Transduction and encoding mechanisms in muscle spindle.

1. Controversy on Spindle potential Katz l ) has recorded a slow depolarizing potential from the sensory nerve terminal of the frog IVth toe muscle during stretch, and has termed this the 'spindle potential'. Since the amplitude of the spindle potential changed almost in parallel with the muscle tension 1) and also with the rate of afferent discharges,2,3) the spindle potential was considered to be equivalent to the receptor potential in other receptors by Ottoson and Shepherd.4) There are found, however, some situations in which the rate of afferent discharges is not always in parallel with the amplitude of the spindle potential (we would employ this term to distinguish it from a genuine receptor potential in the muscle spindle). When muscle spindle is stretched in Ringer solutions with abnormal ionic compositions or osmolalities, no spindle potential is observed or a reversed (hyperpolarizing) spindle potential is found while afferent discharges occur with the same pattern as in the normal Ringer solution. 5 ,6)