M. C. Brown coauthored experiments showing that a muscle spindle's sensory response depends on its recent motor input. In 1969, recordings from the soleus muscle of anaesthetized cats with cut ventral roots paired stimulation of a single motor fibre supplying a muscle spindle with a later stretch. A burst of action potentials that had been absent before stimulation appeared during the testing stretch. The effect could persist for more than a minute, but another stretch abolished it.[ref]
An illuminating feature was the dependence on conditioning length: starting the test from a shorter muscle length delayed the burst until the stretch passed the length used during stimulation. Thus, identical current lengths need not imply identical sensory output when the mechanical histories differ. The authors proposed persistent bonds between actin and myosin inside the muscle spindle; this was an interpretation of the observations, rather than a direct measurement of those bonds.[ref]
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