Response of the Isolated Muscle Spindle to Different Rates of Stretching

The muscle spindle of the frog may be considered as a biological transducer which measures the relative length and the rate of change of length of the muscle, and converts this information into a coded impulse message. Detailed knowledge of the functional properties of the spindle began with the work of Matthews (1931) who recorded from single afferents and showed that during loading of the muscle there is a rapid discharge of nerve impulses, followed by a decline in frequency to a steady level during maintained stretch. The frequency of the initial discharge was found to be proportional to the rate of loading of the muscle. Katz (1950) extended these observations by using stretches of different rates and showed that the impulse discharge was generated by a local graded potential in the sensory endings. Evidence was also presented by Katz (1950) for two distinct components of the spindle potential,