Muscle Tone Physiology and Abnormalities

The simple definition of tone as the resistance to passive stretch is physiologically a complex interlaced network encompassing neural circuits in the brain, spinal cord, and muscle spindle. Disorders of muscle tone can arise from dysfunction in these pathways and manifest as hypertonia or hypotonia. The loss of supraspinal control mechanisms gives rise to hypertonia, resulting in spasticity or rigidity. On the other hand, dystonia and paratonia also manifest as abnormalities of muscle tone, but arise more due to the network dysfunction between the basal ganglia and the thalamo-cerebello-cortical connections. In this review, we have discussed the normal homeostatic mechanisms maintaining tone and the pathophysiology of spasticity and rigidity with its anatomical correlates. Thereafter, we have also highlighted the phenomenon of network dysfunction, cortical disinhibition, and neuroplastic alterations giving rise to dystonia and paratonia.

On theory of motor synergies

Recently Latash, Scholz, and Schöner (2007) proposed a new view of motor synergies which stresses the idea that the nervous system does not seek a unique solution to eliminate redundant degrees of freedom but rather uses redundant sets of elemental variables that each correct for errors in the other to achieve a performance goal. This is an attractive concept because the resulting flexibility in the synergy also provides for performance stability. But although Latash et al. construe this concept as the consequence of a "neural organization" they do not say what that may be, nor how it comes about. Adaptive model theory (AMT) is a computational theory developed in our laboratory to account for observed sensory-motor behavior. It gives a detailed account, in terms of biologically feasible neural adaptive filters, of the formation of motor synergies and control of synergistic movements. This account is amplified here to show specifically how the processes within the AMT computational framework lead directly to the flexibility/stability ratios of Latash et al. (2007). Accordingly, we show that quantitative analyses of experimental data, based on the uncontrolled manifold method, do not and indeed cannot refute the possibility that the nervous system tries to find a unique (optimal) solution to eliminate redundant degrees of freedom. We show that the desirable interplay between flexibility and stability demonstrated by uncontrolled manifold analysis can be equally well achieved by a system that forms and deploys optimized motor synergies, as in AMT.

Ragnar Granit, 30 October 1900 - 11 March 1991

Ragnar Granit was born in Finland in 1900, during the period when Finland was still a Grand Duchy within the Russian Empire. He was the son of an affluent forester, and the grandson of a sea-captain. His father’s family came from the small island of Korpo in the Åboland archipelago, which reaches from Finland to Sweden. His family had Swedish as their native tongue, as most Finns living in the coastal areas around the Baltic have had for centuries. Granit grew up in Helsinki and started school early. He received a classical training in the ‘Swedish Normal Lyceum’ with latin and to some degree greek - a schooling which, according to his own account, fostered logical reasoning. His schooling was interrupted in the spring of 1918, when civil war arose as Finland succeeded in breaking loose from Russia, as a consequence of the Bolshevik revolution. The young man from the lyceum joined the forces trying to make Finland independent, fighting against the socialist forces which strived to instead make Finland into a ‘Soviet republic’. A very bitter civil war followed. For several months Granit was actively engaged as a soldier at the front and was decorated with the Cross of Freedom (Cl. IV, with sword). He mentioned this period not without pride. He then returned to school to complete his training in 1919.