The period of latency before a muscle receptor generates an action potential as a response to a muscle stretch

Six primary (Ia) and seven secondary (II) muscle spindle afferents and eigh t Golgi tendon organ afferents (Ib) from the tibial anterior muscle of the cat, recorded at the dorsal roots, were subjected to a sinusoidal stretch o f the host muscle, the frequency of which increased linearly from 2 to 80 H z over four different lengths of time. Both the amplitude of the sinusoidal stretch and the prestretch of the muscle were varied. The phase of the act ion potentials was determined. The phase of the action potential, driven 1: 1, increased linearly with frequency. From the gradient of the phase of thi s action potential the muscle-muscle receptor latency was determined, i.e., the period of latency between the stretch of the muscle and the occurrence of the action potential at the muscle nerve where it enters the muscle. Th e muscle-muscle receptor latency had values lying between 3 and 8 ms: it wa s dependent on the experimental parameters and became shorter as the conduc tion velocity of the afferent fiber increased. In three experiments the mus cle latency was determined, i.e., the period of latency before the stretch was transferred from the tendon of the muscle to the proximal third of the muscle belly. The muscle was stretched sinusoidally under the same varying parameters as given above. The length changes occurring in the proximal thi rd of the muscle were measured with a piezo element. The muscle latency was determined from the slope of the phase of the zero points of the sinusoida l piezo length changes; the phase increases linearly with frequency. The mu scle latency had values lying between 6 and 15 ms: it was dependent on the experimental parameters. The muscle spindle latency, i.e., the period of la tency between the stretch of the polar parts of the intrafusal muscle fiber s and the recording of the action potentials from the spindle nerve near th e spindle capsule, was determined from 5 Ia fibers and 1 II fiber of isolat ed muscle spindles. The isolated muscle spindle was stretched under the sam e varying parameters as given above. The muscle spindle latency was determi ned from the slope of the phase of the phase-locked action potential. The m uscle spindle latency as measured by our method proved to be 0 ms. The late ncies of the three elements and their dependence on the experimental parame ters are discussed in the light of the transfer properties of the muscle an d the muscle receptors. (C) 1999 Elsevier Science B.V. All rights reserved.