On The Intrafusal Distribution of Dynamic and Static Fusimotor Axons in Cat Muscle Spindles Y. LAPORTE Laboratoire de Neurophysiologie, Coll2ge de France, 75231 Paris (France)
Three kinds of muscle fibre are found in cat spindles: the type 1 nuclear-bag fibres (bag, fibres), the type 2 nuclear-bag fibres (bag, fibres) and the nuclear-chain fibres (Ovalle and Smith, 1972; Banks et al., 1975, 1977; Barker et al., 1976a). Typically, each intrafusal bundle consists of one bag, fibre, one bag, fibre and several (4-6) chain fibres. The bag, fibres are much longer and wider than chain fibres, but their ultrastructure and histochemical profile resemble those of the chain fibres. On the other hand the bag, fibres, which are only slightly shorter and thinner than the bag, fibres, differ markedly from these especially in the intracapsular region where the sarcomeres lack an M line or have a faint double M line. In the equatorial region of the spindle bundle, the bag, fibre and the chain fibres are closely associated, whereas the bag, fibre is situated at some distance from this group of fibres, a disposition that is useful for identifying bag fibres in serial transverse sections. The dynamic fusimotor axons exert their actions through the bag, fibres and the static axons through both chain and bag, fibres. This statement is based on a series of experiments (for details, see the recent review by Laporte, 1978) which started by the demonstration that the motor endings of single static axons (trail endings) lie on bag as well as on chain fibres (Barker et al., 1973). Cinematographical observations of living spindles (mostly tenuissimus spindles) have shown that the stimulation of dynamic y axons elicits a weak and slow contraction in one of the bag fibres, whereas the stimulation of static y axons elicits the contraction of the chain fibres and/or of one of the bag fibres (Bessou and Pagb, 1975; Boyd et al., 1977). As originally shown by Bessou and Pagbs (1975), the contraction of the bag fibre activated by static axons although weaker than that of the chain fibres, is much stronger and faster than that of the bag fibre activated by dynamic axons. The two bag fibres have respectively been named by Boyd et al. (1977) the “dynamic nuclear bag fibre” and the “static nuclear bag fibre”. The distribution of fusimotor axons to intrafusal muscle fibres has also been studied with the glycogen-depletion technique of Edstrom and Kugelberg (1968), which consists of mapping, from serial transverse sections stained for glycogen, the muscle fibres that have been depleted of their glycogen content, following prolonged stimulation of their motor supply. This technique, originally developed to study the motor units of a axons, was first applied to the fusimotor system by Brown and Butler (1973, 1975). More recently, in experiments in which bag1 and b a g fibres were identified, it was found that dynamic y axons consistently induced glycogen depletion
- indicating neural activation - in bagl fibres whereas static axons elicited depletion
not only in chain fibres but also in bag2 fibres (Barker et al., 1976b). As will be discussed later, depletion in bagl fibres was also observed. By itself the demonstration that static y axons supply bags fibres does not prove that these fibres have a static action of their own. Conceivably, bagz fibres might exert a dynamic action which could interact with the static action of chain fibres. However, by taking advantage of the variability in intrafusal distribution of individual static axons, it has been possible to rule out this possibility. In some spindles a given static axon may supply only bag2 fibres whereas in others it supplies either chain fibres alone or both chain and bag2 fibres. Boyd et al. (1977) have reported the static action exerted on primary endings of spindles in which the only activated fibre was a fast-contracting bag fibre. In agreement with this observation Jami and Petit (see Fig. 1) have observed the static action of an axon, which, as shown by the glycogen-depletion technique, only supplied the bag2 fibre of a tenuissimus spindle. All this converging evidence indicates that the bagl fibre may be equated with the dynamic bag fibre and the bag2 with the static bag fibre. im p/s
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Fig. 1. Static action exerted by a single 7 axon which only supplied the bags fibre of a tenuissimus spindle. The left side of the figure shows discharges from the primary ending of the spindle, led from a dorsal root filament and recorded with an instantaneous frequency meter. 1) Passive response to a ramp-and-hold stretch. 2) Response during stimulation of the 7 axon at 100 sec-I. Note the typical static alteration of the response and the irregularity of the firing. The spindle to which the primary ending belonged was located by very localized pressure (technique of Bessou and Laporte, 1962). Its position in the muscle was marked by a thin thread knotted on the side of the muscle. The portion of the muscle containing the spindle was then treated for glycogen detection after the y axon was stimulated in a way known to elicit glycogen depletion in intrafusal muscle fibres (see Harker et al., 1977). On the right, photomicrograph of a transverse section of the spindle showing in the upper left side a totally blanched large-diameter fibre. Examination on serial sections of the whole spindle showed that no other fibre was depleted. The depletion in the bag. fibre occurred in both poles, over a stretch of 250 pm in the distal pole and of 850 pm in the proximal pole. The centres of the depleted zones were in the capsular sleeves, resp. at 800 pm and 1250 pm from the equator. The depleted fibre was identified as ba& fibre on account of its diameter, length and close association with chain fibres in the equator. (From Jami and Petit, unpublished observations.)
5 Two points will be considered in the present review: the distribution of the dynamic y axons that exert a category I1 action; and the innervation of the bag, fibres by static y axons. DISTRIBUTION OF DYNAMIC y AXONS EXERTING A CATEGORY I1 ACTION Emonet-Denand et al. (1977) have recently subdivided the actions exerted by fusimotor axons on primary endings in six categories ranging from apparently “pure” dynamic action to apparently “pure” static action (see Fig. 4). Categories I and I1 are clearly dynamic since in both cases the responses of primary endings to ramp-and-hold stretches show a marked increase in the dynamic index associated with a slow decay of firing after the dynamic phase of the stretch. As illustrated by Fig. 2, the two categories differ by the magnitude of the acceleration of the discharge observed at constant muscle length, and by the variability of the discharge: these are both distinctively larger in category I1 responses. In the peroneus brevis muscle, category I responses are about three times as common as category I1 whereas in the tenuissimus muscle nearly all the responses appear to belong to category I. Most dynamic responses observed in the soleus muscle belong to category 11. Frequency grams of primary endings (see Bessou et al., 1968a) elicited by stimulation of dynamic y axons giving category I1 responses have recently been studied (Emonet-Dknand and Laporte, 1978) in order to obtain some information on the contraction of intrafusal muscle fibres responsible for these responses. A
Fig. 2. Comparison of category I and category I1 dynamic responses. A and D) Passive responses of two primary endings (cat peroneus brevis muscle) to a 2 mm ramp-and-hold stretch followed by a slower release. B and E) Alteration of the responses elicited by the stimulation at 100 sec-’ of two single dynamic axons (each one acting on a different spindle). B and E respectively illustrate category I and category I1 responses. Note in E the stronger excitatory effect of the fusimotor stimulation at the initial length, the irregularity of the discharge and its persistence during muscle release. C and F) Responses to triangular stretching before and during stimulation at 100 sec-’ of the axons. Note in F that the ending continues to fire even when stretching passes through its minimal value. (From Emonet-DBnand et al., 1977.)
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Fig. 3. Frequency grams of a primary ending due to the stimulation of a dynamic y axon giving a category I1 response. Cat flexor hallucis longus muscle. On the left side of the figure records showing the action of the y axon. 1) Action potential of the axon led from the muscle nerve after stimulation of a ventral root filament; conduction velocity: 40 m/sec. 2) Passive response of the ending to a ramp stretch of 0.8 mm. 3-6). The lower bar in each record indicates the stimulation of the axon at 55 sec-'. 3) Same ramp stretch as in 2; a typical category I1 response is observed. 4 , 5 ) Acceleration of the firing observed for two constant lengths Lo and L,. 6) Sinusoidal stretch (1.5 Hz, 1 mm amplitude); a marked increase in the peak-to-peak modulation is observed during the stimulation. On the right side of the figure, frequency grams obtained by superimposition of about 20 records of instantaneous frequency (see Bessou et al., 1968a) while the muscle length remains constant. The points situated at the lower part of each record indicate the stimulation. Note the slower sweep speed used for records 7 and 8 (see text). (From Emonet-DCnand and Laporte, 1978.)
These graphs, as illustrated by Fig. 3, show distinct increments in frequency whose periodicity is equal to that of the stimulation (records 7 and 8) and even "driving" (records 9-10) for rates of stimulation ranging from 9 to 44/sec (indicating a relatively strong and fast intrafusal contraction). These frequency grams differ markedly from those elicited in tenuissimus muscles by dynamic y stimulation which display a smooth contour for rates of stimulation as low as 50/sec (Bessou et al., 1968b). Emonet-DCnand et al. (1977) suggested that category I1 responses resulted from the concomitant activation of a bag, fibre (i.e., the fibre responsible for dynamic action) and of a different functional type of intrafusal muscle fibre, whether a bag, or a chain fibre. This suggestion was supported by the observation that a category I response can be converted into a category I1 response by stimulating a static axon together with the dynamic axon responsible for the category I response, especially when the static axon was stimulated at a lower frequency than the dynamic one (see Figs. 5 and 11 of their paper) The frequency grams shown. in Fig. 3 are probably not incompatible with this assumption, but another possibility should be considered: the contraction responsible
for category I1 action is engendered only in the bag1 fibre but it is relatively strong and/or close to the sensory terminal. This would agree with the observation that strong dynamic effects can be elicited by axons giving category I1 responses for rates of stimulation as low as 20-30/sec, which is not the case for axons giving category I responses. In favour of the latter assumption is also the fact that all grades between category I and I1 responses can be observed. If category I responses were due to bagl fibres alone and category I1 responses to fast contracting intrafusal fibres together with bagl fibres, all responses should be expected to fall into one of two distinct classes. It would be desirable to determine the actual distribution of the axons responsible for category I1 actions by the glycogen-depletion method. However, the precise localization of spindles that is necessary for exact histophysiological correlation can be achieved only in the tenuissimus muscle but unfortunately, in this muscle it is difficult (probably for mechanical reasons) to obtain dynamic and static responses easily classifiable in one of the six categories which have been defined in the peroneus brevis muscle. Barker et al. (1977) have reported the distribution of a peroneus brevis dynamic fl axon whose stimulation activated three primary endings. Glycogen depletion was observed in three spindles of this muscle; in two of them the bagl was the only fibre depleted, whereas in the third spindle a chain fibre was depleted in addition to the bagl fibre. In two spindles the stimulation of the p axon exerted a typical category I action, but the response given by the third spindle (presumably the one in which a chain fibre was depleted in addition to the bagl fibre) was not a category I1 response. It had features of evenly balanced mixed static and dynamic actions and fell in category 111 of Emonet-DCnand et al. (1977) (unclassifiable action). INNERVATION OF BAG1 FIBRES BY y STATIC AXONS As recalled in the introduction, cinematographical analysis of living spindles and mapping of intrafusal distribution of fusimotor axons by the glycogen-depletion technique have given complementary information which leaves little doubt that dynamic axons supply bagl fibres and static axons chain and bags fibres. However, one point remains on which the two methods have not given complementary results, namely, the innervation of some bag1 fibres by static axons. In tenuissimus spindles, Barker et al. (1976b) found that stimulation of single static axons induced glycogen depletion as often in bagl as in bag2 fibres. This result was in line with prior studies in which the two kinds of bag fibre were not distinguished, but in which both bag fibres were found to be depleted in some spindles (Brown and Butler, 1973, 1975). That some static axons, in addition to their statogenic effectors, the chain and bagz fibres, may also supply bag, fibres is supported by the physiological observations reported by Emonet-Dtnand et al. (1977). In their study of the fusimotor innervation they collected all types of response of primary endings during fusimotor stimulation without selecting the most typical ones. They found that the majority of static responses observed during ramp-and-hold stretches could be ascribed to an apparently “pure” static action (category VI responses), but that some of them were suggestive of an admixture of a dynamic action because, on completion of the dynamic phase of stretching, they showed a slow decay of firing comparable to
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Fig. 4. Qtegories of fusimotor actions. Each pair of records shows: on the left, the passive response of a primary ending (cat peroneus brevis muscle) to a 2 mm ramp-and-hold stretch followed a few seconds later by a slower release; on the right, the response during repetitive stimulation of a single y axon, indicated by the bars. I) Category I response. Purely dynamic action, characterized by marked increase in dynamic index with slow decay of firing after the dynamic phase of stretch and regular discharge. 11) Category I1 response. Same dynamic features as in I, but greater excitation at initial length and appreciable variability of discharge. 111) Category I11 response. Unclassifiable. Dynamic and static features equally balanced. IV) Category IV response. Static action modified by dynamic action. Strong static features (considerable excitation at initial length, firing during release, irregularity of the discharge) but slow decay after ramp stretch. V)Category V response. Static action with conceivable dynamicparticipation. Strong static features with slight sign of dynamic action. VI) Category VI response. Purely static action. Considerable excitation with usually a decrease of the dynamic index; no slow decay after ramp stretch, nearly always firing on release; sometimes gross variability in discharge (upper record) sometimes regular discharge (lower record). (From Emonet-D6nand et al., 1977.)
that given by the stimulation of dynamic axons (see Fig. 4). These responses were classified in categories IV and V, respectively called “static action modified by dynamic action” and “static action with conceivable dynamic participation”. Furthermore they readily obtained responses of categories IV and V by combined stimulation of a dynamic axon giving a category I response and of a static axon giving a category VI response. Since these observations were made on the peroneus brevis muscle EmonetDCnand et al. (1978) studied the distribution of static axons in the spindles of this muscle with the glycogen-depletion method. In each experiment a small number of static axons (3-7), previously identified by their actions on the responses of at least one primary ending to ramp-and-hold stretches, were stimulated. The whole muscle was sectioned in order to search for glycogen-depletion in as many spindles as possible. Of 167 spindles examined, 53 contained depleted intrafusal muscle fibres. In 33 spindles (62%) the depletion included only chain and bag, fibres (chain only in 14 spindles, chain and bagz fibres in 18 spindles, bags alone in 1 spindle). However in 20 spindles (38%) the bag1 fibre was depleted in addition to other fibres (with chain fibres in 4 spindles, with chain and bag2 fibres in 13 spindles and with bags only in 3
spindles). Of the 102 static actions classified by Emonet-DCnand et al. (1977), 68 (67%) belonged to category VI (apparently pure static action) and 34 (33%) to categories IV (11%) and V (22%) suggestive of a dynamic participation. These findings are apparently in conflict with the cinematographical observations of Bessou and Pagbs (1975) and of Boyd et al. (1977), who have never seen the same bag fibre (or rather the same pole of a bag fibre) contracting after stimulating either a static axon or a dynamic axon. This negative finding, as already pointed out (Barker et al., 1976b; Emonet-DCnand et al., 1977), may be due to the difficulty of observing a weak contraction in a bagl fibre while a much stronger contraction is taking place in nearly intrafusal fibres such as chain fibres. Another possibility should also be considered, namely, that a bagl pole which is supplied by a collateral of a static axon is not supplied by a dynamic axon. Theinnervation in about one-third of the spindles of bag1 fibres by static axons seems too large to be dismissed as an ontogenic imperfection, but its functional consequence is not yet understood. It has been suggested (Emonet-DCnand et al., 1977) that it could preserve, during strong activation of the static fusimotor system, the responsiveness of the bag, terminals by preventing slackness of the bag, fibres when the dynamicsystem is not active. Further studies of this innervation are obviouslyneeded.
SUMMARY In the first part of this paper the evidence showing that dynamic axons exert their effect through nuclear bagl fibres and static axons through both nuclear chain and nuclear bag2 fibres is briefly reviewed. The second part deals with dynamic y axons giving category I1 responses. Frequency grams of primary endings observed during stimulation of these axons are described. Two interpretations of category I1 responses are discussed. The innervation of some bag1 fibres by static axons is studied in the last part. Recent experiments carried out on cat peroneus brevis muscles with the glycogen-depktion method show that after stimulation of static axons, bagl fibres, in addition to other fibres, are depleted (indicating neural activation) in about one third of the spindles.
ACKNOWLEDGEMENT Recent investigations presented in this review were supported by grants from INSERM (ATP 76-61) and from the Foundation for French Medical Research.
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