M. J. Stacey

Studies of the Histochemistry, infrastructure, Motor Innervation, and Regeneration of Mammalian Intrafusal Muscle Fibres

Publisher Summary This chapter presents studies of the histochemistry, ultrastructure, motor innervation, and regeneration of mammalian intrafusal muscle fibres. In studies, it was found that a direct comparison between the histochemical profile and ultrastructure of an intrafusal muscle fibre could be made by cutting frozen serial transverse sections in batches at about 15 μm, alternating with much thicker ones at about 60 μm. The thin sections could be used for the application of various histochemical techniques, while the thick ones were processed for the observation of ultrastructure in both transverse and longitudinal section. By sectioning according to this sequence, the histochemical and ultrastructural characteristics of each type of intrafusal muscle fibre can be correlated at all levels from equator to extreme pole as it is traced through the spindle. Further, the distribution of static and dynamic γ axons to cat tenuissimus spindles were analyzed using Edstrom and Kugelbergs glycogen depletion technique. This analyses differs in a number of respects from a similar one made by Brown and Butler—the muscle portions containing the activated spindles were quick-frozen and then fixed in absolute ethanol during freeze-substitution in order to avoid the streaming of glycogen granules; sampling of γ static axons was not restricted to those of relatively fast conduction velocity; and the two types of bag fibre were taken into account in the analysis.

Histological analysis of cat muscle spindles following direct observation of the effects of stimulating dynamic and static motor axons.

1. Eleven cat tenuissimus spindles have been analysed mainly by cutting serial, transverse, 1 micrometer thick sections following direct observation of the effects of dynamic motor (gamma or beta) stimulation. 2. Histological results from these spindles were also used to interpret the effects of static fusimotor stimulation of other spindles. 3. Dynamic motor stimulation usually produced contractions seen as convergent movements of sarcomeres in single bag fibres, identified as bag1 fibres for reasons given in the text. 4. In one spindle a single dynamic axon produced a translational movement in one pole of a bag1 fibre and a convergent movement in each pole of a bag2 fibre, together with movements in other unidentified (presumably chain) fibres. Subsequent analysis showed that besides innervating both bag fibres the axon also supplied two chain fibres. 5. Contrary to expectation, motor endings on the bag1 fibres seldom occurred at the sites of convergent movement. Only two cases of coincidence occurred among sixteen foci and twenty‐one motor endings; otherwise focus and nearest ending were separated by distances of 0.85‐‐2.5 mm. 6. Most of the convergent movements of sarcomeres observed in bag1 fibres occurred in a region of the pole that is ultrastructurally distinct from the region where most of the motor endings were located. The possible relevance of this to the production of contractions in the bag1 fibre is discussed. 7. Convergent movement foci in bag2 fibres produced by the stimulation of static axons occurred largely within the same regions of the pole as the motor endings were located, though, whereas foci were observed in both intra‐ and extracapsular regions, most of the endings were intracapsular.

Identification of intrafusal muscle fibres activated by single fusimotor axons and injected with fluorescent dye in cat tenuissimus spindles.

1. Intrafusal muscle fibres of cat tenuissimus spindles have been injected with the fluorescent dye Procion Yellow and identified histologically after recording their changes in membrane potential during 1/sec stimulation of single static or dynamic gamma axons. 2. Thirteen intrafusal muscle fibres innervated by static gamma axons were identified as eight bag2 and five chain fibres. The fact that none proved to be a bag1 fibre is not regarded as significant, for reasons given in the Discussion. 3. In one spindle Procion Yellow was injected into two intrafusal muscle fibres activated by the same static gamma axon; they were identified as a bag2 and a chain fibre. 4. Nine intrafusal muscle fibres innervated by dynamic gamma axons were identified as seven bag1 fibres, one bag2 fibre, and one long chain fibre. 5. In one spindle two bag fibres were injected, one activated by a dynamic gamma axon, the other by a static gamma axon; the former proved to be a bag1 fibre, the latter a bag2 fibre. 6. Stimulation of static gamma axons elicited junctional potentials in seven bag2 fibres and one damaged chain fibre, and action potentials in one bag2 and four chain fibres. In the whole sample of impaled intrafusal muscle fibres (identified and unidentified) activated by static axons, junctional potentials were recorded from twenty‐three (62.2%), and action potentials from fourteen (37.8%). Stimulation of dynamic gamma axons always elicited junctional potentials. 7. In a number of instances it was possible to examine the ultrastructure of motor endings belonging to the stimulated gamma axon. The myoneural junctions of trail endings supplied by static gamma axons to bag2 and chain fibres were both smooth and folded; the deepest and most regular folding occurred on chain fibres. The terminals of p2 plates supplied to bag1 fibres by dynamic gamma axons had smooth myoneural junctions.

A comparative analysis of the encapsulated end-organs of mammalian skeletal muscles and of their sensory nerve endings

The encapsulated sensory endings of mammalian skeletal muscles are all mechanoreceptors. At the most basic functional level they serve as length sensors (muscle spindle primary and secondary endings), tension sensors (tendon organs), and pressure or vibration sensors (lamellated corpuscles). At a higher functional level, the differing roles of individual muscles in, for example, postural adjustment and locomotion might be expected to be reflected in characteristic complements of the various end‐organs, their sensory endings and afferent nerve fibres. This has previously been demonstrated with regard to the number of muscle‐spindle capsules; however, information on the other types of end‐organ, as well as the complements of primary and secondary endings of the spindles themselves, is sporadic and inconclusive regarding their comparative provision in different muscles. Our general conclusion that muscle‐specific variability in the provision of encapsulated sensory endings does exist demonstrates the necessity for the acquisition of more data of this type if we are to understand the underlying adaptive relationships between motor control and the structure and function of skeletal muscle. The present quantitative and comparative analysis of encapsulated muscle afferents is based on teased, silver‐impregnated preparations. We begin with a statistical analysis of the number and distribution of muscle‐spindle afferents in hind‐limb muscles of the cat, particularly tenuissimus. We show that: (i) taking account of the necessity for at least one primary ending to be present, muscles differ significantly in the mean number of additional afferents per spindle capsule; (ii) the frequency of occurrence of spindles with different sensory complements is consistent with a stochastic, rather than deterministic, developmental process; and (iii) notwithstanding the previous finding, there is a differential distribution of spindles intramuscularly such that the more complex ones tend to be located closer to the main divisions of the nerve. Next, based on a sample of tendon organs from several hind‐foot muscles of the cat, we demonstrate the existence in at least a large proportion of tendon organs of a structural substrate to account for multiple spike‐initiation sites and pacemaker switching, namely the distribution of sensory terminals supplied by the different first‐order branches of the Ib afferent to separate, parallel, tendinous compartments of individual tendon organs. We then show that the numbers of spindles, tendon organs and paciniform corpuscles vary independently in a sample of (mainly) hind‐foot muscles of the cat. Grouping muscles by anatomical region in the cat indicated the existence of a gradual proximo‐distal decline in the overall average size of the afferent complement of muscle spindles from axial through hind limb to intrinsic foot muscles, but with considerable muscle‐specific variability. Finally, we present some comparative data on muscle‐spindle afferent complements of rat, rabbit and guinea pig, one particularly notable feature being the high incidence of multiple primary endings in the rat.

Quantitative Studies on Mammalian Muscle Spindles and their Sensory Innervation

It is not surprising, in view of the various functional roles played by skeletal muscles, that each muscle should possess a characteristic proprioceptive innervation. Muscle spindles are relatively easy to count and have been the main subject of quantitative studies. In drawing comparisons between different muscles, most authors have used the number of spindles per gram of adult muscle, or spindle density, as a measure of relative abundance. In both man and cat, where sufficient muscles have been examined, smaller muscles have been found usually to have higher spindle densities than larger muscles (reviewed by Hosokawa, 1961; Voss, 1971; and Barker, 1974). This has frequently led to the suggestion that the higher densities are functionally appropriate to small muscles involved in fine postural adjustment or manipulation, yet it has never been demonstrated that it is justifiable to relate spindle number linearly to muscle mass as a simple density.

Structural aspects of fusimotor effects on spindle sensitivity

The distribution of motor and sensory axons to the three types of intrafusal muscle fibre have been determined using reconstructions of serially sectioned spindles and teased, silver-impregnated, whole spindles.

Form and classification of motor endings in mammalian muscle spindles

The presynaptic features of 234 motor endings supplied to cat hindlimb muscle spindles have been studied in teased, silver preparations, and the postsynaptic features of a further 27 endings have been studied in serial, 1 μm thick, transverse sections. In the presynaptic study motor endings received by the three types of intrafusal muscle fibre were compared with the endings supplied to spindles by the various functional categories of motor axon. Three forms of motor ending were found that had significantly different presynaptic features. These forms correspond closely to those previously identified in the literature as p1 (β), p2 (dynamic γ) and trail (static γ). The results of the postsynaptic study showed that the degree of indentation of the intrafusal muscle fibres by motor axon terminals increases with greater distance from the primary ending, irrespective of muscle-fibre type. We conclude that the postsynaptic form of intrafusal motor endings is determined by distance from primary ending and muscle-fibre type. It is not determined by type of motor axon, and cannot be correlated with presynaptic form so as to produce a unified classification of intrafusal motor endings.

The Innervation of Muscle Spindles in an Intrinsic Muscle of the Hind Foot: The Superficial Lumbrical of the Cat

An important, if largely unrecognised, feature of the design of the mammalian skeletomotor proprioceptive system is the characteristic variation from muscle to muscle in the provision of its components. This applies not only to the relative abundance of muscle spindles, but also to quantitative differences in their sensory ending complement (Banks & Stacey, 1988). Furthermore, Banks (1994) has shown that there is a linear correlation between the numbers of static γ and afferent axons supplied to each spindle, at least in the tenuissimus. Considerably more comparative information of this type is needed for a fuller understanding of the contribution that proprioception makes to particular motor tasks. Here we describe some preliminary observations on the innervation of the superficial lumbrical muscle of the cat’s hind foot.