Crossed segmental inhibition in muscles.

Muscle spindles are critically involved in the reflex regulation of the dynamic and static actions of muscles; the sensitivity of the actions is under strong efferent control2, 8. In the usually studied mammal, the cat, the receptor elements in the muscle spindles convey by two types of fibers (Groups I and II) muscle displacement and velocity information to the central nervous system. The functions of amphibian muscle spindles, usually studied in the frog, appear to be similar to mammalian although the spindles lack a separate system of efferent control and lack the secondary endings of mammalian spindles. Frog skeletal muscle is also similar to mammalian in having twitch fibers, but in an important respect differs in that slow (tonic) muscle fibers are present 6. In contrast with the cat, the hindlimbs of the frog are not as critically involved in postural adjustments. The animal normally sits on the ventral surface of the caudal end of its body with its hind.limbs folded. However, postural adjustments must be maintained during jumping, landing and swimming. The existence of the crossed extension reflex in the frog as in mammals suggests crossed segmental inhibition between homonymous muscles of the hindlimbs 4A2, but rather than reciprocal innervation of crossed extensor and flexor muscles as in the mammal, at least jumping (take-off) and swimming behavior in the frog must be determined by a different kind of central organization. Hindlimb movements in the frog are typically synchronized in extension and flexion. In the present study the properties of frog muscle spindles have been used to establish the quantitative relationships between the gastrocnemius muscles of the two limbs for various stretch velocities and preloadings and to describe the interactions of the limb muscles. The somewhat unique organization of the gastrocnemius muscles in the frog suggests a special functional role.