Reflex responses of fusimotoneurons to sinusoidal muscle stretching

Reflex response of fusimotoneurons to sinusoidal muscle stretching were investigated in decerebrated cats. Nerve impulses of single fusimotoneurons were recorded from thin filaments dissected from otherwise intact nerves to triceps surae muscles. Amplitude of the sinusoidal stretching of these muscles was 3 mm peak-to-peak and the frequencies 0.1-10 Hz. Electric muscle activity was also recorded in some experiments. Fusimotor responses were similar to those of the skeletomotoneurons in that both were advanced in phase with respect to muscle length changes, while their amplitude increased with increase in stretching frequency. Modulation at the frequency corresponding to the second harmonic of the input signal was predominant in fusimotor responses. It is supposed to appear mainly due to the convergence to fusimotoneurons of afferent impulses from different muscle receptors arriving after different delays. Its functional role is discussed.