Bioelectric interactions between cultured fetal rodent spinal cord and skeletal muscle after innervation in vitro.

Electrophysiologic analyses of cultures of various combinations of spinal cord and skeletal muscle tissues demonstrate that characteristic neuromuscular transmission can develop and be maintained for months in vitro. The studies include: (1) cultures of mouse embryo spinal cord with attached myotomes, explanted prior to formation of neuromuscular junctions, and (2) in vitro-coupled fetal rodent cord and muscle explants, in which one tissue is grown in isolation before presentation of the other explant (0.5–1 mm apart). After innervation occurs during the first few weeks in vitro, selective stimulation of spinal cord evokes widespread synchronized twitches in the muscle tissue. Simultaneous microelectrode recordings of cord and muscle responses to local cord stimuli show that muscle action potentials generally occur with latencies of several msec after onset of cord discharges. Rhythmic spontaneous bursts of muscle potentials (and contractions) occur, at times, in close correlation with periodic cord discharges. d-Tubocurarine (1–10 μg/ml) selectively blocks neurally evoked muscle contractions while electric stimuli applied directly to the muscle fibers are still effective. Many apparently non-innervated fibers, on the other hand, show sporadic, asynchronous fibrillations which are not altered by cord stimuli nor blocked by curare. Eserine and strychnine produce characteristic increases in repetitive muscle activities related to the primary site of action of these pharmacologic agents. Cultured neuromuscular tissues can, then, provide a valuable model system for correlative physiologic and cytologic studies of nerve and muscle cells during critical stages of maturation and formation of intercellular relationships under isolated conditions in vitro.