Glycogen depletion elicited in tenuissimus intrafusal muscle fibres by stimulation of static gamma-axons in the cat

The innervation of the muscle spindle: a personal history

I present a brief review of current understanding of the innervation of the mammalian muscle spindle, from a personal historical perspective. The review begins with comparative studies on the numbers of spindle afferents and considers how their relative abundance may best be assessed. This is followed by an examination of the distribution and some functional properties of the motor innervation. The primary ending is the subject of the final section, in particular, I look at what can be learned from serial sectioning and volumetric reconstruction, and present new results on a model and simulations concerning sensory terminal deformation during stretch.

The Stretch-Shortening Cycle

Neuromuscular fatigue has traditionally been examined using isolated forms of either isometric, concentric or eccentric actions. However, none of these actions are naturally occurring in human (or animal) ground locomotion. The basic muscle function is defined as the stretch-shortening cycle (SSC), where the preactivated muscle is first stretched (eccentric action) and then followed by the shortening (concentric) action. As the SSC taxes the skeletal muscles very strongly mechanically, its influence on the reflex activation becomes apparent and very different from the isolated forms of muscle actions mentioned above. The ground contact phases of running, jumping and hopping etc. are examples of the SSC for leg extensor muscles; similar phases can also be found for the upper-body activities. Consequently, it is normal and expected that the fatigue phenomena should be explored during SSC activities. The fatigue responses of repeated SSC actions are very versatile and complex because the fatigue does not depend only on the metabolic loading, which is reportedly different among muscle actions. The complexity of SSC fatigue is well reflected by the recovery patterns of many neuromechanical parameters. The basic pattern of SSC fatigue response (e.g. when using the complete exhaustion model of hopping or jumping) is the bimodality showing an immediate reduction in performance during exercise, quick recovery within 1-2 hours, followed by a secondary reduction, which may often show the lowest values on the second day post-exercise when the symptoms of muscle soreness/damage are also greatest. The full recovery may take 4-8 days depending on the parameter and on the severity of exercise. Each subject may have their own time-dependent bimodality curve. Based on the reviewed literature, it is recommended that the fatigue protocol is 'completely' exhaustive to reduce the important influence of inter-subject variability in the fatigue responses. The bimodality concept is especially apparent for stretch reflex responses, measured either in passive or active conditions. Interestingly, the reflex responses follow parallel changes with some of the pure mechanical parameters, such as yielding of the braking force during an initial ground contact of running or hopping. The mechanism of SSC fatigue and especially the bimodal response of performance deterioration and its recovery are often difficult to explain. The immediate post-exercise reduction in most of the measured parameters and their partial recovery 1-2 hours post-exercise can be explained primarily to be due to metabolic fatigue induced by exercise. The secondary reduction in these parameters takes place when the muscle soreness is highest. The literature gives several suggestions including the possible structural damage of not only the extrafusal muscle fibres, but also the intrafusal ones. Temporary changes in structural proteins and muscle-tendon interaction may be related to the fatigue-induced force reduction. Neural adjustments in the supraspinal level could naturally be operative, although many studies quoted in this article emphasise more the influences of exhaustive SSC fatigue on the fusimotor-muscle spindle system. It is, however, still puzzling why the functional recovery lasts several days after the disappearance of muscle soreness. Unfortunately, this and many other possible mechanisms need more thorough testing in animal models provided that the SSC actions can be truly performed as they appear in normal human locomotion.

Increase in the discharge of muscle spindles during diaphragm fatigue

The discharge of 18 single unit spindles located in the right crural diaphragm was recorded during rhythmic diaphragmatic contractions before the onset of and during fatigue. Spindle discharge was significantly greater (P<0.05) during rhythmic exercise when the diaphragm was fatigued than spindle discharge during rhythmic exercise when the diaphragm was not fatigued. The increase in spindle discharge during diaphragmatic fatigue is inconsistent with the notion that spindles comprise the afferent arm of a fatigue-induced inhibitory reflex originating in the diaphragm.

Reflex and intrinsic changes induced by fatigue of human elbow extensor muscles

Fatigue-induced changes in intrinsic and reflex properties of human elbow extensor muscles and the underlying mechanisms for fatigue compensation were investigated. The elbow joint was perturbed using small-amplitude and pseudorandom movement patterns while subjects maintained steady levels of mean joint extension torque. Intrinsic and reflex properties were identified simultaneously using a nonlinear delay differential equation model. Intrinsic joint properties were characterized by measures of joint stiffness, viscous damping, and limb inertia and reflex properties characterized by measures of dynamic and static reflex gains. Fatigue was induced using 15 min of intermittent voluntary isometric (submaximal) exercise, and a rest period of 10 min was taken to allow the fatigued muscles to recover from acute fatigue effects. Identical experimental and data analysis procedures were used before and after fatigue. Our findings were that after fatigue, joint stiffness was significantly reduced at higher torque levels, presumably reflecting the reduced force-generating capacity of fatigued muscles. Conversely, joint viscosity was increased after fatigue potentially because of the reduced crossbridge detachment rate and prolonged relaxation associated with intracellular acidosis accompanying fatigue. Static stretch reflex gain decreased significantly at higher torque levels after fatigue, indicating that the isometric fatiguing exercise might be associated with a preferential change in properties of spindle chain fibers and bag(2) fibers. For matched pre- and postfatigue torque levels, dynamic reflexes contributed relatively more torque after fatigue, displaying higher dynamic reflex gains and larger dynamic electromyographic responses elicited by the controlled small-amplitude position perturbations. These changes appear to counteract the fatigue-induced reductions in joint stiffness and static reflex gain. The compensatory responses could be partly due to the effects of increasing the number of active motoneurons innervating the fatiguing muscles. This shift in operating point gave rise to significant compensation for the loss of contractile force. The compensation could also be due to fusimotor adjustment, which could make the dynamic reflex gain much less sensitive to fatigue than intrinsic stiffness. In short, the reduced contribution from intrinsic stiffness to joint torque was compensated by increased contribution from dynamic stretch reflexes after fatigue.

Altered reflex sensitivity after repeated and prolonged passive muscle stretching

Experiments were carried out to test the effect of prolonged and repeated passive stretching (RPS) of the triceps surae muscle on reflex sensitivity. The results demonstrated a clear deterioration of muscle function immediately after RPS. Maximal voluntary contraction, average electromyographic activity of the gastrocnemius and soleus muscles, and zero crossing rate of the soleus muscle (recorded from 50% maximal voluntary contraction) decreased on average by 23.2, 19.9, 16.5, and 12.2%, respectively. These changes were associated with a clear immediate reduction in the reflex sensitivity; stretch reflex peak-to-peak amplitude decreased by 84. 8%, and the ratio of the electrically induced maximal Hoffmann reflex to the maximal mass compound action potential decreased by 43. 8%. Interestingly, a significant (P < 0.01) reduction in the stretch-resisting force of the measured muscles was observed. Serum creatine kinase activity stayed unaltered. This study presents evidence that the mechanism that decreases the sensitivity of short-latency reflexes can be activated because of RPS. The origin of this system seems to be a reduction in the activity of the large-diameter afferents, resulting from the reduced sensitivity of the muscle spindles to repeated stretch.

Summation of responses of cat muscle spindles to combined static and dynamic fusimotor stimulation

This is a study of the process of interaction between the responses of muscle spindles to stimulation of two fusimotor fibres. Combined stimulation of a static and a dynamic fusimotor fibre supplying the same muscle spindle in the soleus muscle of the anaesthetised cat gave a response which was larger than from stimulating each fibre separately, but less than their sum. A similar summation process was observed with pairs of static fusimotor fibres. The mean summation coefficient for the responses to stimulation of 14 pairs of static fusimotor fibres was 0.29 (range 0.14-0.52; S.D. 0.09), while for 42 static:dynamic pairs it was 0.30 (range 0.07-0.89; S.D. 0.20). Mechanisms considered for the summation process were probabilistic mixing of impulse traffic from two or more impulse generators within the terminals of the primary ending of the spindle, the spread of generator current from one encoding site to another and mechanical interactions between contracting intrafusal fibres. In an experiment where single static and dynamic fusimotor fibres were stimulated together, and then stimulation of the static fibre stopped, the size of the continuing dynamic response was larger than when the dynamic fibre had been stimulated alone. This finding suggested some kind of mechanical interaction between the contracting intrafusal fibres and implies that static and dynamic fusimotor effects within a spindle cannot be considered to be entirely independent of one another.

Decline in spindle support to alpha-motoneurones during sustained voluntary contractions

1. To address whether the muscle spindle support to alpha-motoneurones is maintained during prolonged isometric voluntary contractions, the discharge of eighteen muscle spindle afferents, originating in the dorsiflexors of the ankle or toes, was recorded from the common peroneal nerve in eight subjects. Isometric contractions were generally sustained for 1 min, usually below 30% of the maximal voluntary dorsiflexion force. 2. Once the afferent had been identified, subjects were instructed to dorsiflex the foot slowly to recruit the spindle ending, to continue the ramp contraction until a predetermined target force was reached, and then to hold that force until requested to relax. 3. Five muscle spindle afferents maintained a constant discharge frequency during the hold phase of the isometric contraction. Following relaxation of the contraction two spindle afferents from tibialis anterior, exhibited a post-contraction discharge despite the absence of detectable electromyographic activity (EMG). 4. The discharge frequency of most of the spindle afferents (72%) declined progressively during the isometric contraction. The mean firing rates had declined to two-thirds of those at the onset of the contraction by 30 s, and to half after 1 min. The decline in spindle firing rate commenced during the ramp phase of the contraction and was statistically significant by 10 s, when force was held constant. The extent of the decline was greater for those units with the higher initial firing rates and for those units studied after many preceding contractions. 5. In the same contractions a progressive increase in EMG was required to maintain force and consequently the change in EMG was inversely related to the change in spindle discharge. While many mechanisms may contribute to the decline in spindle discharge during a sustained isometric contraction, it is argued that the result will be a progressive disfacilitation of alpha-motoneurones, which may contribute to the decline in motor unit firing rates during a sustained contraction.

Tonic vibration reflexes elicited during fatigue from maximal voluntary contractions in man

1. In the present study on human foot dorsiflexor muscles we have examined the effects of high-frequency (150 Hz) muscle vibration on weak or moderate voluntary contractions (maintained by constant effort) and on maximal voluntary contractions (MCVs) of (i) non-fatigued muscles, (ii) muscles fatigued by sustained MVCs and (iii) muscles deprived of gamma-fibre innervation by partial anaesthetic nerve block. The motor outcome of the voluntary dorsiflexion efforts was assessed by measuring the firing rates of single motor units in the anterior tibial (TA) muscle, the mean voltage EMG activity from the pretibial muscles and foot dorsiflexion force. 2. With the subject instructed to exert constant effort in maintaining a weak or moderate contraction, superimposed vibration caused an enhancement of EMG activity and contraction force. 3. Previous claims that muscle vibration has no facilitatory effect on motor output in MVCs were found to hold true for non-fatigued but not for fatigued muscles. Thus, the fatigue-induced decline in EMG activity and motor unit firing rates was counteracted by short periods (less than 10-20 s) of superimposed vibration. However, with longer vibration periods it seemed as if the initial facilitation converted into an opposite effect which accentuated the fatigue-induced decline in motor output and contraction force. 4. Like muscle fatigue, a partial anesthetic block of the deep peroneal nerve, supposedly interrupting transmission in gamma-motor fibres, caused a reduction of MVC motor unit firing rates which could be counteracted by muscle vibration. In prolonged MVCs performed during the block, motor unit firing rates did not show the normal progressive decline from an initially high level, but stayed at a relatively constant low level throughout the contraction period. 5. Even though alternative interpretations are possible, the results agree with the hypotheses (i) that in sustained MVCs, fatigue processes occur not only in extrafusal but also in intrafusal muscle fibres, (ii) that the intrafusal fatigue leads to a reduction of the voluntary drive conveyed to the alpha-motoneurones via the gamma-loop and (iii) that vibration-induced activity in group Ia afferents can act as a substitute for the diminished fusimotor drive.

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.

Resistance to glycogen depletion of motor units in the cat rectus lateralis muscle

In nembutal anesthetized adult cats, intracellular stimulation of single abducens motoneurones was used to elicit glycogen depletion of their muscle units. Stimulation by short trains (13 pulses at 40 Hz) delivered once a second, was applied for 20 to 110 min. The activation of the motor unit was monitored by intracellular recording of motoneurone action potentials and by EMG. After the end of stimulation, the muscle was excised and frozen to be cut in serial sections that were processed for demonstration of either glycogen, ATPases or SDH. In two experiments, a motor unit could be histochemically identified because 10-15 fibres showed zones of complete glycogen depletion measuring about 5 mm in length. All the depleted fibres had the same histochemical profile: ATPases reactions gave dark staining with alkaline preincubation and light staining with acid preincubation whereas SDH activity was low. In other experiments, prolonged stimulation produced either no depletion at all or very limited zones of partial depletion in a few muscle fibres.

Distribution of skeletofusimotor axons in lumbrical muscles of the monkey

The nerve supply to 25 poles of muscle spindles in the monkey was reconstructed by light microscopy of serial 1-micron thick transverse sections of lumbrical muscles. Twenty of 60 motor axons that supplied the spindle poles were identified as skeletofusimotor (beta). Twenty-eight percent of the spindle poles were innervated by beta axons, in addition to gamma axons. Every beta-innervated spindle pole transected an endplate zone of extrafusal muscle. Most beta axons coinnervated extrafusal fibers rich in mitochondria and the nuclear bag1 or nuclear chain intrafusal fibers. All but two beta axons innervated one type of intrafusal fiber only. The intramuscular organization of beta motor system in lumbrical muscles of the monkey was similar to that of the cat tenuissimus muscle. The function of beta-innervated spindles may be preferentially to monitor mechanical disturbances arising from the activity of extrafusal muscle units with which they share motor innervation.