Ultrastructure of Muscle Spindles (Mechanoreceptors) of Rat m. soleus after Support Unloading and Its Combination with Hypergravity

We used electron microscopy to evaluate the effect of support unloading of m. soleus in adult Wistar rats (restrained in antiorthostatic position for 23–24 h/day within 24 days) on the ultrastructure of the intrafusal fibers and motor neuromuscular junctions of the muscle spindles, as well as the efficiency of intermittent hypergravity (+2G_Z; 1 h/day for 19 days in a centrifuge in hypokinetic cages) as a countermeasure used in conditions of support unloading of this muscle. In the absence of support on the hind limbs, most of intrafusal fibers of m. soleus preserved the typical ultrastructure, while the axon terminals of the neuromuscular junctions accumulated a lot of synaptic vesicles (including large vesicles); the coated vesicles were absent due to unloading of the muscle and its muscle spindles (no contractions of the intrafusal fibers). A short-term effects of hypergravity at the background of support unloading of m. soleus mostly induced static loading of the muscle inducing different responses of the intrafusal fibers in different regions of the muscle spindles: local lysis of myofilaments was observed in single intrafusal fibers of the equatorial and intracapsular motor regions, while myofibrils remained intact in most fibers in the intra- and extracapsular regions of the spindles. The revealed adaptive response of the intrafusal fibers is, on the one hand, due to their specific innervation and ultrastructure and, on the other hand, to positive effect of hypergravity on the motor and extracapsular regions of the muscle spindles. Hypergravity decreased the number of synaptic vesicles and induced appearance of the coated vesicles in the axon terminals of the neuromuscular junctions of the intrafusal fibers in the animals restrained in antiorthostatic position (support unloading of m. soleus), which is due to increased functional load of the muscle. The ultrastructure of the muscle spindles adequately reflected the functional status of the postural m. soleus both during support unloading and support unloading combined with hypergravity load.