Search

Ideas about the functions of static gamma-motoneurones are based on the responses of primary and secondary endings to electrical stimulation of single static gamma-axons, usually at high frequencies. We compared these effects with the actions of spontaneously active gamma-motoneurones. In anaesthetised cats, afferents and efferents were recorded in intramuscular nerve branches to single muscle spindles. The occurrence of gamma-spikes, identified by a spike shape recognition system, was linked to video-taped contractions of type-identified intrafusal fibres in the dissected muscle spindles. When some static gamma-motoneurones were active at low frequency (15 Hz) they coupled the firing of group Ia and II afferents. Activity of other static gamma-motoneurones which tensed the intrafusal fibres appeared to enhance this effect. Under these conditions the secondary ending responded at shorter latency than the primary ending. In another series of experiments on decerebrate cats, responses of primary and secondary endings of single muscle spindles to activation of gamma-motoneurones by natural stimuli were compared with their responses to electrical stimulation of single gamma-axons supplying the same spindle. Electrical stimulation mimicked the natural actions of gamma-motoneurones on either the primary or the secondary ending, but not on both together. However, gamma-activity evoked by natural stimuli coupled the firing of afferents with the muscle at constant length, and also when it was stretched. Analysis showed that the timing and tightness of this coupling determined the degree of summation of excitatory postsynaptic potentials (EPSPs) evoked by each afferent in alpha-motoneurones and interneurones contacted by terminals of both endings, and thus the degree of facilitation of reflex actions of group II afferents.

Noradrenaline is known to suppress transmission from group II muscle afferents when locally applied to gamma-motoneurones, and serotonin (5-HT) facilitates the transmission. The purpose of this investigation was to search for evidence of monoaminergic innervation of gamma-motoneurones. Eight gamma-motoneurones were labelled with rhodamine-dextran, and 50 micrometer thick sagittal sections of the spinal cord containing them were exposed to antibodies against dopamine beta-hydroxylase (DBH) and 5-HT. All the cells were directly and/or indirectly excited by muscle group II afferents from the muscle they innervated and/or other muscles. Appositions between monoaminergic fibres and the labelled somata and dendrites were located with three-colour confocal laser scanning microscopy by examining series of optical sections at 1 or 0.5 micrometer intervals. DBH and 5-HT varicosities formed appositions with the somata and dendrites of all the gamma-motoneurones. The mean packing densities for 5-HT (1.12 +/- 0.11 appositions per 100 micrometer(2) for somata and 0.91 +/- 0.07 per 100 micrometer(2) for dendrites) were similar to the densities of contacts reported for alpha-motoneurones. Monoaminergic varicosities in apposition to dendrites greatly outnumbered those on the somata. The density of DBH appositions was consistently lower - corresponding means were 53% and 62% of those for 5-HT on the somata and dendrites, respectively. It is concluded from an analysis of the distribution and density of varicosities in apposition to the gamma-motoneurones compared with the density in the immediate surround of the dendrites that there is indeed both a serotoninergic and noradrenergic innervation of gamma-motoneurones.

1. The aim of this investigation was to identify static gamma-axons which do not drive any Ia afferents at any stimulus frequency in any spindle which they supply, and to determine their occurrence in various hindlimb muscles (peroneus tertius, brevis, longus and tenuissimus). 2. Ia responses to static gamma stimulation were classified as 'non-driven' when the discharge did not follow the stimulation frequency, or its subharmonics, at any time during a linear increase in stimulus frequency up to 150 Hz lasting 2-3 s, and when tested at two muscle lengths--except in the tenuissimus muscle. In almost all experiments, cross-correlograms were used in addition to evaluate the percentage of these 'non-driven' responses in which a time-locking of discharge to stimulus pulses was obscured by irregularity of the Ia discharge. 3. In 104 spindles, out of 347 responses to stimulation of single static gamma-axons 332 (93%) could be characterized, and of these, 57% (183) were of the non-driven type. The mean number of static gamma effects characterized per spindle was 4.1 (fourteen experiments). In the large majority of spindles (79%, 82 out of 104) at least one response was of the non-driven type. 4. Of the static gamma-axons studied 16% were called 'non driving' ('ndr' gamma s-axons) because they elicited non-driven effects, and since they had the same qualitative effect consistently in all spindles whose discharge was modulated by stimulating them they were called specific 'ndr' axons. If axons with non-driven effects, but acting on one spindle were included in the 'non-driving' category the proportion was 23%. Of spindles tested 63% were innervated by at least one 'ndr' axon. 5. Absence of Ia driving during ramp frequency stimulation of gamma s-axons has been equated with selective bag2 contraction. All the non-driven responses identified in this study cannot be attributed to exclusive bag2 involvement because the total number of 'ndr' responses was too high. In fact, in the isolated spindle preparation bag2 and chain co-contraction were shown to elicit non-driven responses, so chain contraction is not detected reliably in all experimental conditions. Possibly chain fibre contraction is sometimes too weak to dominate the response, or can be of a non-driving character.(ABSTRACT TRUNCATED AT 400 WORDS)

1. Extra- or intracellular recordings were made from seventy-six gamma-motoneurones of hindlimb muscles in chloralose anaesthetized cats to re-assess the coupling between secondary muscle spindle afferents (group II muscle afferents) and these neurones. The latencies of a number of responses evoked by group II muscle afferents in gamma-motoneurones were shorter than minimal latencies of responses induced disynaptically in other spinal neurones. These latencies are therefore compatible with monosynaptic coupling between muscle spindle secondaries and gamma-motoneurones. 2. Responses fulfilling criteria for monosynaptically evoked responses were seen in about one third of gamma-motoneurones with input from the group II muscle afferents tested (in 6 of 18 motoneurones recorded intracellularly and in 26 of 74 motoneurones recorded extracellularly). They were usually evoked from only one of the stimulated nerves, stimulation of group II afferents of other nerves being followed by responses at longer latencies. 3. Most gamma-motoneurones were excited by group II afferents from several muscles, both flexors and extensors. However, a comparison of group II input to gamma-motoneurones innervating medial gastrocnemius and four other hindlimb muscles revealed differences in both incidence and sources. 4. This study extends results of previous studies by providing evidence that some synaptic actions of group II afferents, including afferents from the same muscle, are evoked monosynaptically, and may assist in sustaining the activation of gamma-motoneurones by positive feedback.

Nuclear chain fibres are classically regarded as being much shorter and thinner than bag fibres, either bag1 or bag2. However, Barker et al. (1976) noticed that some spindles possessed one or more chain fibres with unusually long poles, often of similar length to the bag fibres. As an estimate of the incidence of these ‘long chain’ fibres they assessed the proportion of spindles containing at least one chain fibre pole that extended for more than 1mm beyond the end of the capsule. Kucera (1980) formalised this practical criterion as a definition of a class of long chain fibres each of which possessed at least one long pole. Since that time, long chain fibres have often been treated as though they formed a distinct type of intrafusal fibre with a specific static beta (βs) innervation (Kucera, 1982, 1984; Barker & Banks, 1986). Here we give evidence in detail of both static β and static γ innervation of long chain fibres.

Histophysiological experiments using the superficial lumbrical muscle of the cat have provided the opportunity to analyse the structure of a large number of spindles (Decorte, Emonet-Denand, Ginape & Laporte, 1986). This muscle was initially chosen because of its small size, spindle content and known motor-unit properties (Emonet-Denand, Hunt, Petit & Pollin, 1988).

Areas in the midbrain where stimulation preferentially affects γs- and γd-motoneurones have been reported (see Appelberg, 1981; Fowle, Taylor, Rodgers & Durbaba, 1992; Taylor, Durbaba, Rodgers & Fowle, 1993). In the tenuissimus muscle of anaesthetised animals we also found these effects in experiments in which the responses of Ia afferents to standard ramp and hold stretches were monitored during midbrain stimulation (Dickson & Gladden, 1990). Static effects tended to predominate. This was also the case in separate experiments in which activity of γ-motoneurones was detected by observing the contractions of the types of intrafusal fibre they control in spindles exteriorised under anaesthesia. Chain and/or bag2 contractions indicate active γs-motoneurones, whereas active γd-motoneurones cause bag1 contractions. In confirmation of Fowle et al. (1992) chain fibres could regularly be recruited with bag2 (pure γs- activation), and these fibres were also recruited together with bag1 (mixed γs- and γd-activation) in 14 experiments, including several in which more than one spindle was isolated. It proved difficult to recruit bag1 fibres alone (pure γd- activation). Spontaneous γs-activity could also be inhibited. Here we document an example in which some γs-motoneurones were activated while others were inhibited.

Pairs of Ia and II afferent fibres supplying primary and secondary endings lying in the same tenuissimus spindles were prepared in barbiturate anaesthetized cats in order to compare the variability in the rhythm of discharge of the two endings during responses elicited by the contraction of different intrafusal muscle fibres, especially by chain fibres. In these spindles, the intrafusal muscle fibres supplied by single static gamma-axons were identified with a recently developed technique based on the types of primary ending activation observed during gamma stimulation at 30 and 100 stimuli/s. The responses of the secondary endings to contraction of chain fibres either alone or with bag2 fibres were smaller and much more regular than the responses of the primary endings lying in the same spindles. This difference is probably related to the position of secondary endings along the intrafusal muscle fibres and to the mechanical properties of the muscle fibre regions on which the terminals lie. The dynamic properties of the encoding site of primary afferent impulses probably contribute to the difference. The different degrees of variability observed among secondary ending responses elicited either by chain fibres alone or by chain and bag2 fibres are not related to the type of activated intrafusal muscle fibres.

The terminations of static gamma-axons on chain fibres may be associated with a muscle surface contour thrown into complex folds at one extreme, ranging through lesser degrees of folding to being apparently uninfluenced by the presence of the terminals. The folding is not necessarily confined to the postsynaptic membrane. As a quantitative indicator of the degree of folding seen in cross-sections, the perimeters of the two halves of the intrafusal fibre with, and without the ending were compared. Both complexity of endplate structure and an undifferentiated appearance could co-exist within individual endings (of six static gamma-axons on thirteen chain fibres), associated with axons that supplied only chain fibres in the spindle isolated. (It should be noted, however, that the type of intrafusal fibres innervated by a particular axon was definitively identified only for the spindle isolated.) Endings of three other static gamma-axons on six chain fibres had a more homogeneous and less complex endplate structure; these supplied bag2 fibres in addition to the chain fibres and their endplates on the bag2 fibres were less folded than those of three axons which supplied bag2 fibres only. A sensory inhibitory influence on folding was not apparent because complexly folded endplates on chain fibres lay close (less than 500 microns) to sensory endings. All the endings studied were functional. In our experimental conditions all nine static gamma-axons which innervated chain fibres, alone, or together with a bag2 fibre, drove the Ia afferent at some stimulus frequencies; none of the three static gamma-axons innervating bag2 fibres alone caused driving at any frequency. These findings are discussed in relation to the concept of a dynamic remodelling of ending structure during life, to the relationship between the motor axon and the intrafusal fibre it innervates, and to the possibility that subgroups of static gamma-motoneurones might exist which could release different amounts of the trophic substances responsible for moulding the endplate structure.

1. The behaviour of nuclear bag and nuclear chain intrafusal fibres in isolated cat muscle spindles with a blood supply, during stimulation of dynamic gamma axons, dynamic beta axons, or static gamma axons in ventral root filaments was observed and recorded on still and moving film. 2. Most spindles were controlled by one dynamic gamma axon (sometimes a beta axon) and three static gamma axons, one of which was often non-selective in distribution. A large majority of fusimotor axons controlled one pole of the spindle only. 3. Dynamic gamma and beta axons produced focal contraction in only one of the two nuclear bag fibres in any spindle and this fibre was never activated by static gamma axons. Maximal tetanic contraction was attained slowly and the primary sensory spiral on this fibre was stretched by a small amount only. This fibre has been named the 'dynamic nuclear bag fibre'. 4. Static gamma axons produced either: (a) focal contraction in the second of the two nuclear bag fibres only; (b) local contraction in the bundle of nuclear chain fibres only; or (c) contraction in one nuclear bag fibre and the nuclear chain fibres together. Maximum tetanic contraction of this nuclear bag fibre stretched its primary sensory spiral considerably and the time to plateau was relatively short. This fibre has been named the 'static nuclear bag fibre'. 5. 'Driving' of the Ia afferent discharge could always be produced by non-selective static gamma axons, frequently by static gamma axons controlling nuclear chain fibres alone, and was probably due to mechanical oscillation in nuclear chain fibres. It was never produced by dynamic gamma axons and on one occasion only by a static gamma axon controlling a nuclear bag fibre alone. 6. The conduction velocities of dynamic gamma and static gamma axons overlapped extensively, though dynamic gamma axons were absent from the lower end, and static gamma axons innervating nuclear chain fibres only were absent from the upper end, of the range of velocities. 7. The observations are correlated with spindle structure and histochemistry. Dynamic and static nuclear bag fibres are shown to correspond with 'bag1 fibres' and 'bag2 fibres', respectively (Ovalle & Smith, 1972). 8. The possible origin of the dynamic and static actions of fusimotor axons and the role of the dynamic and static intrafusal systems in motor control are discussed.

Publisher Summary Nuclear bag fibres that are operated electrophysiologically by either static or dynamic γ axons are iontophoretically labelled with dye. Subsequent histology shows that the nuclear bag fibres operated by static γ axons have thick elastic fibres around their polar regions that continue past the end of the fibres into the muscle. The dynamic nuclear bag fibres have fewer and thinner elastic fibres around their polar regions. This difference is used to correlate indirectly other physiological and structural properties of these fibres with their innervation. In this way, it is inferred that the dynamic nuclear bag fibre is more sensitive to direct application of acetylcholine than the static nuclear bag fibre and that the ATPase of the static nuclear bag fibre is stable in both alkaline and acid conditions, whereas the dynamic nuclear bag fibre contains the acid-stable,—alkalinelabile form of ATPase.

There is both direct and indirect evidence that stretch activation occurs in the dynamic bag1 fibres of the mammalian muscle spindle and that it is responsible for maintaining the high sensitivity of primary sensory endings in stretches great enough to break the resting actomyosin bonds responsible for the short-range stiffness of muscle fibres. However the direct observations of dynamic bag1 fibre behaviour during stretching were made on damaged fibres and during very slow stretches. Preliminary results of experiments employing faster stretches of intact muscle spindles are reported here. An image processing system is being developed to automate and facilitate analysis of sarcomere movements during stretch, release and activation of intrafusal fibres. Unequivocal evidence confirming the development of stretch activation has not yet been found. Boyd (1986a) believed that static bag2 and chain fibres are controlled by separate populations of static gamma motoneurones, while accepting that there is some degree of common innervation. His evidence and the functional implications are discussed.

Results from a new method of studying fusimotor neuron activity are reviewed. The method involves observation of the movement of intrafusal muscle fibres in muscle spindles isolated from the tenuissimus muscle. The muscle is exteriorised with a pedicle containing its nerve and blood supply and the spinal roots are left intact. Observations were made in cats under barbiturate anaesthesia, during stimulation of the sensorimotor cortex and following decerebration and spinalisation. The results suggested that the central control of γd motoneurons influences muscle spindle afferents through dynamic bag1 fibre activity, and that the central control of γs motoneurons influences spindle afferents through static bag2 and nuclear chain activity. Unexpectedly the evidence further suggested a division of γs motoneurons into two groups under separate central control, exerting effects through predominantly static bag2 fibre contraction or predominantly nuclear chain fibre contraction. The development of the method to give quantitative information on γ motoneuron firing rates by intracellular recording from intrafusal muscle fibres is described. Preliminary accounts are given of experiments on the spread of junctional potentials into the poles of dynamic bag1 fibres and static bag2 fibres, and also experiments employing intracellular recording from dynamic bag1 fibres during γs axon stimulation to clarify whether the innervation of these fibres by γd motoneurons, and not γs motoneurons, is as exclusive as observation of the contraction of dynamic bag1 fibres suggests.

Arbuthnott et al. (1982) identified two ultrastructural types of fusimotor ending on typical nuclear chain fibres in cat tenuissimus muscle spindles, ma plates and mc plates, in addition to a distinct md plate on the long chain fibre present in some spindles only and which is not referred to in this paper. The ma plates on chain fibres were identical to fusimotor endings on the static bag2 fibre. Further serial electron microscopy has confirmed the presence of two distinct forms of ending on typical chain fibres.

Publisher Summary In this chapter, a 1 mm region of a muscle spindle, in which a dynamic γ axon is observed to produce focal contraction of one nuclear bag fibre and in which all other intrafusal fibres are operated by a static γ axon, is studied by electron microscopy. Trail endings with negligible junctional folding, presumably the terminations of the static γ axon, are found on one large fibre, one intermediate fibre, and one small fibre. A p 2 plate with definite irregular junctional folding, presumably the termination of the dynamic γ axon, is found on the other large fibre, which is free of trail terminations.

The work presented in this report forms part of a larger research programme in which the behaviour of the muscle spindle is described under conditions when several fusimotor inputs, and length changes are applied simultaneously to the spindle while recording simultaneously from its primary (Ia) and secondary (II) sensory endings. We restrict the current presentation to a discussion of the strength of coupling or association and time delay between the responses of primary and secondary endings from the same tenuissimus muscle spindle during activation of single and multiple fusimotor inputs. A description of this interaction is particularly interesting in the light of a recent study by Edgley and Jankowska (1987) which demonstrated the convergence of group Ia and II afferent axons onto the same interneurones in the midlumbar segment of the spinal cord.

Arbuthnott et al. (1982) classified cat fusimotor endings into four types in terms of ultrastructure. The types are now redefined as follows. m a plates: axon terminals superficial to, or partially indented into, the muscle fibre surface, and none substantially indented; postsynaptic folding absent or rudimentary. m b plates: terminals substantially or partially indented, none lying superficially; postsynaptic folding absent or rudimentary: m c plates: terminals embedded between finger-like processes protruding from the muscle fibre surface; postsynaptic membrane has wide, shallow folds. m d plates: terminals fully indented; numerous deep, narrow postsynaptic folds. The earlier, intermediate, class of m ab plates is eliminated if the above criteria are used. The validity of indentation or postsynaptic folding as criteria for classification has been questioned (Banks, 1981, 1983), on the grounds that indentation may be related to distance of fusimotor endings from the spindle equator, and that substantial variations in form of endings occurs between spindles. A quantitative assessment of these two criteria is given below showing that both are, in fact, of considerable value for the identification of types and locations of fusimotor terminals.