Tipi di fibra muscolare

Lesson #6 This is an in-depth look at lesson #5. If you missed it, here's the link:    • Tessuto muscolare striato   At the beginning of the last century, muscle fibers were classified based on their contraction speed as SLOW (Type I) or FAST (Type II), and this classification also corresponded to the morphological characteristics of the muscle itself. Fast muscles appeared white (or pale), while slow muscles appeared red (or dark) due to their higher myoglobin content and greater capillary distribution. Other differences include fiber size (larger in fast muscles) and mitochondrial content (higher in slow muscles). In the 1960s, histochemical staining for ATPase activity became the most widely used method for characterizing fibers. This method led to the discovery of subcategories of type II fibers. In particular, type IIA and IIB fibers, in addition to the already known type I fibers. IIA fibers showed lower myosin ATPase activity than IIB fibers. Another method, which in those years correlated with ATPase activity, was to identify the isoform of the myosin heavy chain (the MHC type). Four isoforms of the myosin heavy chain were found in mammalian trunk and limb muscles, which took on the same nomenclature used for histochemical staining: MHCI, MHCIIa, MHCIIx, MHCIIb. It was later discovered that the MHCIIb protein is not expressed in humans, and therefore it is the MHCIIx protein that is associated with the histochemical classification of ATPase IIB. There is a good correlation between the two classification methods. Type I ATPase fibers contain MHCI myosins, type IIA fibers contain MHCIIa myosins, and type IIB fibers contain MHCIIx myosins. Over the years, the histochemical staining method for ATPase has allowed us to identify four additional intermediate fiber types, which are I IC IIC IIAC IIA IIAB IIB. It is important to remember that this is a qualitative analysis of the sample's coloration, in response to pH, and therefore rather subjective, and different researchers may classify the fibers differently. Furthermore, many authors still prefer to refer only to the three fiber classes listed above. This further subdivision is also supported by analysis of the myosin heavy chain. In fact, there are not only PURE fibers that contain only one type of myosin, but also HYBRID fibers that contain multiple types. Type C hybrid fibers contain varying amounts of myosins MHCI and IIa, while type IIAB hybrid fibers contain MHCIIa and IIx myosins in varying proportions. However, fibers containing isoforms I and IIx in the absence of isoforms IIa have been found. It should be noted that in addition to the three types of heavy chains I've listed here, the human body expresses others to a lesser extent in certain muscles, such as the extrinsic muscles of the eye and the muscles of mastication, or even at certain stages of life, as demonstrated by neonatal or embryonic MHCs. As you can see, these two classification methods tell us little about the function of these fibers. Another method used is the purely metabolic one: SO (slow oxidative) fibers are slow-twitch fibers that favor aerobic metabolism. These fibers are highly resistant to fatigue. The type I fibers from the classifications discussed above can all be included in this group as well. FG (fast glycolytic) fibers are fast fibers that favor anaerobic metabolism. They can produce force rapidly but deplete energy just as quickly. They have larger glycogen stores but fewer mitochondria and much less myoglobin than SO fibers. FO (fast oxidative) fibers, also called FOG (fast oxidative glycolytic), are intermediate fibers, fast but with a mixed aerobic/anaerobic metabolism. While SO fiber and type I fiber are practically interchangeable terms. The correlation between type IIA fibers and FO fibers or between type IIb fibers and FG is not so obvious. 00:00 Introduction 01:02 Classification by ATPase activity 01:54 Classification by MHC type 03:15 Hybrid fibers 04:47 Other expressed MHC types 05:12 Classification by metabolism