A COMPARATIVE STUDY ON STRUCTURE AND FUNCTION BETWEEN THE EXTRAFUSAL RECEPTOR AND THE SPINDLE RECEPTOR IN THE FROG

Abstract

In the sartorius muscle of the frog, the structure and function of the extrafusal receptors were compared with those of the spindle receptors. There were 5-7 spindle receptors and 10 or more extrafusal ones in a sartorius muscle. The structure of the spindle receptors in the sartorius muscle conformed well with that observed in the small toe muscles by many workers, except that, in the living preparation, myelinated subbranches of the parent axon within the spindle capsule assumed in appearance of chains of ball-like swellings. There was observed some seasonal variation in the size of these swellings. The parent axons supplying the spindle receptors ranged from 7 to 18[mu] in diameter, and the conduction velocity of the impulse travelling along them from 11.4 to 29.7 m/sec. The spindle receptors were stimulated by stretching the muscle at a constant velocity and from the frequency-extension curves their static sensitivity to stretch was calculated as 12 impulses/sec/mm on the average. They generated impulses in response to abrupt focal deformation produced by the displacement of a glass rod. The mean threshold value to the displacement was 17[mu]. In the proximity of the endings the parent axon of the extrafusal receptors usually divides into 2 or 3 branches at every node and finally they form leaf-like expansions of unmyelinated threads, and accordingly its receptive field spreads over the surface of several or more muscle fibres. The parent axons of the extrafusal receptors were 3 to 11[mu] in diameter, and the conduction velocity of the impulse carried along them was between 10.2 and 20.0 m/sec. The extrafusal receptors were distinguished from the motor end-plates for the following reasons Impulse discharges set up from the former inflow exclusively into the dorsal root; no miniature endplate potentials are detected when a micro-electrode is inserted into the muscle fiber in immediate juxtaposition to the former. The extrafusal receptors could be classified into 2 groups; one having threshold at extension of the muscle by 110% of the resting length in situ and another by 140%. The static sensitivity of the former was calculated as 7 impulses/sec/mm and that of the latter no less than the former. As for the threshold to focal displacement, there was virtually no difference between them, the mean value being 52[mu]. The functional significance of afferent discharges from the extrafusal receptor was discussed in comparison with those of group Ib and group III muscle afferent.