Roh et al. (2011)

Modules in the Brainstem and Spinal Cord Underlying Motor Behaviors.

Jinsook Roh, Vincent C K Cheung, and Emilio Bizzi.

Previous studies using intact and spinalized animals have suggested that coordinated movements can be generated by appropriate combinations of muscle synergies controlled by the central nervous system (CNS). However, which CNS regions are responsible for expressing muscle synergies remains an open question. We address whether the brainstem and spinal cord are involved in expressing muscle synergies used for executing a range of natural movements. We analyzed the electromyographical (EMG) data recorded from frog leg muscles, before and after transection at different levels of the neuraxis - rostral midbrain (brainstem preparations), rostral medulla (medullary preparations), and the spinal-medullary junction (spinal preparations). Brainstem frogs could jump, swim, kick, and step, while medullary frogs could perform only a partial repertoire of movements. In spinal frogs, cutaneous reflexes could be elicited. Systematic EMG analysis found two different synergy types: (1) synergies shared between pre- and post-transection states, and (2) synergies specific to individual states. Almost all synergies found in natural movements persisted after transection at rostral midbrain or medulla, but not at the spinal-medullary junction for swim and step. Some of pre-transection- and post-transection-specific synergies for a certain behavior appeared as shared synergies for other motor behaviors of the same animal. These results suggest that the medulla and spinal cord are sufficient for the expression of most muscle synergies in frog behaviors. Overall, this study provides further evidence supporting that motor behaviors may be constructed by muscle synergies organized within the brainstem and spinal cord, and activated by descending commands from supraspinal areas.

J Neurophysiol, 2011 (in press)

Lamy et al. (2009)

Impaired efficacy of spinal presynaptic mechanisms in spastic stroke patients.

Jean-Charles Lamy, Isabelle Wargon, Dominique Mazevet, Zaïd Ghanim, Pascale Pradat-Diehl, and Rose Katz.

Pathophysiological mechanisms underlying spasticity have been the subject of many studies. These studies performed in various kinds of spastic patients have revealed abnormalities in many spinal pathways controlling motoneurone discharge. Unfortunately, the pathophysiological mechanisms responsible for the development of spasticity remains nevertheless largely unknown since most of the previous studies failed to reveal a link between the characteristics of spasticity (severity, time course) and that of the dysfunction of a given perturbed spinal pathway. In the present series of experiments, we focused on the study of presynaptic mechanisms acting at the synapse fibre Ia-motoneurone since monosynaptic reflexes are enhanced in spasticity. Two presynaptic mechanisms have been described in both animals and humans: presynaptic Ia inhibition and post-activation depression. By increasing the number of subjects in comparison with previous studies (87 patients and 42 healthy controls) we have been able to show that these two mechanisms are unequally impaired in stroke patients depending on (i) the duration of the disease (acute, defined as less than 3 months after the causal lesion, or chronic, defined as more than 9 months after the causal lesion), (ii) the side considered (affected or unaffected) and (iii) the severity of spasticity. In this respect, only post-activation depression amount was found to be highly correlated with the severity of spasticity. Although not a definitive proof, this correlation between severity of spasticity and changes in a given spinal pathway lead us to conclude that the impairment of post-activation depression is likely one of the mechanisms underlying spasticity. On the contrary, changes in presynaptic Ia inhibition appear to be a simple epiphenomenon, i.e. a basic correlate of the brain lesions. It is argued that plastic changes develop from the disuse due to motor command impairment in both pathways.

Brain, 2009 vol. 132 (Pt 3) pp. 734-748

Sher et al. (2010)

Spatiotemporal organization of neuronal activity in the cervical cord of behaving primates.

Yoel Sher, Oren Cohen, Nofya Zinger, Ran Harel, Boris Rubinsky, and Yifat Prut.

Spinal neurons operate as a processing link that integrates descending and peripheral information and in turn, generates a specific yet complex muscle command. The functional organization of spinal circuitry during normal motor behavior dictates the way in which this translation process is achieved. Nonetheless, little is known about this organization during normal motor behavior. We examined the spatial organization of neural activity in the cervical spinal cord of behaving primates performing an isometric wrist task by estimating the averaged intraspinal activity of neuronal populations. We measured population response profiles and frequency content around torque onset and tested the tendency of these profiles to exhibit a specific organization within the spinal volume. We found that the spatial distribution of characteristic response profiles was non-uniform; namely, sites with a specific response profile tended to have a preferred spatial localization. Physiologically, this finding suggests that specific spinal circuitry that controls a unique feature of motor actions (with a particular task-related response pattern) may have a segregated spinal organization. Second, attempts to restore motor function via intraspinal stimulation may be more successful when the spatial distribution of these task-related profiles is taken into account.

Front Neurosci, 2010 vol. 4 p. 195