A reexamination of multiaxonal nerve endings innervating intrafusal muscle fibers of the Chinese hamster.

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.

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Intrafusal Muscle Fibers

Encyclopedia of Neuroscience ◽

10.1007/978-3-540-29678-2_2559 ◽

2008 ◽

pp. 2027-2027

Keyword(s):

Muscle Fibers ◽

Intrafusal Muscle ◽

Intrafusal Muscle Fibers

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A new grouping of intrafusal muscle fibers based on developmental studies of muscle spindles in the cat

American Journal of Anatomy ◽

10.1002/aja.1001560112 ◽

1979 ◽

Vol 156 (1) ◽

pp. 115-120 ◽

Cited By ~ 10

Author(s):

Richard Butler

Keyword(s):

Muscle Fibers ◽

Muscle Spindles ◽

Developmental Studies ◽

Intrafusal Muscle ◽

Intrafusal Muscle Fibers

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Histochemical Identification of Three Types of Intrafusal Muscle Fibers in the Cat and Monkey Based on the Myosin ATPase Reaction

Canadian Journal of Physiology and Pharmacology ◽

10.1139/y72-030 ◽

1972 ◽

Vol 50 (3) ◽

pp. 195-202 ◽

Cited By ~ 109

Author(s):

W. K. Ovalle ◽

R. S. Smith

Keyword(s):

Frozen Section ◽

Muscle Fibers ◽

Muscle Spindles ◽

Myosin Atpase ◽

Stable Form ◽

Intrafusal Muscle ◽

Atpase Reaction ◽

Intrafusal Muscle Fibers ◽

Alkaline Conditions ◽

Acid Stable

Serial frozen section of muscle spindles in the lumbrical muscles of the cat and the monkey were tested with a recently described modification of the myosin ATPase reaction (Guth and Samaha 1970. Exp. Neurol 28, 365). This reaction is able to detect acid-stable, alkaline-labile myosin ATPase and acid-labile, alkaline-stable myosin ATPase. The reactions were carried out by preincubation of the sections at pH 10.4 or 4.35 with incubation in a substrate at pH 9.4. In both the cat and the monkey three types of intrafusal muscle fibers could be detected on the basis of their reaction for myosin ATPase. Nuclear bag fibers could be divided into two types: those containing the acid-stable form of myosin ATPase only, and those containing ATPase detectable under both acid and alkaline conditions. Nuclear chain fibers contained the alkaline-stable form of ATPase only.

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