Up to this point, we have concluded that disuse can induce both longitudinal and radial atrophy of muscle fibers. According to our basic guideline, these changes will be mediated by the alterations that occur at the preceding level within the hierarchy of the structural elements. In this case, the preceding level represents the ultrastructure of the muscle, and at this level, one will find that ~80% of the muscle fiber volume is composed of myofibrils [
70,
71]. Given that myofibrils make up the bulk of the muscle fiber volume, it can be reasoned that longitudinal and/or radial atrophy of the muscle fibers will be largely mediated by changes that occur at the level of the myofibrils. Accordingly, in this section, we will explore how disuse induces atrophy at the level of the myofibrils.
6.1. Longitudinal Atrophy of Muscle Fibers
As summarized in the previous section, a compelling body of studies has shown that disuse in a shortened position can induce the longitudinal atrophy of the muscle fibers. Since muscle fibers are filled with myofibrils, and the myofibrils are composed of an in-series repetition of sarcomeres (
Figure 1A), it follows that a decrease in the length of muscle fibers should be driven by a decrease in the length of the sarcomeres and/or the loss of in-series sarcomeres. When considering these two possibilities it is important to recognize that if the decrease in fiber length were mediated by a decrease in sarcomere length, then the shortened sarcomeres would not be able to function at their optimal length for force production [
82,
157]. This point, coupled with the fact that the resting/optimal length of sarcomeres (2.0-2.5 µm) is highly conserved within a given species [
72,
73], suggests that the muscle fibers will be faced with a strong drive to restore the resting/optimal length of its sarcomeres. To accomplish this, the muscle fibers would have to eliminate in-series sarcomeres, and, not surprisingly, this is what has been observed in numerous studies [
41,
82,
84,
126,
127,
129,
148,
158,
159]. For instance, Shah et al. (2001) reported that 28 days of immobilizing the mouse soleus muscle in a shortened position led to a 26% decrease in fiber length and this was accompanied by a 26% decrease in the number of in-series sarcomeres [
129]. Likewise, Tabary et al. (1972) found that 21-43 days of immobilizing the soleus muscle of cats in a shortened position led to an ~34% decrease in fiber length and that this was associated with a ~40% reduction in the number of in-series sarcomeres [
126]. In addition to immobilization, similar decrements in both fiber length and the number of in-series sarcomeres have also been reported to occur in response to hindlimb suspension [
41,
127]. Thus, it can be strongly concluded that when muscles are subjected to disuse in a shortened position it will lead to longitudinal atrophy of the muscle fibers and this effect is largely mediated by the elimination of in-series sarcomeres. With that being said, it bears noting that all of the aforementioned studies were focused on muscles that were subjected to disuse in a shortened position, and whether the same phenomena happens when muscles are subjected to disuse in a neutral position has, to the best of our knowledge, not been addressed.
Having established that disuse can lead to the elimination of in-series sarcomeres, one is faced with the question of how this occurs. Unfortunately, to date, the mechanism(s) that drive this phenomenon remain far from defined. Nonetheless, multiple studies have suggested that the elimination of in-series sarcomeres might occur in regions of myofibrillar disruption that are generally referred to as: (i) central core-like lesions (CCLs) and (ii) segmental necrosis.
CCLs are the most widely reported type of myofibrillar disruption [
42,
43,
47,
54,
146,
160,
161,
162,
163,
164,
165,
166,
167], and conventional CCLs can be described as centrally located circular disruptions of the myofibrils within a cross-section of a muscle fiber (
Figure 5A). However, less widely appreciated patches of irregularly shaped CCL-like disruptions can also be found throughout the entirety muscle fibers that are undergoing disuse-induced atrophy (
Figure 5A-C) [
42,
43,
167]. In both cases, a close examination of these regions reveals that they are enriched with signs of active remodeling including a disarray of the myofilaments, loss of mitochondria, and fragmentation of Z-lines [
42,
43,
54,
160,
162]. Interestingly, a large number of studies have reported that CCLs are very common in muscles that have been subjected to disuse in a shortened position [
42,
43,
47,
54,
146,
163,
164,
165,
166,
167]. Yet, such regions are rarely found in muscles that have been subjected to immobilization in a lengthened position [
47,
54,
164]. This is an important point because when muscles are subjected to immobilization in a lengthened position, they will generally still undergo atrophy, but the atrophic response will not involve the in-series elimination of sarcomeres [
81,
126,
129,
158,
159]. Thus, the presence of CCLs appears to occur specifically in response to atrophic conditions that lead to the in-series elimination of sarcomeres.
The aforementioned points are all consistent with the notion that the elimination of in-series sarcomeres occurs at CCLs. However, numerous studies have also reported that CCLs are largely confined to Type I fibers [
43,
47,
54,
146,
163,
164,
165]. For example, Riley et al. (1990), subjected the soleus muscle of rats to 13 days of hindlimb suspension and found that ~92% of the fibers with CCLs were Type I fibers [
43]. To the best of our knowledge, there is no evidence, or reason to think, that the loss of in-series sarcomeres would be confined to Type I fibers. Thus, although intriguing, the notion that the elimination of in-series sarcomeres occurs at CCLs remains highly questionable and additional studies will be needed to resolve this issue. Moreover, although conventional CCLs and patches of CCL-like disruptions both show signs of active remodeling, they might be involved in distinct biological processes. Indeed, Murakami et al. (2008) reported that irregularly shaped areas of ‘sarcomeric disarray’ (i.e., patches of CCL-like disruptions) were readily apparent in rat soleus muscles after 7 days of hindlimb suspension, but the presence of conventional CCLs only became apparent after 14 days of hindlimb suspension [
167]. The different temporal nature of their appearance suggests that convential CCLs and patches of CCL-like disruptions could represent biologically distinct areas of remodeling, and thus, future studies should take care to classify these areas as different types of myofibrillar disruptions. Consistent with the nomenclature of Murakami et al. (2008), we propose that the centrally located circular disruptions should be classified as conventional CCLs, and the irregularly shaped CCL-like disruptions should be referred to as areas of sarcomeric disarray (SDA).
The other type of myofibrillar disruption that has been implicated in the in-series elimination of sarcomeres is called segmental necrosis [
42,
43,
162,
166,
168,
169]. Unlike CCLs, segmental necrosis typically presents as a destruction of an entire segment of the muscle fiber, and is associated with the infiltration of mononuclear cells and neutrophils as well as signs of elevated phagocytic activity [
162,
168,
169]. Two examples of segmental necrosis are shown in
Figure 5D-E, and it has been proposed that these regions could serve as areas in which large segments of in-series sarcomere are eliminated [
166,
168]. Importantly, just like CCLs, areas of segmental necrosis are generally confined to muscles that have been subjected to disuse in a shortened position, but their appearance is quite rare when compared with CCLs. For instance, Riley et al. (1990) reported that only ~3% of rat soleus muscle fibers possessed regions of segmental necrosis after 10 days of hindlimb suspension, whereas ~30% of the fibers showed signs of CCLs after 13 days [
43]. Undoubtedly, there is still much to learn about the function of CCLs and areas of segmental necrosis, and the mechanisms that give rise to them. The only thing that we can confidently conclude at this point is that additional studies will be needed to determine whether these are truly the sites in which in-series sarcomeres are eliminated during the longitudinal atrophy of muscle fibers.