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Notes: [Auszug] GRINNELL ET AL.1 have demonstrated that when allowing two foreign nerves to innervate a transplanted frog sartorius muscle, control of transmission at neuromuscular junctions on muscle fibres is shared between the two nerves, one being more effective in terms of the quantal content of endplate ...

Notes: Résumé Les vésicules synaptiques granulées se trouvent dans les jonctions neuromusculaires de mammifères, des crustacés, et des araignées. On ne connait pas leur fonction.

Notes: Summary Cross-reinnervations were effected between the extensor digitorum longus and soleus muscles in the cat hind limb. At the same time dorsal root section or ganglionectomy was performed over segments L6-S1. Completeness of the deafferentation was subsequently confirmed either by dissection or by dorsal root recording. The isometric and forcevelocity properties of the muscles were measured. In animals with a unilateral cross plus deafferentation the conversion of the contractile properties of the normally slow-twitch soleus to those resembling a fast-twitch muscle was typical of that seen with an intact afferent supply. In cats with a bilateral cross-reinnervation and unilateral deafferentation there was no significant difference in the degree of transformation between the two sides. It is concluded that at least for the conversion of a slow-twitch to a fast-twitch muscle afferent feedback does not play a major role.

Notes: Summary Normally, γ motoneurones innervate only the intrafusal fibres of muscle spindles. This is a report of sprouting of γ motoneurones to innervate extrafusal muscle fibres following partial denervation of the soleus muscle of kittens. In eight newborn animals, the L7 ventral root was cut on one side under anaesthesia and the animals were then allowed to recover. At approximately 100 days of age animals were reanaesthetised and a study made of mechanical properties of motor units whose axons ran in the S1 ventral root and supplied the partially denervated soleus muscle. Evidence was obtained for sprouting of all surviving α motoneurones. In addition, in four experiments axons conducting within the γ range, on stimulation, produced measurable tension. In one experiment, stimulation of one such γ axon also produced specific fusimotor effects on four afferents identified as coming from primary endings of muscle spindles. The γ axon was therefore a fusimotor axon. The effect observed on stimulation of the γ axon suggested a largely dynamic action. Other examples of γ axons were encountered that on stimulation produced tension, but which could not be specifically associated with spindles. In addition, a number of γ axons that did not develop tension were shown, on stimulation, to have fusimotor effects that were static in action. It is concluded that in extensively denervated muscles γ motoneurones may sometimes sprout to innervate extrafusal fibres. The mechanical properties of the extrafusal fibres innervated by such γ axons were similar to those of ordinary α motor units.

Notes: Abstract  The experiments reported here demonstrate that the mechanical sensitivity of peripheral nerve fibres typically seen after injury can be induced without overtly injuring the nerve, but by simply applying colchicine topically to the nerve. In cats anaesthetised with pentobarbitone sodium, the medial gastrocnemius nerve was exposed and 10 mM colchicine applied topically for 15 min. The animals recovered from the operation normally and showed no subsequent motor deficit. Six days later animals were re-anaesthetised, a laminectomy carried out and responses recorded in single afferents at the level of the dorsal root. It was found that many afferents, particularly those with conduction velocities in the group II–III range, had become sensitive to local mechanical stimulation of the nerve in the region treated with colchicine and showed slowly adapting responses to stretch of the nerve. Many of the smaller fibres exhibited spontaneous activity. Mechanically sensitive afferents exhibited impulse conduction blocks at the colchicine-treated site. Some afferents, which appeared to conduct impulses normally through the treated region, were associated with muscle receptors having normal response properties. However, other muscle receptors were clearly abnormal and were insensitive to muscle stretch or contraction or exhibited only phasic responses. When the nerve was cut proximal to the colchicine-treated site, some, but not all, spontaneous activity was abolished. It was subsequently shown using a collision technique that the activity in some axons had its origin in the cell body in the dorsal root ganglion. In one experiment, it was shown that after nerve section proximal to the colchicine-treated region three of five axons switched their activity from a peripheral to a central origin. It is postulated that colchicine disrupts fast axonal transport of mechanically sensitive or voltage-sensitive ion channels, from the cell body to the peripheral terminals of the axons, leading to an accumulation of these channels at the treated site. This induces mechanical sensitivity and spontaneous activity. It is postulated that interruption of a retrogradely transported signal induces the spontaneous activity in the cell body. These experiments suggest that an important influence is exerted by the cell body on the peripheral terminals of mechanoreceptors to confer on them their normal response properties.

Notes: Summary The fast-twitch flexor digitorum longus (FDL) muscle was partially denervated by a unilateral section of the L7 ventral root. After approximately 100 days the isometric, force-velocity and histochemical properties of single motor units from the partially denervated muscle were determined. The diameter of the nerve axons supplying the muscle were also examined. The extrapolated maximum speed of sarcomere shortening (Vmax) of the enlarged motor units was significantly less than that of the control whole muscle Vmax. It is concluded that the motoneurone is capable of expanding its territory and therefore its tension generating capability without any apparent change in axonal diameter. In addition, the intrinsic properties of the muscle fibres are altered.

Notes: Abstract The effect of initial muscle length on the speed of shortening at different relative loads has been determined for the soleus and flexor digitorum longus (FDL) muscles of the cat. Isometric tetanic force-length relationships for both muscles were similar to those shown previously. The functional length range for FDL occurred at relatively long lengths, from optimum (100%) to 135% of optimum length; however, soleus worked at relatively short lengths from 60% to 110% of optimum length. In FDL the speed of shortening at any given load was relatively constant within the functional range, but at very short muscle lengths the speed of shortening declined. Soleus also showed a decline in the speed of shortening at all loads at short muscle lengths, which included the functional working range of the muscle. Speed of shortening at any given load was maximal at optimum length but tended to decline at low loads and long muscle lengths. It is concluded that in FDL even when the toes are at the extremity of their range, speed of muscle shortening is unaffected. Soleus may be relatively disadvantaged because its functional range extends over short muscle lengths. The results indicate that soleus is capable of making a significant contribution in standing and a slow walk, but that at faster gaits the contribution of soleus may be negligible.

Notes: Abstract Experiments were carried out to compare the stiffness of cross-bridges in amphibian slow and twitch muscle. an isolated iliofibularis muscle was subjected to rapid, small stretches during contraction of either slow or twitch fibres at a number of different isometric tensions. The method of analysis allowed the compliance of the cross-bridges to be distinguished from other sources of compliance. Provided that the muscle was stretched sufficiently rapidly to obtain limiting values of stiffness, little difference was found between the mechanical properties of the cross-bridges in slow and twitch muscle. It is concluded that the difference in observed stiffness of the two muscle types is due to a lower turnover rate of cross-bridges and a smaller number of sarcomeres in slow fibres.

Notes: Abstract These experiments describe the responses of myelinated skin and muscle afferent nerve fibres at a neuroma to stretch, local pressure and vibration in the anaesthetised cat. The sural nerve and the nerve supplying the medial gastrocnemius were studied. Neuroma formation was encouraged by placing the cut end of the nerve in a cuff made of synthetic material (Gore-tex). By 6 days after nerve section, the two nerves contained mechanically sensitive afferents. No motor fibres appeared to be mechanically sensitive. Mechanically sensitive sural afferents responded to ramp stretch of the nerve, applied at the cuff, with a single impulse or brief burst of impulses. The majority of gastrocnemius afferents responded to stretch with slowly adapting trains of impulses. Many muscle group II afferents exhibited a steady resting discharge, while group I afferents had an intermittent or bursting resting discharge or were silent. Those group I axons which showed resting activity had a low stretch threshold and were probably Ia fibres. Many of the silent units were also stretch sensitive. It is proposed that the spontaneously active units and silent units with low stretch thresholds were Ia fibres, while silent units with high stretch thresholds were Ib fibres. Both sural and gastrocnemius afferents responded to locally applied vibration. The mean peak response frequency for sural units was 170 Hz (± 70 Hz SD). For gastrocnemius units it was 325 Hz (± 86 Hz SD). Group I muscle afferents responded to higher frequencies of vibration than group II afferents. In four experiments the nerve was treated at a site a few millimetres proximal to the point of section with the axonal transport blocker colchicine. Twenty-five millimolar colchicine blocked impulse conduction at its point of application. Nevertheless, mechanically sensitive areas developed in the nerve just proximal to the treated region. Ten millimolar colchicine did not block impulse conduction, but led to dispersion of mechanosensitive areas to more proximal regions of the nerve. This result suggests that the disruption of orthograde axonal transport by colchicine leads to development of mechanically sensitive areas in axons further back from their cut ends. Local application of the drugs succinyl choline, tetra-ethyl ammonium and gadolinium had no effect on levels of resting activity or on mechanical sensitivity of afferents in the cuff. The potassium channel blocker 4-aminopyridine, on the other hand, produced an increase in the levels of resting activity and in the stretch responses of afferents. None of these drugs induced any activity in motor axons. It is proposed that mechanical sensitivity is induced at the sprouting tips of sensory axons by a substance or substances transported down the axon from the cell body. Such a conclusion implies that some of the response properties of normal mechanoreceptors in skin and muscle may be the result of influences exerted by the cell body on the peripheral terminal membranes. This conclusion has important implications for understanding transduction mechanisms and the development of somatic receptors, and for interpretation of receptor responses following nerve section and reinnervation or cross-reinnervation.

Notes: Abstract At early stages of muscle development, skeletal muscles contract and relax slowly, regardless of whether they are destined to become fast- or slow-twitch. In this study, we have characterised the activation profiles of developing fast- and slow-twitch muscles from a precocial species, the sheep, to determine if the activation profiles of the muscles are characteristically slow when both the fast- and slow-twitch muscles have slow isometric contraction profiles. Single skinned muscle fibres from the fast-twitch flexor digitorum longus (FDL) and slow-twitch soleus muscles from fetal (gestational ages 70, 90, 120 and 140 days; term 147 days) and neonatal (8 weeks old) sheep were used to determine the isometric force pCa (pCa = −log10[Ca2+]) and forcepSr relations during development. Fast-twitch mammalian muscles generally have a greatly different sensitivity to Ca2+ and Sr2+ whereas slow-twitch muscles have a similar sensitivity to these divalent cations. At all ages studied, the forcepCa and force pSr relations of the FDL muscle were widely separated. The mean separation of the mid-point of the curves (pCa50−pSr50) was ∼1.1. This is typical of adult fast-twitch muscle. The force-pCa and force-pSr curves for soleus muscle were also widely separated at 70 and 90 days gestation (pCa50−pSr50∼0.75); between 90 days and 140 days this separation decreased significantly to ∼0.2. This leads to a paradoxical situation whereby at early stages of muscle development the fast muscles have contraction dynamics of slow muscles but the slow muscles have activation profiles more characteristic of fast muscles. The time course for development of the FDL and soleus is different, based on sarcomere structure with the soleus muscle developing clearly defined sarcomere structure earlier in gestation than the FDL. At 70 days gestation the FDL muscle had no clearly defined sarcomeres. Force (N cm-2) increased almost linearly between 70 and 140 days gestation in both muscle types and there was no difference between the Ca2+- and Sr2+-activated force throughout development.