The pathway for force transmission in the rat anococcygeus muscle: a tale of two tendons

Smooth muscles forming the wall of tissues having conduit and reservoir functions (including blood vessels, intestinal tract, and stomach, gall bladder, urinary bladder, respectively) are organized into sheets or layers. The pathway for force transmission emanates from myosin interaction with actin filaments attached to intracellular dense bodies linked by the cytoskeleton to plasma membrane dense bodies which are adhesion sites for the extracellular matrix. The extracellular matrix is continuous throughout and between muscle layers, facilitating their coordinated function. There are a few instances where smooth muscles are organized in small longitudinal bundles with elastic tendinous ends, such as the pilomotor muscles of skin, the ciliary muscle of the eye, and costo-uterine muscle. In this study, we examine ultrastructure of two tendons that tether the anococcygeus muscle of the rat from the spine to the colon, the former a true tendon (myotendinous junction) and the latter a layer of connective tissue (intramuscular tendon). These regions show morphological specializations in the distribution and thickness of dense bodies, basement membrane, fiber shape and quantity of extracellular matrix. At the plasma membrane between dense bodies are caveolae, flask shaped structures primarily responsible for signal transduction, proliferation and electromechanical coupling. Changes also occur in caveolar regions, where the basement membrane is thickened and attachments to extracellular matrix are seen. Together, both regions of the plasma membrane are designed to facilitate force transmission.