Sensory innervation and intrafusal muscle fibers in tandem muscle spindles of semitendinosus muscle with reference to single type spindles in the frog.

Cipollone (1897) first noted that two or more encapsulated sensory regions occur along the length of the intrafusal bundle in frog skeletal muscle. Similar arrangements have been evident from the reports of Baum (1899) and Barker (1948) on mammals, and termed 'tandem' by Cooper and Daniel (1956). Vihvelin (1932) and Jahn (1959), in their studies of this arrangement in the amphibian muscle spindles, concluded that they consisted of bundles of single spindles lying side by side. Muscle-fiber continuity between capsules also occurs in the spindle systems which Gray (1957) has described as extending throughout the length of the frog's fourth toe extensor muscle.. Barker (1962) stated that G r a y's (1957) definition of a frog spindle system refers to the same arrangement as that described by Cooper and Daniel (1956) as tandem in mammalian spindles. In the frog's toe muscle, Barker and Cope (1962) have verified that the arrangement is more complex than this far a system which consists of two or four tandem spindles, and sometimes one or two single spindles, lying together in parallel to form a bundle of spindle units. This is termed compound spindle I t o and 0 y a m a (1968) found that many tandem muscle spindles and compound spindles were included in the ventral part of the frog's semitendinosus muscle, and that such types of spindle receptors may be provided to respond sensitively to minute changes of the muscle length. They have argued that such high sensitivity may be due to a number of nonmyelinated ramifications of the sensory nerve endings and also to large numbers of intrafusal muscle fibers in comparison with the single type spindles in the sartorius muscle. The results of this investigation are now presented to verify in detail those assumptions.