Effects of the calcium antagonist nifedipine on the afferent impulse activity of isolated cat muscle spindles

The impulse activity of muscle spindles isolated from the cat tenuissimus muscle was investigated under varying concentrations of the L-type calcium channel blocker nifedipine. At a concentration of 25 microM nifedipine impulse activity was clearly diminished in both primary and secondary endings. However, low concentrations of the drug (5-10 microM) exerted unexpected excitatory effects. The dynamic properties of primary endings in particular were augmented; those of secondary endings were also increased, although only slightly. A detailed analysis of the afferent discharge patterns obtained under ramp-and-hold stretches yielded the following effects of 10 microM nifedipine. (1) The initial burst at the beginning of the ramp phase of a stretch was increased in primary endings; (2) the peak dynamic discharge frequency at the end of the ramp phase was considerably increased in most primary endings; (3) the sensitivity of the peak dynamic discharge value to varying amplitudes and velocities of stretch was significantly enhanced in primary endings, and also increased, although only slightly, in secondary endings; (4) the rise in the discharge frequency during the ramp phase of a stretch was augmented in both types of ending, the effect being again stronger in primary endings; (5) the fast adaptive decay of the impulse frequency following the ramp phase of a ramp-and-hold stretch was significantly increased in primary endings, but remained unaffected in secondary endings. The enhanced dynamic properties of primary endings were also observed under small sinusoidal stretch stimuli (10 microm, 40 Hz), where nifedipine induced a significant shift in the position of the 1:1 driven action potentials toward smaller phase values. In view of an increase in tension in the isolated muscle spindle and an increased initial burst in primary endings in the presence of nifedipine, it is suggested that the drug facilitates the attachment of cross-bridges in the poles of the intrafusal muscle fibers. The increase in the dynamic properties of primary endings points to the possibility that the drug preferentially affects the nuclear bag(1) fiber. The inhibitory effect on the afferent discharge rate at high doses of the drug is interpreted as the consequence of a calcium channel block in the membranes of the sensory endings. The membrane potential of sensory endings appears to be highly dependent on sustained Ca(2+) conductance.     

Influence of adequate stimulation on the impulse electrical activity of isolated frog muscle spindles]

Impulse activity of an isolated frog muscle spindle was investigated using mechanical stimulation of various amplitude, rate and form. It is found out that firing frequency during linear-increasing stimulation is determined by the amplitude of the stimulus and alteration of firing frequency by the rate of the stimulation increase. At sinusoidal stimulation the impulsation "repeates" the form of the stimulus, i.e. the muscle spindle is influenced not only by its amplitude or rate, but also by its acceleration. The sensitivity to accelerations may depend on changes in discharge threshold during alteration of the tension rate change.     

Effects of chlorobutanol on primary and secondary endings of isolated cat muscle spindles

The effects of the preservative chlorobutanol on primary and secondary endings of muscle spindles isolated from the tenuissimus muscle of the cat were investigated in this study. Chlorobutanol was applied to the bathing solution in final concentrations of between 10 and 100 microg/ml. It induced a reversible and dose dependent decrease in the discharge frequency of both types of ending without any visible length change in the sensory region of the receptor. The initial activity, the peak dynamic discharge, the maximum static discharge value and the final static discharge value were evaluated from an ending's discharge pattern obtained during ramp-and-hold stretches. These four basic discharge frequencies decreased in parallel with increasing concentrations of chlorobutanol. Their sensitivities to chlorobutanol were similar (mean values: -0.11 to -0.29 imp/s per microg/ml chlorobutanol) and were independent of the amplitude of stretch. The dynamic response and the static response of both primary and secondary endings remained unchanged, indicating that the sensitivity of the spindle to stretch was not influenced by chlorobutanol. Chlorobutanol also reduced the discharge activity of the muscle spindle afferents during sinusoidal stretches. The amplitude of the receptor potential (AC component) remained unchanged under chlorobutanol. With the available recording technique it was not possible to measure slow shifts of the membrane potential. However, a hyperpolarization of the ending's membrane might explain why the afferent discharge frequency is reduced by chlorobutanol. The calcium dynamics of the spindle do not appear to be altered by CB, as the effect exerted on the afferent discharge by a change in the extracellular calcium concentration and a blockage of calcium channels was different from the CB effect. As the inhibitory effect of CB was reduced by ouabain, it is possible that CB activates the electrogenic Na/K pump or affects a mechanism that is closely related to the activity of the pump. The properties of the axonal membrane appear not to be altered, as chlorobutanol did not change the shape of action potentials.     

Spontaneous activity of the passive muscle spindles of the cat triceps surae]

In a slack deeferented m. triceps surae about 45% of 400 stretch receptors under study have a spontaneous discharge. The following facts indicated that some intrafusal fibres in the slack muscles are presumably in the prestretch state: 1) the isotonic contraction of the muscle diminished or abolished the spontaneous discharge; 2) at the beginning of the stretch of the slack muscle the initial slowing in the discharge frequency of the unit sometimes appeared; this slowing did not depend on the type of unit and occured in a unit among many others in the same preparation and under the same conditions of experiment; 3) the dynamic thresholds of spontaneously discharging units were lower than that of units without spontaneous discharge. It is suggested that a static stretch occurs in some muscle spindles, if the distance between two points of big fibres insertion on extrafusal fibres or tendon fascicules is longer than the length of intrafusal fibres. The initial slowing at the beginning of stretch is presumably due to viscous damping forces which must be higher in more viscous extrafusal fibres in comparison to that of intrafusal fibres.     

The innervation of tandem muscle spindles in the cat neck

Patterns of innervation were examined in tandem muscle spindles teased from silver-stained muscles of the cat neck. Each tandem spindle was composed of two or more encapsulated receptors linked in series by a shared bag2 fiber. In most tandem spindles, two different types of encapsulation were identified according to differences in their intrafusal fiber content. One type, the b1b2c unit, contained typical bag1, bag2, and chain fibers and was structurally similar to single spindles described in other cat muscles. Each b1b2c unit contained a single primary sensory ending and 1-6 secondary endings. Fusimotor innervation was supplied by many axons. Some fusimotor axons ended in trail ramifications on bag2 and chain fibers, others ended in plates on the bag1 or long chain fiber. The other type of tandem encapsulation, the b2c unit, had only bag2 and chain fibers in its intrafusal fiber bundle. The b2c unit was usually supplied by only one sensory axon that ended on the nucleated part of the intrafusal fiber bundle. This single ending had a more variable terminal morphology than the primary ending in b1b2c units. A few b2c units (3/49) were also supplied by a secondary ending. The fusimotor innervation of the b2c unit was relatively simple. A single pole of the b2c unit was usually supplied by only one to three axons, all ending in trail ramifications. No plate endings were found in b2c units. These morphological specializations suggest that b1b2c and b2c units in tandem spindles differ in both their transductive and fusimotor mechanisms. Thus, the tandem spindle is a specialized structure that may provide additional proprioceptive information beyond that available from single muscle spindles.     

Effects of calcium on the discharge pattern of primary and secondary endings of isolated cat muscle spindles recorded under a ramp-and-hold stretch

The impulse activity of muscle spindles isolated from the cat tenuissimus muscle was investigated under varying concentrations of external calcium (Ca(2+)). The outer capsule of the muscle spindle represents an effective diffusion barrier for Ca(2+) ions since activity changes were strong and rapid only if the capsule was partly removed from the sensory region of the receptor. The impulse activity of both primary and secondary muscle spindle endings was lowered by an increase in the external Ca(2+) concentration from 1.8 mM (normal Ringer's solution) to 2.7 mM and raised by a decrease in the Ca(2+) concentration from 1.8 to 0.9 mM. Primary endings were generally more strongly affected than secondary endings. With primary endings the firing rate changed by 23-52% when the external Ca(2+) concentration was altered by 0.9 mM. With secondary endings the discharge frequency changed by 15-24%. The afferent discharge patterns were obtained under repetitive ramp-and-hold stretches and were analyzed with regard to influences of external Ca(2+) ions on the static and dynamic components of the endings' responses. The stretch sensitivity and the adaptive response of both types of ending increased in the low Ca(2+) solution and decreased in the high Ca(2+) solution, but a specific effect on a single component of the responses to stretch was not observed. These findings indicate an overall change in excitability when the external Ca(2+) concentration was varied. The mechanical properties of the receptor were probably not affected since changes in the Ca(2+) concentration did not elicit a contraction or relaxation of the intrafusal muscle fibers. On the one hand, the observed effects can be explained according to the surface potential theory by an indirect influence of extracellular Ca(2+) ions on ion channels of the sensory nerve terminals, with Ca(2+) ions binding to negative charged sites at the endings' outer membrane. On the other hand, the results are consistent with the supposition that Ca(2+) ions act directly on ion channels of the sensory membrane of muscle spindle endings.     

The effect of hypertonic solutions on the electrical activity of isolated frog muscle spindles]

Electrical activity of an isolated frog muscle spindle was investigated in hypertonic solutions obtained by adding 400 mM of sucrose, glucose or glycerine to the Ringer solution. In sucrose and glucose hypermedia the frequency of background impulse activity first increased and then decreased up to zero; receptor potentials and evoked impulse activity simultaneously decreased and disappeared. In the glycerine hypermedia impulse activity after some increase returned to normal and then a second rise of frequency to a constant supernormal level was observed. After returning to the Ringer solution from sucrose or glucose the background and evoked impulse activity progressively returned to normal. But after glycerine hypermedia a significant growth in the background impulse activity and a small change in frequency of evoked impulse activity were observed. The observed changes of electrical activity of the muscle spindle in hypermedia are discussed in terms of deformation of sensory endings and intrafusal muscles fibres leading to depolarization of the receptor membrane.     

Response characteristics and classification of muscle spindles of the masseter muscle in the cat

For the classification of primary and secondary spindle endings in jaw-closing muscles, the relationship between physiologic properties of these endings and conduction velocities of their afferent fibers was analyzed. Unitary discharges of afferent fibers from masseter muscle spindles were recorded in themesencephalic trigeminal nucleus (Mes V) by microelectrodes. Conduction velocities of masseter spindle units in the Mes V showed a bimodal distribution with the division at 36 m/s. The units conducting at more than 54 m/s and those conducting less than 36 m/s were categorized as the primary and secondary endings, respectively, on the basis of their responses to muscle stretches, vibratory stimuli, and to suxamethonium (SCh) administration (200 μg/kg, i.v.). On the other hand, spindle units conducting at between 36 and 54 m/s, which were designated as the intermediate spindle unit, showed a great variety in their responses to the same stimuli. In addition, the number of intermediate spindle units was about twice as large as that of the well-defined units. Because the spindle units in the Mes V were rather clearly divied into two groups on the basis of their sensitivity to vibration and to SCh irrespective of their conduction velocities, the intermediate spindle units may effectively be categorized on the same basis. There are several possible reasons to account for the high proportion of intermediate spindles.     

The reinnervation of cat muscle spindles by skeletofusimotor axons

Tests were made to ascertain the numbers of skeletofusimotor axons reinnervating muscle spindles following crush or section of the nerve to peroneus tertius in adult cats. After short periods of recovery there was no change in the proportion of skeletofusimotor axons compared with normal animals.     

Sensory reinnervation of cat peroneus brevis muscle spindles after nerve crush

Results are presented of examining the postcrush sensory reinnervation of cat peroneus brevis muscle spindles previously investigated physiologically by Hyde and Scott. It is shown that primary and secondary endings were successfully restored in their final form in the early stages of recovery. The primary endings were shorter than normal and had fewer transverse bands; 12% were judged to be hyperinnervated. Some secondary endings showed signs of growth through the primary region apparently designed to establish secondary terminals in the opposite pole. This is compared with the collateral regeneration of intact motor axons in partially denervated muscle. It is concluded that the defects observed in the regenerated sensory endings had no effect on their functional recovery.     

Ultrastructural study of motor endings on long chain intrafusal fibers in muscle spindles of the cat

Five complete poles of long nuclear chain intrafusal fibers of cat tenuissimus muscle spindles embedded in a plastic medium were cut in serial, 1 micron thick transverse sections and stained with toluidine blue. Each fiber pole displayed one plate-type motor ending positioned outside the encapsulated part of the spindle. Ultrastructurally, the endings resembled extrafusal end-plates and were more complex, in terms of prominence of sole-plate and degree of post-junctional folding, than any other motor ending present in the spindles. They were identified as the terminals of static (fast) skeletofusimotor axons, which preferentially innervate the longest nuclear chain fibers of cat spindles.     

Intermittent stimulation of fusimotor fibres during slow stretch of muscle spindles in the cat

Responses of muscle spindles of the peroneus tertius muscle of the cat were recorded during intermittent fusimotor stimulation, applied during slow stretch, after muscle conditioning by stimulation of the nerve at the test length or at a length 2.5 mm longer. Some static and all dynamic axons evoked afferent bursts whose amplitude was relatively independent of the level of stretch response. Other static axons produced bursts that grew in size with the stretch.     

The effect of cortical ablation on afferent activity in the cat somatosensory system

The modulatory influence of primary S1 cortex upon afferent activity in the somatosensory system was examined in acute cats. Modulatory influences due both to prior afferent activity and efferent activity on peripherally evoked neural responses in thalamic n. ventralis posterolateralis (VPL) and the dorsal column nuclei (DCN) were analyzed. The right superficial radial nerve was stimulated with suprathreshold pulses applied randomly in time. Evoked activity was recorded from ipsilateral DCN, contralateral VPL, and contralateral S1 cortex before and after ablation of S1 cortex. The data were analyzed using a modified functional power series. The responses were characterized by first- and second-order kernels computed by cross-correlation. S1 ablation reduced or eliminated the second positive peak in the VPL response. Incomplete S1 ablation produced a graded effect on this peak. In addition, S1 ablation eliminated the negative notch on the leading edge of the response observed in the nucleus cuneatus. The results from VPL suggest that the second VPL positive peak is produced by a tonic S1 influence on another region of the central nervous system that is coupled to VPL, rather than by a direct VPL-cortical-VPL reflex loop. The results from the DCN confirm earlier studies suggesting a cuneate-cortical-cuneate reflex loop.     

Conduction in muscle afferent fibres during experimental demyelination in cat nerve

Summary Segmental demyelination has been produced in the dorsal root ganglion region of the eat by the induction of experimental diphtheritic polyneuritis. The axis cylinders, though often damaged, usually remained in continuity. There was a complete block of transmission in some fibres passing through such a region, although in other fibres it appeared that conduction continued, but at a reduced velocity.

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Zusammenfassung Mittels experimenteller diphtherischer Polyneuritis wurde an Katzen im Bereich der Hinterwurzelganglien eine segmentale Entmarkung herbeigeführt. Die Achsencylinder bewahrten dabei trotz häufiger Schädigung ihre Kontinuität. In einigen diesen Bereich durchziehenden Fasern kam es zu einer völligen Leitungsblockade, während sich in anderen Fasern die Leitfähigkeit, wenn auch mit verringerter Geschwindigkeit, als erhalten erwies.

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With 4 figures in the text     

Central projection of nuchal group I muscle afferent fibers of the cat

To investigate the central projection of nuchal group I afferent fibers of nerves which supply the complexus (COM), biventer cervicis (BIV), splenius (SP), and occipitoscapularis (OCC) muscles, experiments were conducted on cats anesthetized with chloralose-urethane. A total of 246 neurons was found to respond to electrical stimulation of those nerves. Of these neurons, 99.2% were located in the ipsilateral external cuneate nucleus (ECN) and 0.8% in the ipsilateral main cuneate nucleus. Ninety-seven neurons responded only to the BIV, 88 neurons only to the SP, and 56 neurons to the OCC alone. The number of monosynaptically activated neurons from group I fibers were found to be 26 in 97 neurons which responded to the BIV, 20 in 88 neurons responded to the SP, and 8 in 56 neurons which responded to the OCC. The ECN contains these neurons organized in a somatotopic manner. Neurons from distal muscle (the OCC) project to medial parts and from proximal muscle (the BIV) to more lateral parts of the nucleus. Forty-three of the 143 neurons in the ECN (30.1%) were activated antidromically to electrical stimulation of the ipsilateral inferior peduncle and anterior lobules IV and V of the cerebellum. Thalamic responses from nuchal muscle afferent fibers were recorded in a very narrow region of nucleus ventralis posterolateralis (VPL) which is situated dorsolaterally to the forelimb muscle afferent (deep radial nerve) projection area. The cortical evoked potentials from these nerve stimulations were observed in the anterior suprasylvian gyrus, areas 2 or 5, which we regarded as a transitional area between the second somatosensory and association areas, and the postcruciate dimple (PCD) or area 3a. Cortical potentials in the PCD were reduced after lesion of the VPL, where the focal potentials evoked by nuchal muscle afferent stimulation were recorded.