Nonlinear properties of stretch reflex studied in the decerebrate cat

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Abstract
Pairs of brief stretches or a series of stretches at random intervals (Poisson process) were applied to a slow (soleus) and a fast (plantaris) muscle in decerebrate cats to analyze the nonlinear effects of 1 stretch on the reflex responses to subsequent stretches. Neural activity, electromyogram (EMG), and force were recorded. The reflex responses due to stretch were compared with reflexes as a result of electrical stimulation of nerves. Nonlinearities of muscle were also examined in the absence of reflexes. Short-latency neural activity produced by the stimuli at all intervals studied was quite constant, so changes in sensory activity cannot account for the nonlinearities. Three phases of nonlinear interactions were observed, and mechanisms for these nonlinearities are suggested. For short intervals (< 100 ms) following a stretch the force and EMG produced by a 2nd stretch is depressed. This early depression could be due to the afterhyperpolarization of the motoneuron cell body or to synaptic mechanisms, since the depression of EMG is seen with electrical stimulation of Ia sensory, but not .alpha.-motor axons. A 2nd stretch can disrupt the reflex contraction produced by the 1st stretch if it occurs at a time when new actomyosin bonds are not readily formed. Because of this force suppression, the total reflex force produced in response to 2 stretches may be less than the response to a single stretch. For intervals of 100-300ms, the force and EMG produced by a 2nd stretch is enhanced. This potentiation is also seen with electrical stimulation of large sensory but not motor axons and could result from a synchronization of motoneuronal excitability cycles. It is more prominent in the homogeneous (soleus) muscle than the mixed (plantaris) muscle, probably because the motoneuron cell bodies will reach a period of high excitability at more nearly the same time in the homogeneous muscle. For longer intervals the force produced by a 2nd stretch is reduced even when the EMG is close to control values. This late depression is also observed with electrical stimulation of cut motor axons and therefore arises from the contractile properties of muscles. With a random series of stretches, the same time course of nonlinear interactions is observed. As the mean rate of the random stretches is increased, the average response of the reflex decreases. The stretch reflex will be most effective in correcting for occasional perturbations to a movement, rather than for continuously varying disturbances.