Response of human muscle spindle afferents to sinusoidal stretching with awide range of amplitudes

1. Impulses of human single muscle spindle afferents were recorded from the m. extensor carpi radialis, while 1 Hz sinusoidal movements for a wide ran ge of amplitudes (0.05-10 deg, half of the peak-to-peak amplitude) were imp osed at the wrist joint. 2. The response was considered as linear when the discharge was approximate ly sinusoidally modulated. The linearity was further checked by a linear in crease in the response with the amplitude and a constancy of the phase and mean level. 3. Fifteen of 25 primary afferents were active at rest with a mean rate of 10.6 impulses s(-1) (median). The linear response to sinusoidal stretching was limited to amplitudes lower than about 1.0 deg. The sensitivity was 5.6 impulses s(-1) deg(-1) (median) in the linear range and decreased at large r amplitudes. The other 10 primary afferents were silent at rest and lacked a linens response at lon amplitudes. 4. Nine secondary afferents were active at rest with a mean rate of 9.5 imp ulses s(-1). The linear range extended up to about 4.0 deg with a sensitivi ty of 1.4 impulses s(-1) deg(-1). 5. In the linear range, the phase advance of the response to sinusoidal str etching was about 50 deg and was similar between the two types of spindle a fferents. In primary afferents, the phase advance increased to nearly 90 de g outside the linear range. 6. The findings suggest that high sensitivity to small stretches is importa nt in determining primary afferent firing during natural movements in intac t humans.