The differentiation of two types of intrafusal fibres in rabbit muscle spindles. An electron microscopic study

Fine structure of rat intrafusal muscle fibers. The equatorial region

An ultrastructural study has been undertaken on the equatorial (sensory) region of the rat muscle spindle. Two kinds of intrafusal muscle fibers, a nuclear bag fiber and a nuclear chain fiber, have been identified in this region on the basis of fiber diameter, nuclear disposition, and M-band appearance. The large-diameter nuclear bag fiber contains an aggregation of tightly packed vesicular nuclei, while the small-diameter nuclear chain fiber contains a single row of elongated, well-separated nuclei. Both muscle fibers contain an attenuated peripheral cylinder of myofilaments surrounding a central core of sarcoplasm. Elements of the sarcotubular system, dilatations of the sarcoplasmic reticulum, and the presence of other sarcoplasmic organelles and inclusions are considerably more abundant in the nuclear chain fiber than in the nuclear bag fiber. Leptomeric organelles and membrane-bounded sarcoplasmic granules are present in both intrafusal fiber types and may be situated between the myofibrils or in intimate association with the sarcolemma. The functional significance of some of these structural findings is discussed.

Fine structure of rat intrafusal muscle fibers. The polar region

An ultrastructural comparison of the two types of intrafusal muscle fibers in muscle spindles of the rat was undertaken. Discrete myofibrils with abundant interfibrillar sarcoplasm and organelles characterize the nuclear chain muscle fiber, while a continuous myofibril-like bundle with sparse interfibrillar sarcoplasm distinguishes the nuclear bag muscle fiber. Nuclear chain fibers possess well-defined and typical M bands in the center of each sarcomere, while nuclear bag fibers contain ill-defined M bands composed of two parallel thin densities in the center of the pseudo-H zone of each sarcomere. Mitochondria of nuclear chain fibers are larger and more numerous than they are in nuclear bag fibers. Mitochondria of chain fibers, in addition, often contain conspicuous dense granules, and they are frequently intimately related to elements of the sarcoplasmic reticulum (SR). Striking differences are noted in the organization and degree of development of the sarcotubular system. Nuclear bag fibers contain a poorly developed SR and T system with only occasional junctional couplings (dyads and triads). Nuclear chain fibers, in contrast, possess an unusually well-developed SR and T system and a variety of multiple junctional couplings (dyads, triads, quatrads, pentads, septads). Greatly dilated SR cisternae are common features of nuclear chain fibers, often forming intimate associations with T tubules, mitochondria, and the sarcolemma. Such dilatations of the SR were not encountered in nuclear bag fibers. The functional significance of these structural findings is discussed.

Skeleto-fusimotor fibres in the rabbit

1. In rabbits, repetitive stimulation of single motor axons to lumbrical muscles elicits both a contraction of extrafusal muscle fibres and an increase in the discharge frequency of spindle primary endings.2. This activation of the sensory endings can be attributed to the contraction of intrafusal muscle fibres because it persists after the contraction of extrafusal muscle fibres has been suppressed by selective curarization of their neuromuscular junctions.3. In non-curarized preparations the frequency of most of the afferent discharges continues to increase when rates of stimulation of motor fibres exceed the tetanic fusion frequency of the extrafusal muscle fibres.4. The effect of repetitive stimulation of motor fibres that supply extrafusal and intrafusal muscle fibres was studied on the responses of primary endings to phasic stretch. Of twelve fibres, eight were found to exert a dynamic effect and four a static one.5. Selective curarization of the extrafusal neuromuscular junctions does not modify the nature of the static and dynamic effects.