Fall in the firing rate of muscle spindles during stretching.

Investigation of the dynamic responses of 500 muscle receptors to stretching of the triceps surae muscle of the de-efferented hind limb of anesthetized cats revealed in 23 cases a transient fall in the firing rate (FFR) of muscle spindle receptors while muscle stretching was in progress. FFR arises in the same receptor during stretching of the muscle in different forms and at different speeds, approximately in the same region of stretching (the "FFR zone"). With an increase in the velocity of muscle stretching the FFR effect is intensified. It is suggested that the FFR effect is due to temporary removal of the load from the area of the spindle where the receptor endings are located. The conditions for this may be created by the nonhomogeneous structure of the extrafusal muscle, and also by the different mechanical properties of the intrafusal muscle fibers.