monoaminergic pathways. Post-transection spinal rhythmicity then becomes more dependent on glutamatergic mechanisms. Finally, we argue that the mid-lumbar spinal segments evolve to play a crucial role in the elaboration of spinal locomotion as their inactivation abolishes spinal locomotion. In summary, the above findings suggest that the recovery of spinal locomotion is determined by a number of factors, each of which must now be more fully understood in the ever-continuing effort to improve the rehabilitation of spinal-cord-injured subjects.

Effects of hypodynamia-hypokinesia on somatosensory evoked potentials in the rat.

The aim of this study was to determine if a prolonged period (7 or 14 days) of hypodynamia-hypokinesia (HH) affects the conduction of afferent input and the cortical and spinal responsiveness. Acute recordings of cortical and spinal somatosensory evoked potentials (SEPs) were performed after stimulation of the sciatic nerve in control rats and in rats submitted to 7 or 14 days of HH. HH was obtained by unloading the hindquarter. HH induced some subtle modifications in the SEP characteristics. Latency was increased for the spinal and cortical SEPs after 7 days of HH, and restored after 14 days of HH. A decrease in the amplitude was observed after 14 days of HH for the cortical SEP only. At the end of the experiment, the compound action potential of the sciatic nerve was recorded in vitro in order to evaluate the mean conduction velocity. Results indicate that the nerve velocity was reduced after 14 days of HH. The results also suggest that sensory conduction and/or cortical and spinal excitability are changed after HH.