Growth and degeneration of motor end-plates in mammalian skeletal muscle

Abstract

A detailed quantitative study of the morphological variations of extrafusal motor end-plates of normal cat hindlimb muscles as seen in teased silver impregnated preparations was undertaken. The principal aim was to examine the hypothesis that end-plates undergo growth and degeneration in a process of cyclic degeneration and renewal ('replacement'). A new classification of the morphological complexity of end-plates was devised and the three muscles studied (peroneus brevis, peroneus digiti quinti and soleus) show characteristic variations in the proportions of complex endings, and in the size relationships of end-plates and muscle fibres. These variations are not related to differences in the proportions of muscle fibres of the three principal histochemical types. The existence of growth and degeneration of end-plates was confirmed, but no new end-plates are formed. Hence the 'replacement' hypothesis is regarded as untenable. Instead, it is suggested that growth leads to the formation of end-plates of greater complexity ('elaboration'), and degeneration to the loss of muscle fibres commonly observed in ageing mammals. The examination of muscles from cats aged 6 - l8 years supports this concept because there is a striking increase in the proportions of complex end-plates and the number of muscle fibres is reduced. By analogy with the formation of end-plates, it is suggested that 'elaboration' during ageing is a response to the increasing functional demands indicated by the increased body weights and muscle fibre diameters. An attempt to test the effects of use and disuse in kittens was unsuccessful. However, surgically-induced hypertrophy of soleus in a young adult cat resulted in an increased proportion of larger, more complex end-plates with more numerous, longer axon terminals. This strongly suggests that 'elaboration' is indeed a response to functional demand. The observations are discussed in relation to current views of the neuromuscular system.