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.     

Characterization of spindle afferents in rat soleus muscle using ramp-and-hold and sinusoidal stretches

The discharge properties of 51 afferents were studied in the rat soleus muscle spindles. Under deep anesthesia using a pentobarbital sodium solution (30 mg/kg), a laminectomy was performed and the right L(4) and L(5) dorsal and ventral roots were transected near their entry into the spinal cord. In situ, the minimal (L(min)) muscle length [3 +/- 0.08 (SE) cm] of the soleus was measured at full ankle extension. Unitary potentials from the L(5) dorsal root were recorded in response to ramp-and-hold stretches applied at 3 mm (S3) and 4 mm (S4) amplitudes and four stretch velocities (6, 10, 15, and 30 mm/s), sinusoidal stretches performed at four amplitudes (0.12, 0.25, 0.5, and 1 mm) and six stretch frequencies (0.5, 1, 2, 3, 6, and 10 Hz), and vibrations applied at 50-, 100-, and 150-Hz frequencies. These two kinds of stretches were performed at three different muscle lengths (L(min+10%), L(min+15%), and L(min+20%)), whereas vibrations were applied at L(min+20%) muscle length. Conduction velocity of the fibers was calculated but did not allow to discriminate different fiber types. However, the mean conduction velocity of the first fiber group (43.3 +/- 0.8 m/s) was significantly higher than that of the second fiber group (33.9 +/- 0.9 m/s). Three parameters allowed to differentiate the responses of primary and secondary endings: the dynamic index (DI), the discharge during the stretch release from the ramp-and-hold stretches, and the linear range and the vibration sensitivity from sinusoidal stretches. The slope histogram of the linear regression based on the DI and the stretch velocity was clearly bimodal. Therefore the responses were separated into two groups. During the stretch release at a velocity of 3 mm/s, the first response group (n = 26) exhibited a pause, whereas the second (n = 25) did not. The linear range of the second ending group (0.12-1 mm) was broader than that of the first (0.12-0.25 mm). The first ending group showed a higher sensitivity to high-vibration frequencies of small amplitude than the second. In comparison with the literature, we can assert that the first and the second ending groups corresponded to the primary and secondary endings, respectively. In conclusion, our study showed that in rat soleus muscle spindles, it was possible to immediately classify the discharge of Ia and II fibers by using some parameters measured under ramp-and-hold and sinusoidal stretches.     

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.     

An analysis of muscle spindle behavior using randomly applied stretches

The behavior of mammalian muscle spindles to relatively large amplitude randomly applied stretches is compared to the linear behavior elicited by small amplitude sinusoidal stretches. The results disclose two apparently independent sources of nonlinear behavior. One is a static nonlinearity affecting both primary and secondary endings in which the sensitivity decreases with stretch amplitude up to about 1% stretch. The other is a ‘dynamic’ nonlinearity affecting only primary endings that is manifest as a charge in the ratio of rate to proportional sensitivity depending on the duration of an applied stretch.     

The response to stretch of human intercostal muscle spindles studied in vitro.

1. The discharge properties of human muscle spindles have been studied in vitro in a preparation based on the biopsied external intercostal muscle. 2. The static and dynamic responsiveness of thirty-six endings in twenty visualized and histologically identified spindles have been investigated using amplitudes and velocities of stretch likely to encompass those occurring in vivo. 3. The dynamic index, measured at a stretch velocity of 3 mm/sec, ranged from 3 to 40 impulses/sec and was distributed bimodally, consitent with the presence of primary and secondary endings. 4. The relationship between the dynamic index and the velocity of stretch was approximately linear both for primary and secondary endings up to the maximum velocity tested (10 mm/sec). 5. The frequency/extension relationship was approximately linear for both primary and secondary endings. The mean values of the slope for primary and secondary endings were 16-1 +/- 8-3 S.D. of the observation and 12-1 +/- 6-5 impulses/sec per five per cent extension. 6. The slopes of the frequency/extension relationship for endings lying in the same spindle were positively correlated, significant at the 10% level. 7. It was estimated from the results in vitro that the position sensitivity of human intercostal spindles in vivo ranges from 2 to 21 impulses/sec per millimetre.     

Alteration of proprioceptive messages induced by tendon vibration in man: a microneurographic study

Summary The activities of single proprioceptive fibres were recorded from the lateral peroneal nerve using transcutaneously implanted tungsten microelectrodes. Unitary discharges originating from muscle spindle primary and secondary endings and Golgi tendon organs were identified by means of various physiological tests. The sensitivity of proprioceptors to mechanical vibrations with a constant low amplitude (0.2–0.5 mm) applied at various frequencies to the tendon of the receptor-bearing muscle was studied. Muscle spindle primary endings (Ia fibres) were found to be the most sensitive to this mechanical stimulus. In some cases their discharge could be driven in a one-to-one manner up to 180 Hz. Most of them also fired harmonically with the vibration up to 80 Hz and then discharged in a subharmonic manner (1/2–1/3) with increasing vibration frequencies. Muscle spindle secondary endings (II fibres) and Golgi tendon organs (Ib fibres) were found to be either insensitive or only slightly sensitive to tendon vibration in relaxed muscles. The effects of tendon vibration on muscle spindle sensory endings response to muscle lengthening and shortening induced by imposed constant velocity or sinusoidal movements of the ankle joint were studied. Modulation of the proprioceptive discharge frequency coding the various joint movement parameters was either completely or partly masked by the receptor response to vibration, depending on the vibration frequency. Moreover, vibrations combined with sinusoidal joint movements elicited quantitatively erroneous proprioceptive messages concerning the movement parameters (amplitude, velocity). The sensitivity of the Golgi tendon organs to vibration increased greatly when the receptor-bearing muscle was tonically contracted. These data confirm that vibration is able to preferentially activate the Ia afferent channel, even when the vibration amplitude is low. They define the frequency sensitivity of the muscle spindle primary and secondary endings and the Golgi tendon organs. They also show that the physiological messages triggered by ongoing motor activities undergo a series of changes during the exposure of muscles to vibration.     

Physiological properties of muscle spindles in dorsal neck muscles of the cat.

1. Single-fiber recording was used to examine the properties of 107 spindle endings in cat biventer cervicis (BC) and complexus (CM) muscles. Responses of receptors were examined following muscle contraction and ramp and hold stretch. Twenty-two endings in splenius (SP) were also examined, but their responses could not be quantitated because the anatomy of SP prevented the application of appropriate stretches. 2. Conduction velocitites of spindle afferents ranged from 13 to 90 m/s. Endings with primary response patterns usually had faster conduction velocities than secondary endings, but there was overlap in the conduction velocity ranges of the two subgroups. 3. Most neck spindle afferents could be classified as either primary or secondary by a constellation of physiological criteria including dynamic response pattern, dynamic index, and variability of resting discharge frequency. However, 22 of 107 endings from BC and CM had responses with characteristics intermediate between primary and secondary responses. The possible sources of these characteristics are discussed. 4. Despite the similarity in properties between spindles of different neck muscles, the length sensitivities of CM spindles were high compared to those of BC spindles. CM spindles showed length-related modulation of firing frequency over a more restricted range of initial muscle lengths than did BC spindles. 5. Eight Golgi tendon organs (GTO) were identified by their characteristics responses. Conduction velocities obtained for five GTO afferent nerves ranged from 50 to 67 m/s. Recordings were also made from receptros in deep muscles surrounding the vertebrae. These receptors had properties characteristic of muscle spindles.     

Function and structure of atypical muscle spindles after neonatal nerve crush in rats

Summary During the early postnatal period, the differentiation and maturation of muscle spindles in the rat is still dependent on their sensory innervation. When a nerve is crushed during this period, most spindles in the denervated muscles degenerate and after reinnervation only occasional spindles of atypical structure are to be found in these muscles. We determined the basic functional properties of these atypical spindles in adult rats and attempted to correlate them with their structural characteristics. The discharge rates of 13 afferent units from the soleus or lateral gastrocnemius muscles were evaluated in response to stretch. These units were capable of a slowly adapting response to 2–4 mm stretches. Their mean discharge frequencies at any point of the ramp-and-hold stretch were, however, on an average 50% lower than normal values. The conduction velocities of afferents from the atypical spindles were in the range of 10–40 m/s. Histological examinations revealed that 90% of the atypical muscle spindles found in the soleus or lateral gastrocnemius muscles had only 1 or 2 intrafusal fibres without any nuclear accumulations as compared to four intrafusal fibres in normal muscle spindles in the rat. The proportional decrease of the discharge rate in both the dynamic and static part of the response of these atypical spindles could be due to the decreased synaptic area between the sensory terminals and the intrafusal fibres and/or to altered structural properties of the intrafusal fibres.     

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).     

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.     

Effects of hypodynamia-hypokinesia on the muscle spindle discharges of rat soleus muscle

The aim of this study was to determine whether Ia and II fiber discharges of soleus muscle spindles were modified after a 14-day period of hypodynamia (absence of weight bearing) and hypokinesia (reduction of motor activity). Fifty-one and 38 afferent fibers were studied, respectively, in control and hypodynamia-hypokinesia (HH) groups. Under deep anesthesia (pentobarbital, 30 mg/kg), a L3-L6 laminectomy was performed. Unitary potentials from the L5 dorsal root were recorded in response to ramp-and-hold stretches applied at two stretch amplitudes (3 and 4 mm) and four stretch velocities (6, 10, 15, and 30 mm/s) and to sinusoidal stretches applied at four stretch amplitudes (0.12, 0.25, 0.5, and 1 mm) and six stretch frequencies (0.5, 1, 2, 3, 6, and 10 Hz). In both animal groups, the Ia fibers showed higher dynamic index values, smaller linear range, and higher vibration sensitivity than the II fibers. They also exhibited a pause in their discharges during the stretch release contrary to II fibers, which displayed no pause in their responses. After HH, our results showed that for both fiber types all parameters measured under ramp-and-hold stretches (except the static sensitivity) were significantly increased and under sinusoidal stretches, the vibration sensitivity increased, and the response amplitude only increased at 0.12-mm stretch amplitude. The linear range of Ia afferents was limited to 0.12 mm, whereas it was unchanged for the II fibers. After HH, the stretches could be better transmitted to the muscle spindles, probably resulting from changes in passive mechanical properties of the soleus.     

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.     

Postembedding light- and electron microscopic immunocytochemistry of amino acids: description of a new model system allowing identical conditions for specificity testing and tissue processing

Summary Responses of soleus muscle afferents were studied in anaesthetised kittens, after intravenous injection of succinyl choline (SCh), a drug which in adult spindles, at low doses, produces a contracture preferentially of bag₁ intrafusal muscle fibres and so mimics the effect of dynamic fusimotor stimulation. Following injection of SCh in kittens aged between 1 and 57 days the integrated discharge recorded in large portions of dorsal root during muscle stretch showed a smaller increase than in the adult. Recordings from functionally single afferents of muscle spindles showed that in the youngest animals SCh induced a resting discharge from all spindles, which were normally silent. The response to a ramp-and-hold stretch showed an abrupt rise in firing rate at the start of the stretch followed by a slow decline. The abrupt fall at the end of the ramp, typical of the adult dynamic response, was almost absent in spindles of animals 2–5 days old. In the younger animals SCh allowed primary and secondary spindle endings to be distinguished on the basis of their response to stretch. Conduction velocity of developing Group I nerve fibres was found to increase from 9.5 ms^–1 on day 2 to 51.6 ms^–1 on day 57, an increase of 0.77 ms^–1 per day, while Group II increased from 5.0 ms^–1 to 23.2 ms^–1 over the same period, an increase of 0.33 ms^–1 per day. The two groups appeared to be separate even in the youngest animals. It is concluded that although spindles in the newborn kitten respond vigorously to SCh, the pattern of discharge is quite different from that seen in older animals.     

Muscle afferent responses to isometric contractions and relaxations in humans

1. One hundred and two single afferents from the finger extensor muscles of humans were studied with the microneurography technique. 2. The afferents were provisionally classified as primary muscle spindle afferents (62/102), secondary spindle afferents (22), and Golgi tendon organ afferents (18) on the basis of their responses to four tests: 1) ramp-and-hold stretch, 2) 20- and 50-Hz small-amplitude sinusoidal stretch superimposed on ramp-and-hold stretch, 3) maximal isometric twitch contraction, and 4) stretch sensitization. 3. The response profiles of the three unit types were analyzed during slowly rising isometric contraction terminating with an abrupt relaxation. About 75% (61/84) of all muscle spindle afferents increased their discharge during isometric contraction, whereas the discharge was reduced for the remaining afferents. All Golgi tendon organs increased their discharge during the contraction. 4. The level of extrafusal contraction at which a spindle afferent increased its discharge rate often varied from trial to trial, speaking against a fixed fusimotor recruitment level of the individual spindle ending. 5. In 70% of the spindle afferents, a distinct burst of impulses appeared when the subject rapidly relaxed after the isometric contraction. The burst was more common and usually much more prominent with primary than secondary afferents, often reaching instantaneous discharge rates well above 100 Hz. 6. Whereas all Golgi tendon organ afferents displayed an increased discharge during the contraction phase, only one of them exhibited a rate acceleration close to the relaxation phase. However, this response could clearly be identified as being of different nature than the spindle bursts.(ABSTRACT TRUNCATED AT 250 WORDS)     

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 responses of human muscle spindle endings to vibration of non-contracting muscles.

1. In micro-electrode recordings from the human peroneal and tibial nerves, the responses of thirty-two primary spindle endings, thirteen secondary spindle endings and three Golgi tendon organs were studied during vibration of the tendons of the receptor-bearing muscles in the leg. The amplitude of the applied vibration was 1-5 mm and the frequency was varied from 20 to 220 Hz. As checked with e.m.g. and torque measurements, the muscles of the leg were relaxed during the sequences analysed. 2. Providing that the vibrator was accurately applied, all endings responded with discharges phase-locked to the vibration cycles, the discharge rates being at the vibration frequency or at subharmonics of that frequency. The response to vibration was of abrupt onset and offset, was maintained for the duration of vibration, and was not subject to fluctuation with changes in attention or with remote muscle contraction. 3. The maximal discharge rate that could be achieved varied from one ending to the next, and increased with the length of the receptor-bearing muscle. For endings driven at their maximal rate an increase in vibration frequency produced a decrease in discharge rates as the ending changed to a subharmonic pattern of response. The converse occurred on decreasing vibration frequency. 4. For any given muscle length, primary endings could generally be driven to higher rates than secondary endings but there was a wide range of responsiveness within each group and a significant overlap between the groups. At medium muscle length, the most responsive primary endings could be driven up to 220 Hz but secondary endings did not reach discharge rates higher than 100 Hz. 5. With combined vibration and passive movements, primary endings exhibited maximal vibration responsiveness during the stretching phases, sometimes firing twice per vibration cycle. During the shortening phases, however, they usually ceased responding to the vibratory stimulus. The vibration responsiveness of secondary endings was not potentiated to the same extent by on-going muscle stretch or reduced to the same extent by on-going muscle shortening. Thus, during shortening, secondary endings may be more responsive than primary endings. 6. The responses of primary endings to tendon taps were reduced during muscle vibration, a reduction which probably contributes to vibration-induced suppression of tendon jerks. Additionally, as the muscle shortened after tendon percussion, there was a transient pause in the response to vibration.     

Responses of muscle spindles following a series of eccentric contractions

To investigate the effects of eccentric exercise on the signalling properties of muscle spindles, experiments were done using the medial gastrocnemius muscle of cats anaesthetised with 40 mg/kg sodium pentobarbitone, i.p. Responses were recorded from single afferent nerve fibres in filaments of dorsal root during slow stretch of the passive muscle and during intrafusal contractions at a range of lengths, before and after a series of eccentric contractions. The sensitivity to slow stretch was measured as the average firing rate between muscle lengths 10.5 and 9.5 mm shorter than the physiological maximum (L_m), during stretch at 1 mm/s over the whole physiological range. The mean sensitivity of both primary and secondary spindle endings increased slightly, but not significantly, after a series of 20–150 eccentric contractions consisting of a 6 mm stretch, at 50 mm/s, to a final length of between L_m –7 mm and L_m, during stimulation of the whole muscle or sometimes of single fusimotor fibres. Discharges were recorded from primary endings during fusimotor stimulation at 100–150 pulses/s, and from secondary endings during static bag intrafusal contractures produced by i.v. injection of 0.2 mg/kg succinyl choline. Spindle responses were recorded, over a range of muscle lengths, in steps covering the whole physiological range. About half of the responses showed a peak in the relation between length and net increase in firing rate, while the remainder either progressively increased or progressively decreased over the physiological range. No large or consistent changes were seen after the eccentric contractions. It is concluded that the intrafusal fibres of muscle spindles are not prone to damage of the kind seen in extrafusal fibres after a series of eccentric contractions.     

The relative sensitivity to vibration of muscle receptors of the cat

1. Longitudinal vibration was applied to the de-efferented soleus muscle of anaesthetized cats while recording the discharge of single afferent fibres from the proprioceptors within the muscle. Conditions were defined under which vibration can be used to excite selectively the primary endings of muscle spindles without exciting the secondary endings of muscle spindles or Golgi tendon organs.2. Frequencies of vibration of 100-500 c/s were used. The maximum amplitude of vibration which the vibrator could produce fell with increasing frequency; it was 250 mu (peak to peak) for 100 c/s and 20 mu for 500 c/s.3. Primary endings of muscle spindles were very sensitive to vibration. Most could be ;driven' to discharge one impulse for each cycle of vibration over the whole of the above range of frequencies, provided the initial tension was moderate (20-200 g wt.). The amplitude of vibration required to produce driving usually varied by less than a factor of two over the whole range of frequencies. The most sensitive endings could be driven by vibrations of below 10 mu amplitude.4. Stimulation of single fusimotor fibres, whether static or dynamic fusimotor fibres, increased the sensitivity of primary endings to vibration. Contraction of the main muscle, produced by stimulating alpha motor fibres, reduced the sensitivity of primary endings even when fusimotor fibres were also being stimulated.5. The secondary endings were very insensitive to longitudinal vibration and with the amplitudes available not one of twenty-five endings could be driven at 150 c/s or above; one ending could be driven at 100 c/s by vibration of 250 mu amplitude. Stimulation of single fusimotor fibres, probably all of which were static fusimotor fibres, made them slightly more sensitive to vibration but none of them approached the sensitivity of the primary endings.6. The Golgi tendon organs were as insensitive as the secondary endings when the muscle was not contracting and none could be driven at any frequency in spite of quite high tensions in the muscle. However, when the muscle was made to contract by stimulating alpha fibres in ventral root filaments the tendon organs became appreciably more sensitive, the degree of sensitization increasing approximately with the strength of the contraction. They never became as sensitive as the primary endings, and with the amplitudes of vibration available none was driven at frequencies of over 250 c/s.7. When the amplitude of vibration was somewhat below that required to produce driving of an ending it still produced some increase in its mean frequency of discharge. However, amplitudes of vibration of 25-50 mu applied to a non-contracting muscle, whether with or without fusimotor stimulation, produced driving of nearly all primary endings without any significant increase in the mean frequency of firing of secondary endings or Golgi tendon organs. Such vibration can therefore be used as a specific stimulus for the primary endings in order to investigate the central effects or repetitive discharge of the Ia afferent fibres from them.8. Experiments on endings in the peroneus longus muscle showed that these behaved similarly to those in soleus.     

The sensory reinnervation of hind limb muscles of the cat following denervation and de-efferentation.

The sensory reinnervation of muscle spindles following lesions of the peripheral nerve was studied in hind limb muscles of the cat. Earlier results reporting complete redevelopment of both primary and secondary endings were confirmed.However, after section of the ventral roots reinnervation of muscle spindles was impaired in that many primary endings did not develop the spiral-like structures and their appearance remained abnormal for up to 120 days. The response to stretch in two-thirds of such de-efferented regenerated primary endings was also abnormal. Although the phasic and vibration responses were present, the slowly adapting part of the response to maintained stretch was defective or absent in many of the primary endings.From these results it appears that motor innervation of the muscle is important for the normal redevelopment of the complex structure and function of the primary ending of the muscle spindle during reinnervation. The results do not indicate whether de-efferentation causes a permanent impairment or only a delay in redevelopment.     

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.