Golgi tendon organs can be activated by either muscle stretch or muscle contraction due to their in-series relationship with the muscle. However, the actual stimulus sensed by the Golgi tendon organ is the force that develops in the linked tendon. Therefore, the response to stretch is influenced by the spring-like nature of the muscle, which follows Hooke's law where the force on a spring is proportional to its stretch.

To differentiate between the responsiveness of muscle spindles and Golgi tendon organs, the firing patterns of group Ia and group Ib fibers are compared when a muscle is stretched and held at a longer length. The firing rate of group Ia fibers continues to increase until the stretch is reversed. On the other hand, group Ib fibers initially show a significant increase in firing, indicating the increased tension caused by the stretch. However, their firing rate gradually returns to its initial level as the tension on the muscle decreases due to cross-bridge recycling and subsequent lengthening of sarcomeres.

Golgi tendon organs signal force, while spindles signal muscle length. This distinction is further supported by the fact that group Ib firing is correlated with force level during isometric contraction, even when muscle length and group Ia activity remain unchanged.

Group Ib afferent fibers branch upon entering the spinal cord and terminate on interneurons. There are no direct connections to alpha motor neurons. Instead, group Ib afferent fibers synapse with two types of interneurons: inhibitory interneurons that inhibit alpha motor neurons supplying the homonymous muscle (e.g., rectus femoris muscle) and excitatory interneurons that activate alpha motor neurons for the antagonist muscle (e.g., semitendinosus muscle). Due to the two synapses in series within the central nervous system, this reflex arc is considered disynaptic. This connectivity explains why group Ib reflex is also known as the inverse myotatic reflex.
Because of their in-series relationship to the muscle, Golgi tendon organs can be activated either by muscle stretch or by muscle contraction. In both cases, however, the actual stimulus sensed by the Golgi tendon organ is the force that develops in the tendon to which it is linked. Thus the response to stretch is the result of the spring-like nature of the muscle (i.e., by Hooke’s law, the force on a spring is proportional to how much it is stretched).
To distinguish between the responsiveness of the muscle spindles and Golgi tendon organs, the firing patterns of group Ia and group Ib fibers can be compared when a muscle is stretched and then held at a longer length (Fig.
9.8 ). The firing rate of the group Ia fibers maintains its increase until the stretch is reversed. In contrast, the group Ib fiber shows an initial large increase in firing, reflecting the increased tension on the muscle caused by the stretch, but then shows a gradual return toward its initial firing rate as the tension on the muscle is lowered because of cross-bridge recycling and the resultant lengthening of the •Fig. 9.7 Reflex Arc of the Inverse Myotatic Reflex. Theinterneuronsincludebothexcitatory(white) andinhibitory(black) interneurons.Thisisanexampleofadisynapticreflex.
•Fig. 9.8 Changes in Group Ia and Group Ib Firing Rates When Muscle Is Stretched to a New Length. Afteratransientburst,thefiringrateofthegroupIafiberremainsconstantatanewhigherlevelthatisproportionaltotheincreaseinlength(compareblue lines in upper and lower graphs).Incontrast,thegroupIbfibershowsaninitialrapidincreaseinfiringfollowedbyaslowdecreasebacktowarditsoriginallevel(lower graph, red line) andhasafiringprofilethatmatchesthetensionlevelinthemusclecausedbythestretch(upper graph, red line).
sarcomeres. Therefore, Golgi tendon organs signal force, whereas spindles signal muscle length. Further evidence of this distinction is that group Ib firing is correlated with force level during isometric contraction even though muscle length and therefore group Ia activity are unchanged.
CHAPTER 9 Organization of Motor Function •Fig. 9.9 The Reflex Arc of the Flexion Reflex. Black interneuronsareinhibitory,andwhite onesareexcitatory.FRA,flexionreflexafferentfiber.++––FRASemitendinosusNociceptorFlexionExtensionRectusfemoris
The group Ib afferent fibers branch as they enter the spinal cord and end on interneurons. There are no mono-synaptic connections to α motor neurons. Rather, the group Ib afferent fibers synapse onto two classes of interneurons: interneurons that inhibit α motor neurons that supply the homonymous muscle (in this case the rectus femoris muscle) and excitatory interneurons that activate α motor neurons to the antagonist (the semitendinosus muscle). Because there are two synapses in series in the CNS, this is a disynaptic reflex arc. Because of these connections, group Ib fiber activity should have the opposite action of the group Ia stretch reflex during passive stretch of the muscle, which explains the group Ib reflex’s other name, the inverse myotatic reflex.