Neurovascular alignment in adult mouse skeletal muscles

Objective: Muscle blood flow increases with motor unit recruitment. The physical relationships between somatic motor nerves, which control muscle fiber contraction, and arterioles, which control microvascular perfusion, are unexplored. The authors tested the hypothesis that motor axons align with arterioles in adult skeletal muscle. Methods: Transgenic mice (C57BL/6 background, n = 5; 10 months of age) expressing yellow fluorescent protein in all motor nerves underwent vascular casting (Microfil). Excised epitrochlearis, gracilis, gluteus maximus, and spinotrapezius muscles were imaged at 380x and 760x and a computer-integrated tracing system (Neurolucida) was used to acquire 3-dimensional digital renderings of entire arteriolar and neural networks within each muscle. Results: Arteriolar networks were typically similar to 3-fold longer than neural networks. Nerves coursed with arterioles until terminating at motor endplates. Across muscles, proximity analyses revealed that similar to 75% of total nerve length (9.8-48.8 mm) lay within 200 mu m of the nearest arteriole (diameters of 15-60 mu m).