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.     

Visco-elastic properties of the rapidly adapting stretch receptor muscle of the crayfish

The visco-elastic properties of the receptor muscle associated with the rapidly adapting stretch receptor organ of crayfish (Pacifastacus Leniusculus) were studied by recording the tension responses to various length changes. Steady-state length changes resulted in a non-linear tension development in the receptor muscle. The tension increased slowly for small extensions and more rapidly when extension increased. Muscle tension responses to ramp-and-hold extension were characterized by a transient peak followed by a gradual non-exponentional decline in tension. At the onset of the ramp the tension increased rapidly, similar to what has been observed in the muscle of the slowly adapting receptor (SM). The steeper rise in tension during the first part of the ramp indicating higher initial stiffness, resulted in a ‘hump’ when large extensions (> 15%) were applied. The results show that the rapidly adapting receptor muscle has a more pronounced dynamic component; the ratio between the amplitude of the peak and the steady state response was larger in the rapidly than in the slowly adapting receptor muscle. Accordingly, different values for the elements of a visco-elastic model of the muscle had to be set for the two types of receptors. The different properties of the rapidly and slowly adapting receptor muscles are in line with the differences in the overall adaptive behaviour of the organ and give further support to the idea that mechanical factors contribute to the adaptive properties.     

Unilateral suppression of pharyngeal motor cortex to repetitive transcranial magnetic stimulation reveals functional asymmetry in the hemispheric projections to human swallowing

Age-related physiological and morphological changes of muscle spindles were examined in rats (male Fischer 344/DuCrj: young, 4–13 months; middle-aged, 20–22 months; old, 28–31 months). Single afferent discharges of the muscle spindles in gastrocnemius muscles were recorded from a finely split dorsal root during ramp-and-hold (amplitude, 2.0 mm; velocity, 2–20 mm s⁻¹) or sinusoidal stretch (amplitude, 0.05–1.0 mm; frequency, 0.5–2 Hz). Respective conduction velocities (CVs) were then measured. After electrophysiological experimentation, the muscles were dissected. The silver-impregnated muscle spindles were teased and then analysed using a light microscope. The CV and dynamic response to ramp-and-hold stretch of many endings were widely overlapped in old rats because of the decreased CV and dynamic response of primary endings. Many units in old rats showed slowing of discharge during the release phase under ramp-and-hold stretch and continuous discharge under sinusoidal stretch, similarly to secondary endings in young and middle-aged rats. Morphological studies revealed that primary endings of aged rat muscle spindles were less spiral or non-spiral in appearance, but secondary endings appeared unchanged. These results suggest first that primary muscle spindles in old rats are indistinguishable from secondary endings when determined solely by previously used physiological criteria. Secondly, these physiological results reflect drastic age-related morphological changes in spindle primary endings.     

Changes in motor commands, as shown by changes in perceived heaviness, during partial curarization and peripheral anaesthesia in man

1. The responses to stretch have been studied in living, isolated Golgi tendon organs (GTOs) from tail muscles of cat. Experiments were performed in vitro and consisted of subjecting single GTOs to controlled ramp-and-hold stretch while recording the response from their sensory axons raised in oil. 2. The threshold force required for sustained afferent discharge was measured directly, and found to be between 8 and 170 dynes at 24 degrees C for nine GTOs tested. Beyond threshold, the discharge frequency is approximately proportional to applied static tension over a wide range. Sensitivy to tension varies among different GTOs and appears to be inversely correlated with mechanical stiffness. 3. With impulse activity blocked by tetrodotoxin, graded receptor potentials could be recorded whose amplitude varied in proportion to applied static tension. All GTOs examined showed in addition a dynamic response, which became larger with increasing velocity of ramp stretch. This dynamic sensitivity appears in the receptor potential and is then augmentd by an apparent accommodative process at the impulse initiating site. 4. Based on the above findings, possible mechanical models are discussed for the sensory transduction mechanism.     

Modelling the mechanoreceptor's dynamic behaviour

All sensory receptors adapt, i.e. they constantly adjust their sensitivity to external stimuli to match the current demands of the natural environment. Electrophysiological responses of sensory receptors from widely different modalities seem to exhibit common features related to adaptation, and these features can be used to examine the underlying sensory transduction mechanisms. Among the principal senses, mechanosensation remains the least understood at the cellular level. To gain greater insights into mechanosensory signalling, we investigated if mechanosensation displayed adaptive dynamics that could be explained by similar biophysical mechanisms in other sensory modalities. To do this, we adapted a fly photoreceptor model to describe the primary transduction process for a stretch‐sensitive mechanoreceptor, taking into account the viscoelastic properties of the accessory muscle fibres and the biophysical properties of known mechanosensitive channels (MSCs). The model's output is in remarkable agreement with the electrical properties of a primary ending in an isolated decapsulated spindle; ramp‐and‐hold stretch evokes a characteristic pattern of potential change, consisting of a large dynamic depolarization during the ramp phase and a smaller static depolarization during the hold phase. The initial dynamic component is likely to be caused by a combination of the mechanical properties of the muscle fibres and a refractory state in the MSCs. Consistent with the literature, the current model predicts that the dynamic component is due to a rapid stress increase during the ramp. More novel predictions from the model are the mechanisms to explain the initial peak in the dynamic component. At the onset of the ramp, all MSCs are sensitive to external stimuli, but as they become refractory (inactivated), fewer MSCs are able to respond to the continuous stretch, causing a sharp decrease after the peak response. The same mechanism could contribute a faster component in the ‘sensory habituation’ of mechanoreceptors, in which a receptor responds more strongly to the first stimulus episode during repetitive stimulation.     

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.     

The visco-elasticity of resting intact mammalian (rat) fast muscle fibres

Summary Tension responses induced by ramp stretches (amplitude of 1–2% fibre length and speeds of 0.01–15 L _o s^-1) were examined in resting intact muscle fibre bundles isolated from the extensor digitorum longus (a fast muscle) of the rat; sarcomere length of a 2 mm region was monitored near the tension transducer end by means of a He-Ne laser diffractometer. The experiments were done at 10°C. During a ramp stretch, the tension rose rapidly (P₁) and then slowly (P₂) to reach a peak; after completion of the ramp, the tension decayed in a complex manner to a steady level (P₃) at approximately constant sarcomere length. At stretch velocities higher than 1–2 L _o s^-1, P₁ tension increased in direct proportion to stretch velocity, indicating that it is due to viscous resistance; in a half sarcomere, the viscous resistance to filament sliding may be about 5 × 10⁸ N s m^-3. The steady tension level after the ramp (P₃ tension) was independent of stretch velocity indicating that it represents an elastic tension. The amplitude of the slow tension rise (P₂ tension corrected for P₃) increased with stretch velocity up to a plateau (as in a visco-elastic component); the calculated relaxation time was 5–13 ms. Amplitudes of all three components were larger at longer sarcomere length (range 2.4–3 m). The presence of 5–10 mm BDM which abolished the twitch and markedly depressed the tetanic responses, produced little or no change in the tension components. Our results show that none of the tension components to stretch in relaxed mammalian muscle fibres is due to active, cycling cross-bridges; the possibility that the resting sarcomeric visco-elasticity (net P₂) resides in the connectin (=titin) containing gap filament is discussed.     

Small-signal analysis of response of mammalian muscle spindles with fusimotor stimulation and a comparison with large-signal responses.

1. Response dynamics of primary and secondary muscle spindle endings to small-amplitude sinusoidal stretches were found to be unaltered by tonic repetitive stimulation of fusistatic or fusidynamic fibers. 2. Overall sensitivity of these receptors is decreased by fusistatic stimulation and either unchanged, increased, or decreased by fusidynamic stimulation at rates of 75/s or greater. 3. In the case of primary endings, the results obtained with small-amplitude sinusoidal stretches are not compatible with the response of these receptors to large-amplitude ramp stretches. The difference is explained by dependence of receptor dynamics on stretch amplitude. Fusistatic stimulation tends to prevent those changes in dynamics, whereas fusidynamic stimulation tends to enhance them. 4. In the case of secondary endings, the results obtained with small- and large-amplitude stretches appear to be compatible with a linear model for this receptor (i.e., one with dynamics independent of input parameters). 5. By modulating the frequency of stimulation applied to fusimotor fibers and comparing the resulting afferent response to the receptor response to stretch dynamic characteristics of intrafusal muscle contraction can be deduced. The results suggest that the dynamics of fusiastatic and fusidynamic contraction are the same and, furthermore, that they are the same as those of extrafusal muscle. We note that the result is incompatible with measurements of the time course of twitch and tetanus development and suggest, therefore, that muscle dynamics are a function of contractile state.     

Force responses to fast ramp stretches in stimulated frog skeletal muscle fibres

Force responses to fast ramp stretches at various velocities were recorded from single muscle fibres isolated from either lumbricalis digiti IV or tibialis anterior muscle of the frog (Rana esculenta) at sarcomere length between 2.15 and 3.25 μm at 15° C. Stretches were applied at rest, at tetanus plateau and during the tetanus rise. Stretches with the same velocity but different accelerations were imposed to the fibre to evaluate the effect of fibre inertia on the force responses. Length changes were measured at sarcomere level with either a laser diffractometer or a striation follower apparatus. The force response to a fast ramp stretch could be divided into two phases. The initial fast one (phase 1) lasts for the acceleration period during which the stretching velocity rises up to the steady state. The second slower phase (phase 2) lasts for the remainder of the stretch and corresponds to the well-known elastic response of the fibre. Most of this paper is concerned with phase 1. The amplitude of the initial fast phase was proportional to the stretching velocity as expected from a viscous response. This viscosity was associated with a very short (about 10 μs) relaxation time. The amplitude of the fast phase increased progressively with tension during the tetanus rise and scaled down with sarcomere length approximately in the same way as tetanic tension and fibre stiffness. These data suggest that activated fibres have a significant internal viscosity which may arise from crossbridge interactionThis revised version was published online in July 2005 with corrections to the Cover Date.     

After-effects of fusimotor stimulation on the response of muscle spindle primary afferent endings

1. The experiments were performed on the soleus muscle of the anaesthetized cat in which the ventral roots had been cut.2. A short period of repetitive stimulation of a single fusimotor fibre which influenced a particular spindle primary ending invariably caused a characteristic alteration in the response of the same ending to a subsequently applied ramp stretch of the muscle. The change consisted in the appearance of a burst of impulses at the beginning of the stretch where none had been present before. Occasionally, such an ;initial burst' was spontaneously present; it was then enhanced following fusimotor stimulation.3. This after-effect of fusimotor stimulation was abolished by a subsequent stretch of the muscle, but otherwise persisted for over a minute.4. When the muscle was released to below the length at which the spindle had been facilitated and a testing stretch applied from the new initial length there was no burst of impulses at the beginning of stretch. There was, however, a burst as the muscle was stretched through the length at which the fusimotor fibre had been stimulated.5. These effects are suggested to be due to the persistence of stable bonds between the actin and myosin filaments of the intrafusal fibres, so that their previously activated regions were ;stuck' at the length they were when the fusimotor stimulation was applied.6. Such effects were produced both by static and by dynamic fusimotor fibres. The effects of the two kinds of fusimotor fibre, however, appeared to be mediated by different intrafusal muscle fibres. This was shown by stimulating one kind of fibre with the muscle slightly stretched, then releasing the muscle a few mm and stimulating the other kind of fibre to the same spindle. A subsequent testing stretch then elicited two bursts, one at the beginning and one in the middle of the stretch.     

Comparison of the tension responses to ramp shortening and lengthening in intact mammalian muscle fibres: crossbridge and non-crossbridge contributions

We examined the tension responses to ramp shortening and lengthening over a range of velocities (0.1–5 L ₀/s) and at 20°C and 30°C in tetanized intact fibre bundles from a rat fast (flexor hallucis brevis) muscle; fibre length (L ₀) was 2.2 mm and sarcomere length ∼2.5 μm. The tension change during ramp releases as well as ramp stretches showed an early transition (often appearing as an inflection) at 1–4 ms; the tension change at this transition and the length change at which it occurred increased with velocity. A second transition, indicated by a more gradual reduction in slope, occurred when the length had changed by 14–28 nm per half-sarcomere; the tension at this transition increased with lengthening velocity towards a plateau and it decreased with shortening velocity towards zero tension. The velocity dependence of the time to the transitions and the length change at the transitions showed some asymmetries between shortening and lengthening. Based on analyses of the velocity dependence of the tension and modelling, we propose that the first transition reflects the tension change associated with the crossbridge power stroke in shortening, or with the reversal of the power stroke in lengthening. Modelling shows that the reduction in slope at the second transition occurs when most of the crossbridges (myosin heads) that were attached at the start of the ramp become detached. After the second transition, the tension reaches a steady level in the model whereas the tension continues to increase during lengthening and continues to decrease during shortening in the experiments; this continuous tension change is seen at a wide range of initial sarcomere lengths and when active force is reduced by the myosin inhibitor, BTS. The continuous tension decline during shortening is not abolished by caffeine, but the rate of decline is reduced when the active force is depressed by BTS. We propose that stiffening of non-crossbridge visco-elastic elements upon activation contributes to the continuous tension rise during lengthening and the release of such tension and Ca-insensitive deactivation contribute to the tension decline during shortening in muscle fibres.     

The effects of muscle cooling and stretch on muscle spindle secondary endings in the cat.

1. The objectives of the investigation were to identify the muscle spindle endings which respond to cooling of the relaxed muscle and to study their response to stretch. 2. The discharge of single afferents from 162 de-efferented muscle spindles in the relaxed medial gastrocnemius muscle of the anaesthetized cat was studied in vivo during cooling of the muscle from 37 to 24 degrees C. Temperature measurements were made at the inner surface of the muscle, while cooling (never below 15 degrees C) was applied at the skin over the muscle. 3. The endings were classified as primary or secondary endings on the basis of their conduction velocity, the dividing line being set at 70 m/sec. A response to cooling was obtained only from endings with afferents conducting at velocities of 20-70 m/sec. These fifty-six endings (CR) represented 65% of the secondary endings studied; the remaining secondary endings (NCR) and the primary endings showed no activity during cooling of the relaxed muscle. 4. During maintained stretches of 4-12 mm, activity of the NCR and primary endings decreased when the muscle was cooled. Cooling affected the CR endings in the same way, but only if the muscle was stretched 6 mm or more. During a smaller maintained muscle stretch, cooling caused an increase in CR activity, superimposed on the response to stretch. 5. The response to a 10 mm stretch at velocities of 10-70 mm/sec was studied in twenty-six CR, eleven NCR and twenty-one primary endings. 6. The dynamic responses of CR endings were intermediate between those of the primary endings and NCR endings. For any velocity of stretch the mean dynamic index of the CR endings was significantly greater than that of the NCR endings but significantly less than that of the primary endings. 7. The mean static responses of the CR and primary endings, measured 0-5 sec after the end of ramp stretch, were the same and significantly greater than that of the NCR endings. 8. The results indicate that cooling of the relaxed mammalian muscle may be used to differentiate between primary endings and about two-thirds of the secondary endings. The remaining secondary endings can be recognized by their small dynamic and static response to stretch.     

After-effects following responses of a muscle stretch receptor of the shore crab, Carcinus maenas

Stretch evoked responses of the thoracic-coxal muscle receptor organ in the crab are shown to be time dependent. Thus, by varying the interstretch interval, the duration of the T fibre dynamic component of the response to a ramp-and-hold stretch was found to be directly related to the inter-stretch interval. With long interstretch intervals (e.g. 10 min), the T fibre dynamic component of the response to stretch was undistinguishable from the response when pre-conditioned with stimulation of the receptor motor nerve. These observations are consistent with the hypothesis of spontaneous cross-bridge formation when the receptor is quiescent. Such properties lead to a source of ambiguity of the muscle afferent response, which are presumably overcome by maintaining tonic efferent activation of the receptor.     

Evidence for a link between the extra-retinal component of random-onset pursuit and the anticipatory pursuit of predictable object motion

During pursuit of moving targets that temporarily disappear, residual smooth eye movements represent the internal (extra-retinal) component of pursuit. However, this response is dependent on expectation of target reappearance. By comparing responses with and without such expectation during early random-onset pursuit, we examined the temporal development of the extra-retinal component and compared it with anticipatory pursuit, another form of internally driven response. In an initial task (mid-ramp extinction), a moving, random-velocity target was initially visible for 100 or 150 ms but then extinguished for 600 ms before reappearing and continuing to move. Responses comprised an initial visually driven rapid rise in eye velocity, followed by a secondary slower increase during extinction. In a second task (short ramp), with identical initial target presentation but no expectation of target reappearance, the initial rapid rise in eye velocity was followed by decay toward zero. The expectation-dependent difference between responses to these tasks increased in velocity during extinction much more slowly than the initial, visually driven component. In a third task (initial extinction), the moving target was extinguished at motion onset but reappeared 600 ms later. Repetition of identical stimuli evoked anticipatory pursuit triggered by initial target offset. Temporal development and scaling of this anticipatory response, which was based on remembered velocity from prior stimuli, was remarkably similar to and covaried with the difference between mid-ramp extinction and short ramp tasks. Results suggest a common mechanism is responsible for anticipatory pursuit and the extra-retinal component of random-onset pursuit, a finding that is consistent with a previously developed model of pursuit.     

Viscoelastic properties of the slowly adapting stretch receptor muscle of the crayfish

The viscoelastic properties of the muscle associated with the slowly adapting stretch receptor organ of the crayfish (Astacus astacus) were studied by recording the tension response to various length changes. When steady-state length changes were applied to the muscle, the tension developed in a non-linear way, increasing slowly for small extensions and rapidly when extension increased. Muscle tension responses to ramp-and-hold extensions were characterized by a transient peak followed by a gradual decline in tension. At the onset of the ramp the tension increased rapidly, similar to the response seen in resting skeletal muscle. The relation between peak dynamic tension and extension was non-linear. In a log-log plot the relation was linear with a mean slope of 1.4. At small extensions (less than 5%) the slope seemed to be lower. The experimental results have been analysed in relation to a viscoelastic model consisting of a Voigt element in series with a non-linear spring. The model could describe both the static length-tension relation and the dynamic response, but different parameters for the springs had to be used for the two cases. When the measured tension response was transformed by an exponential function of the squared tension, in accord with recent findings on stretch-activated channels, a good agreement was obtained with the time course of the receptor currents. Adaptation is thus likely to be caused by both the mechanical properties of the receptor muscle and the characteristics of stretch-activated channels of the neuron.     

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.     

Transition in sensitivity of spindle receptors that occurs when muscle is stretched more than a fraction of a millimeter.

We studied the responses of 34 deefferented spindle receptors to slowly applied ramp stretches (0.01-1 mm/s) of small (0.02-0.2 mm) and intermediate (0.2-1 mm) amplitudes. The afferent discharge from primary and secondary endings was recorded from filaments of dorsal root in anesthetized cats. 1. Responses of most endings to ramps of intermediate amplitude showed abrupt changes in slope (discontinuities) which were highly repeatable. Discontinuities occurred more nearly at constant stretch (in the range 50-400 mum for different receptors) than at constant discharge rate. They were less pronounced in the case of secondary endings. 2. Changes in sensitivity occurred when the degree of stretch exceeded a transitional amplitude which ranged from 50 to 200 mum. These changes were studied by constructing plots based on a family of responses to a family of ramps which were scaled versions of each other. The plots indicated that reductions in sensitivity occurred both during stretch and during adaptation; the reductions were more marked for primary than for secondary endings. 3. Responses were modified considerably by preceding changes in muscle length. When the last change was an increase of a few millimeters, discontinuities became more pronounced and other changes in the appearance of the dynamic response occurred, particularly in the case of primary endings. These changes could last for several minutes, but were abolished by a single test stretch of intermediate amplitude. 4. The resetting of high sensitivity that occurs when muscle length is changed, the discontinuities, the transitions in sensitivity, nonlinear adaptation, and the effects of previous length change appeared to be related phenomena. They can all be accounted for by the hypothesis that polar zones of intrafusal muscle fibers possess a frictionlike property, one analogous to that which has been described for whole muscle. A simple nonlinear model which shows these features is presented. 5. The adequate stimulus for a change in primary ending discharge is a small change in muscle length, relatively independently of its velocity. The dynamic response arises mainly from a changing sensitivity to length itself, which is a nonlinear property.     

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.     

Neural control: novel evaluation of stretch reflex sensitivity

We evaluated the stretch reflex activities of the elbow flexor and extensor muscles considering the relationship between the reflex electromyographic (EMG) responses and their corresponding standardized muscle stretch velocities. Specifically, muscular stretch velocity was estimated by using ultrasonograms. Stretch reflex EMG responses were elicited in the biceps brachii, brachioradialis and triceps brachii with a ramp-and-hold rotation at the elbow joint, which consisted of various angular velocities for the extension- or flexion-direction. The whole muscle stretch velocity induced by each ramp-and-hold rotation was calculated on the basis of fibre length changes associated with the elbow joint angle. A linear regression equation was fitted to the relation between the whole muscle stretch velocity and the reflex EMG responses, and the variables from the equation were used to quantify sensitivity of each reflex EMG component. The reflex EMG responses were increased as the ramp-and-hold rotational velocity increased. There were no significant differences in the recorded magnitudes of reflex EMG responses with equivalent joint rotational velocity between the brachioradialis and the triceps brachii medial head. These muscles showed the highest reflex responses in the flexor and extensor muscles, respectively. To the contrary, the reflex EMG response elicited by the standardized muscle stretches was significantly greater in the extensor muscles, indicating a higher reflex sensitivity. This was because of the lower muscle stretch velocity of the triceps brachii with an equivalent elbow joint rotation. The stretch reflex sensitivity in both the elbow flexor and extensor muscles might be regulated so as to make the reflex responses the same when the equivalent joint rotational velocity is applied to these muscles.     

Tension changes during and after stretch in frog muscle fibres

1. Small fibre bundles from the semitendinosus muscles of the frog were stretched during tetanic stimulation, and the resulting tension changes were studied over a wide range of stretch velocities from 0.1 to 150 cm/sec (0.1-100 length/sec). The experiments were performed within the range of fibre lengths where the resting tension was negligible.2. With stretch velocities of more than 30 cm/sec (20 length/sec), the tension rose abruptly at first, and then started to fall while the stretch still continued, indicating the ;slip' of the contractile component. When the fibres were stretched at 80-150 cm/sec (70-100 length/sec), the tension fell quickly below the initial isometric level at the end of the stretch, and then began to rise again to the initial isometric value.3. Following stretches of 30-60 cm/sec (20-50 length/sec), the tension showed a delayed transient rise. The delayed rise of tension became more marked as the amount of stretch was increased.4. In some preparations, oscillatory tension changes were observed following stretches of 50-100 cm/sec (40-70 length/sec).5. The tension developed above the isometric level during moderate-velocity stretches of less than 15 cm/sec (10 length/sec) increased by lowering temperature, and showed a tendency to decay when the stretch velocity was suddenly reduced during a stretch.6. These results are discussed in relation to the sliding filament hypothesis, which provides an explanation for the findings of the present work.