Structural and functional asymmetries of myelinated branches in the frog muscle spindle.

1. Most of the axons innervating single-type muscle spindles in the frog sartorius muscle divide into myelinated branches at a node; one of the branches subdivides (Bs) and the other does not (Bn).2. Electrical responses at the afferent nerve terminal of the isolated spindles were recorded extracellularly and intracellularly with micro-electrodes, concurrently with the terminal responses by the paraffin gap method.3. By recording spontaneous orthodromic and provoked antidromic impulses and by stimulating the nerve terminals with micro-electrodes and inactivating the terminals with micro-application of potassium ion, it was deduced that propagated and abortive spikes are almost always generated at the terminal of Bn (Tn), not at the terminals of Bs (Ta and Tb).4. The site of impulse initiation, as determined in (3), was found, after measurement of the lengths of the myelinated branches in each preparation, to be essentially the terminal nearest the parent axon.5. Selective inactivation of Tn by far-ultra-violet light removed both propagated and abortive spikes. Both types of spikes survived after irradiation of Ta and Tb, although the relationship between spike frequency and spindle stretch was altered. The same results were obtained after transection of the branches with a laser microbeam.