Properties of stretch receptors in cat extraocular muscles.

1. The properties of fifty-two stretch receptors in the extraocular muscles were studied in thirty cats. Between ten and twenty-eight receptors were observed in each preparation.2. In four preparations, spontaneously discharging receptors were observed, with afferent fibre conduction velocities ranging from 16.9 to 41.1 m/sec.3. In the remaining twenty-six cats, the minimal threshold receptors ranged from 3 to 130 g, with a peak distribution between 10 and 20 g, and afferent fibre conduction velocities ranging from 6.5 to 52.0 m/sec, the peak being between 10 and 15 m/sec. Of these receptors, nineteen were quickly adapting and seven were slowly adapting.4. 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 fibres comparable to the equatorial region of the intrafusal muscle fibres.5. All of the receptors were located in the muscle; none was located in the tendon. Forty-seven of forty-nine receptors were in parallel and two receptors were in series with the contractile elements.6. 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.