No abstract

8. Responses to large amplitude sinusoidal stretching were also studied in relation to our classification.

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.

SUMMARY1. The distribution of fusimotor axons to bag1, bag2 and chain muscle fibres in cat tenuissimus spindles has been studied using a modification of the glycogen-depletion technique of Edstr6m & Kugelberg (1968). Single fusimotor axons were stimulated intermittently at 40-100/sec for long periods (30-90 sec) during blood occlusion. Portions of muscle containing the activated spindles were quick-frozen, fixed in absolute ethanol during freeze-substitution, and then embedded in paraffin wax. Serial transverse sections were stained for glycogen using the periodic acid-Schiff method, and examined for depletion.2. Dynamic y axons (i.e. those that increase the dynamic index of primary-ending responses to ramp stretches of large amplitude) depleted bag1 fibres almost exclusively.3. Static y axons (i.e. those that reduce or abolish the dynamic index) depleted both bag and chain fibres. Bag1 and bag2 fibres were depleted about equally. 4. A single static y axon may activate both bag and chain fibres in one spindle (the most common pattern), chain fibres only in another, and bag fibres only in a third spindle.5. Static y axons with conduction velocities less than 25 m/sec also had a non-selective distribution, but no depletion was observed in bag2 fibres.6. The zones of depletion produced by dynamic y axrons were distributed more or less equally in the intra-and extracapsular parts of spindle poles, whereas those produced by static y axons were mainly intracapsular.7. The results are compared with the glycogen-depletion studies of Brown & Butler (1973 and our own study of the distribution of

SUMMARY1. Tenuissimus muscles of the cat were prepared in which the motor innervation was reduced to a single y axon by cutting all the other motor axons and allowing them to degenerate during a period of 7-12 days. The function of the surviving y axon was then determined, and the distribution of its endings ascertained in teased, silver preparations.2. In the ten muscles successfully prepared the function of the surviving y axon was static and the motor innervation distributed to the spindles consisted of trail endings. The conduction velocities of the axons ranged from 33 to 48 m/sec. 3. A detailed histological analysis was made of thirty spindles innervated by six of the surviving static axons.4. The six static axons distributed trail endings to both bag and chain muscle fibres in the poles of thirty spindles with about twice the frequency of supplying them to poles in which the distribution was restricted exclusively to one type of muscle fibre or the other.5. The density of trail innervation supplied to the bag fibres, in terms of the mean number of terminals per fibre, was typically from one and a half to twice that supplied to the chain fibres. On the other hand, whereas the number of bag fibres supplied with trail endings in a spindle pole was seldom more than one, the number of chain fibres innervated was usually two in a range of one to four.

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.

1. The types of intra‐ and extrafusal muscle fibre innervated by dynamic skeleto‐fusimotor (beta) axons were determined by using a modification of the glycogen‐depletion method of Edström & Kugelberg (1968) combined with histochemical tests for various enzyme reactions. A single beta axon was prepared in each of the experiments, which were carried out on six peroneus brevis and two tenuissimus muscles. 2. The intrafusal distribution of dynamic beta axons is almost exclusively restricted to bag1 fibres. The bags fibre was depleted in each of twenty‐four beta‐innervated spindle poles; the only fibres of a different type depleted intrafusally were a bag2 fibre in one pole and a long chain in another. 3. Depletion in the bag1 fibres was usually restricted to one zone in one pole, generally in a mid‐polar location. 4. The extrafusal muscle fibres depleted by dynamic beta axons belong to the slow oxidative type as defined by Ariano, Armstrong & Edgerton (1973). The number of such fibres in each motor unit could not be accurately determined, but is almost certainly small. 5. The slow oxidative muscle fibres innervated by dynamic beta axons were not depleted over their entire length. Since there is no reason to assume that they are not twitch fibres, it would seem that the localized depletions result from the conditions required to obtain glycogen depletion, i.e. long periods of motor stimulation applied during the occlusion of the muscle's blood supply. Under similar experimental conditions depletion of glycogen was also restricted to portions of fibres in fast oxidative‐glycolytic motor units, but extended over most of the length of the fibres in fast glycolytic units.

4. The dynamic action of six ft axons was observed only after the contraction of extrafusal muscle fibres was selectively suppressed. 5. Tendon organs can be activated by ft motor units.