Receptor potential and spike initiation in two varieties of snake muscle spindles.

1. Receptor potentials, in response to ramp-and-hold stretch, have been recorded from two varieties of snake muscle spindles. 2. The two types of spindles have a similar sensitivity of impulse discharge to amplitude of receptor potential during the static phase of stretch. 3. Receptor potentials from short-capsule spindles show a high dynamic sensitivity to velocity of stretch. Amplitude of dynamic receptor potentials is well related to frequency of dynamic discharge except beyond a certain velocity of stretch where the frequency deviates progressively more than expected from linearity. 4. Receptor potentials from long-capsule spindles show a low dynamic sensitivity to velocity of stretch and amplitude of dynamic receptor potentials is well correlated with dynamic firing frequency. 5. The threshold level of receptor potential for initiating spike discharge varies with the velocity of stretch, the relation being similar for the two types of spindles. 6. It is concluded that the basis for functional differentiation of snake spindles may lie in the mechanism by which deformation of sensory endings is transformed into receptor potential. 7. Late adaptation of impulse discharge, a characteristic feature of the response of the short-capsule spindle to maintained stretch, has been related to length changes of the sensory region measured directly with Nomarski optics. The linear relation found between the slow adaptive fall of impulse discharge and the simultaneous shortening of the sensory region strongly suggests a mechanical basis for the late adaptation.     

Ultrastructural changes of the sensory nerve terminals in frog muscle spindle during dynamic stretch

Summary The sensory ending of the frog muscle spindle consists of bulbous swellings interconnected by thin, tube-like axonal branches. This study was made to determine if the bulb or thin tube regions are deformed to the same degree during dynamic stretch, by comparing spindles prepared in the relaxed and stretched states. Isolated muscle spindles were rapidly frozen, either in a completely relaxed state or at the end of dynamic stretch to 130% of thein situ length. Longitudinal sections for ultra-high-voltage electron microscopy revealed that dynamic stretch caused a decrease of 61.7% in bulb diameter, an increase of 88% in bulb length and an increase of 34.6% in bulb volume. The cross-sectional areas of the nonmyelinated terminals in the reticular and compact zones measured in electron micrographs decreased by 79.2% in the bulb region and 66.7% in the tube region during dynamic stretch.     

Mammalian muscle spindle: peripheral mechanisms

The responses of sensory endings of the muscle spindle to stretch are produced by transduction in the sensory terminals and by impulse initiation in the sensory axon, both of which appear to be largely linear and non-time-dependent processes. The marked nonlinearity of spindle responses to length, the processes of gain compression, and the aftereffects of fusimotor activity and of stretch appear to reside mainly in the mechanical properties of the intrafusal fibers. Although the basis of the dynamic sensitivity of the primary ending in the passive spindle is still not well understood, dynamic fusimotor effects have been shown to depend on activation of the bag 1 fiber. Static fusimotor actions result from contraction in the bag 2 and/or chain fibers. Certainly, a great deal is known about the muscle spindle at the level of changes in sensory discharge to variations in muscle length and to fusimotor stimulation, although new insights continue to arise from experiments of this type. However, there is a need for further quantitative information that will lead to greater understanding of transduction mechanisms, impulse initiation, and intrafusal fiber contractile activation.     

Slowing of the discharge of secondary endings of cat muscle spindles during fusimotor stimulation

Summary Responses of secondary endings of muscle spindles of the peroneus tertius muscle of the anaesthetized cat have been recorded during repetitive stimulation of functionally single fusimotor fibres that produced slowing of the discharge. In a sample of 125 pairs of single fusimotor fibres and secondary spindle afferents 5 examples of slowing were seen. The amount of slowing became less at longer muscle lengths. Conditioning the spindle by stimulating the muscle nerve at fusimotor strength, at a length 2.5 mm longer than the test length, and then returning to the test length 3 seconds later led to a greater degree of slowing of the discharge than after conditioning stimulation at the test length. With one exception, responses to muscle stretch were reduced during stimulation of a fusimotor fibre that produced slowing. On two occasions stimulating a fusimotor fibre that produced slowing of the response of one secondary ending, led to excitation of two other endings. Two possible explanations for the generation of slowing responses have been considered. The first is that the slowing is the result of contraction of the region of intrafusal fibre directly underlying the secondary sensory ending. The second, which we favour since it accounts for the facts more adequately, is that slowing is the result of shortening of the region of nuclear chain fibres on which the sensory ending lies, produced by movement in an adjacent nuclear bag fibre.     

A model of spindle afferent response to muscle stretch

1. A unified model of the properties of stretch responses of mammalian spindle endings is proposed. This model encompasses the disparity between sensitivity of spindle endings to small and to large stretch of the muscle as well as the disparity in their dynamic responsiveness for different amplitudes of stretch. 2. In the model the mechanical properties of intrafusal fibers include a property akin to friction, which is hypothesized on the basis of reported observations on amphibian muscle. Transducer and encoder processes are modeled in the light of recent observations on isolated spindles. The model involves five unknown parameters whose values are selected by reference to certain reported observations on deefferented primary and secondary endings. The model can be used to predict responses to length changes of arbitrary time course. 3. Predicted responses to large ramp-and-hold stretch are quantitatively comparable to observations over a wide range of stretch velocities. The quantities compared include the increment in response during ramp stretch as well as the dynamic index, which is a measure of adaptation at stretch plateau. 4. At a fixed frequency of sinusoidal stretch, the relation between amplitudes of stretch and response is predicted in quantitative agreement with measurements. As the frequency of stretch is decreased, the predicted phase lead decreases and then increases, while the sensitivity decreases monotonically, in accord with observations. 5. In the model the high sensitivity for small stretch is not specific to any particular length of the muscle. When stretch is large, the region of high sensitivity is gradually reestablished at the new length, a phenomenon referred to as resetting. The dynamic response to a large stretch can be seen as arising, for the most part, from the dynamic process of resetting. 6. The influences of static or dynamic fusimotor activation on stretch responses of the primary ending are simulated by modifying the parameter values in the model. The modifications are such that static (dynamic) fusimotor activity speeds up (slows down) the resetting of the high-sensitivity region. The predictions mimic qualitatively the observed fusimotor effects not only on the response to large ramp stretch but also the contrasting effects seen with smaller, sinusoidal stretch.     

Responses of primary and secondary endings of isolated mammalian muscle spindles to sinusoidal length changes.

1. Responses of primary and secondary endings of isolated cat spindles to sinusoidal length changes have been recorded before and after block of impulse activity by tetrodotoxin. 2. Primary endings may discharge with each cycle of sinusoidal stretch at 25-50 Hz, with stretch amplitudes applied to the spindle poles as small as 1 micron. Thresholds are higher at lower frequencies. 3. In primary endings, amplitude of the receptor potential varies with frequency and magnitude of sinusoidal stretch. At a given stretch amplitude, the receptor-potential response increases markedly between 1 and 10 Hz. At a fixed frequency, for example, at Hz, the response to graded amplitude of sinusoidal stretch is highly nonlinear, sensitivity decreasing with large amplitudes. 4. Secondary endings show a much higher threshold than primary endings to sinusoidal stretch. Thus, at 25 Hz, secondary endings required stretch amplitudes of 50-100 micron to evoke discharge. Relatively large amplitudes of stretch were also required to evoked detectable receptor potentials. Over the range studied, the receptor potential varied more linearly with stretch amplitude in secondary than in primary endings.     

A determination of static mechanical properties of intrafusal muscle in isolated cat muscle spindles.

The sensitivity of mammalian muscle spindles to stretch is greater in a stretched muscle than in a slack one. We have investigated this behavior in isolated muscle spindles removed from cat tenuissimus muscle. We measured the steady-state strain of intrafusal muscle in sensory and non-sensory regions and found that there is a proportional relationship between sensory strain and receptor sensitivity; both increase with spindle length. By comparing intrafusal strain of sensory and non-sensory areas with and without an intact spindle capsule, we conclude that capsule does not contribute to the non-linear sensitivity. Measurements of steady-state tension indicate that the striated portions of the intrafusal muscle exhibit a non-linear stiffness which can quantitatively account for the observed behavior.     

The muscle spindles in slow and twitch skeletal muscle of the lizard

1. Responses from stretch receptors, identified as muscle spindles, were recorded in filaments of the nerve supplying a twitch muscle, semimembranosus, and a slow muscle, semitendinosus in the lizard Tiliqua.2. While recording afferent discharges in one filament of the motor nerve, several adjacent filaments were each in turn stimulated repetitively until one was encountered which on stimulation produced a powerful increase in spindle firing. Such an effect of the motor stimulus was interpreted as resulting from intrafusal contraction. Any interference with spindle firing patterns from extrafusal contraction produced by the motor stimulation was removed by differentially blocking the contraction with the drug curare.3. Discharge patterns of spindles in response to a slow stretch of the muscle were compared with the response to the same stretch, but during repetitive stimulation of the motor nerve filament which produced an intrafusal contraction.4. At the initial length, the firing rate of spindles in the twitch muscle was greatly increased by the motor tetanus. There was little further increase in the response during and following stretch of the muscle.5. While the spindles in the slow muscle were only moderately excited by the motor tetanus at the initial length of the muscle, a large increase was recorded during the dynamic component of the stretch. At the new length, the steady-state firing continued at a rate well above that for the initial length.6. The effect of the motor tetanus on the response to stretch of muscle spindles in the slow muscle could be mimicked by adding succinyl choline (5 mug/ml.) to the perfusion solution. Spindles in the twitch muscle did not show a sustained sensitivity to the drug.7. It is suggested that while the different effects of motor stimulation on the responses to stretch of spindles in slow and twitch muscle can be explained by propositions based on the sliding filament theory of contraction, the sustained elevation, at the new length, of firing frequencies of spindles in slow muscle might require an additional explanation.     

A `late supernormal period' in the recovery of excitability following an action potential in muscle spindle and tendon organ receptors

1. Discharge patterns have been recorded from five types of stretch receptor; frog muscle spindles, lizard tendon organs, cat soleus tendon organs and primary and secondary endings of cat soleus muscle spindles.2. The fully adapted discharge of each type of receptor is irregular, especially for frog spindles and primary endings of cat spindles as compared with the other three types (the ;regularly firing' receptors). Frog spindles and some cat spindle primary endings would maintain a discharge at very low mean rates (1/sec or less) while the remaining receptors would stop suddenly, as soon as their rate of discharge fell below a critical value characteristic for each individual ending.3. This pattern of discharge suggests that there is a peak in the excitability of ;regularly firing' receptors at a time following a preceding impulse, which corresponds to the intervals between impulses at each particular receptor's slowest rate of maintained firing, and that the excitability subsequently falls again. Primary endings of cat muscle spindles also showed some evidence of such a ;late supernormal period', but frog spindles did not.4. Direct evidence for the ;late supernormal period' was obtained from experiments in which a maintained discharge was restarted by an antidromic action potential in a receptor which had stopped firing, and to which had been applied a stretch just too small to restart the discharge.5. It is shown in an Appendix that a model receptor in which the recovery of excitability following an impulse has a hyperbolic time course, and in which Gaussian distributed noise is superimposed on the generator potential, can have a discharge pattern very closely resembling that of a frog spindle (cf. Buller, 1965).6. After addition of a late supernormal period to the model, its discharge pattern could mimic closely that of a lizard or cat tendon organ, or of a secondary ending of a cat spindle.     

Length changes within isolated frog muscle spindle during and after stretching

1. The length changes within the frog muscle spindle during stretch have been studied by stroboscopic photomicroscopy. Attention was focused on the length changes within the central reticular zone and these changes were related to the features of the receptor potential.2. It was found that the length changes of the central reticular zone closely followed the applied stretch in time course and magnitude. The results suggest that the length changes of the polar zones are generally similar to those in the central zone.3. There was no evidence of a relative shortening of the central zone in the early phase of maintained stretch, corresponding to the decline of the receptor potential from its dynamic peak to the static level.4. Following release of stretch the central zone returned to its original resting length within a few msec. The rapid return of the spindle was in sharp contrast to the relatively slow exponential decay of the receptor potential. With strong or prolonged stretches the return became slower and resting length was not completely restored until 100-150 msec after release of stretch. No corresponding change in the decay of the receptor potential was seen.5. The results suggest that the early adaptive fall of the receptor potential is not related to differential length changes between the central zone and the polar zones. It seems more likely that the contribution of mechanical factors to the early adaptation of the frog spindle have to be sought at the ultrastructural level.6. The finding that the length changes closely follow the applied stretch suggests that the stimulus in terms of lengthening is transmitted to the endings with little distortion.7. The results suggest that the elastic elements play a dominant role for the transmission of the stimulus to the endings and for the return of the spindle to resting length after release of stretch.     

Changes in size of the stretch reflex of cat and man attributed to aftereffects in muscle spindles

1. This is a report of experiments carried out on the cat and on man, which demonstrate that conditioning of a muscle by contraction and movement can lead to changes in amplitude of stretch reflexes elicited in that muscle. 2. In triceps surae of the cat, the reflex response to a brief stretch was recorded after conditioning with a whole-muscle contraction followed by a pause at a length either 5 mm longer or shorter than the length at which the reflex was elicited. Following conditioning at the long length the reflex response was less than half as large as that following conditioning at the short length. 3. The changes in reflex amplitude could be correlated with an altered stretch responsiveness of muscle spindles in the soleus muscle. When the muscle had been held long during conditioning, a subsequent brief stretch applied at an intermediate length elicited fewer impulses in primary endings of spindles than after conditioning at a short length. 4. The same kind of experiment was then carried out on adult human subjects. When a tendon tap was applied to the Achilles tendon after a voluntary contraction and relaxation of triceps surae with the muscle at a long length, (foot dorsiflexed) the reflex was frequently less than half the size it had been after a contraction at a short length (foot plantarflexed). It was concluded that the same kind of spindle aftereffects as observed for cat soleus spindles were responsible for the changes in reflex amplitude. 5. It was found both in the cat and in human subjects that the changes in reflex amplitude after conditioning became progressively less as the test length was made longer. 6. The explanation put forward to account for these observations is that stable cross-bridges form between actin and myosin filaments of passive intrafusal (and extrafusal) fibers. When the muscle is shortened several seconds after a contraction at a long length, the intrafusal fibers, stiffened by the presence of cross-bridges, fall slack. Slack does not develop after a contraction at a short muscle length, as the fiber is stretched to the test length. Since any slack must first be taken up by the test stretch, there is a smaller afferent response and consequently a smaller reflex contraction in response to a tendon tap after conditioning at a long length.(ABSTRACT TRUNCATED AT 400 WORDS)     

The responses of muscle spindles in the kitten to stretch and vibration

Summary Discharges of muscle spindle afferents from the soleus muscle were studied in kittens aged 1–21 days and in adult cats. Vibration applied longitudinally to the tendon elicited one impulse for each cycle of vibration over the range 1–200 Hz for the kittens and up to 450 Hz for the adult. Threshold amplitudes were generally higher in the kitten than in the adult. In response to large ramp and hold stretches applied at long muscle lengths kitten spindles showed rate saturation during the length change. Dynamic index, that is the peak rate during the length change minus the rate at the final length became progressively smaller at longer muscle lengths. No sign of saturation was seen at comparable muscle lengths in the adult. It is suggested that in the newborn the bag₁ intrafusal fibre is not functional and that the dynamic response is produced only by the afferent terminals on the bag₂ fibre. Another difference between kitten and adult was the length sensitivity measured under dynamic conditions. This increased much more steeply with stretch rate in the kitten. One possible explanation for the higher dynamic length sensitivity is a lack of elastic fibres surrounding intrafusal fibres of immature spindles.     

Ultrastructural identification of the primitive muscle spindle in the Xenopus laevis larvae

Summary The initial formation of muscle spindles was studied with electron microscopy using the toe muscle of Xenopus laevis. At the larval stage 57 (Nieuwkoop and Faber 1967), muscle spindles were first identified primarily by the presence of sensory endings associated with a thin bundle of myotubes, e.g. intrafusal (IF) myotubes which were partly invested by a single cellular layer. The number of IF myotubes per spindle was 5 to 6; the adult complement. IF-and extrafusal (EF) myotubes were almost identical in their size and structure. A few thinner IF myotubes with scaree myofibrils were also present. The reticular zone had been undeveloped. Sensory endings were smaller in size and in number per spindle than those in the adult, forming irregular beaded chains with occasional tubular expansions. The endings and IF myotubes were rarely in direct contact, being frequently interposed by a satellite cell and its process. Incipient fusimotor endings were widely distributed from the juxta-equatorial to the polar region. Large cored vesicles resembling the neurosecretory vesicles occurred in sensory and motor endings as well as in intramuscular nerve fibers. The vesicles may be involved in the neuronal influence upon the spindle differentiation.The results were compared with the formative process of mammalian spindles.     

An ultrastructural study of nerve terminal degeneration in muscle spindles of the tenuissimus muscle of the cat

Summary Muscle spindles in the tenuissimus muscle of the cat were studied between 12 and 168 h after cutting or freezing the nerve to this muscle. Degenerative changes in sensory and motor nerve terminals on intrafusal muscle fibres were observed using the electron microscope. Comparisons were made with spindles from unoperated or sham-operated cats.The earliest degenerative changes were seen in sensory and motor terminals at 20–24 h after the lesion. No nerve endings were seen by 114 h after denervation. The most consistent initial signs of degeneration were: (1) the presence of abnormal mitochondria and dense bodies in sensory terminals, and (2) a decrease in the number and clumping of synaptic vesicles combined with an increase in glycogen and neurofilaments in motor endings. Intrafusal fibres participate in the removal of degenerating sensory endings. Schwann cells phagocytose degenerating motor terminals. The disappearance of nerve terminals precedes the complete degeneration of preterminal myelinated fibres within the muscle spindle.     

Initial burst of primary endings of isolated mammalian muscle spindles.

The initial burst has been studied in primary endings of isolated mammalian muscle spindles subject to controlled ramp-and-hold stretch. Near the onset of ramp stretch the primary ending discharges at a frequency dependent on stretch velocity. The initial burst is reduced or abolished by repetitive stretch. After block of impulse activity by tetrodotoxin, the receptor potential of primary endings shows an initial component, a rapid depolarization which occurs near the onset of ramp stretch at the same time as the initial burst. This initial component depends, in rate of rise and amplitude, on stretch velocity. It is also reduced or abolished by repetitive stretch. Recording of tension development by the isolated spindle in response to ramp-and-hold stretch shows an early rise in tension associated with the initial burst and the initial component of the receptor potential. This tension rise is also dependent on stretch velocity and is reduced or abolished by repetitive stretch. The results provide direct evidence that the initial burst results from mechanical factors, probably from cross bridge formation between thick and thin filaments as has been suggested (3).     

Fusimotor axons in the kitten

In kittens 1- to 23-days old growth of axons in the soleus nerve has been studied using the structural parameters nerve length, internodal length, and axonal diameter. In addition, single functional fusimotor axons were isolated in lumbosacral ventral roots, and the responses of muscle spindles in soleus were studied during fusimotor stimulation. While nerve length over the soleus nerve to lumbar spinal root increased from 41 to 76 mm during the 22 days, mean internodal length increased from 250 to 410 microns. Mean axon diameter increased from 2.1 to 4.1 microns. In the youngest animals values for both internodal length and axon diameter were distributed uniformly about the mean. From day 11 onward the distributions became bimodal, including a growing number of new axons in the small-myelinated range. Filaments of ventral root were isolated that on repetitive stimulation had a specific excitatory effect on the discharge of muscle spindles. The responses could be attributed to axons that were not associated with measurable tension and were therefore likely to be fusimotor fibers. Measurements of the conduction velocity of skeletomotor and fusimotor axons showed that conduction speed increased progressively with age for both groups, but the rate of increase was more than three times faster in the most rapidly conducting skeletomotor axons compared with the fusimotor axons. The distribution of conduction velocities for fusimotor fibers showed two peaks, one in the range typical for conduction in unmyelinated fibers, 0.5-1.0 m/s, the second at 3-4 m/s. The small number of values in the range of 1-2 m/s was attributed to the process of myelination. It is suggested that conduction speed increases discontinuously over this part of the range as impulse conduction changes from continuous propagation to saltatory transmission. Eighteen fusimotor axons could be classified as having either a static or a dynamic action on spindle discharge. Repetitive stimulation of fusimotor fibers during a ramp-and-hold stretch of the muscle produced a characteristic response. Fibers classified as dynamic had little effect on the response of the spindle when the muscle was held at a particular length but greatly increased the response during a length change. Static fusimotor fibers, on stimulation, increased the response of the spindle at constant length but did not evoke a selective increase during the length change. Both kinds of fusimotor effects were characterized by overall low firing rates.     

The contribution of mechanical factors to the early adaptation of the spindle response

1. Adaptation in terms of the early fall of the receptor potential was studied in isolated frog spindles. The contribution of gross mechanical changes to the decline of the response was determined by comparing the responses obtained under constant length and under constant tension.2. It was found that the early adaptation under constant stretch increased with increasing lengthening of the spindle for stretches up to 25-30% of the resting length and decreased with still stronger stretches. When the spindle was stretched by 100% or more the static phase of the receptor potential reached nearly the same height as the dynamic peak and the early adaptation approached zero.3. The early adaptation decreased with decreasing velocity of linearly rising stretch and approached zero for stretches below about 0.5 mm/sec.4. For different strengths of a steplike stretch the amount of early adaptation was linearly related to the fall in tension over the same period. The relative amount of tension fall, however, was always less than the corresponding fall of the response.5. The early adaptation was 15-20% smaller under constant tension than under constant length for stretches below the level giving the maximum dynamic peak.6. The results suggest that a comparatively small amount of the early adaptation of the spindle response to constant stretch is related to gross alterations in length in different regions of the spindle. The main part of the adaptive fall of the response is probably related to functional properties of the sensory membrane and to the ionic mechanism underlying the production of the receptor potential.     

Impulse rates and sensitivity to stretch of soleus muscle spindle afferent fibers during locomotion in premammillary cats

Impulse from soleus muscle afferents were recorded in premammillary cats that were walking on a treadmill. In normal walking the effects of gamma-motoneurons on impulse rates of muscle spindle afferents are confounded by the effects of the large length changes that occur. To isolate the effects of gamma-motoneurons the leg was fixed in place for recording and denervated except for soleus muscle. Because gamma-motoneurons produce marked effects on the stretch sensitivity of muscle afferents, soleus muscle was oscillated about a present length so the stretch sensitivity of its afferents could be determined. The impulse rate of secondary muscle spindle afferents in soleus muscle was generally increased at all phases of the step cycle. The mean rate approximately doubled during walking (82 imp/s), compared with nonwalking (rest) periods (44 imp/s). The sensitivity to sinusoidal length changes was generally reduced throughout the step cycle (mean reduction = 33%). Primary muscle spindle afferents also showed an increased mean rate during walking (47 imp/s) compared with rest (24 imp/s). The impulse rate peaked after the muscle reached its maximum force and often showed a second peak before the maximum electromyogram (EMG) activity. The sensitivity to sinusoidal stretches varied cyclically during locomotion. During the extension phase it sometimes exceeded the resting value, but was greatly reduced during the flexion phase (mean reduction = 49% over whole cycle). Control experiments were carried out in which static and dynamic gamma-motoneurons were stimulated and activity from muscle spindle afferents was recorded in anesthetized cats. With the amplitude and frequency of stretch applied, stimulation of dynamic gamma-motoneurons usually increased and stimulation of static gamma-motoneurons usually decreased the sensitivity of primary muscle spindle afferents to sinusoidal stretch. The patterns observed in muscle spindle afferents suggest a strong, maintained activation of static gamma-motoneurons throughout the step cycle and a phasic activation of dynamic gamma-motoneurons, which is consistent with previous direct recordings from gamma-motoneurons. With this pattern of activating gamma-motoneurons, the secondary muscle spindle afferents will provide a good feedback signal of the large length changes that normally occur in the muscle during locomotion. The changes in sensitivity of primary muscle spindle afferents will complement central changes so the gain of the stretch reflex from extensors is high during extension (when required to help support the weight of the body) and low during flexion (when a high gain would be counterproductive).     

Impulse activity and receptor potential of primary and secondary endings of isolated mammalian muscle spindles.

1. An isolated muscle spindle preparation from a tail muscle of cat is described. The afferent response to a ramp-and-hold stretch was recorded in individual axons from identified primary and secondary endings. 2. Primary endings exhibit a prominent dynamic response, including an initial burst. They also show a well-maintained static discharge. Secondary endings also show a well-sustained static discharge but generally have a much lower dynamic sensitivity. The response of primary and secondary endings of the isolated spindle are similar to the typical responses seen in vivo in groups Ia or group II afferent fibres respectively. 3. Following impulse blockade by tetrodotoxin, the receptor potential was recorded from primary and from secondary endings in response to ramp-and-hold stretch. 4. During the dynamic phase the receptor potential of primary endings consists of a depolarization which has two components. (a) An initial component occurs early during ramp stretch, depends in rate of rise and amplitude on velocity of stretch and is reduced on repetitive stretch; it appears to be responsible for the initial burst. (b) A late dynamic component, which follows, is also dependent on stretch velocity and produces the late dynamic discharge. At the end of ramp stretch the receptor potential falls, and may undershoot, the static level. There is a subsequent adaptive fall during hold stretch, then a maintained static level of receptor potential. On release from stretch the membrane is hyperpolarized. 5. Secondary endings usually show a smaller dynamic response, lacking the initial component seen in primary endings. They also generally lack an undershoot following the ramp and have less of a post-release hyperpolarization. 6. Static levels of receptor potential in both primary and secondary endings are related to amplitude of stretch. 7. The receptor potentials of primary and secondary endings account for the major features of the impulse responses of these endings to ramp-and-hold stretch. In primary endings the dynamic frequencies may also depend upon a sensitivity of the impulse initiating site to rate of change of receptor current.     

The responses of frog muscle spindles during stimulation of slow motor axons

Summary Responses of muscle spindles of the iliofibularis muscle of the frogLitoria aurea have been recorded during single shock and repetitive stimulation of single functional motor axons. Repetitive stimulation of axons which innervated slow muscle, and on four occasions, axons which innervated twitch muscle, produced a large increase in the dynamic response of the spindle to a ramp-and-hold stretch. While extrafusal slow muscle did not respond to a single motor volley, some spindles did, especially if at the same time the muscle was being stretched. In an explanation of the effect of muscle stretch on responses of spindles to slow motor volleys it was proposed that stretch acted to reduce the internal motion in muscle fibres produced by a non-uniform distribution of sarcomere lengths. It was proposed that this kind of effect may account for dynamic fusimotor actions in all vertebrate spindles.Financial assistance was provided by a grant from the National Health and Medical Research Council of Australia, Grant No. 78/5721