Properties of stretch receptors in cat extraocular muscles*

Abstract

The properties of 52 stretch receptors in the extraocular muscles were studied in 30 cats. Between 10 and 28 receptors were observed in each preparation. In 4 preparations, spontaneously discharging receptors were obsenred, with afferent fiber conduction velocitiesrangingfrom 16.9 to 41.1 m/sec. In the remaining 26 cats, the minimal threshold receptors rangedfrom 3 to 130 g, with a peak distribution between 10 and 20 g, and afferent fiber conduction velocities ranging from 6.5 to 52.0 m/sec, the peak being between 10 and 15 m/sec. Of these receptors, 19 were quickly adapting and 7 were slowly adapting. The dynamic and static indexes of all the receptors were essentially similar; they both increased markedly on increasing the initial length. This suggests that the receptors do not lie in contact with regions of reduced viscosity on the muscle fibers comparable to the equatorial region of the intrafusal muscle fibers. All of the receptors were located in the muscle; none was located in the tendon. Forty-seven of 49 receptors were in parallel and 2 receptors were in series with the contractile elements. The properties of all the receptors studied appeared to be similar, suggesting that a single type of stretch receptor is located in the inferior oblique muscle of cats.