Marie-Hélène Canu

A 3D analysis of hindlimb motion during treadmill locomotion in rats after a 14-day episode of simulated microgravity

This study describes the effect of simulated microgravity in rat on kinematics and electromyographic activity during treadmill locomotion. The analysis was performed in rats submitted to 14 days of hindlimb unloading (HU), in rats submitted to hindlimb unloading and then authorized to recover for 7 days (REC), and in aged-matched control rats (CON). Movements of the right hindlimb were measured with a 3D-optical analyzer (SAGA3 system) and five small infrared-reflective disks positioned on the skin, recorded by three CCD cameras. Results showed that HU rats exhibited hyperextensions at the end of the stance phase. By contrast, during the major part of the step, the ankle was less extended than CON. Possible origins of the changes are discussed. This leads to the question of how important is sensory input in the regulation of the locomotor pattern after HU. Data obtained in REC animals showed that 1 week of recovery allowed the restoration of a good locomotor performance. However, the limb motion remained abnormal, and at contrary to HU rats: higher extension during the step, except at push-off when the limb was in hyperflexion. We concluded that simulated microgravity involves a dual adaptive process: a first one during unloading, and a second one during the period of recovery, which is not a simple return to initial characteristics of the locomotor pattern.

Effect of hindlimb unloading on locomotor strategy during treadmill locomotion in the rat

Electromyographic activity (EMG) was recorded from the soleus muscles of adult rats during treadmill locomotion after 7 and 14 days of hindlimb unloading, and after 7 days of recovery. Observation of the rats indicated that treadmill locomotion was disrupted after unloading since the animals had some difficulty in moving. Soleus muscle EMG analysis was performed. Onset and offset of bursts of activity were determined, and the relationships between step duration and cycle duration were analysed. Our main results were as follows: firstly, mean cycle duration was increased after 14 days of hindlimb unloading when walking at low speed. At high speed, no difference was observed. Secondly, linear regression analysis indicated that the relationships between step duration and cycle duration were altered after 7 days of unloading. Thirdly, adaptation occurred, since the normal slope and correlation coefficient were restored after 14 days of unloading. Fourthly, when speed increased, no variation of mean EMG was demonstrated after hindlimb unloading whereas an increase occurred in normal rats. Fifthly, video analysis showed that the rats presented frequent hyperextension of the hindlimb after unloading. These abnormal steps were more numerous when walking at low speed. These data would indicate that a transitory disruption of the soleus muscle motor pattern occurred after 7 days of unloading. This disruption depended on the treadmill belt speed. Possible origins of these modifications are discusssed.

A 3D analysis of fore- and hindlimb motion during locomotion: Comparison of overground and ladder walking in rats

The locomotor pattern, generated by the central pattern generator, is under the dependence of descending and peripheral pathways. The afferent feedback from peripheral receptors allows the animal to correct for disturbances that occur during walking, while supraspinal structures are important for locomotion in demanding situations such as ladder walking. Such walking, by regards to the control needed for accuracy of movements, is now widely used for description of consequences of nervous system dysfunction on motor performance. It is important to have a good knowledge of the changes in kinematic parameters according to walking conditions, since it might reflect different neural mechanisms. The aim of this work was to perform a 3D kinematic analysis of both hind- and forelimb during overground and ladder walking, to study qualitative and quantitative locomotor characteristics in different modes of locomotion. The analysis was performed on 5 rats. Movements of the right hind- and forelimb were evaluated using a 3D optical analyser, and EMG of the soleus and tibialis anterior muscles was synchronously recorded. Results indicate that kinematic and electromyographic characteristics of locomotion are dependent on the type of support. Changes were more obvious for hindlimb than for forelimb. Velocity, stride length and tibialis anterior burst duration were lower on ladder than on runway. In addition, during ladder walking, a protraction was noticed, rats bring their feet more rostral at the end of the swing phase. All these changes constitute an adaptive strategy to allow a better tactile activity with forelimbs and to avoid foot misplacement.

Short-term reorganization of the rat somatosensory cortex following hypodynamia-hypokinesia.

This study was performed to determine if hypodynamia-hypokinesia (HH) could induce a reorganization of the rat somatosensory cortex. The cortical hindpaw representation was determined by stimulating the limb and recording multi-unit cortical activity. The size of the cutaneous receptive fields was also measured. After 14 days of HH, the size of the cortical hindpaw representation was decreased. The proportion of small cutaneous receptive fields decreased while the large ones increased. After 7 days of HH, no change in the two studied parameters was noticed in five animals. In the other rats, a number of sites unresponsive to cutaneous stimulation or with high thresholds was observed. This study provides evidence of a plasticity of the somatosensory cortex induced by a situation that reduces both sensory and motor functions. The cortical reorganization occurs in two stages.

Effect of hindlimb unloading on interlimb coordination during treadmill locomotion in the rat

Abstract Effects of hindlimb unloading on interlimb coordination were examined in adult rats walking on a treadmill at moderate speed. In the first group of animals, the electromyographic activity (EMG) of soleus muscle of both hindlimbs was recorded after 7 and 14 days of unloading. In the second group, the EMG was recorded daily until the 14th day of unloading. The general organization of locomotion was preserved in the two groups whatever the duration of the unloading. The step cycles of the two hindlimbs were always strictly alternating. However, the locomotor pattern was very irregular. A lateral instability was observed. It was accompanied by an abduction of the hindlimbs, and frequent hyperextensions of the ankle when walking. The EMG analysis showed an increase in step cycle duration and in coactivation duration of the soleus muscles (i.e. in the double stance duration). In the rats recorded daily, mean EMG was dramatically reduced the 1st day of unloading, suggesting a decrease in the neural drive. Taken together, these data indicate that 14 days of hindlimb unloading can alter the neuromuscular pattern during locomotion. It is proposed that these changes are related to changes in the peripheral sensory information.

Activity-dependent regulation of myelin maintenance in the adult rat

Hindlimb unloading (HU) is known to induce changes in the neuromuscular system. However, no data describing the effects of HU on morphological characteristics of peripheral nerve have been reported so far. Therefore, we used soleus and radial nerves obtained from control and rats submitted to 14 days of HU to study the consequences of a decrease (soleus) or an increase (radial) in neural activity on its morphology. The mean number of fibers was not changed after HU. The soleus nerve axon diameter was weakly affected after HU, whereas the myelin thickness was reduced. For the radial nerve, both axon and fiber diameter were increased, and the myelin thickness and internodal distance were higher in HU rats. These results suggest that regulation of myelin maintenance undergoes plastic mechanisms. Neural activity and/or neural pattern might be essential in the maintenance of myelin sheath in adults.

A 14-day period of hindpaw sensory deprivation enhances the responsiveness of rat cortical neurons

Hypodynamia-hypokinesia (HH) is a model of hindpaw sensory deprivation. It is obtained by unloading of the hindquarters during 14 days. In this situation, the feet are not in contact with the ground and as a consequence, the cutaneous receptors are not activated; the sensory input to the primary somatosensory cortex (SmI) is thus reduced. In a previous study, we have shown that HH induced a cortical reorganisation of the hindlimb representation. The understanding of the mechanisms involved in cortical map plasticity requires a close examination of the changes in response properties of cortical neurons during HH. The aim of the present study was thus to study the characteristics of neurons recorded from granular and infragranular layers in hindlimb representation of SmI. A total of 289 cortical neurons were recorded (158 from control rats and 131 from HH rats) in pentobarbital-anaesthetized rats. Cutaneous threshold, cutaneous receptive fields, spontaneous activity (discharge rate and instantaneous frequency) and activity evoked by air-jet stimulation (response latency and duration, amplitude) were analysed. The present study suggests that activity-dependent changes occur in the cortex. The duration of the spike waveform presented two populations of spikes: thin-spike cells (<1 ms, supposed to be inhibitory interneurons) and regular cells (>1 ms). Thin-spike cells were less frequently encountered in HH than in control rats. The analysis of regular cells revealed that after HH (1) spontaneous activity was unchanged and (2) cortical somatosensory neurons were more responsive: the cutaneous threshold was reduced and the response magnitude increased. Taken together, these results suggest a down-regulation of GABAergic function.

Hypodynamia--hypokinesia induced variations in expression of fos protein in structures related to somatosensory system in the rat.

There have been many reports describing modifications of the sensory and motor cortex following various types of disuse. Hypodynamia--hypokinesia is characterized by the absence of weight-bearing and by a decrease in motor activity. We have shown a reorganization of the cortical cartography after hypodynamia--hypokinesia. In order to give an anatomical account for this cortical plasticity, we set out to determine whether cerebral and spinal structures exhibited variations of their neuronal activation. For this purpose, immunocytochemical detection of Fos protein was performed in the rat brain and spinal cord. Following stimulation of the sciatic nerve, Fos protein was detected in the primary and secondary somatosensory cortex in control rats and in rats submitted to an episode of 14 days of hypodynamia--hypokinesia. Results showed that the stimulation of the sciatic nerve induced an increase in the number of Fos-immunoreactive neurons in all these structures. Moreover, after hypodynamia--hypokinesia, the number of Fos-immunoreactive neurons was increased in the primary and secondary somatosensory cortex and in the spinal cord. These results provide evidence for a higher activation of cortical cells after hypodynamia--hypokinesia in comparison to controls. These data support the hypothesis that hypodynamia--hypokinesia contributes to the development of functional plasticity.

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.

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.