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)     

Dynamic response of human muscle spindle afferents to stretch

1. One hundred and twenty-four muscle afferents from the finger extensor muscles were recorded from the radial nerve in human subjects. 2. The afferents were provisionally classified as muscle spindle primary (78/124) and secondary afferents (25/124), and Golgi tendon organ afferents (21/124), on the basis of their response to 1) maximal twitch contractions, 2) 20- and 50-Hz sinusoids superimposed on ramp-and-hold stretches, 3) stretch sensitization, and 4) isometric contractions and sudden relaxations. 3. Ramp-and-hold stretches at two velocities, 10 and 50 degrees/s, were applied to the appropriate metacarpophalangeal (MCP) joint while the parent muscle remained relaxed. For each unit three discrete parameters were assessed: the presence or absence of 1) an initial burst at the commencement of the ramp stretch, 2) a deceleration response at the beginning of the hold phase, and 3) a prompt silencing at muscle shortening. In addition, two kinds of dynamic indexes were calculated for 79 of the muscle spindle afferents. 4. Most spindle afferents responded readily to stretch, whereas the Golgi tendon organ afferents produced very poor stretch responses. All of them lacked a static response, whereas the dynamic response, when present at all, consisted of only a few impulses. 5. The dynamic index was higher for spindle primaries than for secondaries, and this difference was statistically significant although the distribution was unimodal for spindle afferents as a group. Hence, this parameter was a poor discriminator. 6. Initial bursts, deceleration responses, and silences during imposed shortening were more common in spindle primaries than in secondaries. The differences were significant in all these respects. 7. The three discrete parameters were statistically pairwise independent for the spindle afferents, justifying the combination of the three into a useful battery for discrimination between primary and secondary spindle afferents and the use of this battery as a partial data base for a probability approach towards a solid classification of human muscle afferents.     

In-parallel and in-series behavior of human muscle spindle endings

1. The responses of 62 putative muscle spindle afferents innervating the pretibial flexor muscles of normal human subjects were studied during graded twitch contractions of the receptor-bearing muscle to search for possible in-series coupling between spindle endings and motor units. 2. The majority of afferents (n = 57) responded unequivocally in an in-parallel manner to the twitch contractions, regardless of contraction strength. There were two patterns of in-parallel response: afferents without background activity discharged during the relaxation phase of the twitch; afferents with a background discharge were transiently silenced during the contraction phase and resumed their discharge on the relaxation phase. 3. Evidence of in-series coupling was found for five afferents during submaximal twitch contractions, to which each afferent responded in a mixed "biphasic" manner, with increases in discharge during both the contraction and relaxation phases of the twitch. Background discharge, response to stretch, and response during isometric voluntary contractions suggested that four of the afferents innervated primary spindle endings and the fifth a secondary spindle ending. 4. It is argued that the five atypical spindle endings responded in an ambiguous manner during twitch contractions of the receptor-bearing muscle because there was an in-series mechanical coupling between motor units and the spindle. The incidence of demonstrable in-series responses has serious implications for the mechanisms of spindle activation during normal motor behavior, but has only minor implications for the use of the twitch test to identify muscle spindle endings.     

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.     

Muscle spindle discharge in response to contraction of single motor units

1. In human subjects, microelectrode recordings were made from 25 muscle spindle afferents and two tendon organ afferents coming from muscles innervated by the peroneal nerve. 2. Stimulation at low intensity through the recording microelectrode activated efferent axons innervating motor units in close proximity to the muscle spindle or tendon organ. There was a clear alteration in the discharge of 17 afferents (15 muscle spindle, 2 tendon organ) in response to twitch contractions that involved only one, two, or three motor units. With three other afferents there was a less overt but statistically significant alteration in discharge rate by the twitch contraction of a single motor unit. 3. The sensitivity of 21 receptors (20 spindles, 1 tendon organ) to twitch contractions of anatomically close motor units was contrasted with their sensitivity to twitches of more remote motor units in the muscle. In no instance was the sensitivity to the contraction of remote motor units greater than that to the contraction of local motor units stimulated through the microelectrode; with remote stimulation many units usually had to be activated before the resulting twitch contraction altered the discharge of an afferent. 4. It is concluded that muscle spindles as well as tendon organs can play a role in monitoring the activity of motor units anatomically close to the receptor.     

A functional analysis of the components of the mesencephalic nucleus of the fifth nerve in the cat

1. The mesencephalic nucleus of the trigeminal nerve has been studied using extracellular micro-electrode recording and the constituent cell types identified.2. Two types of unit were found, namely, muscle spindle first order afferents of ipsilateral jaw-closing muscles and mechanoreceptor afferents of ipsilateral maxillary and mandibular teeth.3. No evidence was found for representation of extra-ocular muscle stretch receptors, of temporo-mandibular joint receptors or of tendon organs of jaw muscles.4. Spindle units of each of the jaw-closing muscles were recorded in all parts of the nucleus and there was no evidence of their segregation according to muscle of origin.5. Attempts to classify spindle units by their dynamic response to ramp stretches, their following of high frequency vibration and their interspike interval variability at constant length gave no indication of two populations when fusimotor activity was suppressed.6. Following the injection of suxamethonium, however, units fell into two groups according to their dynamic index. Their behaviour resembled that described for primary and secondary spindle afferents. In data pooled from all of the jaw-closing muscles there were approximately equal numbers of units in each group.     

Synaptic and mechanical coupling between type-identified motor units and individual spindle afferents of medial gastrocnemius muscle of the cat

Experiments were performed to test the possibility that motor unit-muscle spindle pairs that are coupled especially strongly mechanically will also be coupled especially strongly synaptically ("weighted ensemble input": Ref. 4). Synaptic and mechanical coupling between one or two individual muscle spindle afferents and individual motor units of the medial gastrocnemius (MG) muscle were measured in barbiturate-anesthetized cats. Synaptic coupling was assessed by measuring the amplitude of single-fiber monosynaptic excitatory postsynaptic potentials (EPSPs) generated in motoneurons by individual spindle afferents. Mechanical coupling was assessed by measuring the alteration in discharge rate of these spindle afferents caused by tetanic activation of the same motor units. Afferents were classified as primary or secondary on the basis of conduction velocity and response to muscle stretch and contraction. Motor units were classified as slow twitch (S); fast twitch, fatigue resistant (FR); fast twitch, intermediate fatigue resistance (FI); and fast twitch, fatigue sensitive (FF) on the basis of twitch contraction time and resistance to fatigue. In 85% of 138 motor unit-primary afferent interactions tested, tetanic activation of the single motor unit unloaded (i.e., decreased the discharge rate of) the primary afferent. A very weak though significant correlation was found between tetanic contraction strength and primary afferent unloading. In 66% of 155 motor unit-secondary afferent interactions tested, tetanic activation of the single motor unit unloaded the secondary afferent. Again, afferent unloading was but weakly related to tetanic contraction strength. Single-fiber EPSPs generated by primary or secondary muscle spindle afferents were recorded in type-identified motor units. EPSPs generated by primary afferents were significantly larger in oxidative (S + FR) than in glycolytic (FF) motor units. No such differences were seen for EPSPs generated by secondary afferents. The magnitude of the EPSP generated in a motoneuron by a spindle afferent was compared to the magnitude of the unloading of that afferent by tetanic activation of the corresponding motor unit. Overall, no relationship was found between these measures.(ABSTRACT TRUNCATED AT 400 WORDS)     

Properties of cat neck muscle spindles and their excitation by succinylcholine

Summary The sensitivity to sinusoidal stretching and small-amplitude vibration, and the variability of the resting discharge rate of de-efferented muscle spindles in the neck extensor muscle biventer cervicis (b.c.) of the cat have been studied. The effects of intra-arterial infusion of succinylcholine (SCh) on the response of the receptors to ramp stretches of the muscle were also determined. When activated by SCh, one group of afferents showed only a slow facilitation of their discharge rate, similar to that of spindle secondary sensory endings in hind-limb muscles. A second group of afferents developed a large dynamic response and a marked increase in their static discharge rate and were presumed to originate in normal b₁b₂c spindles in the b.c. muscle. A third group of afferents developed only a marked increase in static discharge, without potentiation of the dynamic response, suggesting an origin in the b₂c units of tandem spindles which are relatively common in the neck muscles. On the basis of their passive characteristics alone, afferents from b₁b₂c units could not be readily distinguished from those from b₂c units. The characteristics of these receptors, and their differences from the well-studied hind-limb spindle afferents, are discussed.     

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).     

The ‘initial burst’ of human primary muscle spindle afferents has at least two components

Ten muscle spindle primary afferents from the extensor digitorum communis muscle of man were studied with single unit afferent recordings. Responses to slow test stretches with three different pre-history conditions were assessed to investigate the contribution of rapid stretches to the stretch sensitization phenomenon. In two of the conditions, the slow test ramps were preceded by rapid stretch after which the parent muscle of the receptor was either (a) kept short for 5 seconds or (b) kept long for 3.2 seconds and then returned to the short muscle length for 5 seconds. The third condition (c) consisted of a slow stretch from short to long muscle length followed by a rapid return to the short muscle length, in turn followed by 5 seconds at the short muscle length. Afferent responses were depressed when the muscle had been kept at the long length after the rapid stretches (condition b) and enhanced when the muscle had been kept at the short length (conditions a & c). A prominent ‘initial burst’ was only present in the afferent discharge when the parent muscles of the primary endings had been kept short (condition a). A second, more prolonged burst was present for conditions (a) and (c) but was lacking or inconspicuous when the muscle had been kept long after rapid stretches (condition b). The rapid stretches in the stretch sensitization paradigm appear to be a primary factor not only for the enhanced responses of sensitized primary afferents but also for the depressed responses of desensitized primary afferents.     

The 'initial burst' of human primary muscle spindle afferents has at least two components

Ten muscle spindle primary afferents from the extensor digitorum communis muscle of man were studied with single unit afferent recordings. Responses to slow test stretches with three different pre-history conditions were assessed to investigate the contribution of rapid stretches to the stretch sensitization phenomenon. In two of the conditions, the slow test ramps were preceded by rapid stretch after which the parent muscle of the receptor was either (a) kept short for 5 seconds or (b) kept long for 3.2 seconds and then returned to the short muscle length for 5 seconds. The third condition (c) consisted of a slow stretch from short to long muscle length followed by a rapid return to the short muscle length, in turn followed by 5 seconds at the short muscle length. Afferent responses were depressed when the muscle had been kept at the long length after the rapid stretches (condition b) and enhanced when the muscle had been kept at the short length (conditions a & c). A prominent 'initial burst' was only present in the afferent discharge when the parent muscles of the primary endings had been kept short (condition a). A second, more prolonged burst was present for conditions (a) and (c) but was lacking or inconspicuous when the muscle had been kept long after rapid stretches (condition b). The rapid stretches in the stretch sensitization paradigm appear to be a primary factor not only for the enhanced responses of sensitized primary afferents but also for the depressed responses of desensitized primary afferents.     

Tendon organs of cat medial gastrocnemius: responses to active and passive forces as a function of muscle length.

The responses of 13 Golgi tendon organs to graded force development of 29 motor units in medial gastrocnemius of the cat have been studied in five experiments. Of the 13 tendon organs, 11 were responsive to passive stretch within the physiological range of muscle length and 5 were "spontaneously" active at very short lengths where no passive tension could be recorded. The relationship between passive force and the firing rates of the various afferents ranged from a linear one to a power relation (Y = Axb + c) with b, a widely varying exponent. Results support the general conclusion that although many Ib afferents respond to passive force within the physiological range of muscle stretch, this form of stimulus is not a particularly effective one. The statis responses of Golgi tendon organs to active force development produced by single motor units was studied at different muscle lengths. In all cases the apparent sensitivity (change in firing rate per active force developed) decreased as muscle length approached Lo. The static responses of Golgi tendon organs to force developed by single motor units were also studied during fatiguing contractions. The data suggest a sigmoid relationship between force developed at the tendon and the Ib response. The collective response of all 13 tendon organs to active and passive forces at different muscle lengths was also examined. This analysis offered further support for the viewpoint that active motor unit contractions provide themost significant excitatory input to tendon organs and that changes in passive force during muscle stretch have comparatively little effect on the collective tendon organ response. The interaction between active and passive force inputs to the Golgi tendon organs is discussed in relation to the complicated nature of the relationship between forces measured at the tendon and those acting within the receptor capsule. When these complications were taken into account it was possible to explain the differences in responsiveness of a given tendon organ to active contraction of several motor units and to passive force in terms of a single force-firing rate curve for the receptor. It is concluded that changes in the force of contraction of single motor units result in relatively small changes in Ib afferent firing and that during normal muscle contractions, changes in the number of motor units acting on a single receptor must produce far more significant changes in firing rate than changes in the amount of force developed by any single unit. Changes in dynamic Ib sensitivity to single motor unit contractions are also shown to depend on length and in a similar way to the changes in static Ib sensitivity. During fatiguing contractions, a sigmoid relation was found between the dynamic Ib response and the rate of force development by single motor units.     

Neural mechanisms underlying the clasp-knife reflex in the cat. I. Characteristics of the reflex

1. The goal of this study was to characterize the clasp-knife reflex by the use of stretch and isometric contraction of ankle extensor and flexor muscles in decerebrated cats with bilateral dorsal hemisections of their spinal cords at segment T12. 2. Stretch of an extensor muscle evoked inhibition in both homonymous and synergistic extensor muscles. The similarities between homonymous and synergistic inhibition suggest that similar neural mechanisms were responsible. 3. Homonymous and synergistic clasp-knife inhibition showed several characteristic features: 1) inhibition was evoked only by large stretches that produced significant muscle force. Short stretches that did not produce large forces evoked only excitation; 2) the magnitude of clasp-knife inhibition increased with increasing initial motor output, as reflected in the level of rectified EMG; 3) the time course of reflex inhibition evoked by ramp-and-hold stretch was characterized by segmentation of EMG during ramp stretch, dynamic overshoot of inhibition at the end-of-ramp stretch, and slow but usually complete decay of inhibition during maintained stretch; 4) inhibition persisted beyond the termination of stretch, and 5) inhibition showed adaptation to repeated stretch. 4. Isometric contraction of the soleus or medial gastrocnemius, produced by electrical stimulation of the muscle nerve, also evoked powerful synergistic-reflex inhibition via similar mechanisms as stretch-evoked, clasp-knife inhibition. Stretch evoked a greater degree of inhibition than did contraction, indicating that receptors responsive to both stretch and contraction contribute to clasp-knife inhibition. 5. The reflex effects produced by stretching the soleus or medial gastrocnemius were not confined to the homonymous and close synergistic muscles. Extensor muscles were inhibited and flexor muscles were excited throughout the hindlimb, which paralleled the pattern of a flexion-withdrawal reflex evoked by cutaneous stimulation. 6. Stretch of a flexor muscle, the tibialis anterior, evoked the same spatial pattern and time course of reflex action as stretch of an extensor muscle--inhibition of extensor muscles and excitation of flexor muscles throughout the hindlimb, including homonymous excitation of the tibialis anterior. 7. We conclude that neither Golgi tendon organs nor secondary spindle afferents are likely to contribute significantly to clasp-knife inhibition because their responses to stretch and isometric contraction differ from the reflex actions evoked by stretch and contraction.(ABSTRACT TRUNCATED AT 400 WORDS)     

Static and Dynamic Properties of Golgi Tendon Organs in the Anterior Tibial and Soleus Muscles of the Cat

The responses of tendon organs of the anterior tibial muscle have been studied during twitch contraction, passive stretch, fused tetanic contractions and sinusoidal stretching in Nembutal anesthetized cats. The majority of the tendon organs had similar thresholds to active contraction and passive stretch tension. Some tendon organs were found to discharge spontaneously in the relaxed muscle. An approximate power function relationship has been found between steady state impulse frequency and muscle tension. The receptor properties are compared with those of soleus tendon organs. It is concluded that static and dynamic properties of Golgi tendon organs in the anterior tibial and the soleus muscles fall within the same range of variation, and that apparent discharge dissimilarities can be explained as due to differences in muscle mechanics.     

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.     

Responses of cortical neurons (areas 3a and 4) to ramp stretch of hindlimb muscles in the baboon.

1. A study was made of the response of single cortical units in areas 3a and 4 to electrical stimulation of hindlimb muscle nerves and to ramp stretch of hindlimb muscles in baboons anesthetized with chloralose.2. Stimulation of hindlimb muscle nerves revealed a group I projection primarily to area 3a but with some input into adjacent area. 4. A major group II projection was found in area 4 adjacent to area 3a. A small number of area 3a neurons receive convergence from both group I and group II muscle afferents.3a. On the basis of their response pattern to ramp stretch, units were classified into one of six categories and their cytoarchitectonic location was determined. Units in area 3a had hynamic sensitivities equivalent to that of the primary spindle afferents. Although the discharge of some area 3a neurons also reflected differences in muscle length, most area 3a neurons had low position sensitivities. One unit type in area 3a did not respond to maintained muscle stretch and signaled only velocity of stretch.4. Units in area 4 had position sensitivities equivalent to that of primary and secondary spindle afferents. Although the discharge of some area 4 units reflected different velocities of muscle stretch, these units had dynamic sensitivities similar to those of secondary spindle afferents rather than those of primary afferents. One type of unit in area 4 had no dynamic component to muscle stretch and signaled only muscle length.5. The results demonstrate that there is a transfer of dynamic and position sensitivity from spindle afferents to cortical neurons. Furthermore, data processing has occurred because some units respond only to the steady-state length of muscle, while other units encode only the dynamic phase of stretch. This behavior is different from the responses to ramp stretch of either group I or group II muscle afferents in the baboon.6. The results demonstrate that single units in cerebral cortex can encode the information transmitted to the central nervous system by muscle spindle afferents. The purpose for which this information is used remains undetermined.     

Spindle model responsive to mixed fusimotor inputs and testable predictions of beta feedback effects

Skeletofusimotor (beta) motoneurons innervate both extrafusal muscle units and muscle fibers within muscle spindle stretch receptors. By receiving excitation from group Ia muscle spindle afferents and driving the muscle spindle afferents that excite them, they form a positive feedback loop of unknown function. To study it, we developed a computationally efficient model of group Ia afferent behavior, capable of responding to multiple fusimotor inputs, that matched experimental data. This spindle model was then incorporated into a simulation of group Ia feedback during ramp/hold and triangular stretches with and without closure of the beta loop, assuming that gamma and beta fusimotor drives of the same type (static or dynamic) have identical effects on spindle afferent firing. The effects of beta feedback were implemented by driving a fusimotor input with a delayed and filtered fraction of the spindle afferent output. During triangular stretches, feedback through static beta motoneurons enhanced Ia afferent firing during shortening of the spindle. In contrast, closure of a dynamic beta loop increased Ia firing during lengthening. The strength of beta feedback, estimated as a "loop gain" was comparable to experimental estimates. The loop gain increased with velocity and amplitude of stretch but decreased with increased superimposed gamma fusimotor rates. The strongest loop gains were seen when the beta loop and the gamma bias were of different types (static vs. dynamic).     

Muscle spindle afferent studies in the baboon

1. The muscle spindle afferent conduction velocity and response to muscle twitch and stretch in young baboons has been recorded in order to find a conduction velocity that can be used to separate primary and secondary afferents.2. A number of the features of the response of the spindle afferents to stretch were examined. It was found that none could distinguish between primary and secondary afferents with greater certainty than the conduction velocity.3. Spindle afferents with conduction velocities below 50 m/sec can be classified as secondary and those with conduction velocities above 68 m/sec can be classified as primary with a reasonable degree of certainty.4. The spindle afferents with conduction velocities between 51 and 67 m/sec are most likely not a separate intermediate group but represent a region of overlap between the two groups.5. Comparisons of the difference between the response of primary and secondary afferents to overstretch suggested a mechanism to explain the saturation of the primary afferent response at frequencies far below those at which it is capable of firing.     

Reduced short and long latency reflexes during voluntary tracking movement of the human wrist joint

In six healthy human subjects we compared changes in the strength of Hoffmann (H), short latency (30-55 ms) and long latency (55-100 ms) stretch reflexes of flexor carpi radialis (FCR) muscle during movement and isometric contractions. In one set of experiments, stretches were imposed to the wrist during voluntarily tracked sinusoidal movement and during matched isometric contractions to compare short and long latency stretch reflex responses. In the second set, H-reflexes were compared during similar matched conditions. All reflexes decreased significantly (P < 0.05) during the voluntary tracking movement. The H-reflex was reduced during the wrist flexion, on average, by 33% of its value obtained during the isometric condition. Compared with their values during isometric conditions, the short latency stretch reflex and long latency stretch reflex during movement were reduced by 52 and 40%, respectively. From the pattern changes of the stretch reflexes and the H-reflex, a movement-induced presynaptic inhibition combined with pronounced muscle spindle unloading is proposed to play an important role in decreasing the strength of the stretch reflexes during the tracking task as compared with a matched isometric contraction.     

Effects of UTP on Na+, Cl− and K+ transport in primary cultures from human sweat gland coils

In six healthy human subjects we compared changes in the strength of Hoffmann (H), short latency (30–55 ms) and long latency (55–100 ms) stretch reflexes of flexor carpi radialis (FCR) muscle during movement and isometric contractions. In one set of experiments, stretches were imposed to the wrist during voluntarily tracked sinusoidal movement and during matched isometric contractions to compare short and long latency stretch reflex responses. In the second set, H-reflexes were compared during similar matched conditions. All reflexes decreased significantly (P < 0.05) during the voluntary tracking movement. The H-reflex was reduced during the wrist flexion, on average, by 33% of its value obtained during the isometric condition. Compared with their values during isometric conditions, the short latency stretch reflex and long latency stretch reflex during movement were reduced by 52 and 40%, respectively. From the pattern changes of the stretch reflexes and the H-reflex, a movement-induced presynaptic inhibition combined with pronounced muscle spindle unloading is proposed to play an important role in decreasing the strength of the stretch reflexes during the tracking task as compared with a matched isometric contraction.