The structural and functional development of muscle spindles and their connections in fetal sheep

In this paper we have studied the structural and functional development of hindlimb muscle receptors and the connections of their afferent fibres in fetal sheep (n = 26) from 67–143 days of gestation (term = 146 days). By recording extracellular discharges in dorsal root ganglia (L7, S1) we have shown that muscle spindle afferents first respond to a ramp-and-hold stretch at mid-gestation ( 75 days). Silver-stained preparations of muscle spindles revealed that afferent fibres are just beginning to form annulospiral windings at this age. It therefore appears that the annulospiral formation is not a necessary requirement for the generation of the response. By 87–92 days some receptors had developed a discharge at resting muscle length. Discharges were generally more robust and easier to elicit and static and dynamic components could be identified in the response to stretch. Although static sensitivity was generally low it was more evident than dynamic sensitivity. By 107–115 days it was possible to clearly distinguish between muscle and tendon afferents and to tentatively classify muscle responses as originating from primary or secondary afferent spindle endings. With increasing gestational age there was a progressive increase in the length and complexity of the spindle innervation in parallel with the maturation of functional activity. Biocytin injections into the dorsal root ganglia revealed afferent projections to the motoneuron pools by 67 days. Silver-staining of muscles showed that innervation of extrafusal fibres was also present by this age. We therefore conclude that the neural pathways necessary for reflex activity involving muscle spindles are present and functional from early in gestation and could contribute to early fetal movements.     

Observations on static and dynamic responses of muscle stretch receptors in kittens

In 5-19-day-old kittens anesthetized with Saffan, the discharges of de-efferented triceps surae muscle receptors were recorded from afferent fibers in dorsal root filaments. The conduction velocities of the afferent fibers ranged between 7 and 30 m/s. Receptors were identified as spindles on the basis of their response to muscle stretch and, whenever possible, the pause in their discharge during muscle contraction. Spindles responding with sustained discharges to muscle extensions of less than 1 mm could be found in 5-8-day-old kittens, provided the muscles was left 'in situ', with intact skin, tendon and aponeuroses. However, such responses were rare before 10 days, and the majority of receptors essentially displayed phasic responses to muscle stretch, in agreement with previous observations. In responses to sinusoidal muscle stretching of 0.1-0.5-mm amplitude, kitten receptors were easily driven to discharge one impulse per cycle at frequencies of 10-15 Hz. But unlike adult spindles, they could not follow higher frequencies unless the amplitude of stretch was increased. The maturation of dynamic responses is slower than that of static responses in kitten spindles. In the first postnatal weeks, small changes in muscle length are poorly signaled to the central nervous system.     

Development of the sensory innervation of muscle spindles in the kitten.

This is a report of the changes observed in the pattern of sensory innervation of muscle spindles in hindlimb muscles of kittens during the first four weeks of life. The structural analysis, made on teased, silver-stained preparations, was complemented by a series of recordings of afferent responses of kitten spindles during ramp-and-hold stretches of the muscle. The primary endings of spindles from newborn animals showed a large degree of variability in their branching pattern and branches formed a network across the intrafusal fibres. In older animals there was less variability and lateral branches of stem axons began to encircle the intrafusal fibres. The process of maturation was characterized by a more uniform shape of the endings and more complete, evenly spaced, annulospiral terminals. Recordings of the responses of primary endings of spindles during muscle stretch showed that several features of the adult response were already present in the newborn, although the overall rate of discharge was very much lower. It was concluded that the changes observed in the structure of the sensory endings of kitten spindles did not have clearly identifiable physiological correlates. It appears that an annulospiral shape of the sensory terminals is not a necessary prerequisite for the generation of stretch responses. The predominant factor which appears to determine the responses of spindles to stretch is the maturity of the intrafusal fibres, in particular, the bag2 fibre.     

About the origin of cerebral somatosensory potentials evoked by Achilles tendon taps in humans

This study examines the effects of ischemic hypoxia and cooling of the leg, muscle contraction and vibration on cerebral potentials evoked by Achilles tendon taps and posterior tibial nerve stimulation to obtain indirect evidence leading to the identification of receptors activated by tendon taps. Experiments performed during ischemia of the leg showed that these receptors lie between the ankle and the knee. Cooling of the leg showed that they are located deep in muscles or bone. Experiments performed during vibration and muscle contraction suggest that muscle stretch receptors provide the afferent input responsible for Achilles tendon tap evoked potentials. All of these experiments point to primary muscle spindles in the proximal gastrocnemius-soleus muscle belly as the main source of afferent input for evoking cerebral potentials to Achilles tendon taps in humans.     

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.     

Muscle afferent potential (`A-wave'') in the surface electromyogram of a phasic stretch reflex in normal humans

The experiments reported in this paper tested the hypothesis that the afferent potential elicited by a tendon tap in an isometrically recorded phasic stretch reflex can be detected in the surface EMG of normal humans when appropriate techniques are used. These techniques involved (1) training the subjects to relax mentally and physically so that the EMG was silent before and immediately after the diphasic MAP which reflects a highly synchronous discharge of afferent impulses from low threshold muscle stretch receptors after a tendon tap, and (2) using a data retrieval computer to summate stimulus-locked potentials in the EMG over a series of 16 samples using taps of uniform peak force and duration on the Achilles tendon to elicit the tendon jerk in the calf muscles. A discrete, diphasic potential (`A-wave') was recorded from EMG electrodes placed on the surface of the skin over the medial gastrocnemius muscle. The `A-wave' afferent potential had the opposite polarity to the corresponding efferent MAP. Under control conditions of relaxation the `A-wave' had a latency after the onset of the tap of 2 msec, the peak to peak amplitude was of the order of 5 μV and the duration was in the range of 6 to 10 msec. Further experiments were conducted to show that the `A-wave' (1) was not an artefact of the instrumentation used, (2) had a threshold at low intensities of stimulation, and (3) could be reliably augmented by using a Jendrassik manoeuvre compared with the potential observed during control (relaxation) conditions. The results support the conclusion that the `A-wave' emanates from the pool of muscle spindles which discharges impulses along group Ia nerve fibres in response to the phasic stretch stimulus because the primary ending of the spindles is known to initiate the stretch reflex and the spindles can be sensitized by fusimotor impulses so that their threshold is lowered as a result of a Jendrassik manoeuvre. The finding has important implications for the investigation of the fusimotor system in intact man.     

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 Jendrassik manoeuvre on muscle spindle activity in man.

Twenty-eight mechanoreceptive units identified as primary or secondary spindle afferents were sampled from muscle nerve fascicles in the median, peroneal, and tibial nerves of healthy adult subjects. The responses of these units to sustained passive muscle stretch, to passive stretching movements, to tendon taps, and electrically-induced muscle twitches were studied while the subject performed repeated Jendrassik manoeuvres involving strong voluntary contractions in distant muscle groups. The manoeuvres had no effect upon the afferent spindle discharges as long as there were no EMG signs of unintentional contractions occurring in the receptor-bearing muscle and no mechanotransducer signs of unintentional positional changes altering the load on that muscle. Unintentional contractions in the receptor-bearing muscle frequently occurred during the manoeuvres, however, and then coactivation of the spindle afferents was observed. Multiunit afferent responses to Achilles tendon taps, led off from tibial nerve fascicles, were in a similar way uninfluenced by the Jendrassik manoeuvres, even when these resulted in marked reinforcement of the calf muscle tendon jerk. The results provide no evidence for fusimotor sensitization of spindles in muscles remaining relaxed during the Jendrassik manoeuvre, and reflex reinforcement occurring without concomitant signs of active tension rise in the muscles tested is presumed to depend upon altered processing of the afferent volleys within the cord.     

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.     

Morphological and functional demonstration of rat dura mater mast cell-neuron interactions in vitro and in vivo

The effects of changes in temperature on primary and secondary endings of isolated cat muscle spindles were investigated under ramp-and-hold stretches and different degrees of pre-stretch. Temperature-induced alterations of the discharge frequency were compared over a temperature range of 25–35°C. Both primary and secondary endings responded to warming with increasing discharge frequencies when the spindle was pre-stretched by 5–10% of its in situ length. The following differences between the temperature effects on primary and secondary endings were observed: (1) The temperature coefficients (Q₁₀) obtained from the discharge frequencies during the dynamic and static phase of a stretch were similar for endings of the same type, but they were larger in primary endings (range of Q₁₀: 2.3–3.3; mean: 2.9) than in secondary endings (range of Q₁₀: 1.6–2.2; mean: 2.0); (2) With primary endings, but not with secondary endings, the temperature sensitivity (imp s⁻¹ °C⁻¹) was larger during the dynamic phase than during the static phase of a stretch; (3) In primary endings, the fast and slow adaptive components occurring in the discharge frequency during the static phase of a stretch clearly increased with warming while in secondary endings, the slow decay was less affected, and the fast decay showed no change; (4) In relaxed spindles, the excitatory effect of warming was overlaid by a strong inhibitory effect as soon as the temperature exceeded about 30°C, resulting in an abrupt cessation of the background activity in most secondary endings, but not usually in primary endings. In general, warming induced an enhanced stretch sensitivity in both types of ending, and additionally an inhibitory effect that is obvious only in secondary endings of relaxed spindles. The different effects of temperature on the discharge frequency of primary and secondary afferents are assumed to be caused by different properties of their sensory membranes.     

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.     

Orofacial motor control impairment in Parkinson's disease

Motor impairments in lip, jaw, and tongue muscles in patients with idiopathic Parkinson's disease (PD) were quantified. These orofacial muscle groups are nonuniformly endowed with muscle afferents, thus permitting evaluation of the hypothesis that certain PD motor symptoms are due to aberrations in muscle afferent function. Tongue muscles, devoid of stretch reflexes, were most impaired, while jaw-closing muscles, with numerous spindles and a monosynaptic stretch reflex, manifested the least impairment. Seemingly, PD motor impairments are independent of fusimotor or muscle afferent dysfunctions.     

Effect of ethanol on stretch response of muscle receptors in cats

The response of muscle spindles and tendon organs to steady and sinusoidal muscle stretch was investigated at different blood alcohol concentrations (BAC). After initial anesthesia (pentobarbital), cats were spinalized, the lumbar ventral roots cut and the gastrocnemius muscles of one hindleg prepared for controlled stretching. The animals were paralyzed and artificial respiration was applied. Action potentials from isolated Ia/Ib/II afferent fibers could be recorded.Under steady stretch conditions, all fibers responded to an increasing BAC with an increase in firing rate. This could be observed already at 0.8 mg/ml BAC. The increase in discharge rate reached at the most 80 imp/s. During intoxication the regularity of firing was higher than in the no-alcohol situation. At blood alcohol concentrations higher than 5 mg/ml, the neuronal activity suddenly dropped to zero, exhibiting an irregular impulse pattern. The increase in discharge rate at steady stretch is regarded to be of minor significance in the explanation of the impairment of motor performance under ethanol.When sinusoidal stretch was applied, the increase in the mean discharge rate was smaller than at steady stretch conditions. Up to about 10 mg/ml BAC the periodical modulation of firing rate during sinusoidal stretch of a large amplitude remained mainly unchanged. After the discharge rate had dropped to zero for the steady stretch condition at high BAC, elicitation of action potentials was always possible using dynamic stretch.     

Static and dynamic fusimotor action on the response of spindle primary endings to sinusoidal stretches in the cat

In anaesthetized cats, the discharge of single spindle primary endings of flexor and extensor muscles was studied during sinusoidal stretches of the muscle (3–4 mm peak-to-peak, 1–5 Hz). In the response to each stretch, two main components could be distinguished; they were related respectively to the speed and to the amplitude of lengthening. By analogy with the usual definitions of the ‘dynamic index’ (d) and ‘position sensitivity’ (s) during ramp stretches of the same amplitude at a velocity of 35–60 mm/sec, a ‘sinusoidal dynamic index’ (D) and a ‘sinusoidal position sensitivity’ (S) were defined. Stimulation of dynamic fusimotor fibres increased D and d, S and s, while stimulation of static fusimotor fibres decreased D and d and had a less consistent effect on S and s. The total frequency variation during a sinusoidal stretch was always increased by a dynamic fusimotor action while it could be either decreased or increased by a static fusimotor action, depending on the stretch frequency and on the rate of fusimotor stimulation. These results were obtained in various muscles, without any significant difference related to their flexor or extensor function. It is concluded that in order to identify a fusimotor effect as static or dynamic during sinusoidal muscle stretch, it is necessary to use, as here, stretch parameters which elicit a dynamic component in the primary afferent response.     

Summing responses of cat soleus muscle spindles to combined static and dynamic fusimotor stimulation(1)

This is a study of the summation of responses of primary endings of muscle spindles to combined static and dynamic fusimotor stimulation in the soleus muscle of the anaesthetised cat. Summation, expressed as a summation coefficient, K, was measured under a variety of conditions including (1) at several, fixed muscle lengths using steady rates of stimulation, (2) using ramp-shaped increases in stimulation rates, (3) during passive stretches after muscle conditioning, and (4) during combined stretch plus stimulation. The predominant effect observed was occlusion, that is, the combined response was less than the sum of the two individual responses. The calculated mean K value for responses at fixed length was 0.156 (+/-0.005 S.E.M.). It was hypothesised that summation arose from electrotonic spread of generator current between the afferent terminals, either directly, or as a result of mechanical interactions between the contracting intrafusal fibres. Summation for responses from pairs of static fusimotor fibres gave a larger mean K value, 0.340 (+/-0.020 S.E.M.). These findings were interpreted in terms of a model of the muscle spindle where responses to dynamic fusimotor stimulation arise at one impulse generating site, and static fusimotor responses arise at another.     

Discharge characteristics of spindle receptors located in different parts of the soleus muscle in the cat

A procedure is described for precise location of the capsula of a muscle spindle within the muscle by a restricted coagulation which abolishes the discharge of the spindle afferent. The point for coagulation was approximately determined by careful stretching of a little portion of muscle fibres, by slight pressure on the external and internal surface of the muscle, by intramuscular and external electric stimulation. The analysis of the dynamic and static sensitivity of 48 units shows that the differences in responses to stepwise stretching existing among both primary and secondary endings do not depend on the location of muscle spindles in different parts of m. soleus, but probably reflect variations in the density of afferent terminal contacts in 3 types of intrafusal muscle fibres.     

The responses of muscle spindles to small, slow movements in passive muscle and during fusimotor activity

We have previously shown that movement detection thresholds at the human elbow joint were less than a degree of joint rotation in the passive limb but were higher if they were measured while subjects co-contracted elbow muscles [A.K. Wise, J.E. Gregory, U. Proske, J. Physiol., 508 (1998) 325-330]. Here we report observations on the responses of muscle spindles of the soleus muscle of the anaesthetised cat to determine their ability to signal small length changes in the passive muscle and during a contraction, under conditions resembling those of the human experiments. After appropriate conditioning of the muscle to control for history effects, primary endings of muscle spindles showed thresholds to ramp stretch at 20 micrometers s-1 of between less than 5 micrometers and 15 micrometers, which translates to 0.05 degrees -0.15 degrees of human elbow joint rotation. Thresholds were much higher following conditioning to introduce slack in the muscle. Since during a voluntary contraction there is likely to be alpha:gamma co-activation, responses of spindles were also recorded during slow stretches (100 micrometers at 20 micrometers s-1) during static fusimotor stimulation, dynamic fusimotor stimulation, combined fusimotor stimulation and fusimotor plus skeletomotor stimulation. Invariably, responses to passive stretch were larger than during motor stimulation. It is concluded that spindles are sensitive enough to signal fractions of a degree of elbow joint rotation and that the rise in threshold observed during a voluntary contraction may be accounted for by the actions of fusimotor and skeletomotor axons on spindle stretch responses.     

Reinnervation and recovery of cat muscle receptors after long-term denervation

After nerve injury muscles remain denervated until axons return to begin reinnervation and recovery. The delay between injury and recovery in human limb nerves averages 13 weeks after crush, and 16 weeks after transection and suture. In order to assess the effects of such long denervation periods on the recovery of cat muscle receptors, we crushed the common peroneal nerve and denervated peroneus brevis for 10 to 134 days; 39 days were allowed for reinnervation in each experiment. After 50 days denervation, the mean number of terminal bands in the regenerated spindle primary endings was 10.3 compared with a normal mean of 29.0. After 134 days, the mean was 0.6 and spindles were severely atrophied. Despite this most spindle afferent fibers continued to respond normally to ramp-and-hold stretch, abnormal responses being recognized as those that failed to maintain firing during the held phase of the ramp. After 50 days, 21% of spindle afferent fibers responded abnormally and about this proportion did so after all the longer denervation periods. Maximum afferent firing rates were all significantly lower than normal, and many afferent fibers fatigued more rapidly. Tendon organs were atrophied after 113 and 134 days and received fewer terminals, but their afferents fired apparently normally during muscle twitch. These results imply that the consequences of long-term denervation on human muscle spindles would be unlikely to affect the overall pattern of response to stretch of any Ia or II afferent fibers reinnervating them, though the quality of their response might be impaired.     

Adaptation to long term stretch in the passive discharge of muscle spindles.

The effect of long-term stretch on the discharge of muscle spindles was studied by placing a cast on one hindlimb of cats so as to keep the calf region under moderate extension and after 2 to 6 weeks comparing responses of gastrocnemius afferents on casted and unrestrained sides to step wise extension of the muscle. The mean slope of the frequency-length relationship for populations of afferent fibers from immobilized limbs showed mixed effects. The slope was less in three animals, greater in three, and not significantly different in four. Three cats in which the calf region had been under stretch for only 3 to 5 days also demonstrated reduced sensitivity. In contrast, all cats in an earlier series in which the leg had been casted for 2 to 7 weeks with the calf muscles at resting length, showed greater stretch sensitivity. It is concluded that the spindle receptor adapts to long-term stretch of the muscle with a decrease in stretch sensitivity.     

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.