Facilitation effect of auxiliary noise stimuli on response of isolated frog muscle spindle to sinusoidal movements

Abstract

Receptor potentials and impulse patterns were recorded from isolated frog muscle spindles using sinusoidal and superimposed random stretches as stimuli with different sinus-to-noise ratios. The entire dynamic amplitude range of the spindle receptor was evaluated by measuring the sensory response at different levels of static stretch. Auxiliary random stimuli provoked rectified fast depolarizing receptor potential transients; their amplitude and slope grew larger with increasing intensity of the noise stimulus and with increasing prestretch level. Due to this strongly nonlinear behavior of the transducing site the frequency and size of the receptor potentials evoked by the auxiliary input signal increased during the stretching phase of the sinusoidal movement. Since the fast depolarizing receptor potential transients provided a powerful trigger for the action potential encoding site, auxiliary random stimuli effectively enhanced the afferent discharge rate, especially during the stretching phase of the sinusoidal movement. Auxiliary noise stimuli could even activate the afferent discharge to an otherwise subthreshold sinusoidal stretch. It is assumed that by the same mechanism the transfer characteristic of the receptor is broadened towards higher frequencies. Since auxiliary random stimuli increased the nonlinear properties of all receptor response components, a "linearizing" approximation technique only partially describes the receptor's transfer properties. The facilitation effect recorded in the differentiated muscle spindle when random stimuli were superimposed on sinusoidal displacements closely resembled the excitation of afferent firing when passive stretching interacted with active fusimotor innervation. A hypothesis is proposed to explain both effects by the same mechanism acting upon the transducing sensory endings: Since passive random stretches as well as active twitching of the intrafusal muscle fibers exhibited almost the same range of frequency components, we propose that both stimuli also generate the same kind of receptor potentials; namely, those fast-rising depolarization transients of the receptor potential, which vigorously drive the encoding site. In general, these experiments explain how the specific response of a neuron can be facilitated by an additional unspecific (noisy) input.