Miniature end-plate potentials in the frog muscle spindle

Abstract Spontaneous miniature end-plate potentials (m.e.p.p.s) were recorded from the intrafusal muscle fibers in the muscle spindles isolated from the frog sartorius and semitendinosus muscles by means of microelectrode (intracellularly) and across a vaseline gap made transversely on the intrafusal bundle (extracellularly). The latter method was available to record simultaneously the m.e.p.p.s and the afferent terminal activities, of which the latter was easily distinguishable from the former by concomitant occurrence with the axonal responses. Simple type spindles in the sartorius muscle have a restricted end-plate region along the intrafusal bundle on either side of the capsule, while complex type spindles in the semitendinosus muscle are innervated by plural regions which are found on both sides of the capsule in most preparations. Application of 2× hypertonic Ringer's solutions with NaCl or sucrose induced an increase in the rate of the m.e.p.p.s preceding a decrease in the rate of afferent discharges from the spindle.