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.     

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)     

Classification of human muscle stretch receptor afferents: a Bayesian approach

1. A sample of 124 human muscle afferents originating from the finger extensor muscles were recorded from the radial nerve in the upper arm. A method is described to formalize the classification of units in muscle spindle primary and secondary afferents and Golgi tendon organ afferents on the basis of a few, nonrigorous assumptions. The classification was based on experimental data that largely have been described in a series of previous papers, although some additional data were collected in the present study. 2. The units were subjected to five tests providing identification data: twitch contraction test, ramp-and-hold stretch, small-amplitude sinusoidal stretches superimposed on ramp stretch, stretch sensitization, and isometric contraction/relaxation. From these five tests the following eight response features were extracted: response to maximal isometric twitch contractions, type of stretch sensitization, correlation between discharge rate and contractile force, response to sudden isometric relaxation, presence or absence of an initial burst, deceleration response, prompt silencing at slow muscle shortening, and driving by small-amplitude sinusoidal stretches. 3. A Bayesian decision procedure was adopted to classify the units on the basis of the eight discriminators. As a first step, units were provisionally classified into muscle spindle primary and secondary afferents, and Golgi tendon organ afferents, by intuitively weighting their responses to the identification tests. Prior probabilities were estimated on the basis of the provisional classification. The eight response features were analyzed and tabulated for all afferents, and the likelihood functions of the tests were directly calculated on the basis of these data.(ABSTRACT TRUNCATED AT 250 WORDS)     

Responses of Ia and spindle group II afferents to single motor-unit contractions

1. The responses of deefferented Ia and spindle group II afferents to electrically activated twitch contractions of randomly selected motor units of the cat tibialis posterior muscle have been studied. Each afferent was paired with from 8 to 20 of the muscle's 60 motor units, and each afferent-motor unit interaction was recorded to two muscle lengths. 2. Cross-correlation histograms were compiled for each afferent-motor unit interaction studied as well as the average twitch tension produced by the motor unit. A numerical "coupling index" was computed for the histogram distributions to quantitate the extent of mechanical coupling between the receptor and the single motor units. 3. Qualitatively, no consistent differences were noted in the responses of Ia and spindle group II afferents to single motor-unit contractions. However, Ia afferents were responsive to a higher percentage of motor units with which they were tested (89%) and, on the average, displayed a significantly larger magnitude of response (mean coupling index, 0.72 +/- 0.04 SE) than the spindle group II afferents (66% of motor units; mean coupling index, 0.51 +/- 0.03). 4. The extent to which a motor-unit contraction altered the discharge pattern of a spindle afferent was not strictly related to the amount of force generated by the unit, nor to its contraction time. 5. Muscle length exerted a strong influence on both the qualitative and quantitative features of many of the motor unit-muscle receptor interactions. 6. These results suggest that the degree of "mechanical coupling" between a receptor and a motor unit is largely dependent on anatomical arrangements and reinforce the possibility that muscle receptors generate a "sensory partitioning" of the motor-unit population within a muscle.     

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)     

The tendon organs of cat medial gastrocnemius: significance of motor unit type and size for the activation of Ib afferents.

1. Histological and histochemical studies suggest that each tendon organ in a mixed mammalian muscle should be particularly responsive to the contraction of a discrete number of motor units (ca. ten to fifteen), each with differing mechanical properties. This report describes physiological experiments that demonstrate this arrangement for the tendon organs of cat medial gastrocnemius. 2. No correlations could be found between the intensity of discharge of a single tendon organ and the contraction strengths of motor units whose contraction excited the receptor. Tendon organs were found to be as responsive to contraction of small slow twitch units as they were to contraction of larger fast twitch units. Taking the data as a whole, the apparent sensitivity of the receptors during motor unit contractions (pps/force recorded at the tendon) was inversely related to the contraction strengths of the motor units. 3. These findings are discussed in relation to recent evidence on the territory of single motor units in medial gastrocnemius and the force producing capabilities of their individual muscle fibres. It is concluded that in general each motor unit, whose contraction excites a given receptor, contributes one muscle fibre to the receptor capsule. Further, it appears that the various excitatory effects of those muscle fibres inserting into a given receptor capsule are not simply related to their relative contraction strengths but also depend on the details of the mechanical coupling between each fibre and the Ib afferent receptor endings. 4. The results of an ensemble analysis show that despite the lack of correlation between the intensity of tendon organ discharge and the force developed at the tendon during contraction of different motor units, a correlation does appear when the responses of several tendon organs and the forces developed by the motor units which excite them are summed progressively. This finding has implications for the recruitment order of motor units in that the profile of the collective Ib response is shown to differ according to whether motor unit forces are summed randomly or in order of increasing contraction strengths.     

The tendon organs of cat soleus: Static sensitivity to active force

The static force sensitivity of soleus tendon organ (Ib) afferents has been studied by noting their responses to graded force development produced by isometric contractions of either the whole muscle or single motor units. Data included responses of 23 Ib afferents to contraction of 8 whole muscles (8 experiments) and 16 Ib afferents to contraction of 30 motor units (5 experiments). Tendon organ responses of varying magnitude to contraction of the whole muscle or several of its individual motor units could be explained by differences in the number of muscle fibers that insert into each receptor's capsule and by differences in the contraction strength of these fibers. This finding suggests that soleus tendon organs have similar absolute sensitivities to static force development. An estimate was made of this absolute sensitivity and the value obtained (314 pps/g of force actually coupled to the receptor) is 2 orders of magnitude greater than those previously reported indices that simply relate Ib firing rate to force as measured at the tendon. The relationship between force exerted on a tendon organ's capsule and Ib firing rate is whown to be curvilinear and in keeping with a possible saturation effect that reduces the receptor's responsiveness to active contractions at relatively long muscle lengths.     

The response of Golgi tendon organs to single motor unit contractions.

1. Cross-correlation analysis has been used to quantify the responses of cat soleus tendon organs to repetitive twitch contractions of: (a) different motor units within the muscle, (b) single motor units at different muscle lengths, and (c) single motor units when the pulse-train pattern of stimulation delivered to the motor unit axon was altered. 2. Ib afferents were observed which responded to each of several hundred successive motor unit twitches with identical numbers of spikes and with relatively invariant latencies. 3. The present results show that tendon organs are sensitive to subtle alterations in motor unit twitch wave form and amplitude, and that this sensitivity is reflected in the precise timings of their afferent discharge. 4. Examination of these tendon organ responses indicates that the forces produced by single motor units couples to the receptor capsule are well above threshold. Calculations based on these results, and earlier soleus motor unit and muscle fibre data, suggest that the absolute force threshold for tendon organs may be as little as 4 mg, which is less than the estimated minimum twitch force generated by individual soleus muscle fibres. 5. Considering the number of tendon organs in a muscle, and the likelihood that every motor unit is connected with at least one receptor, the sensitivity of tendon organs ensures that every twitch of every motor unit will be reflected in the population of afferent signals projecting to the spinal cord.     

Effects of muscle shortening on the responses of cat tendon organs to unfused contractions

1. The discharges from individual Golgi tendon organs of peroneus tertius and brevis muscles were recorded in anesthetized cats. Responses to unfused isometric contractions of single motor units and combinations of motor units were compared with responses to contractions eliciting muscle shortening (i.e., shortening contractions). 2. In 75% of the examined instances, the effect of muscle shortening during unfused contractions was a slight decrease in tendon organ activation, in keeping with the reduction of contractile tension recorded at the muscle tendon. In other instances there was either no change in tendon organ response or, in less than 10% of instances, a slight increase For two motor units eliciting similar activation of a given tendon organ under isometric conditions, the effect of shortening contraction was not necessarily the same. 3. The reductions observed in tendon organ discharges upon muscle shortening were less than proportional to the reductions of contractile tension and difficult to correlate with the properties of motor units, as determined under isometric conditions. The present observations suggest three main reasons for this lack of relation. 4. The first reason depended on the properties of motor units, in that the relation between length changes and tension changes was not the same for all units. Two motor units developing similar isometric tensions did not necessarily produce the same degree of muscle shortening. Some units produced relatively significant shortening without much loss of tension. 5. Second, the dynamic sensitivity of tendon organs is known to exert a major influence on their responses to isometric unfused contractions, accounting for 1:1 driving of discharge during tension oscillations and high frequency bursts upon abrupt increase of tension. Although less tension was produced and the rate of tension development was slower in shortening contractions, similar manifestations of the dynamic sensitivity of tendon organs were observed. In such cases, the responses of tendon organs were the same whether or not the muscle shortened during contraction. 6. Third, when several motor units were stimulated in combination, the unloading influences of in-parallel units were facilitated by muscle shortening so that unloading effects, which were hardly visible under isometric conditions became evident during shortening contractions.     

Relations between identified tendon organs and motor units in the medial gastrocnemius muscle of the cat

Summary In one medial gastrocnemius muscle of each of several cats, the response was recorded of a single tendon organ to the contraction of a single motor unit which strongly excited the receptor. The motor unit was depleted of its glycogen and the depleted muscle fibres identified in PAS-stained transverse sections. The site of maximum tendon organ sensitivity was marked and the tendon organ identified in the same sections. Five pairs of tendon organs and motor units were studied completely. Each tendon organ was found to have one or two (mean 1.6) depleted muscle fibres attached to it, included in the bundle of fibres attached to the end (mean no. 14.4) and side (mean no. 5.6) of the tendon organ. A correlation was found between tendon organ discharge rate and the tension calculated from cross-sectional area measurements of the depleted muscle fibres attached to the tendon organ, with variation between individual pairs of tendon organs and motor units. One estimate of the average sensitivity of the sample was 28 imp/s/mN. A nearly linear discharge rate vs. tension relation was found for single tendon organ and motor unit pairs when tension was graded during a series of fatiguing contractions. Under these conditions the sensitivity, measured as the slope of the relation between discharge rate and motor unit tension recorded at the common tendon, varied between 0.11 and 0.30 imp/s/mN for 6 pairs.     

Organization of group I activated cells in the main and external cuneate nuclei of the cat: Identification of muscle receptors

Summary Extracellular recording was made from 77 primary afferent fibres, 106 cells in the external cuneate nucleus, and 60 cells in the main cuneate nucleus, all activated by slowly adapting muscle stretch receptors. The nature of the muscle receptors responsible for the activation was determined by various types of receptor stimulation.Primary group I afferents from muscle spindles and tendon organs in distal forelimb muscles showed complete overlap of conduction velocities and thresholds to electrical stimulation. Both types of group I afferents as well as group II muscle spindle afferents were shown to ascend through the dorsal funiculus to the level of the cuneate nuclei.Three groups of cells were identified in the external cuneate nucleus, activated by group I muscle spindle afferents, tendon organ afferents and group II muscle spindle afferents, respectively.Almost all group I activated cells in the main cuneate nucleus, including all 34 cells identified as cuneo-thalamic relay cells, received their afferent input from muscle spindle afferents. Three cells were activated by tendon organ afferents.     

Correlation analysis of muscle spindle responses to single motor unit contractions.

1. Cross-correlation techniques have been used to study the responses of muscle spindle afferents from the soleus muscle of the cat to twitch contractions of single motor units. 2. Cross-correlograms (post-stimulus time histograms) were used to give the frequency of occurrence of a receptor spike at various times following the initiation of a motor unit contraction together with a display of the average twitch tension wave form. 3. The cross-correlograms revealed that the contraction of a single motor unit can be an effective stimulus to a spindle receptor and may induce afferent firing pattern alterations similar to those observed with whole muscle contraction. 4. The cross-correlograms also revealed quantitative differences in the response of a receptor to contraction of different motor units and to contraction of the same motor unit at different lengths. These differences reflect subtle changes in receptor deformation developed by the twitch of a motor unit under different conditions and by the twitches of different motor units. The results are consistent with anatomical data on the number and distribution of motor units and receptor organs in cat soleus. 5. These findings emphasize that rather than simply acting as generalized force or length sensors for the muscle as a whole, each receptor's spike train carries information about the state of a particular set of motor units.     

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.     

Ensemble discharge from Golgi tendon organs of cat peroneus tertius muscle

1. The responses of individual tendon organs of the cat peroneus tertius muscle to motor-unit contractions were recorded in anesthetized cats during experiments in which all the Ib-afferent fibers from the muscle had been prepared for recording in dorsal root filaments. This was possible because the cat peroneus tertius only contains a relatively small complement of approximately 10 tendon organs. 2. Motor units of different physiological types were tested for their effects on the whole population of tendon organs in the muscle. Effects of unfused tetanic contractions were tested under both isometric and anisometric conditions. Each motor unit activated at least one tendon organ, and each tendon organ was activated by at least one motor unit. Individual slow-type units were found to act on a single or two receptors, whereas a fast-type unit could activate up to six tendon organs. 3. In one experiment, the effects of 8 motor units on 10 tendon organs were examined. One fast-twitch, fatigue resistant (FR)-type unit acted on six tendon organs, of which four were also activated by another FR unit. The contraction of each unit, on its own, elicited a range of individual responses, from weak to strong. The discharge frequencies of individual responses displayed no clear relation with the strength of contraction, nor did they accurately represent the shape of force profiles. But when all the discharges were pooled, a fairly good correspondence appeared between variations of contractile force and variations of averaged discharge frequencies.(ABSTRACT TRUNCATED AT 250 WORDS)     

The discharge of cat tendon organs during unloading contractions

Summary Tendon organs respond preferentially to contractions of a select set of motor units. If a tendon organ is given a resting discharge by raising the passive tension in the muscle, other motor units can be identified which on contraction interrupt the discharge, presumably by unloading the passive tension. These experiments investigate the possibility that stimulation of motor units with unloading effects can reduce the response of a tendon organ to a loading contraction. We find that over most of the range of muscle lengths the unloading contraction produces only a transient lowering of the firing rate. Only at short muscle lengths where further shortening is accompanied by a steep fall in tension is the unloading contraction able to induce a sustained reduction of afferent discharge.     

Absence of somatotopic projection of muscle afferents onto motoneurons of same muscle

During weak voluntary contractions, muscle afferents from a restricted region of the tibialis anterior of humans were activated with mechanical or electrical stimuli while electromyographic recordings (EMG) were made from pairs of motor units. One motor unit of a pair was located in the region of muscle from which the afferents arose; the other was at least 10 cm distal. The territories of the motor units of each pair did not overlap. All motor units were of low threshold, recruited at less than 5% of maximal voluntary strength. Direct recordings of muscle afferent activity, using a microelectrode in the nerve fascicle innervating the tibialis anterior, showed that taps delivered to the muscle belly activated submaximally a discrete population of receptors, restricted to the site of the taps. The spread of the disturbance set up by the taps or by vibration at 100 Hz applied to the muscle belly was measured with an accelerometer and was also found to be restricted to the site of application of the vibrator tip. With each of 12 pairs of motor units, vibration at 100 Hz was applied to the muscle near the motor unit of higher threshold for recruitment in a voluntary contraction. The vibration produced detectable reflex effects but did not alter recruitment order within any pair of motor units. During voluntary contractions involving 10 pairs of motor units, taps were applied to the muscle belly near each motor unit of a pair. The taps produced, at short latency, an increase in the probability of discharge of each motor unit. This change in probability is related to a composite excitatory postsynaptic potential (EPSP) caused by dynamically responding mechanoreceptors near the tap site and is generated through monosynaptic/oligosynaptic pathways. Taps near one motor unit of a pair did not selectively or preferentially affect the discharge of that motoneuron. During a voluntary contraction of tibialis anterior, electrical stimuli just below threshold for efferent axons, and so probably above threshold for afferent axons, were delivered singly and in brief trains through a microelectrode in a nerve fascicle innervating that muscle. Such stimulation had no detectable effect on the discharge pattern of four pairs of motor units. These findings suggest that the feedback from a few receptors in the muscle has a negligible effect on the motoneuron pool, in comparison with the total excitatory drive present during a voluntary contraction.(ABSTRACT TRUNCATED AT 400 WORDS)     

Interaction of recurrent inhibitory and muscle spindle afferent feedback during muscle fatigue.

Mammalian skeletal motor units have differing properties including their different susceptibility to fatigue. The question discussed in this paper is whether and to what extent proprioceptive feedback via muscle spindles can contribute to shape the firing patterns of motor units so as to minimize their loss of force during fatiguing contraction. The firing of a skeleto-motoneuron dispatches signals which are fed back to the same and homonymous as well as synergistic motoneuron. Two feedback pathways are of concern here: one via the related muscle unit and muscle spindle afferents (proprioceptive path), and one via recurrent motor axon collaterals and Renshaw cells (recurrent inhibitory path). It is suggested that the contraction of a motor unit or a small group of adjacent ones is signalled to the homonymous alpha-motoneurons via proprioceptive afferents, the signal being filtered and enhanced by spinal recurrent inhibition. This is effected by timed correlation of the signals which are propagated through the two feedback loops. The effects of the correlation can be strengthened by (i) topographical order of the feedback connections, (ii) heterosynaptic interaction, and (iii) tendencies towards synchronous discharge between motoneurons. These mechanisms render the possibility more likely that information about the unfused contractions of a muscle unit (or a small group of them), mediated via proprioceptive afferents, play a role in shaping the precise discharge pattern of the innervating motoneuron(s). These mechanisms could be used to optimize the force output of large fatiguing motor units during long activation, during which their activation rates normally decrease (adapt) over time. Our results show that during adapting motoneuron firing Renshaw cells and muscle spindle afferents may show discharge patterns which at least in part are in keeping with such an hypothesis.     

Effects of temperature on the discharges of muscle spindles and tendon organs

1. In anaesthetized cats the effects of temperature on the nervous outflow from skeletal muscle via thick myelinated afferent fibres were studied. Single unit recordings were made from afferents of muscle spindles and tendon organs during slow and fast temperature changes of the medial gastrocnemius muscle which was deefferented by ventral root section and prestretched to a tension of 100 p.
2. Group I afferent units from muscle spindles were activated by warming and depressed by cooling, the effect of warming being much more pronounced than that of cooling. Afferents from secondary spindle endings with a high background discharge behaved similar to Ia fibres, whereas those with a low initial discharge rate showed an activation by cooling and a depression (mostly to cessation of firing) by warming. The discharges of group I afferents from tendon organs varied; an activation by warming was the most frequently observed reaction.
3. Some of the afferents from muscle spindles and tendon organs showed signs of a dynamic sensitivity to thermal stimulation, but in general the dynamic component in the responses to temperature changes was only small.
4. The results suggest that the afferent outflow via thick myelinated fibres from a resting, moderately prestretched muscle strongly depends on temperature. At raised intramuscular temperatures (about 42°C) the nervous outflow is characterized by an increased activity in all of the I a and many of the I b afferents, while the majority of group II spindle afferents will be depressed. In contrast, in a cold muscle (about 29°C) the nervous outflow via afferents from primary spindle endings will be reduced, while the net activity from secondary spindle endings will be increased and no marked changes are expected to occur in the discharges of I b fibres.

     

The “size principle” and synaptic effectiveness of muscle afferent projections to human extensor carpi radialis motoneurones during wrist extension

The question of whether muscle spindle afferents might control human motoneurone activity on the basis of the “size principle” during voluntary contraction was investigated by recording the discharge of single motor units (n=196) in wrist extensor muscles while stimulating the homonymous muscle spindles by means of tendon taps. The mechanical stimuli were delivered with a constant post-spike delay of 80 ms so that the resulting afferent volleys could be expected to reach the motoneurones towards the end of the inter-spike interval (mean±SD duration: 124.7±11.9 ms). In the six subjects tested, the response probability was found to be significantly correlated with the motor units' functional parameters. Differences in twitch rise times, twitch amplitudes, recruitment thresholds and macro-potential areas were found to account for 18%, 9%, 6% and 2% of the differences in the response probability observed within the whole population of motor units tested. These differences could not be due to differences in firing rate for two reasons: first, the motor units were found to discharge with a similar range of inter-spike intervals whatever their functional characteristics; secondly, the weak positive correlation observed between the response probability and the motor unit firing rate showed parallel regression lines between the late-recruited fast-contracting motor units and the first-recruited slowly contracting motor units, but they-intercept was significantly higher in the latter case. This confirmed that the responses of the first-recruited slowly contracting motor units tended to be larger whatever the firing rates. In most of the pairs tested in the same experiment, the motor units which had the lowest recruitment thresholds, longest contraction times, smallest contraction forces or smallest motor unit macro-potentials tended to produce the largest responses, which also had the longest latencies. Taking the response latency to be an index of a motoneurone's conduction velocity and therefore of its size, the data obtained with this index — and with other functional indices such as the twitch rise times and amplitudes, the macro-potential areas and the recruitment thresholds — can be said to be fully consistent with the “size principle”, as previously found in anaesthetized animals. It can be inferred that the presynaptic inhibition which is liable to take action during voluntary contraction does not seem to alter the graded distribution of the muscle afferent projections to human wrist extensor motoneurones.     

Response characteristics of muscle afferents in the domestic duck

1. Response patterns of 116 muscle stretch receptor units isolated from the sciatic nerve of the duck have been studied, and the units classified as muscle spindles and tendon organs.2. Units classified as spindles had low threshold tensions for maintained discharge. From conduction-velocity measurements, the calculated fibrediameter spectrum appears to be unimodal, ranging from 5 to 11-12 mum.3. Spindle units showed essentially ;in parallel' behaviour, though increase in initial tension often led to the appearance of ;in series' responses. Although apparent ;alpha-excitation' during maximal tetanic contractions was a common occurrence, no direct evidence of alpha-innervation of spindles was obtained.4. Evidence has been obtained for motor innervation of spindles by fibres distinct from those constituting the alpha supply to extrafusal muscle fibres. Afferent response attributable to this fusimotor innervation is influenced by initial tension and stimulus-frequency. Electrical thresholds for fusimotor responses ranged from 1.1 to 4.03 times alpha maximum.5. Tendon organ units consistently showed ;in series' response patterns during muscle contractions. They were not influenced by stimulation of the high-threshold efferent nerve supply to the muscles.6. Threshold tensions required for maintained discharge in tendon organ units from m. gastrocnemius pars lateralis were characteristically high; however, many units from m. flexor perforans et perforatus d. 3 had unexpectedly low mechanical thresholds. The calculated fibre-diameter spectrum for tendon organ units is unimodal, ranging from 4-7 to 10-11 mum. As in mammals, they contribute to the coarse-fibre component in the muscle nerve and include the fastest fibres present.