We know that actin and myosin filaments are contained within sarcomeres and a lot of sarcomeres in series create myofibrils. Myofibrils are contained in the cell of the muscle, the muscle fiber.
A lot of visual representation of the extracellular matrix (ECM) show that integrins are anchored both outside of the cell and inside in the actin cytoskeleton (e.g. http://jonlieffmd.com/wp-content/uploads/2013/11/ECM-INTEGRIN-.jpg ). There is also evidence to suggest integrins are able to relay signals bidirectionally, to the inside or the outside of the cell.
Do integrins and in extend the entire ECM interact only with the cytoskeleton of the myofibrils that lie towards the outside of the muscle fiber or is there some kind of structure to reach also the deeper ones?
Moreover the sarcoplasmic reticulum is shown to be spread underneath the sarcolemma. is there any evidence to suggest that it also interacts with the ECM ? ( https://upload.wikimedia.org/wikipedia/commons/thumb/6/6f/Blausen_0801_SkeletalMuscle.png/300px-Blausen_0801_SkeletalMuscle.png )
I guess it sort of depends on your definition of "extracellular". Extracellular fluid is found within the transverse tubules (T-tubules) deep within the fiber. The T-tubules are responsible for conduction of the action potential deep within the fiber. Though I don't know for sure if the extracellular matrix is associated with T-tubules, I'm guessing it would be to provide structural support..
Broadly speaking, desmin interlinks deeper myofibrils and multi-molecular protein complexes involving dystrophin links peripheral myofibrils to the sarcolemma and from there to the ECM. I suggest to read myofascial force transmission studies by our group (a review is attached) and from Dr. Huijing and some further look into the literature (the reference below is quite relevant).
Berthier C, Blaineau S. Supramolecular organization of the subsarcolemmal
cytoskeleton of adult skeletal muscle fibers. A review. Biol. Cell.
1997; 89:413Y34.
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