Twitch contraction for identification of human muscle afferents

Functional properties of human muscle spindles

Muscle spindles are ubiquitous encapsulated mechanoreceptors found in most mammalian muscles. There are two types of endings, primary and secondary, and both are sensitive to changes in muscle length and velocity, with the primary endings having a greater dynamic sensitivity. Unlike other mechanoreceptors in the somatosensory system, muscle spindles are unique in possessing motor innervation, via γ-motoneurons (fusimotor neurons), that control their sensitivity to stretch. Much of what we know about human muscles spindles comes from studying the behavior of their afferents via intraneural microelectrodes (microneurography) inserted into accessible peripheral nerves. We review the functional properties of human muscle spindles, comparing and contrasting with what we know about the functions of muscle spindles studied in experimental animals. As in the cat, many human muscle spindles possess a background discharge that is related to the degree of muscle stretch, but mean firing rates are much lower (~10 Hz). They can faithfully encode changes in muscle fascicle length in passive conditions, but higher level extraction of information is required by the central nervous system to measure changes in muscle length during muscle contraction. Moreover, although there is some evidence supporting independent control of human muscle spindles via fusimotor neurons, any effects are modest compared with the clearly independent control of fusimotor neurons observed in the cat.

Microneurographically Recorded Ia Discharge from the Tibial Nerve Mainly Transmits the Angular Velocity of the Ankle Joint in Humans

Investigations of the Ia afferent discharge in clarifying problems in disused and malused skeletal muscles have been carried out mainly in muscles of the upper extremities. However, such problems actually occur more frequently in the antigravity muscles of the lower extremities, such as the triceps surae muscle. An analysis of microneurographically recorded Ia discharges from the tibial nerve innervating the triceps surae muscle during dynamic movement of the ankle joint indicated that they mainly transmitted information on the angular velocity of the joint. However, the information on the position sense of the joint was not as well transmitted through Ia discharges. There was no correlation between the joint angle and the static response. However, the dynamic response of a Ia afferent was well correlated to the angular velocity. It is concluded that the human proprioception of the triceps surae muscle was not dependent on the position of the ankle joint, but largely on its movement by the stretching of the muscle.

The methodology and scope of human microneurography

Microneurography was introduced in 1967 and has developed into an invaluable tool for investigating human somatosensory, motor and cardiovascular physiology and pathophysiology. It involves percutaneous insertion of a metal microelectrode into fascicles of limb and facial nerves. This review covers the procedures and equipment necessary for microneurography and provides a current circuit for a preamplifier. Evidence is presented that (i) most recordings from myelinated axons involve an effective penetration of the myelin by the electrode; (ii) based on physiological criteria, microstimulation through the electrode can be used to activate single axons although the probability of this is relatively low and (iii) despite 'micro' lesions caused by the electrode insertion into the nerve and its fascicles, the morbidity with the procedure is acceptably low.

Coding of pulsatile motor output by human muscle afferents during slow finger movements

1. Impulse activities of thirty-eight muscle spindle and tendon organ afferents from the finger extensor muscles were recorded in the radial nerve of human subjects while the subjects performed voluntary flexion and extension finger movements at a single metacarpophalangeal joint. 2. The afferent firing was analysed in relation to the 8-10 Hz discontinuities which previously have been shown to characterize these movements. Spike-triggered averaging and frequency domain analyses demonstrated that all Ia muscle spindle afferents and a large proportion of group II spindle afferents responded in close association with local peaks in the joint acceleration. During muscle lengthening the impulses appeared during phases of rapid muscle stretch, whereas they appeared during the phase of minimal speed during muscle shortening. 3. The Golgi tendon organ (Ib) afferents displayed a reverse pattern of activity in relation to the discontinuities, i.e. the impulses tended to appear in the phase of minimal speed during lengthening movements and close to maximal shortening speed during shortening movements. Hence, their firing often coincided with the phasic increases of the parent muscle activity which account for the 8-10 Hz discontinuities. 4. A close analysis of the time relations between spindle firing and the kinematics of the 8-10 Hz discontinuities revealed that the population spindle response was too delayed and too dispersed to support the hypothesis that the discontinuities are accounted for by the stretch reflex. 5. If, as suggested in a previous paper, the 8-10 Hz discontinuities are produced by a pulsatile descending motor command, the coding of the periodic but tenuous kinematic events by the population of proprioceptors may have a role in relation to an alleged pulsatile command generator.

Efferent responses to twitch tests used in identifying human muscle afferents

The response of a muscle afferent to the mechanical stimulus produced by a twitch contraction of the receptor-bearing muscle is an important test for differentiating between muscle spindle endings and Golgi tendon organs. The present study demonstrates that active alpha-motor axons can behave in a similar manner to spindle afferents, presumably responding not to the mechanical event per se, but reflexly to the change in afferent discharge created by the mechanical event. alpha-Motor axons were unequivocally identified during microneurography using spike-triggered averages of EMG. Caution is required when the twitch test is utilized to assist in the classification of muscle afferents during an intentional or unintentional voluntary contraction.

Human muscle spindle afferent activity in relation to visual control in precision finger movements

1. Impulse activities of muscle spindle afferents from the finger extensor muscles were recorded in the radial nerve of human subjects. In addition to single unit activity, surface EMG was recorded as well as finger joint position and angular velocity. 2. All units were studied under two conditions of voluntary finger movements. In the visual condition, the subject tracked ramp and hold sequences at a single metacarpophalangeal joint. In the non-visual condition the subject was asked to produce the same movement while visual control was denied altogether. 3. With sixteen units, detailed statistical analyses failed to reveal significant differences in muscle spindle afferent activity between the visual and the non-visual task. However, with two group Ia units, impulse rate was marginally but significantly higher in the visual task even when differences in average movement velocity, velocity variability and EMG level had been factored out. 4. The findings suggested that access to visual information for movement control did not produce any large-scale differences in spindle afference, although a small effect of an increased and independent gamma-activation emerged in the statistical analysis in 11% of the units.

Microneurography and applications to issues of motor control: Fifth Annual Stuart Reiner Memorial Lecture

Among the hypotheses regarding fusimotor functions based on earlier animal experiments some are inconsistent, others are in conformity with microneurographic observations in man. The human data provide evidence against the following two theories: (1) the length follow-up servo theory; and (2) the theory that fusimotor neurons can be selectively activated to produce spindle sensitization and stretch reflex reinforcements. The human data support the theory of alpha-gamma coactivation. In particular, in the early phase of isometric voluntary contractions fusimotor-driven afferent spindle activity assists in autogenetic activation of alpha motoneurons and in reciprocal relaxation of antagonists. As muscle fatigue develops, the autogenetic reflex drive via the fusimotor route declines. The fusimotor bias during contraction provides for maintenance of spindle sensitivity to minute perturbations and for load-compensating reflex adjustments to such perturbations. Reflex overcorrections may lead to uncontrollable oscillations of the type seen in enhanced physiological tremor.

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.

Dual response from human muscle spindles in fast voluntary movements

Single-unit impulses were recorded from the radial nerve of attending human subjects using the microneurography technique. The discharge of muscle spindle afferents from the extensor digitorum muscles was analysed while subjects performed fast lengthening and shortening voluntary movements as well as movements of moderate speed at a single metacarpophalangeal joint. Opposing or assisting loads of moderate size were added in some tests. Fast lengthening movements were, in practically all units, associated with acceleration of spindle discharge. However, the responses were modest and in many primary afferents it was of similar size as their response to small irregularities during slower movements. During shortening movements, most spindle afferents stopped firing altogether, whereas some afferents exhibited a distinct burst of impulses at the onset of active shortening followed by silence during the main part of the movement. This initial shortening responses was sometimes more prominent when the parent muscle worked against an opposing load. It was interpreted as a result of fusimotor drive associated with the building up of force in the contracting muscle. The initial shortening response from the contracting muscle and the stretch response from the antagonist constitute a dual signal, describing accurately the onset of joint movement as seen from the two muscles. It remains to be clarified which role this pattern of afferent responses may have in the design of the current motor output and in the capturing of nature and size of the external load.

Human muscle spindle response in a motor learning task

1. Impulse discharge of single muscle spindle afferents from the finger extensor muscles was recorded in the radial nerve of conscious human subjects, during a motor learning task engaging the metacarpo-phalangeal joint of a single finger, using the microneurography technique. 2. Subjects were requested first to pay attention to a complex sequence of imposed single joint movements, and immediately afterwards to reproduce actively the same sequence. No external load was added to the finger and visual control was denied altogether so that subjects relied on mechanoreceptor input exclusively for the sampling and reproduction of movement. In addition, sequences of imposed movements were delivered while subjects were not attending in order to allow analysis of the attention effect. 3. The response of the individual unit was uniform in repeated tests. There were clear differences between spindle firing rate in imposed and actively reproduced movements with most units. However, the difference was complex during the individual sequence, in that firing rate was usually higher during periods of reproduced movements when the muscle was relatively short whereas it was identical when the muscle was relatively long. 4. The hypothesis that reproduction and verification of an imposed movement may be based on simple matching between identical spindle firing in imposed and active movements, was difficult to reject altogether because identical spindle input was present during considerable sections of the movement sequence. It may be speculated that agonists and antagonists cover different ranges of joint excursion, with identical spindle firing rates in imposed and reproduced movements. 5. Attention to imposed movements was associated with a minute and inconsistent increase of spindle firing rate in some afferents and then usually with a slight increase of EMG activity of the parent muscle as well. 6. It was concluded that focusing attention on the kinaesthetic input during imposed movement was not associated with a consistent increase of fusimotor drive.

Role of the human fusimotor system in a motor adaptation task

1. Single-unit activity was recorded with the microneurographic technique from the radial nerve of attending human subjects. During active finger movements, impulses in spindle afferents from the extensor digitorum muscle were analysed along with joint movements, size of imposed load and EMG activity of the receptor-bearing muscle. 2. In a simple motor adaptation task the subjects were requested to perform ramp-and-hold movements of prescribed amplitudes and velocities at a single metacarpo-phalangeal joint. A test run consisted of a series of movement cycles when the flexor muscle was continuously loaded with a constant torque, immediately followed by cycles when this load was abruptly decreased during the flexion movement, producing a fast stretch of the receptor-bearing muscle. The subjects' task was to strive for movements of constant velocity and particularly to minimize the effect of the disturbance. In order to allow prediction on the basis of immediately preceding cycles, the disturbance was always injected at the same angular position in a number of successive cycles. 3. Motor adaptation was manifested as a successive decrease of the perturbation amplitude, usually associated with the development of a continuous and growing EMG activity in the parent muscle and a growing reflex response of long latency (60 ms). Short-latency reflexes were not seen. 4. The main mechanism accounting for the improved performance was a co-contraction of the agonist-antagonist muscle pair during voluntary movements, producing an increased muscular stiffness. The reflex did not contribute to the motor adaptation because it was not fast enough to curtail the perturbation. 5. The development and the growth of the reflex were not due to a growing fusimotor drive during adaptation, because spindle discharge actually decreased when the reflex increased. The size of spindle response was related to the amplitude of perturbation rather than to the amplitude of the reflex. These findings suggest that reflex modifications were due to central excitability changes which paralleled the muscle contraction. 6. Spindle firing rate during active movements was generally higher in disturbed cycles compared to undisturbed cycles, indicating a higher fusimotor drive. Since muscle contraction was present mainly in the former, this finding may simply represent a case of fusimotor activation along with skeletomotor activation. No indication of an independence between the two was found. 7. The findings lend no support for the view that the size of the stretch reflex in a behavioural task is adjusted by selective changes of the fusimotor drive.

Stretch sensitization of human muscle spindles

1. Sixty-seven afferents from the finger extensor muscles were consecutively recorded by microneurography. 2. The units were classified as primary or secondary muscle spindle afferents or Golgi tendon organ afferents on the basis of their responses to ramp-and-hold stretches, sinusoidals superimposed on ramp-and-hold stretches, maximal twitch contractions and isometric contractions and relaxations. 3. The muscle was repeatedly stretched and then either kept short or long for a few seconds followed by a slow ramp stretch. The responses of the muscle afferents to the slow stretch were compared under the two conditions. 4. Thirty out of thirty-eight units classified as primary spindle afferents and four out of eleven units classified as secondary afferents showed an enhanced response to the slow ramp when the muscle had been kept short compared to the response when the muscle had been kept long. 5. None of the eighteen Golgi tendon organ afferents showed any difference in this respect. 6. It is concluded that stretch sensitization does occur in human muscle spindles and, when present, constitutes firm evidence of the afferent originating from a muscle spindle rather than a Golgi tendon organ. In addition, due to differences in the response characteristics of primaries and secondaries, the test may aid in separating muscle spindle primary afferents from secondary afferents.