Bulk modulus of poly(N-isopropylacrylamide) microgels through the swelling transition.

We report measurements of the bulk modulus of individual poly(N-isopropylacrylamide) microgels along their swelling transition. The modulus is determined by measuring the volume deformation of the microgel as a function of osmotic pressure using dextran solutions. We find that the modulus softens through the transition, displaying a nonmonotonous behavior with temperature. This feature is correctly reproduced by the theory of Flory for polymer gels, once the concentration dependence of the solvency parameter is properly incorporated.

Comparison of the effects of stimulating groups of static gamma axons with different conduction velocity ranges on cat spindles.

In cat peroneus tertius muscles, static gamma axons were prepared in groups of three to four according to the conduction velocity of their axons (fast, intermediate, or slow). Effects of stimulating these groups (at 20, 30, and 50 Hz) on spindle ensemble discharges during sinusoidal stretch (peak-to-peak amplitude, 0.5 mm; frequency linearly increasing from 0.5 to 8 Hz in 10 s) were compared. Ensemble discharges were obtained by digital treatment of the discharges in afferent fibers from all the spindles in peroneus tertius as recorded from the muscle nerve. Stimulation of each group prevented ensemble discharges from falling to very low levels during shortening phases. However, this effect was clearly larger when the group of fast-conducting axons was stimulated. In view of the known effects of the activation of bag(2) and chain fibers (either separately or together) on single primary ending discharges during comparable sinusoidal stretches, this stronger effect supports the view that static gamma axons with faster conduction velocities are more likely to supply more bag(2) fibers than slower ones. Possibly the proportions of bag(2) and chain fibers activated during motor activity are determined by a recruitment of static gamma motoneurons related to their size.

Testing the classification of static gamma axons using different patterns of random stimulation.

The possibility of using randomly generated stimulus intervals (with a Poisson distribution) to identify the type(s) of intrafusal fiber activated by the stimulation of single static gamma axons was tested in Peroneus tertius muscle spindles of anesthetized cats. Three patterns of random stimulation with different values of mean intervals [20 +/- 4. 47, 30 +/- 8.94, and 40 +/- 8.94 (SD) ms] were used. Single static gamma axons activating, in single spindles, either the bag2 fiber alone or the chain fibers alone or both types of intrafusal fiber were prepared. Responses of spindle primary endings elicited by the stimulation of gamma axons were recorded from Ia fibers in cut dorsal root filaments. Cross-correlograms between stimuli and spikes of the primary ending responses, autocorrelograms, interval histograms of responses, and stimulations were built. The characteristics of cross-correlograms were found to be related not only to the type of intrafusal muscle fibers activated but also to the parameters of the stimulation. Moreover some cross-correlograms with similar characteristics were produced by the activation of different intrafusal muscle fibers. It also was observed that, whatever the type of intrafusal muscle fiber activated, cross-correlograms could exhibit oscillations after an initial peak, provided the extent in frequency of the primary ending response was small; these oscillations arise in part from the autocorrelation of the primary ending responses. Therefore, cross-correlograms obtained during random stimulation of static gamma axons cannot be used for unequivocally identifying the type(s) of intrafusal muscle fiber these axons supply.

Comparison of static fusimotor innervation in cat peroneus tertius and longus muscles.

Static fusimotor innervation was compared in cat peroneus longus and tertius muscles because the gamma to spindle ratio is considerably higher in the longus (approximately 60 gamma axons for 17 spindles) than in the tertius (approximately 24 gamma axons for 14 spindles). Single gamma axons were identified as static (gamma(s)) by their typical effects on the response of primary ending to ramp stretch. The intrafusal muscle fibers that single gamma(s) axons activated in the spindles they supplied were identified by the features of cross-correlograms between Ia impulses and stimuli, at 100 Hz, and by those of primary ending responses during stimulation at 30 Hz. In each experiment, a large proportion of the gamma population was tested on about nine spindles. A statistical analysis was used to estimate the number of spindles supplied by single gamma(s) axons and the proportion of gamma(s) axons that supply only one spindle among those the stimulation of which had activated either bag2 or chain fibers alone in a single spindle. In peroneus longus, nearly all gamma(s) axons supply one or two spindles, whereas in peroneus tertius, the majority of gamma(s) axons supply from three to six spindles. The proportion of nonspecifically distributed gamma(s) axons, i.e., of axons that supply both bag2 fibers and chain fibers either in the same or in different spindles, is much lower (56%) in the longus than in the tertius (83%) as previously observed on a population of gammas axons that supplied from three to six spindles. Correspondingly, the proportion of specific axons is much higher in the longus (44%) than in the tertius (17%). In none of the two muscles was a strict relationship observed between the conduction velocity of gamma(s) axons and their intrafusal distribution (specific bag2, specific chain fibers, nonspecific). However, gamma(s) supplying bag2 fibers either specifically or in combination with chain fibers tended to have faster conduction velocities, which suggests that, in various motor acts, the proportion of activated bag2 and chain fibers may be related to the proportions of activated fast and slow gamma(s) axons.

Functional consequences of bag2 and chain fiber coactivation by static gamma-axons in cat spindles.

A study of the distribution in cat peroneus tertius spindles of 42 single static gamma-axons was recently carried out with a physiological method for identifying the intrafusal muscle fibers supplied by single gamma-axons. It was found that 35 axons (83%) supplied both slow-contracting bag2 fibers and fast-contracting chain fibers. The distribution of these axons generally varied from one spindle to another among all the spindles that each of them supplied (bag2 and chain fibers together, bag2 alone, chains alone). To find some functional consequences of this coactivation, responses of primary endings to sinusoidal stretch of the muscle (amplitude 0.5-1 mm, frequency linearly increasing from 0.6 to 8-9 Hz in 12 s) were recorded at different average muscle lengths (0.5, 1.0, and 1.5 mm shorter than maximal physiological length) in nembutalized cats during repetitive stimulation at 10, 20, and 30 Hz of single gamma-axons previously shown to supply bag2 and chain fibers in the spindles bearing the primary endings. These responses were compared with responses elicited in passive spindles and during activation of either bag2 fibers or chain fibers alone. Several records of discharge frequency were averaged. During stimulation at 30 Hz of gamma-axons coactivating bag2 and chain fibers, the averaged discharge of primary endings became continuous (that is, without interruption during each shortening phase as occurs in passive spindles) over the whole range of stretch frequencies. The modulation of the discharge was roughly sinusoidal, with an amplitude that increased with the stretch frequency. Stimulation at 30 Hz of gamma-axons activating bag2 fibers alone elicited a modulation of comparable shape and amplitude but only in the range of sinusoidal stretch from 0.6 to 3-4 Hz. Stimulation at 30 Hz of gamma-axons activating chain fibers alone elicited for each cycle in the range of 0.6 to 5-6 Hz a distorted modulation of large amplitude with a minimal frequency close to that of the stimulation. The average muscle length did not significantly influence these various responses. In summary, the coactivation of bag2 and chain fibers, at presumed physiological frequencies, enables primary endings to continuously signal changes of length over a large range of stretch velocities independently of the average muscle length.

Brown-Séquard and the discovery of the vasoconstrictor nerves.

In August 1852, Brown-Séquard who had left Paris for Philadelphia at the beginning of the same year published in the Medical Examiner a description of the effects he observed in various animals after electrical stimulation of the distal part of severed cervical sympathetic chains. The blood vessels of the face and ear contracted and the temperature of the tissues decreased. After stimulation ceased, all the phenomena observed by Claude Bernard after sectioning the chain reappeared, especially vasodilation and hyperthermia. Brown-Séquard concluded that the sympathetic chain sends motor nerve fibres to many of the blood vessels of the head and that vasodilation followed by hyperthermia resulted from the section of these fibres. This view was challenged by Claude Bernard who had assumed the presence of "calorific" fibres in the sympathetic chain. The controversy between the two physiologists is related in the article.

Primary and secondary afferent discharges from the same spindle during chain fibre contraction in cat tenuissimus muscle.

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.

Distribution of static gamma axons in cat peroneus tertius spindles determined by exclusively physiological criteria.

1. The intrafusal muscle fiber(s) activated in cat peroneus tertius spindles by single static gamma (gamma s) axons were identified by exclusively physiological criteria based on the different contractile properties of chain and bag2 fibers. 2. The identification rested both on the features of primary ending discharges observed during gamma s electrical stimulation at a rate of 30 pulses per second (stimulation at 30/s) and on cross-correlograms constructed during stimulation at 100/s. Three types of primary ending activation could be distinguished. 3. Type F (fast) activations are characterized, at 30/s, by either a 1-to-1 driving or a very irregular increase in firing arising from a level close to the frequency of stimulation and by the presence in cross-correlograms of significant peaks. They are ascribed to chain fibers whose contractions, at 30/s, present large oscillations and, at 100/s, are still incompletely fused. 4. Type S (slow) activations are characterized, at 30/s, by a sustained and generally regular increase in firing and by the absence of significant peaks in cross-correlograms constructed during stimulation at 100/s. They are ascribed to bag2 fibers whose contractions are nearly fused at 30/s and completely fused beyond 60-70/s. 5. Type M (mixed) activations are characterized, at 30/s, by an irregular increase of discharge above a level distinctly higher than the frequency of stimulation and by the presence of significant peaks in cross-correlograms. They are ascribed to the coactivation of chain and bag2 fibers for two reasons: first, they have some features of both type F and type S activations; and second, they are readily reproduced by stimulating together two axons supplying the same spindle, one exerting a type F activation, the other a type S activation. 6. In seven experiments the distribution of 42 single gamma s axons was determined by observing the type of activation they exerted on several spindles (from 3 to 6). Thirty-five axons (83%) were classified "nonspecific" because the type of activation (F, S, or M) varied from one spindle to the other. Seven axons (17%) were classified "specific" because the type of activation was the same in all spindles: either type F for five axons (12%) or type S for two axons (5%). A statistical analysis of the distribution of all activations showed that the proportions of specific axons were not significantly different from those predicted by chance.

Comparison of skeleto-fusimotor innervation in cat peroneus brevis and peroneus tertius muscles.

1. The skeleto-fusimotor or beta innervation was compared in cat peroneus brevis and peroneus tertius muscles, which differ in their composition of fatigue-resistant motor units; the slow (S) units predominate in brevis and the fast units (FR) in tertius. 2. In four brevis muscles, of thirty-four beta-axons (from a total of 114 axons supplying extrafusal muscle fibres) twenty-nine were dynamic (beta D) and only five static (beta S). In contrast, in three tertius muscles, of twenty-five beta-axons (from a total of 82 axons) twelve were static and thirteen dynamic. 3. In a population of thirty-five brevis and thirty tertius spindles, the proportion of beta D-innervated spindles was greater in the brevis (68.5%) than in the tertius (50%) whereas that of beta S-innervated spindles was greater in the tertius (40%) than in the brevis (17.1%). In a population of thirty-two brevis and twenty-seven tertius spindles in which the presence of bag1 fibres was deduced from the existence of a dynamic innervation, the proportion of spindles innervated by beta D-axons was 80% in the brevis and 62% in the tertius. 4. In both muscles, the number of beta D effects was greater than that of beta S effects. beta S-axons were rarely found to supply more than one spindle whereas beta D-axons supplying more than one spindle (up to four) were common. Spindles were often coinnervated by beta D- and beta S-axons.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of tetanic contraction of motor units of similar type on the initial stiffness to ramp stretch of the cat peroneus longus muscle.

1. The stiffness during the initial portion of a ramp stretch was measured in cat peroneus longus muscle at rest and during maximal tetanic contractions produced by increasing numbers of motor units of the same type [slow (S), fast fatigue resistant (FR), or fast fatigable (FF)]. 2. This initial ramp stiffness was defined as the ratio between tension and length change over the limited range of constant velocity extension during which tension rose linearly with length change. This stiffness was reduced by tetanic contraction of a number of motor units while other units remained inactive. The reduction had different characteristics in contractions produced by S, FR, or FF units. 3. Two brief ramp (triangular) stretches were applied at short intervals to evaluate the contribution of stable cross bridges to the changes in ramp stiffness. When the amplitude of the first stretch exceeded the presumed elastic limits of the stable cross bridges, the second ramp stretch showed a reduction of 20-60% in initial stiffness. This was seen both in passive muscles and in muscles in which several motor units were contracting. 4. When increasing numbers of motor units of the same type were activated, the initial ramp stiffness to the second of a pair of triangular stretches delivered during contraction increased almost linearly with the developed tension. The slope of this increase was 2.5 times steeper for S units than for FR units. This reflects the fact that contraction produced by S units causes a proportionally greater resistance to stretch than that of fast units.

Summation of tension in motor units of the soleus muscle of the cat.

In the cat soleus muscle which is exclusively composed of slow motor units the discrepancy between the sum of individual tensions and the tension on combined stimulation of several motor units was found to be much less than previously reported for slow motor units of peroneus longus. In peroneus the tension on combined stimulation was systematically larger than the value predicted from the sum of individual tensions. For both muscles it was possible to reduce the difference between observed and expected values by comparing the tension on combined stimulation with the sum of tensions, not of single motor units, but of groups of units. It is concluded that whenever tension is measured for single motor units, especially slow units in mixed muscles, the values obtained may be modified by frictional forces. The size of the effect appears to vary from one preparation to the next.

Individual differences in multiple-bag spindles of cat superficial lumbrical muscles.

A total of 791 spindle poles was analysed with regard to intrafusal fibre composition in the first and second superficial lumbrical muscles from the right and left hindfeet of 9 male and 5 female adult cats. Bag and chain muscle fibres were identified by their myofibrillar ATPase staining profile in the B region, after either acid or alkaline preincubation. A high proportion of the spindle pole population (43.2%) was observed to contain three or more (up to 5) bag fibres; those poles were classified as multiple-bag spindle poles. In the 334 muscle spindles in which both poles were studied, 42 bag fibres (12.6%) were found to be of the 'mixed' type, that is a fibre in which the two poles differ in their ATPase staining profile (either bag1/bag2 or bag/chain). The variability of the intrafusal fibre content observed in spindles of these muscles has been studied in relation to individual characteristics such as sex, weight and side of the animal. In general, multiple-bag spindles are more frequent in male than in female cats and in right as compared to left side muscles. Nearly all 'mixed' bag intrafusal fibres (38 out of 42) were observed in spindles containing 3 or more bag fibres. In 3-bag spindles the proportion of 'mixed' bag spindles is approximately the same in male and female cats. The ratio of 'dynamic' (mean polar bag1 content) to 'static' (mean polar bag2 plus chain fibre content) intrafusal effectors per muscle tends to increase in spindles of right side muscles and to decrease in the heaviest animals. The quantitative and qualitative differences in fibre content of spindles observed in first lumbrical muscles of different animals suggest that the spindle fibre composition, especially that of the 'dynamic' bag1 fibre, may be related to individual predetermined and/or acquired factors.

Changes in muscle stiffness produced by motor units of different types in peroneus longus muscle of cat.

1. The effects of maximal tetanic contractions of varying numbers of motor units of the same type [slow (S), fast fatigue-resistant (FR), or fast fatigable (FF)] on the mechanical responses to muscle stretch were studied in the peroneus longus muscle of anesthetized cats. 2. Two types of stiffness measurements were made: 1) an average stiffness, defined as the tension change from the beginning to end of a 0.5-mm ramp stretch; and 2) a dynamic stiffness, defined as the ratio of peak-to-peak tension to amplitude of a maintained 85-microns sinusoidal stretch at frequencies of 10-80 Hz. 3. Contractions of slow and fast units elicited different increases in average stiffness. Type S units, although developing much smaller tetanic tensions than fast ones, produced a resistance to stretch comparable with or greater than that of fast units developing much higher tensions. 4. For comparable tetanic tensions, slow units also elicited a greater dynamic stiffness than fast units. During sinusoidal stretch, changes in muscle tension led changes in muscle length during contraction of S units, but the reverse was observed for frequencies 30-50 Hz during contraction of FF units. This suggests that the latter perform oscillatory work on the driving apparatus. 5. Type S units, whose low-threshold motoneurons are the first to be recruited, appear well adapted to play a role in posture and in slow movements because of the resistance they offer to forces tending to change joint position or to oppose the progression of slow movements.

Does stimulation of sympathetic axons elicit an increase in cat spindle afferent discharge detectable by the antidromic collision technique?

Averaged antidromic action potentials of Group I and Group II fibres elicited in cats by stimulation of L7 and S1 dorsal roots were recorded from intact tibial nerves (near the ankle) either in absence of or during repetitive stimulation of the ipsilateral lumbar sympathetic chain. This was done to test the suggestion that stimulation of noradrenergic sympathetic axons may elicit, in spindles of foot muscles, a substantial increase in the firing rate of secondary endings, capable of reducing the size of afferent antidromic volleys by collision with orthodromic impulses. We found that potentials recorded during sympathetic stimulation were identical to those recorded in absence of stimulation. The reduction in size of a component of the compound action potentials led from the intact tibial nerve during stimulation at 10-20 Hz of the sciatic nerve with C strength pulses, as described by Grassi, Filippi and Passatore (Brain Research, 435 (1987) 15-23), was observed in certain conditions of stimulation. However this reduction cannot be ascribed to antidromic collision, because it is still observed after severing the tibial nerve distal to the recording electrode.

High incidence of long-chain fibers in spindles of cat superficial lumbrical muscles.

Forty-two complete spindle poles of cat superficial lumbrical muscles were analyzed with particular regard to the length and the diameter of intrafusal fiber types. Poles were reconstructed from serial transverse sections of fresh-frozen muscles. The staining module, which was repeated throughout the whole muscle, comprised sections treated for glycogen detection and sections treated for detecting myofibrillar ATPase activity after preincubation at three different pH's (see METHODS). The identification of intrafusal fiber types was essentially based on the ATPase activity of the B region of the intrafusal fibers. Long-chain fibers, i.e., chain fibers that have at least one pole that extends by more than one millimeter beyond the end of the spindle capsule (6), were very commonly observed. Of 42 spindle poles analyzed, 30 (71%) contained at least one long-chain fiber (one in 17 spindle poles, 2 in 11 poles, and 3 in 2 poles). Of 246 poles of chain fibers, 45 (18%) were "long". In four spindles, in which both poles could be completely examined, 10 long-chain fibers were observed. In eight of these, only one pole was long; the opposite pole ended either intracapsularly or at a short distance outside the capsule. Since long-chain fiber poles, presently considered to be among the effectors of static skeletofusimotor (beta) axons, are present in a large proportion of muscle spindles of lumbrical muscles, it would be of particular interest to reevaluate the beta-supply of these muscles by physiological methods.

[Summation of maximal tetanic tensions developed by slow or fast motor units of the peroneus longus muscle in the cat]. / Sommation de tensions tétaniques maximales développées par des unités motrices lentes ou rapides du muscle long péronier du chat.

The maximal tetanic tension developed by the simultaneous contraction of several slow motor units (measured tension) is always greater than the sum of the tensions developed by each unit individually (cumulated tension). Generally, the same holds true for fast units but, for measured tensions of same value, the difference between measured tensions and cumulated tensions is much smaller. It is sometimes negligible or even negative.

[Comparison of the increase of stiffness produced during sinusoidal stretch of the long fibula muscle in the cat by the contraction of slow or fast motor units]. / Comparaison de l'augmentation de raideur produite au cours de l'étirement sinusoïdal du muscle long péronier du chat par la contraction d'unités motrices lentes ou rapides.

The increase in stiffness of cat peroneus longus muscle elicited by the contraction of homogeneous groups of motor units (slow, fast fatigue-resistant and fast fatigable) was measured during sinusoidal stretches (20-80 Hz) of small amplitude (40-100 micron). For comparable tetanic tensions, slow units increase muscle stiffness more than fast unit.

Afferent fibres in cat ventral roots: electrophysiological and histological evidence.

Electrophysiological experiments using averaging techniques, as well as anatomical experiments using horseradish peroxidase staining, have provided further evidence of afferent axons in lumbosacral ventral roots of cats. Recording from dorsal root filaments in L7, S1 or S2, following stimulation of the companion ventral root close to the dura, often shows action potentials of slow conduction velocity belonging to the A delta or C group. Stimulation applied to the proximal part of the ventral root failed to evoke such responses. Recording from multiple sites along a centrally cut ventral root filament shows responses of two types: action potentials of long latency to peripheral nerve stimulation which are seen at all recording locations and which are not seen following dorsal root stimulation. These appear to be afferent fibres which enter the cord via the ventral root; action potentials which follow dorsal root stimulation and which are usually seen only at the most distal ventral root recording site. Some of these were also activated by stimulation of some skin or muscle nerves. At appropriate intervals collision of impulses from dorsal root or peripheral nerve can be demonstrated. Such axons appear to have a recurrent course in the ventral root. Section of the spinal nerve at points progressively closer to the dorsal root ganglion abolishes the dorsal to ventral root continuity of most recurrent type axons at 2 mm distal to the ganglion. Following application of horseradish peroxidase to crushed ends of distal stumps of cut dorsal roots, thin fibres marked by the enzyme are observed in the distal part of companion ventral roots. U-turns of axons have been observed in the distal part of ventral roots and in the spinal nerve near the pole of the ganglion.

[Glycogen depletion produced in intrafusal nuclear bag muscle fibers by brief large-amplitude muscle stretches]. / Déplétion glycogénique produite dans les fibres musculaires intrafusales à sac nucléaire par des étirements musculaires brefs de forte amplitude.

The glycogen content of the three types of intrafusal muscle fibre was studied with histochemical techniques in cat muscle spindles of superficial lumbrical muscles after a very large number of brief large stretches. Zones of glycogen depletion were observed in a high proportion of nuclear bag fibres, notably in bag 1 fibres, but not in chain fibres. These observations suggest that stretching of bag fibres by itself may activate these fibres.