The effect on sarcomere organization of stretching intact single skeletal muscle fibres by 50% of their optimum length (Lo) during ten consecutive short tetani was investigated. Stretch reduced tetanic force to 36 ± 4% of the pre‐stretch condition. Sarcomere organization was analysed using both electron and confocal microscopy. For confocal microscopy the striation pattern was examined by fluorescently staining F‐actin with rhodamine–phalloidin. Electron microscopy revealed that fibres which had been stretched during contraction contained areas of severe sarcomere disorganization, as well as adjacent sarcomeres of normal appearance. Confocal images of stretched fibres, which had been fixed and stained with rhodamine–phalloidin, showed focal regions of overstretched sarcomeres and regions where sarcomeres of adjacent myofibrils were out of alignment with each other. Analysis of all sarcomeres along the length of fibres showed regions of sarcomere inhomogeneity were distributed throughout the fibre length and cross‐section. Fibres were microinjected with the fluorescent [Ca2+]i indicator fura‐2 before being stretched. Conventional wide‐field fluorescence imaging microscopy showed that the tetanic [Ca2+]i was reduced after stretching but remained uniformly distributed. This study confirms the finding that stretch‐induced muscle injury has components caused by disorganization of the myofibrillar array and by failure of tetanic Ca2+ release. The structural damage is spatially heterogeneous whereas the changes in Ca2+ release appear to be spatially homogeneous.

SUMMARY1. Changes in the dimensions of the sarcoplasmic reticulum in frog sartorius muscles exposed to hypertonic and hypotonic solutions have been studied with the electron microscope.2. The volume of the sarcoplasmic reticulum has been found to be linearly related with a negative slope to the reciprocal of the osmotic pressure. Over the range 075 to 3-5 x normal osmotic pressure the reticulum volume has been calculated to change from 11x5 to 18-5 % of normal cell volume.3. These changes in sarcoplasmic reticulum volume correspond to the calculated changes in the volume of the intra-fibre sucrose compartment, postulated by earlier workers on the basis of studies on changes in cell volume with changes in osmotic pressure in living muscles.4. To explain these and other related findings on the distribution of electrolytes in muscle, it is proposed that the sarcoplasmic reticulum of skeletal muscle is an extracellular compartment.5. The significance of this hypothesis for the mechanism of excitationcontraction coupling is discussed.

This study tested the usefulness of Schwann cells in the repair of a severed nerve with a biosynthetic bridge or guide. Reinforced collagen nerve guides were used to bridge an 18 mm gap in the sciatic nerve of 21 young adult rats. The animals were divided into three groups and the guides were filled with: (i) more than 0.5 x 10(6) cultured syngeneic adult Schwann cells (group L, n = 12); (ii) less than 0.5 x 10(6) Schwann cells (Group S, n = 6); and (iii) phosphate buffered saline (control, n = 3). Schwann cells were pre-labelled with Hoechst dye. Regeneration was assessed functionally and histologically at 1, 2, 3 and 6 + months after surgery. Group L animals showed numerous regenerated axons surrounded by implanted Schwann cells within the first month. The total number of myelinated fibers (12.5 x 10(3)) remained above normal unoperated values (7 x 10(3)) in long-term animals. Regenerated axons were found in Group S in the third month, but no Hoechst labelled cells were found. The number of myelinated fibers (3.9 x 10(3)) remained below normal values in long-term animals. Control guides failed to support axonal regeneration. Functional recovery was evident at week 20 (Group L) and week 30 (Group S) after surgery, with no difference in function between the two groups by the end of the study. Supplementing guides with Schwann cells enhances regeneration of peripheral axons over a distance normally prohibitive. This effect is greatest in the early stages of regeneration (1-3 months) and is dependent on the number of cells implanted.

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(1) The effects of glycerol-treatment on the ultrastructure, tension, and electrical properties of rat sternomastoid muscle fibers are described. (2) The effect upon the ultrastructure of fibers differed from that previously reported for amphibian fibers, in that the sarcoplasmic reticulum, as well as the T-system, was disrupted. (3) Tension (tension (tetanus and K-contracture) was abolished when preparations were returned to normal Krebs after exposure to a glycerol-Krebs solution (exposure periods were 1 hr in 200--350 mM-glycerol or 10--60 min in 350 mM-glycerol), although fibers had normal resting membrane potentials and action potentials. (4) Fibers treated for 1 hr with 350 mM-glycerol were detubulated when returned to normal Krebs. Specific membrane capacity was reduced and exogenous horseradish peroxidase (HRP) did not penetrate the T-system. (5) Fibers were not detubulated after treatment for 1 hr with 200 to 300 mM-glycerol or after treatment for 10 to 30 min with 350 mM glycerol. Specific membrane capacity and resistance were normal and HRP entered the T-system. (6) Ultrastructural disruption of the triad junction became progressively more extensive with increasing glycerol concentration used and may be responsible for uncoupling.