Identifications of the intrafusal endings of skeletofusimotor axons in the cat

Skeletofusimotor (beta) innervation of proximal and distal forelimb muscles of the cat

The innervation of muscle spindles by skeletofusimotor (beta) axons has been compared in two long digit extensor muscles with three elbow extensor muscles of the cat forelimb. The proportion of muscle spindle poles with p1-endplates was analysed in silver impregnated teased material. The proximal and distal muscles displayed a significant difference in their proportion of muscle spindle poles with p1-endplates. The distal muscles had an estimated proportion of more than 70% beta innervation of their muscle spindles, the proximal muscles of 41-47%. This difference is discussed with respect to the different circuitry of the motoneurones. We suggest that the high proportion of beta-innervated spindles in distal muscles obtained with the absence of a recurrent inhibitory system in the innervating motoneurones [18] serves the execution of manipulative movements.

Scanning electron microscopy of teased intrafusal muscle fibers from rat muscle spindles

Scanning electron microscopic (SEM) studies on intrafusal muscle fibers and their various sensory and motor endings are not available thus far although SEM greatly increases the resolution power for analyzing surface structures in comparison to light microscopic preparations. From a total of 68 isolated muscle spindles in lumbrical muscles of Wistar rats, only one primary sensory ending could be isolated. Secondary sensory endings were of an annulospiral, a flat, cuff-shaped, or a more complicated branched type; they were always completely surrounding intrafusal muscle fibers. Motor terminals were more focally arranged not surrounding the muscle fibers. Three types could be distinguished. Collagen fibrils were seen to insert on capsular elements as well as on the surface of intrafusal muscle fibers. Thus SEM helps understanding the most complicated electromechanical system for motor control.

Discharge patterns of gamma motoneurone populations of extensor and flexor hindlimb muscles during walking in the thalamic cat

Two monopolar recordings of the whole activity in a fine nerve branch innervating the gastrocnemius lateralis (GL) or the sartorius medialis (SM) muscle were obtained during spontaneous walking in thalamic cats. Using a special electronic device, the potentials of several groups of efferent (alpha and gamma) and afferent (I and II) fibres constituting the whole nerve activity were separated. In the present paper we compare the data obtained for an ankle extensor (GL) and a hip-knee flexor (SM) during the step cycle. In both muscles the gamma motoneurone population is activated in parallel with the alpha motoneurone population. Usually, between the cyclic locomotor discharges, the GL gamma neurones are tonically active whereas the SM gamma neurones are silent. During muscle contraction, the group I and II afferent discharges are both length and gamma dependent, but the prevailing factor is the muscle shortening for the GL afferents and the cyclic gamma drive for the SM afferents. Both dynamic and static fusimotor efferents appear to be activated during muscle contraction, but on indirect evidence it is suggested that dynamic action prevails in GL spindles whereas static action dominates in SM spindles.

Factors that determine the form of neuromuscular junctions of intrafusal fibers in the cat

The form of terminations of fusimotor (gamma) and skeletofusimotor (beta) axons on intrafusal fibers was analyzed in serial sections of 20 spindles of the cat tenuissimus muscle. Seven synaptic features were assessed either qualitatively or quantitatively from electron micrographs of transverse sections of 184 intrafusal and 30 extrafusal endings. Features were compared among endings that were terminations of gamma or beta axons on different types of intrafusal fiber at different distances from the spindle equator. These comparisons indicated that interactions of several factors, and not the motor axon alone, determine the form of motor endings. Intrafusal muscle fiber type is dominant to the motor axon in regulation of the number and depth of postsynaptic folds. Separation of the influence of the motor axon from the muscle fiber was less clear with respect to the size of ending. Complete expression of the muscle fiber-motor axon interaction reflected by the form of motor endings is dependent upon location of the ending relative to the sensory region. Both depth of the primary synaptic cleft and size of the soleplate of motor endings increased with increasing distance of the ending from the spindle equator. A system of classification of cat intrafusal motor endings that reflects the multiplicity of factors that determine the form of endings, and one that simplifies the current terminology, is proposed.

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 micron 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 (beta), p2 (dynamic gamma) and trail (static gamma). 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.

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.

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.

Physiological identification of static beta axons in primate muscle

Skeletofusimotor (beta) axons exerting a static action on spindle primary endings were identified in the peroneus brevis and tertius muscles of the cynomolgus monkey. In most of the identified static beta motor units, the extrafusal portion was of the fast-contracting fatiguable type.

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.

The ultrastructure of cat fusimotor endings and their relationship to foci of sarcomere convergence in intrafusal fibres

1. Six muscle spindle poles, five from experiments in which foci of sarcomere convergence had been observed during stimulation of fusimotor axons, were serially sectioned for light and electron microscopy. Every somatic motor terminal was studied in ultrathin sections at several levels.2. In all six poles static gamma axons, or presumed static gamma axons, supplying the static bag(2) fibre and/or chain fibres had no terminations on the dynamic bag(1) fibre. In five poles, the dynamic bag(1) fibre was selectively innervated by dynamic gamma or beta axons save in one case where a dynamic gamma axon also innervated one chain fibre.3. Seventy-seven motor endings were of four distinct ultrastructural types: ;m(a) plates' lay superficially on the surface of static bag(2) or chain fibres; ;m(b) plates' were deeply indented into dynamic bag(1) fibres; in ;m(c) plates', found on chain fibres only, the muscle surface was thrown into projecting fingers between which the axon terminals were embedded; one type ;m(d) plate' was found, fully indented into a long chain fibre. A few plates of intermediate form (m(ab)) were variants of m(a) and m(b) plates.4. The muscle membrane beneath both m(a) and m(b) plates was smooth, or had a few wide, shallow folds; m(c) plates usually had wide, shallow subjunctional folds; numerous deep, narrow folds were characteristic of the m(d) plate. The length of unmyelinated pre-terminal axon or the number of sole plate nuclei were not useful diagnostic features.5. Obvious foci of sarcomere convergence in the capsular sleeve region of dynamic bag(1) and static bag(2) fibres coincided with the location of motor plates. Additional contraction foci were observed in the extracapsular region of dynamic bag(1) fibres where there was no motor innervation; contraction occurs principally in the outer half of these fibres. No foci of contraction or motor plates were observed in the extracapsular region of static bag(2) fibres; contraction in these fibres is typically mid-polar.6. In some poles local contraction of chain fibres centred on the location of m(c) plates. In others, very localized contraction occurred distal to the sites of m(a) plates. Both m(a) and m(c) plates were never found on the same pole of a chain fibre.7. Dynamic gamma or beta axons end in m(b) plates, probably equivalent to p(2) plates. The concept of distinctly different p(1) and p(2) plates on dynamic bag(1) fibres, supplied by dynamic beta and gamma axons, respectively, is not supported by ultrastructural evidence.8. Some static gamma axons end in multiple m(a) plates which correspond with ;trail endings', or in single large m(a) plates, on static bag(2) or chain fibres. The m(c) plates are the terminations of other static gamma, or occasionally dynamic gamma, axons on chain fibres. Static beta axons probably end in m(d) plates on long chain fibres which may correspond with p(1) plates.9. It is proposed that there are two types of static gamma motoneurone, one terminating in m(a) plates and the other in m(c) plates, possibly directed preferentially towards static bag(2) fibres and chain fibres, respectively.

Development of immunohistochemical characteristics of intrafusal fibres in normal and de-efferented rat muscle spindles

Intrafusal muscle fibres in adult muscle spindles differ in their myosin composition. After selective motor denervation intrafusal muscle fibres develop mature ultrastructural characteristics. In order to evaluate the role of fusimotor innervation on the maturation of the myosin composition of intrafusal muscle fibres we have examined with immunohistochemical techniques i) the postnatal development of muscle spindles in new-born rats and in 7-21 day old rats; ii) muscle spindles in the EDL of 21-day-old rats de-efferented at birth. For the characterization of myosins in intrafusal fibres we used three myosin antisera: antipectoral myosin, antiheart myosin and antiheart myosin adsorbed with muscle powder from the soleus muscle of guinea pig. We show in this study that during development intrafusal fibres change immunoreactivity and that in the absence of motor innervation bag fibres do not fully develop the myosin characteristics of control spindles. We conclude that the maturation of bag1 and bag2 fibres apparently requires next to the inductive influence of sensory axon terminals the presence and activity of fusimotor axons.

A study of motor nerve terminals on cat nuclear bag1 intrafusal muscle fibers using the ChE staining technique

Muscle spindles were traced in serial transverse sections of cat tenuissimus muscles. Histochemical staining for "myofibrillar" adenosine 5'-triphosphatase was employed to identify nuclear bag1 intrafusal muscle fibers. Staining for cholinesterases (ChE) was used to demonstrate the termination sites of motor axons along the fibers. Several types of ChE deposits could be distinguished along the bag1 fibers based on intensity of staining and morphological characteristics. Most ChE deposits could be classified as either the "pale" or the "nonpale" plates. Some ChE active areas fitted neither of these two categories. Among 328 ChE "plates" encountered on 192 bag, fiber poles, 197 (60%) were of the "pale" and 27 (8%) of the "nonpale" type with 104 (32%) remaining unclassified. These histochemical observations are discussed with regard to the current structural and functional concepts of motor innervation of the nuclear bag1 fiber. It is suggested that the histochemical (ChE staining intensity) and morphological (length and form) characteristics of bag1 fiber motor endings are not determined solely by the nature of the corresponding motor axons.

Morphometric studies on tenuissimus muscle spindles in the cat

Muscle spindles were studied histochemically in serial transverse sections of 42 cat tenuissimus muscle specimens. Staining for myofibrillar adenosine triphosphatase was employed to identify nuclear bag 1, nuclear bag 2, and nuclear chain intrafusal muscle fibers. The nuclear chain fibers were further subdivided into three categories according to their polar length and the intensity of their staining for nicotinamide adenine dinucleotide tetrazolium reductase. A total of 430 spindle poles were surveyed. The mean spindle content of bag 1, bag 2, and chain fibers was established. The mean polar length of intrafusal fibers as well as that of the intracapsular and extracapsular spindle regions was determined. A cholinesterase (ChE) staining technique was used to demonstrate the termination sites of motor axons along intrafusal fibers. Two types of circumscribed ChE deposits. The "rim" and the "plate," occurred on the fibers. The nuclear chain fibers usually carried both the ChE rims and plates, while most nuclear fibers displayed only the plates. The ChE plates were assessed in term of their appearance, staining intensity, length, and location along the fibers. The mean number of ChE plates found along the fibers was established for each of the various intrafusal fiber types. These histochemical observations are discussed with regard to the current concepts of cat spindle morphology and motor innervation. The results suggest a degree of predictability in the spindle fiber content and in the distribution of motor nerve terminals along intrafusal muscle fibers, at least in the tenuissimus muscle.

A quantitative study of skeletofusimotor innervation in the cat peroneus tertius muscle

1. Physiological tests were used to identify skeletofusimotor or beta axons to the cat peroneus tertius muscle in order to assess the proportion of beta axons in the motor supply to this muscle. 2. Static beta axons (beta S) were identified by: (a) observation of a delay between the complete block of extrafusal contraction and the failure of spindle activation upon prolonged stimulation, (b) increase of spindle excitation with stimulation frequencies above that eliciting maximal extrafusal contraction, (c) observation of 'unfused' frequencygram of spindle primary afferent discharge during stimulation of the axon at frequencies above that eliciting complete fusion of extrafusal contraction and (d) static action exerted on the response of the spindle afferent to ramp stretch. 3. Dynamic beta axons (beta D) were identified by the persistence of spindle activation after selective block of extrafusal neuromuscular junctions and by their dynamic action on spindle primary endings. 4. The actions of 116 motor axons (conduction velocity 56-104 m/sec) on ninety-five spindle afferents (fifty-seven from primary and thirty-eight from secondary endings) were examined in ten experiments. Thirty-six beta axons (31% of the total sample) were identified: twenty-four beta S (conduction velocity 69-104 m/sec) and twelve beta D (conduction velocity 56-91 m/sec). 5. Twenty (35%) primary endings were activated by a beta S and sixteen (28%) by a beta D axon. Nineteen (45%) secondary endings were activated by a beta S and five (13%) by a beta D axon. Convergence of beta D and beta S axons on the same spindle occurred in 10% of instances. beta-innervated spindles were also supplied by gamma axons. 6. Most of the beta S motor units were of the fast-fatigue resistant (FR) type, with a few units of the fast-fatigable (FF) type, and nearly all the beta D motor units were of the slow (S) type.