An evaluation of length and force feedback to soleus muscles of decerebrate cats.

IT IS NOW WELL KNOWN that contraction of a muscle is reflexly excited by responses of its spindle receptors to stretch (32, 34) and is reflexly inhibited by responses of its Golgi tendon organs to contraction (10, 31). Many experimental techniques have been used to confirm these observations (7, 12, 23). Nevertheless, the actual importance of each of these reflexes in the gradation of contraction remains obscure because of the lack of an experimental approach which is capable of estimating quantitatively their respective influences (39). Formerly it was believed by many that tendon organs responded and inhibited contraction on1 .y when muscular forces became excessive. Recent studies (19, 26) have cast doubt on this hypothesis by demonstrating for these receptors a much lower threshold to must ular contract ion than was previ not ouslv t .h assure ought. This find .ing alone does a con tinuo us regulation of muscular force by signals from tendon organs since impulses must be transmitted through one or two interneurons before they may inhibit homonymous motoneurons (7, 31). Studies have shown that these Ib pathways transmit impulses more effectively in spinal than in decerebrate cats (8). It is therefore likely that the gain of this reflex pathway is not constant but, rather, is subject to control by signals from various regions of the nervous system. For example, Ib pathways are facilitated by signals transmitted from the red nucleus (IS). As a result, the relative importance of tendon organs in the regulation of contraction would depend on the particular state of the experimental animal. We have attempted to determine whether or not this regulation of force is significant in decerebrate cats.