The influence of synovial inflammation and hyperplasia on symptomatic outcomes up to 2 years post-operatively in patients undergoing partial meniscectomy

OBJECTIVE

Synovitis is associated with pain and other symptoms in patients with knee osteoarthritis (OA), and in patients with meniscal tears even in the absence of radiographic OA. Patients undergoing arthroscopic partial meniscectomy were followed for 2 years to determine whether synovitis predicts post-operative symptoms.

DESIGN

Thirty-three patients scheduled for arthroscopy were recruited for this pilot study. Symptoms were assessed using a knee pain scale, the Lysholm score, and the short form-12 (SF-12(®)) pre-operatively and at 16 weeks, 1 year and 2 years post-operatively. Synovial inflammation and hyperplasia were graded on surgical biopsies. Linear mixed effects models were tested to determine whether inflammation or hyperplasia is associated with outcome scores over time.

RESULTS

Lysholm scores and SF-12(®) physical component sub-scores were worse pre-operatively in patients with inflammation (Lysholm: 52.42 [95% confidence interval (CI) 42.37, 62.47] vs 72.38 [66.03, 78.72], P < 0.001; SF-12: 36.81 [28.26, 45.37] vs 46.23 [40.14, 52.32], P < 0.05). Up to 2-years post-operatively, patients with inflammation achieved mean scores similar to those without inflammation. As a result, the mean improvement in Lysholm scores was 13.01 [1.48-24.53] points higher than patients without inflammation, P = 0.03. 33% (4/12) of patients with inflammation still had fair to poor Lysholm scores 2 years after surgery compared to 7% (1/15, P=0.14) without inflammation. No association between hyperplasia and symptoms was noted.

CONCLUSIONS

In this pilot study of patients undergoing partial meniscectomy, synovial inflammation was associated with worse pre-operative symptoms, but not with poorer outcomes in the first 2 years post-arthroscopy. Larger cohorts and longer follow-up should be pursued to confirm this relationship, and determine if the initial response is sustained.

Magnetic resonance imaging of relative glycosaminoglycan distribution in patients with autologous chondrocyte transplants

RATIONALE AND OBJECTIVES

Autologous chondrocyte transplantation (ACT) is a potential treatment for full-thickness chondral lesions in the knee. Delayed gadolinium-enhanced magnetic resonance imaging of cartilage (dGEMRIC) has recently been developed as a sensitive and specific measure of cartilage glycosaminoglycans (GAGs). Under the conditions of dGEMRIC, T1 is directly related to the GAG concentration. Our aim for this study was to demonstrate the potential of dGEMRIC to evaluate ACT implants.

METHODS

Eleven ACT implants were studied 2 to 24 months postoperatively by dGEMRIC. T1 values from three regions of interest were obtained to examine GAG content (1) in the implant, (2) in native cartilage adjacent to the implant, and (3) in native cartilage further removed from the implant (as "control").

RESULTS

One implant failed and therefore was not included. Four of the implants were studied between 2 and 6 months postoperatively and showed low T1 (GAG), less than 80% of the control native cartilage. Five of the six implants studied between 12 and 24 months postoperativley showed T1 (GAG) comparable to (>80%) of control. One 18-month graft showed low T1 comparable to the surrounding native cartilage, with normal GAG seen in cartilage far from the graft site. The GAG index (T1 values of the graft normalized to control) from the group of implants 6 months or less was 59% +/- 5% of control, whereas those at 12 to 24 months were 91% +/- 18% of control. The two groups were statistically different with a P value of 0.005.

CONCLUSIONS

The GAG level in grafts that were implanted for less than 12 months appeared to be lower than that in the remote cartilage. At 12 months or greater, the grafts in this study had GAG levels that were comparable to both the adjacent and remote cartilage. This preliminary study of ACT implants has shown that it is feasible to apply the dGEMRIC technique in patients with ACT as a way to obtain information related to the composition of grafts. These results provide motivation and the pilot data with which to design further clinical studies.

Human immunodeficiency virus type 1 shedding pattern in semen correlates with the compartmentalization of viral Quasi species between blood and semen

High levels of human immunodeficiency virus (HIV) type 1 have been detected in semen at all stages of disease. However, it is not clear whether HIV-1 is shed in semen continuously or intermittently. In a prospective longitudinal study, viral RNA was measured weekly for 10 weeks in semen and blood of HIV-seropositive subjects. Results showed three different patterns of HIV-1 shedding in semen: none (28%), continuous (28%), and intermittent (44%). In contrast, there was no change in blood plasma virus load during the study period. Phylogenetic analysis of the envelope sequences of HIV-1 RNA in semen and blood revealed distinct virus populations in semen and blood of intermittent shedders but similar virus populations in the semen and blood of continuous shedder. These results indicate for the first time that HIV-1 is shed primarily in an intermittent manner and that shedding patterns of HIV-1 in semen are related to compartmentalization of HIV-1 between semen and blood.

Coupling between human muscle spindle endings and motor units assessed using spike-triggered averaging

The electromyographic activity (EMG) generated by voluntary contraction of a muscle was averaged using the potentials from 18 identified muscle spindle afferents as a trigger. In post-spike averages of 1000-10,000 sweeps, no evidence of reflex excitation of the homonymous motoneurone pool was detected. In pre-spike averages there was no evidence of a motor-unit EMG potential that was closely correlated to the trigger spike. A single spindle afferent has only a weak reflex effect on an active motoneurone pool and must be part of a synchronized volley to affect motoneurone discharge significantly. No evidence was found for spindle activation via beta-motoneurones in weak voluntary contractions.

Monosynaptic and oligosynaptic contributions to human ankle jerk and H-reflex

Studies were undertaken in normal subjects to determine whether it is possible for oligosynaptic reflex pathways to affect motoneuron discharge in the ankle jerk and H-reflex of the soleus. It is argued that if the rising phase of the increase in excitability of the soleus motoneuron pool produced by tendon percussion or by electrical stimulation of the peripheral nerve lasts more than a few milliseconds and if the increase in excitability takes several milliseconds to reach the threshold for motoneuron discharge, these reflexes are unlikely to be exclusively monosynaptic. In relaxed subjects, changes in excitability of the soleus motoneuron pool produced by tendon percussion and by electrical stimulation of the tibial nerve were examined using conditioning stimuli just below threshold and a test H-reflex just above threshold for a reflex response. The increase in excitability due to tendon percussion had an average rise time of 10.8 ms and a total duration of approximately 25 ms. With electrical stimulation the rising phase appeared shorter, but it could not be measured accurately due to afferent refractoriness. In single motor units, the rise times of the composite excitatory postsynaptic potentials (EPSPs) set up by subthreshold tendon percussion and by subthreshold electrical stimulation of the tibial nerve were estimated from changes in the probability of discharge of voluntarily activated single motor units. Rise times were significantly longer with tendon percussion (mean +/- SD, 7.1 +/- 2.3 ms; n = 34) than with electrical stimulation (2.4 +/- 1.4 ms; n = 32). In four experiments in which a number of motor units were studied using identical mechanical and identical electrical stimuli, the poststimulus time histograms (PSTHs) for each stimulus were pooled to provide an estimate of the rise time of the excitability change in the motoneuron pool. The mean rise times of these four samples were 10.5 ms with mechanical stimulation and 4.5 ms with electrical stimulation. The spontaneous variability in latency of reflexly activated single motor units was 0.8-3.1 ms (average SD, 0.34 ms) in the tendon jerk, and 0.6-1.4 ms (average SD, 0.19 ms) in the H-reflex. Comparison of these figures with the measurements of rise time given above suggests that the composite EPSPs are larger than the background synaptic noise. With six motor units, the timing of reflex discharge in the tendon jerk when the subject was relaxed was compared with the timing of the change in probability of discharge due to apparently identical percussion when the units were activated voluntarily.(ABSTRACT TRUNCATED AT 400 WORDS)

Reproducibility of the visual evoked potential using a light-emitting diode stimulator

The intersubject and intrasubject reproducibility of the VEP was evaluated using two different methods of producing pattern reversal--a mirror/projector system and a light-emitting diode (LED) system. Intersubject reproducibility was determined in 100 normal subjects (50 males, 50 females). Ten subjects were studied on ten different occasions over 11 months to establish intrasubject reproducibility. The two methods gave comparable results. It is concluded that the mirror/projector system has no advantages over the LED system, which is cheaper, more robust and more convenient. Most of the intersubject variability was found to be due to subject variables and it is concluded that technical improvements are unlikely to reduce further the variability of the test. A variability in the latency of P100 of up to 7.7 ms was recorded in the serial studies on the ten subjects, indicating that in longitudinal studies on patients, changes in latency must exceed 9-10 ms to be significant. With both stimulus systems, there were significant sex-related differences in latency (P50 and P100 but not N70) and amplitude (N70-P100). The extent of the difference was such that the upper limit of normal latency for P100 (2.5-3 SD from the mean) was 4.2-4.7 ms longer for males than females-a value which exceeds 1 SD. Separate control values for males and females are advisable whichever method is used to produce pattern reversal.

The projection of muscle afferents from the hand to cerebral cortex in man

This study provides direct evidence that muscle afferents from the human hand project to the cerebral cortex. Electrical stimulation within a purely muscle fascicle of the median nerve at the wrist produced a cerebral potential, the latency of which (to the initial negative deflection, N1) was the same as, or shorter than, the latency with stimulation of cutaneous fascicles. The average latencies to N1 for muscle afferent and cutaneous afferent inputs were 19.0 and 20.3 ms, respectively. This difference could not be accounted for by differences in peripheral conduction velocity. The site of the maximal initial negative deflection of the cerebral potential for muscle afferents was posterior to that for cutaneous afferents. This locus for muscle afferents corresponded to that for mixed afferent inputs. Muscle afferents contribute to, and may dominate, the cerebral potential produced by stimulation of the mixed median nerve at the wrist.

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)

The afferent volleys responsible for spinal proprioceptive reflexes in man

1. To define the neural volleys responsible for the Achilles tendon jerk and the H reflex, muscle afferent activity was recorded using micro-electrodes inserted percutaneously into appropriate fascicles of the tibial nerve in the popliteal fossa.2. The response of soleus muscle afferents to tendon percussion consisted of a dispersed volley, starting 3.5-7.0 ms after percussion, increasing to a peak over 6.5-11.0 ms, and lasting 25-30 ms, depending on the strength of percussion. Electrical stimuli to the sciatic nerve at a level adequate to evoke an H reflex but subthreshold for the M wave produced a more synchronized volley, the fastest fibres of which had conduction velocities of 62-67 m/s, and the slowest 36-45 m/s.3. The wave of acceleration produced by percussion subthreshold for the ankle jerk spread along the skin at over 150 m/s. Midway between the bellies of the gastrocnemii it consisted of a damped oscillation with four to five separate phases and maximum amplitude approximately one-twentieth of that recorded on the Achilles tendon.4. With ten primary spindle endings, tendon percussion subthreshold for the ankle jerk elicited two to five spike discharges per tap, the shortest interspike intervals being 4-7 ms. Tendon percussion elicited single discharges from two Golgi tendon organs, and altered the discharge pattern of a single secondary spindle ending. The degree of dispersion of the multi-unit muscle afferent volley can be explained by the pattern of discharge of primary spindle endings.5. Percussion on the Achilles tendon evoked crisp afferent volleys in recordings from nerve fascicles innervating flexor hallucis longus, tibialis posterior, the intrinsic muscles of the foot and the skin of the foot. Electrical stimuli delivered to the tibial nerve in the popliteal fossa at a level sufficient for the H reflex of soleus produced either a volley in muscle afferents from the intrinsic muscles of the foot or a volley in cutaneous afferents from the foot.6. For comparable stimuli in the two positions, the H reflex was inhibited but the Achilles tendon jerk enhanced when the ankle was dorsiflexed from 105 degrees to 90 degrees .7. The duration of the rise times of the excitatory post-synaptic potentials (e.p.s.p.s) produced in soleus motoneurones by electrical stimulation, and by tendon percussion subthreshold for the H reflex and the ankle jerk respectively, was estimated from post-stimulus time histograms of the discharge of voluntarily activated single motor units in soleus. The mean e.p.s.p. rise times were 1.9 ms for electrical stimulation and 6.6 ms for tendon percussion. There was evidence that the duration of the electrically evoked e.p.s.p. was curtailed by an inhibitory post-synaptic potential (i.p.s.p.) of only slightly longer latency than the e.p.s.p.8. The mechanically induced and electrically induced afferent volleys are not homogeneous volleys in group I a afferents from triceps surae. The afferent volleys differ in so many respects that it is probably invalid to compare the H reflex and tendon jerk as a measure of fusimotor activity. It is suggested that neither reflex can be considered a purely monosynaptic reflex.

The effect of warning and prior instruction on short-latency cerebral potentials produced by muscle afferents in man

This study examined the effect of a warning and of prior instruction on the early components of the cerebral potential produced by stimulation of the left posterior tibial nerve at the ankle. Early components of this potential are dominated by the activity in muscle afferents from the small muscles of the foot. In 50% of presentations, the stimulus to the posterior tibial nerve was preceded by an auditory cue. In some sequences subjects were required to move toes on the left foot immediately the stimulus was detected. Although subjects responded more rapidly to shocks which were preceded by a warning there was no statistically significant effect of the warning on the amplitude or latency of the early components of the muscle afferent cerebral potential. Prior instruction to respond to the stimulus also failed to change the cerebral potentials. This study suggests that the ability to respond more rapidly to "warned" stimuli is due to events "upstream" of the sensorimotor cortex rather than to enhancement of the volley arriving at the cortex.

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.

The relationship between the size of a muscle afferent volley and the cerebral potential it produces

This study examined the relationship between the size of an afferent neural input produced by electrical stimulation of the posterior tibial nerve at the ankle and the size of the early components of the evoked cerebral potential. For five of six subjects the first peak of the afferent neural volley recorded in the popliteal fossa was uncontaminated by either motor efferents or cutaneous afferents. This was established by measuring the conduction times of motor fibres in the posterior tibial nerve and cutaneous fibres in the sural and posterior tibial nerves over the ankle to popliteal fossa segment. It is likely therefore that the first peak of the afferent volley contained predominantly, if not exclusively, activity in rapidly conducting afferents from the small muscles of the foot. The size of the two earliest components of the cerebral potential did not increase in direct proportion to the size of the afferent volley which produced it. The early components of the cerebral potential reached a maximum when the responsible muscle afferent volley was less than 50% of its maximum.

Interactions between cutaneous and muscle afferent projections to cerebral cortex in man

In order to demonstrate interactions between cutaneous and muscle afferent volleys in the ascending somatosensory pathways, different nerves of the lower limb were stimulated together in a conditioning-test paradigm, the changes in the earliest component of the cerebral potential evoked by the test stimulus being taken to indicate such an interaction. It was first confirmed that the cerebral potential evoked by stimulation of the posterior tibial nerve at the ankle is derived from muscle afferents in the mixed nerve and has shorter latencies than the cerebral potential evoked by purely cutaneous volleys in the sural nerve (see Burke et al. 1981). Complete suppression of the cerebral potential evoked by stimulation of muscle or cutaneous afferents was produced by conditioning volleys in a different nerve or in a different fascicle of the same nerve. The major factors determining the degree of suppression were found to be the relative sizes of the conditioning and test volleys and their timing, rather than whether the volleys were of cutaneous or muscular origin. It is concluded that the transmission of cutaneous or muscle afferent volleys to cortex can be profoundly altered in normal subjects by conditioning activity. The possibility that normal background afferent activity can similarly modify afferent transmission has implications for diagnostic studies, particularly when they are performed under non-standard conditions, such as in the operating theatre or intensive care unit. It is also concluded that, although a subject may perceive cutaneous paraesthesiae when the posterior tibial nerve is stimulated at the ankle, there may be no cutaneous component to the evoked cerebral potential.

Dependence of the Achilles tendon reflex on the excitability of spinal reflex pathways

Muscle afferent activity from the triceps surae was recorded during experimentally induced alterations in amplitude of the Achilles tendon jerk. No changes in the neural afferent response to tendon percussion or in the background level of neural activity occurred when the reflex response was altered by discomfort, distraction, changes in attention, or changes in the rate of tendon percussion. Reinforcement of the Achilles tendon jerk by forceful contraction of the forearm muscles did not alter the relationship between intensity of the tendon tap and amplitude of the evoked neural afferent volley. Nevertheless, such maneuvers lowered the reflex threshold and raised reflex sensitivity so that a smaller afferent volley was required to produce a tendon jerk, and an increase in the afferent volley produced a disproportionately greater increase in reflex electromyographic activity than would have occurred at rest. Reinforcement maneuvers potentiated the H-reflex but did not alter the electrically induced afferent volley or the background level of neural activity. It is concluded that these changes in reflex responsiveness occurred through intrinsic spinal mechanisms independent of the fusimotor system.

The irrelevance of fusimotor activity to the Achilles tendon jerk of relaxed humans

In two normal subjects the sciatic nerve was blocked completely using concentrated lidocaine. The muscle afferent and reflex electromyographic responses to reproducible percussion of the Achilles tendon were recorded while the blocks developed. The intensity of percussion was sufficient to produce an Achilles tendon jerk in one subject when at rest and in the other during reinforcement. The block did not alter the muscle afferent response to tendon percussion in either subject. It is concluded that background fusimotor activity is not a prerequisite for the tendon jerk and that, during complete relaxation, there may be no significant fusimotor drive directed to the triceps surae. The varying ease with which tendon jerks can be elicited in different normal subjects or in different muscles of the same subject appears to be related not to fusimotor activity but to differences in the "central excitability state."

Component of muscle spindle discharge related to arterial pulse

1. In human subjects microelectrode recordings were made from 25 muscle spindle afferents from the pretibial muscles. 2. The spike discharges of three endings were locked in time to the arterial pulse. With 17 of the remaining endings, there was a significant pulse-related modulation of discharge rate. For these 20 endings the latency to the onset of the pulse-related influence was 200-310 ms. 3. The time course of the modulation of discharge rate was similar to that of arterial blood flow, as estimated using a Doppler flowmeter. With four endings occlusion of blood flow using a sphygmomanometer cuff reduced any modulation. 4. For five endings the contribution by the arterial modulation to the variance of discharge of the ending was 3-54%. For the population of endings there was no significant relationship between the depth of modulation and coefficient of variation. 5. It is concluded that the arterial pulse can be significant contributor to the variability of muscle spindle discharge. The pulsatile effects seen in the responses of single afferents are unlikely to be eliminated in the summed activity forming the population response. This could constitute a limitation of the information capacity of the population of muscle spindle afferents.

Cerebral potentials evoked by muscle stretch in man

The cerebral potentials evoked by passive plantar flexion movements of the ankle were recorded by scalp electrodes in normal human subjects. The potential consisted of a biphasic positive wave (P45, P65), a prominent negative wave (N90), and a subsequent positive-negative-positive complex (P120, N145, P190). The components of the potential were of largest amplitude at the vertex, small displacements of the active recording electrode resulting in a marked decrease in amplitudes. The amplitudes of the components were also profoundly affected by changes in the repetition rate of the stimulus and by changes in the acceleration of the stretching movements; they were little affected by changes in the extent or peak velocity of stretch provided that acceleration remained constant. Potentials of similar morphology but of slightly shorter latency could be evoked by percussion of the tendon of tibialis anterior and by electrical simuli delivered through a microelectrode inserted into a pure muscle nerve fascicle of the peroneal nerve innervating one of the pretibial flexor muscles. The results of nerve block experiments (ischaemic/pressure block above the ankle; pressure block and local anaesthetic block of the peroneal nerve at the fibular head) suggest that the afferents responsible for evoking the cerebral potential arose from muscle mechanoreceptors. It is concluded that information about muscle stretch arising from muscle mechanoreceptors (probably muscle spindle endings) reaches the cerebral hemispheres of man at short latency and that the resulting potentials can be readily detected using scalp electrodes.

Anticipation and fusimotor activity in preparation for a voluntary contraction

1. Multi-unit and single-unit recordings were made of muscle spindle afferent activity from the pretibial muscles of human subjects who were initially relaxed. The muscles were subjected to a stretching perturbation of 1 s duration, occurring irregularly, on average once every 5 s. In test sequences, an auditory or visual warning was provided 1 . 06 s before some of the perturbations. Subjects were required to oppose every perturbation by contracting the receptor-bearing muscle as rapidly as possible. 2. Following the warning all subjects sometimes tensed the receptor-bearing muscle unintentionally in preparation for the perturbation. In these contractions, the discharge of a spindle ending accelerated only if the contraction strength exceeded the ending's threshold for activation, established in control voluntary contractions performed under isometric conditions. 3. When the receptor-bearing muscle did not contract in the interval between warning and perturbation, there was no detectable change in the multi-unit recordings of spindle activity or in recordings from twelve of thirteen single spindle afferents. The thirteenth spindle afferent discharged prior to the perturbation in the absence of detectable e.m.g. in response to (only) three of twenty-three warning stimuli. However, this ending had been so responsive during isometric voluntary contractions that a contraction level at which it did not respond could not be established, and it is suggested that the findings with this ending resulted from its low threshold rather than from selective activation of the fusimotor system. 4. When subjects were warned of the perturbations, the dynamic response of spindle endings to the perturbations was not increased in size or altered in latency. 5. The motor response to perturbations without warning generally contained only long-latency (volitional) e.m.g. activity occurring 107--200 ms after the onset of the perturbation. When a warning was given, short-latency (reflex) e.m.g. activity was also recorded, beginning 46--76 ms after the onset of the perturbation. 6. It is concluded that anticipation of the need to contract a muscle does not result in selective activation of fusimotor neurones in preparation for the contraction. The change in stretch reflex gain that occurs as a result of 'anticipation' occurs through a central process which does not involve the fusimotor system.

Induced changes in the thresholds for voluntary activation of human spindle endings

1. Recordings were made from forty-two muscle spindle afferents from the pre-tibial muscles of human subjects. For each afferent, the contraction level at which its discharge accelerated (its 'threshold for activation') was defined using isometric voluntary contractions of the receptor-bearing muscle. The effects on these thresholds of various manoeuvres designed to activate descending pathways or segmental sensory inputs were studied to determine whether the balance between the skeletomotor and fusimotor drives to the contracting muscle could be altered. 2. The Jendrassik reinforcement manoeuvre raised the threshold for voluntary activation of one spindle ending slightly but had no significant effect with seven other endings. It is concluded that the Jendrassik manoeuvre does not excite fusimotor neurons selectively or even preferentially. 3. Caloric vestibular stimulation altered the thresholds for voluntary activation of eight of eleven spindle endings, the most common effect being lowering of the threshold during the irrigation and for 1--1 . 5 min after its cessation. 4. Cutaneous stimulation by vibration lowered the thresholds for voluntary activation of some spindle endings when applied to the dorsum of the foot, and raised thresholds when applied to the lateral plantar surface. 5. Passive stretch of the receptor-bearing muscle by plantar flexion of the ankle and passive stretch of synergistic muscles in isolation raised thresholds for spindle activation; passive stretch or vibration of triceps surae lowered thresholds. 6. These studies suggest that the relationship between the skeletomotor and fusimotor drives to a muscle during a voluntary contraction is not rigidly fixed, but can be varied appropriately with the changing motor role demanded of the muscle by supraspinal drives and with the changes in sensory feed-back generated by the movement itself. 7. It is concluded that, provided the skeletomotor effects of a stimulus are known, changes in the threshold for spindle activation in a voluntary contraction can provide a means of determining the effects of that stimulus on fusimotor neurons even when the stimulus alone is not adequate to alter fusimotor drive.

Identification of muscle spindle afferents during in vivo recordings in man

A preamplifier for use in microelectrode recordings from human muscle afferents is described. Electrical stimuli can be delivered through the microelectrode to produce the twitch contractions necessary for afferent identification while still recording the activity of the afferent fibre.