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.     

Static sensitivity of the primary endings of muscle spindles

Static discharges have been studied in 75 primary endings of the muscle spindle at the stepwise stretch of m. triceps surae in anesthetized cats. The sampled afferents possessed the conduction velocity between 72 and 115 M/s, high dynamic sensitivity and the static thresholds below 8 mm. The main firing rates (MFR) of the discharges were computed during 30 s after each step of the muscle stretch at 0.8 mm from complete slack up to 10 mm. The measurements of the MFR of the discharge were started 40 s after completing a dynamic phase of stretch. The following criteria of the "good" position sensitivity of the receptors were used: high probability of an increase in the MFR per step of stretch, a linear "muscle length-MFR" relationship, a wide range of position detection and high value of static and differential static responses. Only 30% of the analyzed primary endings are characterized by linear (up to 4.5 pps/mm) or by a monotonous increase of the MFR (up to 40 pps) with a probability of more than 0.7 per step of muscle stretch. The remaining primary endings detected the change in the stationary muscle length "poorly" because of the presence of nonsensitive areas which are conditioned either by high static thresholds or by lack of an increase in the MFR. The primary endings did not possess obvious length sensitivity.     

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.     

Effects of changes in pH on the afferent impulse activity of isolated cat muscle spindles

Muscle spindle activity has been shown to decrease in the sustained contracting muscle. The effect has been assumed to result from a declining fusimotor drive. Since accumulation of metabolites including H(+), lactate and CO(2) might also affect the receptor in the fatiguing muscle, the impulse activity of muscle spindles isolated from the cat tenuissimus muscle was characterized under varying degrees of extracellular pH, thus excluding any effect on fusimotor activity, blood supply and extrafusal muscle fibers. The isolated receptor was exposed to bathing fluids of pH 6.4, 7.4 and 8.4, and afferent discharge activity was recorded from the spindle nerve. Both primary and secondary endings responded similarly to changes in pH. Resting discharge frequency usually decreased with decreasing pH and increased with increasing pH. A sudden break-off in activity was observed with about 40% of primary endings and about 30% of secondary endings at pH 6.4. Experiments with slow stretch stimulation indicated that this effect was caused by a rising threshold of firing at the encoder site of the endings. With brief ramp-and-hold stretches, we tested the effects of changes in pH on the dynamic and static sensitivity of primary and secondary endings. When pH was reduced from 7.4 to 6.4, the initial burst activity at the beginning of the ramp phase increased in primary and secondary endings and the dynamic response increased in secondary endings, demonstrating that the dynamic properties of muscle spindle endings were usually augmented in the acidic milieu. The static properties rose as well because the static index of both types of ending increased significantly. By contrast, dynamic and static properties of both primary and secondary endings decreased significantly, when pH was increased from 7.4 to 8.4. The amplitude of tension that was measured during the passive stretch stimuli very slightly decreased in the acidic solution and very slightly increased in the alkaline solution. The decrease in the resting discharge activity at low pH supports those previous observations, which demonstrate a reduced peripheral input from muscle spindle afferents to the spinal motor nuclei during fatigue in the isometric contracting muscle. The present finding indicates that an attenuated afferent discharge is not only caused by a decreasing central activation of gamma-motorneurons, but may additionally be supported by a direct effect of protons on the muscle receptor itself. The accompanying augmentation of stretch sensitivity is suggested to correspond to the well-known increase in physiological tremor during exhaustive exercise.     

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.     

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.     

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.     

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 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.     

Attenuated dynamic responses of primary endings of muscle spindles: a basis for depressed tendon responses in acrylamide neuropathy.

Depressed or lost tendon reflexes commonly observed in patients with peripheral neuropathies may result partly from attenuation of the dynamic discharge from the primary endings of muscle spindles. This possibility was investigated in cats with an experimental neuropathy induced with acrylamide (30 mg/kg/day intramuscularly). Achilles tendon reflexes and the dynamic discharge from primary muscle spindles were evaluated after five or ten injections of acrylamide. After five injections the animals were moderately impaired neurologically, the Achilles tendon reflex was difficult to elicit in 2 of 5 animals, and the dynamic responses of primary endings of soleus muscle spindles to stretch were depressed. Following ten injections of acrylamide the cats were severely impaired neurologically; tendon responses were either absent or difficult to elicit, and the dynamic responses of their muscle spindles to stretch were reduced by 50% (p less than 0.01). Additionally, the spindels responded to stretch with only 20 to 30% the normal number of afferent impulses. These data suggest that lost tendon responses in acrylamide neuropathy result in part from inadequate activation of motoneurons by spindle afferent discharge.     

Crossed and uncrossed projections to cat sacrocaudal spinal cord: II. Axons from muscle spindle primary endings

The terminal fields of primary afferent fibers from tail muscle spindle primary endings were mapped within cat sacrocaudal spinal cord (S3-Ca7), using intra-axonal recording and horseradish peroxidase staining techniques. We sought to determine the ipsilateral and contralateral projection patterns and to relate these to the fibers' muscles of origin. Fifty-three group Ia fibers were successfully stained. Segmental collaterals originated from either the ascending or descending branch within the dorsal columns. Collaterals coursed rostromedially within the dorsal columns and traversed the medial aspect of the dorsal horn. Ipsilateral terminations were similar for all fibers. Within the ventral horn, boutons were consistently observed in the medial or central portions of lamina VII. In lamina VIII, a variable number of boutons was seen on fine branches emerging from larger fibers coursing ventrally. Clusters of terminals were plentiful in the regions of motoneurons, i.e., lamina IX and the nucleus commissuralis. Terminals were found in the adjacent white matter. In addition to ipsilateral terminations, some group Ia fibers (20 of 53) had collateral branches that crossed ventrally to the central canal, terminating within the midline ventral gray commissure and/or the contralateral ventral horn. Crossed projections always originated in medial (dorsal or ventral), but not lateral, muscles of the tail. These data suggest that ipsilateral projections of group Ia fibers make connections on sacrocaudal motoneurons, on neurons mediating segmental reflex functions and on neurons conveying ascending information. It is speculated that crossed and uncrossed connections between group Ia fibers from medial muscles and bilateral dendritic trees of motoneurons subserve synchronized co-contraction of synergistic muscles located on the two sides of the body, such as with dorsal or ventral flexion of the tail. Group Ia projections from lateral muscles, that are entirely ipsilateral, would be involved with lateral movements of the tail.     

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.     

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.