Variability and plasticity of motor unit contractile properties
The motor unit contractile properties respond to changes in activity and in motor innervation of muscles. The contractions of isolated motor units evoked by the electrical stimulation of very thin fi laments of the ventral root were studied in anesthetized animals. Changes in the distribution of the three basic physiological types of motor units (fast fatigable, fast resistant and slow) in the rat medial gastrocnemius muscle and plasticity of their contractile properties as: contraction and relaxation time, force and fatigability were studied in ageing process, as effects of total and partial spinal cord injury, locomotor and vibration training. Moreover, distribution and variability of motor unit properties as well as innervation of skeletal muscles in male and female muscles were investigated. The comparison of motor unit properties in rat and cat muscle revealed considerable differences in twitch time and force parameters, distribution of motor unit types in muscles of the two species whereas the fatigability, ability to potentiate the force, sag and tetanic phenomenon revealed numerous similarities.