Anatomical evidence for multiple sources of action potentials in the afferent fibers of muscle spindles

Probable sites of origin for action potentials were identified in the distal portions of afferent fibers in neuromuscular spindles of cats and frogs (both primary and secondary afferents were examined in cats). In every case, each of the several terminal heminodes reacted with a ferric ferrocyanide cytochemical stain, indicating that these might all be initiators of action potentials. The staining results imply that in spindles the frequency response of afferent action potentials may result from competition among multiple encoders, rather than from integration of all the terminals by a single encoder.