Effect of hindlimb suspension on activation and MHC content of triceps brachii and on the representation of forepaw on the sensorimotor cortex.

The aims of this work were to analyze the effects of a chronic (14 days) increase in the functional demand imposed on the triceps brachii and to evaluate the changes of the cortical representation of forelimb to this increased activity. The activation of triceps brachii was obtained by the hindlimb unloading (HU) model. Electromyographic activity changed from a phasic to a tonic pattern. Response amplitude increased during the first days of hyperactivity and then stabilized at an intermediate level. A transient decrease (-13% to -36% on day 2) in the mean frequency of motor units was observed. Content in myosin heavy chain of muscle fibers showed a reduction in IIb+IIx fibers in HU rats, whereas IIa+IIx fibers were more numerous. Thus, fibers tend to be more resistant to fatigue. Taken together, these observations reveal a dual plastic process. First, the nervous system reacts immediately to an environmental change, and second it reorganizes its motor command to impose a pattern of activity that is more adapted to a postural function. The extent of the cortical forelimb representation was delimited by oxidase histochemistry. No differences were detectable between control and HU animals for the period corresponding to enlarged receptive fields in the HU condition. Our observation lends support to our hypothesis that activation patterns contribute to the maintenance of neuronal properties in the somatosensory cortex. Moreover, the new tonic pattern resulting from the long contact of the paw with the floor may contribute to the adaptation of the central control of motoneuronal activity.