Changes in the electron microscopic structure of slow and fast skeletal muscle fibres of the mouse after the local injection of botulinum toxin

Abstract

A sublethal dose of botulinum toxin was injected into the calf muscles of one leg in mice and caused paralysis of those muscles within 24 hr. Recovery from paralysis took place after several weeks. Electron-microscopic studies were made of muscle fibres of soleus (a “slow” muscle) and of the superficial part of gastrocnemius (composed of “fast” fibres) from animals surviving from 4 days to 33 weeks after the injection of toxin. Muscle fibres from both regions became atrophied during the first 2–3 weeks and folds of redundant basement membrane became dissociated from the plasma membranes of the atrophied fibres. The atrophied fibres contained lysosomes and multivesicular bodies but their glycogen content was much reduced. By the 4th–6th week the fibres of soleus became restored to normal in size and abnormally abundant glycogen was then found. Fibres of gastrocnemius became more severely atrophied and showed many changes which were not seen in soleus fibres. These included focal disintegration of myofilaments, tubular aggregates, “helical complexes” and dilatation of cisternae. Recovery of gastrocnemius fibres began only after the 5th–6th week. In later stages there was splitting of muscle fibres in gastrocnemius and tubular aggregates, disoriented myofibrils forming annulets and centrally-located nuclei were common. Some fibres became considerably larger than normal but others showed extreme atrophy and disorganization of their internal structure many months after the injection of the toxin.