Sensory terminals on extrafusal muscle fibres in myotendinous regions of developing rat muscles.
Axon terminals were observed to form neuromuscular contacts with extrafusal muscle fibres in myotendinous regions of developing rat muscles up to 5 days after birth. These neuromuscular contacts are found in fascicles of muscle fibres connected with differentiating Golgi tendon organs. Axon terminals establishing these contacts are obviously sensory, since they do not degenerate after de-efferentation performed in neonatal rats. The terminals contain mainly clear and dense core vesicles and form neuromuscular connections resembling developing motor endplates, with a cleft about 60 nm wide and basal lamina interposed between the axolemma and the sarcolemma. Each terminal, however, also forms a close contact in a restricted region where the basal lamina is missing; there the cleft is reduced to 20 nm and the axolemmal and sarcolemmal membranes are linked by desmosome-like attachment plaques. After the fifth postnatal day, axon terminals become detached from muscle fibres and are only found among collagen bundles of the tendon organ. The functional significance of these temporary neuromuscular contacts is not clear.
D000818:Animals; D001369:Axons / Q000648:ultrastructure; D003896:Desmosomes / Q000648:ultrastructure; D024510:Muscle Development; D009132:Muscles / Q000294:innervation* / Q000648:ultrastructure; D009469:Neuromuscular Junction / Q000648:ultrastructure*; D009940:Organoids / Q000648:ultrastructure; D051381:Rats; D012508:Sarcolemma / Q000648:ultrastructure; D013710:Tendons / Q000294:innervation / Q000648:ultrastructure