Y. Laporte

On the subdivision of static and dynamic fusimotor actions on the primary ending of the cat muscle spindle.

1. Using large ramp and triangular stretches a survey has been made of the effect of stimulating single gamma fusimotor fibres on primary endings of muscle spindles in the peroneus brevis to see whether intermediate types of fusimotor action could be recognized, falling between the well known static and dynamic types. 2. Responses were classified into six groups, as detailed on pp. 844‐846, ranging from apparently pure dynamic action (category I) to apparently pure static action (category IV). Models for a putative mixed action were produced by combining the stimulation of a static and of a dynamic fibre to the same spindle. The clearest sign of static action was firing on the releasing phase of the stretch. The essential sign of dynamic action, which survived combination with the more dominant static action, was the low adaptive decay of firing with a time constant of about 0‐5 sec that occurs on the plateau of the ramp stretch. 3. Out of 153 responses, each elicited from a primary ending on stimulation of a single fusimotor fibre, 67% were apparently pure examples of dynamic and static action. The remaining 33% of responses were to some degree suggestive of an admixture, in various proportions, of static and dynamic actions. For only 18% of them was there firm indication of such admixture. 4. When a given fibre was tested on more than one ending then, with one exception out of thirty‐six instances, its action always proved to be either predominantly static or predominantly dynamic. There was no special tendency for an axon with a mixed action on one spindle to have a similarly mixed action on other endings so that individual fusimotor fibres were best classified as static or dynamic without intermediate grades. 5. Simultaneous stimulation of two fusimotor fibres eliciting apparently pure static and dynamic actions, could mimic all the intermediate types of action. 6. The results are discussed in relation to recent studies, especially those based on glycogen depletion. It was concluded that dynamic action arises from activation of the bag1 intrafusal muscle fibre, and that static action arises from the bag2 and chain fibres, whether acting individually or collaboratively. The intermediate actions are suggested to arise from an overlap of motor innervation to contrasting types of intrafusal muscle fibre. 7. On the basis of effects on the regularity of the afferent discharge the findings support the view that a given static action axon can innervate bag2 and chain fibres in various proportions in different spindles, so that they do not provide separable effector pathways. 8. Responses to large amplitude sinusoidal stretching were also studied in relation to our classification.

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)

Skeleto-fusimotor fibres in the rabbit.

1. In rabbits, repetitive stimulation of single motor axons to lumbrical muscles elicits both a contraction of extrafusal muscle fibres and an increase in the discharge frequency of spindle primary endings.

Brown‐sequard 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.

Observations on the effects on spindle primary endings of the stimulation at low frequency of dynamic β-axons *

Stimulation of single dynamic beta-axons at presumed physiological frequencies (10-30/s) significantly increases the sensitivity of spindle primary endings of cat peroneus brevis muscle to ramp and sinusoidal stretches. This action becomes greater as the amplitude of stretch and the initial muscle length is increased. Stimulation at low frequency of two dynamic beta-axons supplying the same spindle greatly enhances the effect elicited by each axon individually.

Frequencygrams of rabbit spindle primary endings elicited by stimulation of fusi‐motor fibres

1. Frequencygrams of rabbit spindle primary endings elicited by stimulation of single fusimotor axons were obtained by the method described by Bessou, Laporte & Pagès (1968a) which gives indirect information on the contraction of intrafusal muscle fibres.

Effects of slow muscle stretch on the responses of primary and secondary endings to small amplitude periodic stretches in de-efferented soleus muscle spindles.

The responses of primary and secondary endings of de-efferented soleus spindles to small amplitude periodic stretches superimposed on slow ramp stretches have been studied. For primary endings, the responses become progressively larger especially during the last period of the ramp whereas for secondary endings, after an initial period of moderate growth, the amplitude of the responses display a relative reduction precisely when the responses of the primary endings are the largest. These differences can be interpreted in terms of progressive increase in stiffness of the striated polar portions of the intrafusal muscle fibers.

Does stretch excite the bag1 fibre

The observations supporting the view that bag1 fibres of cat muscle spindles can be excited by stretch are briefly reviewed in this paper.

THE SKELETOFUSIMOTOR INNERVATION OF CAT MUSCLE SPINDLES

Publisher Summary The muscle fibres of cat spindles, in addition to their specific innervation by fusi-motor axons, may be supplied by collaterals from motor axons that innervate ordinary or extra-fusal muscle fibres. The motor axons that supply both extra-fusal and intra-fusal fibres are called skeleton–fusimotor axons. They are also referred to as β axons to distinguish them from exclusively skeleton–motor or α axons and exclusively fusimotor or γ axons. Initially, the term β motor axon was introduced because nearly all the skeleton–fusimotor axons that were identified in early studies had conduction velocities approximately within the β range. At present, it is conveniently used to qualify skeleton–fusimotor axons regardless of their conduction velocities, which in fact range from 35m/s up to 105m/s. Fast conducting skeleton–fusimotor axons were first demonstrated in the peroneus tertius muscle by the glycogen depletion method. After prolonged stimulation of several motor axons to this muscle, with conduction velocities above 85m/s, spindles were found in which some intra-fusal fibres were depleted of their glycogen content, indicating the neural activation of these fibres by the collaterals of the stimulated axons.

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.