[Muscular phenotypes in relation to the specific differentiative influences of motor innervation]

Vertebrate skeletal muscles are classified into fast-twitch and slow-twitch muscles according to the intrinsic speed of contraction. These physiological characteristics of the muscles are ontogenetically determined by epigenetic influences arising from the specific motor innervation. The evidence for a neural control on gene expression comes mainly from the demonstration that myosin is present in two different molecular forms in fast and in slow muscles. It is known from previous work in several laboratories that the two myosin isozymes differ with respect to both the primary structure of the heavy chains and the subunit composition of the light chains. Fast muscle myosin is characterized by a three-bands electrophoretic pattern in SDS-gels, whereas the myosin from slow muscle contains only two, distinct types of subunits. Our results show that the tripartite band pattern of the light chains is a common characteristic for the myosin of the fast-white muscles with intermittent-phasic activity (e.g. rabbit adductor) and the fast-red muscles with sustained-phasic activity (e.g. rat masseter and pigeon pectoralis). These results lend support to the view that the neural control on gene expression in skeletal muscles is mediated by specific influences somehow arising from the pattern of activity and which are independent from the total input of nerve impulses.