Reconstruction of the nerve supply to a human muscle spindle

Distributions of one sensory and 13 motor axons to intrafusal fibers in a human spindle from the biceps brachii muscle were reconstructed from serial, 1 micron thick transverse sections. The primary afferent was distributed predominantly to nuclear bag fibers. Motor innervation in the human spindle was characterized by the presence of shared innervation among different types of intrafusal fiber, long unmyelinated preterminal segments of axon, and numerous short motor endings on both bag1 and bag2 fibers. These neuroanatomical features differ grossly from those in the cat tenuissimus spindles and may reflect a major functional difference.

Rat muscle spindles deficient in elements of the static system

Motor nerve supplies to 15 poles of rat lumbrical spindle were reconstructed from serial, 1-micron transverse sections of muscle embedded in resin. Neural and muscular elements associated with the modulation of static sensitivity of afferents were deficient in these spindles relative to cat tenuissimus and rat soleus spindles. Rat lumbrical spindles contained fewer static fusimotor axons, fewer static chain intrafusal fibers, fewer motor-innervated static bag2 and chain fibers and fewer secondary afferents. The sparsity of static elements in spindles of the rat lumbrical muscle may correlate with the distal location or with the delicate motor tasks performed by the muscle.

Motor innervation of intrafusal fibers in rat muscle spindles: incomplete separation of dynamic and static systems

Distributions of 53 motor axons to different types of intrafusal fibers were reconstructed from serial 1-micron-thick transverse sections of 13 poles of spindles in the rat soleus muscle. The mean number of motor axons that innervated a spindle pole was 4.1. Approximately 60% of motor axons lost their myelination prior to or shortly after entry into the periaxial fluid space of spindles. Motor innervation to the juxtaequatorial portion of nuclear bag fibers (particularly the bag1) consisted of groups of short, synaptic contacts that were terminations of thin, unmyelinated axons. In contrast, motor endings on both the bag1 and bag2 fibers were platelike in the polar intracapsular region. Chain fibers had a single midpolar platelike ending. The ratio of motor axons that innervated the bag1 fiber exclusively to axons that innervated bag2 and/or chain fibers was 1:1. However, one-fourth of motor axons coinnervated the dynamic bag1 fiber in conjunction with static bag2 and/or chain fibers. Thus the complete separation of motor control of the dynamic bag1 and static bag2 intrafusal systems observed in cat tenuissimus spindles is neither representative of the pattern of motor innervation in all other species of mammals nor essential to normal spindle function.

Histological study of motor innervation of nuclear bag1 intrafusal muscle fibers in the cat

The nerve supply to spindles of the cat tenuissimus muscle was reconstructed with light and electron microscopy of serial transverse sections. Fifty-two poles of the nuclear bag1 intrafusal muscle fiber were examined for motor innervation. The fiber poles were supplied by 71 myelinated motor axons that either terminated on bag1 fibers exclusively (93%) or coinnervated a chain fiber of the same intrafusal bundle (7%). No axons coinnervated both the bag1 and bag2 fibers. The unmyelinated preterminal segments of the axons were frequently short. Lengths and pre- and postsynaptic features of motor endings on bag1 fibers were variable. These features did not permit reliable classification of the endings into more than one morphological category. Moreover, the terminals of fusimotor (gamma) and skeletofusimotor (beta) axons on bag1 fibers appeared similar in cross-section. The degree of indentation of axon terminals into the surface of bag1 fibers increased with increasing distance from the spindle equator. However, cross-sectional areas of sole plates and axon terminals were relatively constant regardless of distance from the equator. The subjunctional membranes of both gamma and beta bag1 endings were typically smooth in contour. Bag1 endings differed from those on bag2 and typical chain fibers in having a thicker sole plate, frequently indented axon terminals, and unfolded subjunctional membranes. None of the bag1 endings resembled an extrafusal end plate. These observations indicated that (1) the dynamic (bag1) and static (bag2 and chain) intrafusal systems of the cat spindle are under separate motor control, and (2) the type of intrafusal fiber and the distance of the motor ending from the equator have a greater influence on the form and structure of bag1 endings than do supplying axons.

Shared motor innervation between dynamic and static intrafusal muscle fibers in the monkey

Distributions of 25 motor axons to 60 intrafusal muscle fibers of 10 poles of monkey spindle were reconstructed from serial 1 micron thick transverse sections of lumbrical muscles. About 44% of motor axons co-innervated two or more types of intrafusal fiber. The (dynamic) bag1 fiber shared motor innervation with the (static) bag2 or chain fibers in about 50% of spindle poles. Activation of single intrafusal fibers independent of the other fibers of the same intrafusal bundle occurs to a lesser degree in spindles of monkeys than in spindles of cats. Functional implications of this pattern of motor innervation are discussed.

Distribution of skeletofusimotor axons in lumbrical muscles of the monkey

The nerve supply to 25 poles of muscle spindles in the monkey was reconstructed by light microscopy of serial 1-micron thick transverse sections of lumbrical muscles. Twenty of 60 motor axons that supplied the spindle poles were identified as skeletofusimotor (beta). Twenty-eight percent of the spindle poles were innervated by beta axons, in addition to gamma axons. Every beta-innervated spindle pole transected an endplate zone of extrafusal muscle. Most beta axons coinnervated extrafusal fibers rich in mitochondria and the nuclear bag1 or nuclear chain intrafusal fibers. All but two beta axons innervated one type of intrafusal fiber only. The intramuscular organization of beta motor system in lumbrical muscles of the monkey was similar to that of the cat tenuissimus muscle. The function of beta-innervated spindles may be preferentially to monitor mechanical disturbances arising from the activity of extrafusal muscle units with which they share motor innervation.

Ultrastructure of dynamic and static skeletofusimotor endings in a cat muscle spindle

Endings of four beta skeletofusimotor axons in a spindle of the cat tenuissimus muscle were examined in semithin (1-micron thick) and ultrathin transverse serial sections. Two (dynamic) beta axons terminated on the nuclear bag1 intrafusal muscle fiber and on extrafusal fibers of the dark type. Two (static) beta axons terminated on the nuclear chain intrafusal fibers and extrafusal fibers of the intermediate type. The degree of indentation of axon terminals into the muscle surface, thickness of the sole plate and extent of folding of subjunctional membranes differed among intrafusal and extrafusal terminations of the same axon. Endings of beta axons on the bag1 and chain fibers were also morphologically dissimilar. Motor axons may not determine ending morphology. Rather the form and structure of a beta bag1 or chain ending may be determined by the type of intrafusal fiber on which the ending lies and the ending's distance from the primary sensory axon.

Nonselective motor innervation of intrafusal fibers in muscle spindles of the rat

Distributions of motor axons to different types of intrafusal fiber were reconstructed from serial 1-micron thick transverse sections of six poles of muscle spindle in the rat soleus. Motor axons innervated (dynamic) bag1 fibers, or (static) bag2 fibers in conjunction with chain fibers. However, approximately forty percent of axons that supplied the spindles synapsed on both bag1 and bag2 or bag1 and chain fibers. The significance of this co-innervation of dynamic and static intrafusal fibers is discussed relative to the general organization and function of mammalian spindles.

Nonselective motor innervation of nuclear bag1 intrafusal muscle fibers in the cat

The motor nerve supply to cat nuclear bag1 intrafusal muscle fibers was reconstructed from light and electron microscopy of serial transverse sections of spindles in the tenuissimus muscle. Twenty-six of thirty poles of bag1 fibers that were examined received motor innervation. Every innervated bag1 pole received at least one (range 1-3) selective motor axon that supplied this fiber type only. Four of the innervated bag1 poles (15%) received additional motor supply from a nonselective motor axon that also innervated one nuclear chain fiber in the same spindle pole. The chain fibers co-innervated with bag1 fibers were among the longest chain fibers although they were shorter than two long chain fibers also present in the spindle poles. In cross-sections stained with toluidine blue they displayed 1-3 equatorial nuclei side by side, and there were fewer intermyofibrillar granules in their polar regions than in most of the other chain fibers. The endings of nonselective motor axons on the bag1 and chain fibers were morphologically and ultrastructurally dissimilar. It is suggested that instances of common innervation of the (dynamic) bag1 fiber and a (static?) chain fiber represent an integral and, presumably, functionally meaningful part of the motor pattern in some cat spindles.

Ultrastructure of extrafusal and intrafusal terminals of a (dynamic) skeletofusimotor axon in cat tenuissimus muscle

Terminals of a histologically identified beta skeletofusimotor axon in cat tenuissimus muscle were examined in semi-thin (1 micrometer thick) and ultrathin transverse sections. The beta axon supplied one extrafusal endplate and two motor endings on the nuclear bag1 intrafusal muscle fiber. The staining properties of the co-innervated bag1 and extrafusal fibers were dissimilar. The beta intrafusal and extrafusal terminals were also dissimilar by both light and electron microscopy , differing in length, mitochondrial content of the sole-plate, and folding of the post-synaptic membrane.

Histological identification of (static) skeletofusimotor innervation to a cat muscle spindle

Two skeletofusimotor (beta) axons were identified histologically in cat tenuissimus muscle. The axons supplied the distal poles of 3 intrafusal muscle fibers of the nuclear chain type and two extrafusal fibers of the fast twitch type. The observations strengthen the case for the existence of static beta-innervation to cat muscle spindles.

Evaluation of infusion sets by biological and chemical testing

Some of the biological and chemical tests used in toxicological evaluation of synthetic polymers were correlated. The individual tests (sperm cell motility, tissue culture inhibition, intracutaneous test, optical absorption at 220-360 nm, reducing impurities) did not mutually correspond. The final estimation of the tested device is assessed as a sum of these units. Therefore, it is necessary to evaluate the results of the individual tests in arbitrary units.

Ultrastructural study of motor endings on long chain intrafusal fibers in muscle spindles of the cat

Five complete poles of long nuclear chain intrafusal fibers of cat tenuissimus muscle spindles embedded in a plastic medium were cut in serial, 1 micron thick transverse sections and stained with toluidine blue. Each fiber pole displayed one plate-type motor ending positioned outside the encapsulated part of the spindle. Ultrastructurally, the endings resembled extrafusal end-plates and were more complex, in terms of prominence of sole-plate and degree of post-junctional folding, than any other motor ending present in the spindles. They were identified as the terminals of static (fast) skeletofusimotor axons, which preferentially innervate the longest nuclear chain fibers of cat spindles.

Multiple-bag-fiber muscle spindles in tenuissimus muscles of the cat

Over 300 complete and incomplete cat muscle spindles were examined in serial transverse sections of tenuissimus muscles in search of spindles with more than two nuclear bag intrafusal muscle fibers. Several histochemical and histological stains were used to identify the intrafusal fibers and assess their motor and sensory innervation. About 13% of the spindles contained either three or four bag fibers rather than the usual two. Every multiple-bag-fiber spindle possessed at least one nuclear bag1 and one nuclear bag2 fiber. The supernumerary bag fibers were either another bag1 and/or bag2 fiber, or a mixed bag fiber. The extra bag fibers had the usual morphologic and histochemical properties of cat nuclear bag fibers. All multiple-bag spindles received primary sensory innervation, and most had secondary sensory endings in addition. Their motor pattern was similar in the number, appearance and disposition of intrafusal motor endings to that of the usual two-bag-fiber spindles. Bag fibers of the same kind shared motor nerve supply in three multiple-bag spindles in which tracings of individual motor axons were obtained histologically. It is unclear whether any functional advantage is conveyed to a muscle spindle by its having more than one bag1 and one bag2 fiber.