Morphological and Physiological Aspects of Length Adaptations of the Soleus Muscle in Spinalized Rats.

Experimental evidence has demonstrated that length adaptations in normal skeletal muscles can be induced readily in experimental animals. Clinical evidence implies that this is also true in humans. Although muscles are capable of making length adaptations in the absence of nervous system influences, they do exhibit modifications of this potential following alterations in neural input. Patients with spasticity due to central nervous system disorders appear to be particularly susceptible to abnormalities of muscle length which result in muscle contractures. This study examines the effect of spinal cord transection on length adaptations of the soleus muscle in rats. Casting of the ankles of adult female rats was used to manipulate the muscular environment. Changes in muscle fiber lengths were measured morphologically by determining the number of sarcomeres/muscle fiber. Physiological length-tension studies were used to determine functional changes in muscle length. The presence of spinal cord transection was found to be not only permissive of adaptive increases of muscle and muscle fiber lengths, but was found to enhance the animals' adaptive responses to immobilization in a lengthened muscle position. This response was demonstrated for both the shortened muscles of muscle contractures and for muscles of normal initial length. When muscles had been immobilized in a shortened position, little of the adaptation occurred in the muscle fibers in spite of the development of substantial contractures. Neither the morphological nor the physiological measurement tools used in this study were sufficient to elucidate the nature of the adaptations which resulted in the contractures. The findings of this study confirm the hypothesis that after spinal cord transection and subsequent development of spasticity, rats will retain the capacity to increase the length of their soleus muscle fibers in response to chronic changes in muscle length.PhDMorphologyUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/161279/1/8702783.pd