Structure and function of the motor unit. Effects of immobilization, monoparesis or paraplegia

Single motor units have been studied in the medial gastrocnemius muscle in cat following acute or chronic unilateral hind limb immobilization or antero-lateral spinal cord section. Immobilization produces muscle atrophy by 5 weeks and in the chronic phase (4-6 months) is characterized functionally by reduced twitch and tetanus tensions and structurally by fiber atrophy without type grouping or preponderance. Antero-lateral cord section with minimal motor dysfunction does not produce muscle atrophy or change in tension but in the chronic animals there is a slight decrease in twitch-tetanus ratios. Changes in the functional properties of motor units reflect changes in structure and both may result from secondary effects (i.e., type and amount of muscle activity, muscle length, pain, etc.) of a neuromuscular disease on muscle.