The tendon organs of cat soleus: static sensitivity to active force.

The static force sensitivity of soleus tendon organ (Ib) afferents has been studied by noting their responses to graded force development produced by isometric contractions of either the whole muscle or single motor units. Data included responses of 23 Ib afferents to contraction of 8 whole muscles (8 experiments) and 16 Ib afferents to contraction of 30 motor units (5 experiments). Tendon organ responses of varying magnitude to contraction of the whole muscle or several of its individual motor units could be explained by differences in the number of muscle fibers that insert into each receptor's capsule and by differences in the contraction strength of these fibers. This finding suggests that soleus tendon organs have similar absolute sensitivities to static force development. An estimate was made of this absolute sensitivity and the value obtained (314 pps/g of force actually coupled to the receptor) is 2 orders of magnitude greater than those previously reported indices that simply relate Ib firing rate to force as measured at the tendon. The relationship between force exerted on a tendon organ's capsule and Ib firing rate is whown to be curvilinear and in keeping with a possible saturation effect that reduces the receptor's responsiveness to active contractions at relatively long muscle lengths.