Abstract
The instantaneous sarcomere length (SL) is regarded as an important indicator of the functional properties of striated muscle. Previously, we found greater sarcomere elongations at the distal end compared to the mid-portion in the mouse tibialis anterior (TA) when the muscle was stretched passively. Here, we wanted to see if SL dispersions increase with activation, as has been observed in single myofibrils, and if SL dispersions differ for different locations in a muscle. Sarcomere lengths were measured at a mid- and a distal location of the TA in live mice using second harmonic generation imaging. Muscle force was measured using a tendon force transducer. We found that SL dispersions increased substantially from the passive to the active state, and were the same for the mid- and distal portions of TA. Sarcomere length non-uniformities within a segment of ~30 serial sarcomeres were up to 1.0 µm. We conclude from these findings that passive, mean SLs obtained from a single location are not necessarily representative of the distribution of SL in active muscle, and thus may be misinterpreted when deriving muscle mechanical properties, such as the force-length relationship. In view of these findings, it seems crucial to determine how SL distributions within a muscle relate to the most fundamental properties of muscle, such as the maximal isometric force.
Highlights
Skeletal muscles produce forces through tens of thousands of sarcomeres that are hierarchically organized within a complex network of connective tissues
There is an intricate structural connective tissue network in the whole muscle that can provide stability to the sarcomeres[20,21]. It is unknown if the behavior of sarcomere length non-uniformities observed in fibers and myofibrils is representative of sarcomeres in whole muscle or it is just an artifact in isolated fibers and myofibrils that is not present in entire muscles
We found that the sarcomere length (SL) measured in an active muscle gave better predictions of the maximal isometric force than the corresponding SLs measured in the passive muscle[25]
Summary
Skeletal muscles produce forces through tens of thousands of sarcomeres that are hierarchically organized within a complex network of connective tissues. The contractile behaviour of sarcomeres has been studied extensively using single myofibers[3,9,10] and myofibrils[11,12,13,14] in an attempt to unravel fundamental properties of muscle contraction and function. An important observation in isolated myofibers and myofibrils is the non-uniform sarcomere lengths in active in vitro preparations[7,15,16]. With recent advances of non-linear laser microscopy, individual sarcomeres can be visualized and their lengths measured accurately in intact whole muscles[21,22,23,24,25]. We hypothesized that sarcomere contractile behaviour depends on the location of the sarcomeres within the TA muscle
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