Assessment of fusimotor contribution to reflex reinforcement in humans

Standardization of the Jendrassik maneuver in Achilles tendon tap reflex

Objective

For many decades, the Jendrassik maneuver (JM) has been used as a reinforcement for stretch reflexes, although the underlying mechanism of this reinforcement is still not fully understood. Moreover, the term JM has been used for many different muscle contraction strategies as there is no fixed movement for the maneuver in the literature. In this study, we aimed to investigate the effects of clenched hand pull, teeth clenching, and their combined effects to reach standardization.

Methods

Achilles tendon tap reflex responses in the soleus were recorded during rest (R), hand pull (HP), teeth clench (TC), and HP + TC combined, hereafter referred to as the JM.

Results

Reflex response amplitudes significantly increased during JM, HP, and TC in the soleus. HP and JM significantly changed the background activity in the soleus, but TC alone did not.

Conclusion

These results suggest that dominantly presynaptic disinhibitory mechanisms may be responsible for the increase in the tendon tap reflex during HP, TC, and JM.

Significance

Because the findings indicate that HP increases the background activity of the soleus, we suggest that researchers should use only TC during the Jendrassik maneuver to avoid any confounding background activity change.

Fusimotor drive may adjust muscle spindle feedback to task requirements in humans

The aim of the present study was to investigate whether the fusimotor control of muscle spindle sensitivity may depend on the movement parameter the task is focused on, either the velocity or the final position reached. The unitary activities of 18 muscle spindle afferents were recorded by microneurography at the common peroneal nerve. We compared in two situations the responses of muscle spindle afferents to ankle movements imposed while the subject was instructed not to pay attention to or to pay attention to the movement, both in the absence of visual cues. In the two situations, three ramp-and-hold movements were imposed in random order. In one situation, the three movements differed by their velocity and in the other by the final position reached. The task consisted in ranking the three movements according to the parameter under consideration (for example, slow, fast, and medium). The results showed that paying attention to movement velocity gave rise to a significant increase in the dynamic and static responses of muscle afferents. In contrast, focusing attention on the final position reached made the muscle spindle feedback better discriminate the different positions and depressed its capacity to discriminate movement velocities. Changes are interpreted as reflecting dynamic and static gamma activation, respectively. The present results support the view that the fusimotor drive depends on the parameter the task is focused on, so that the muscle afferent feedback is adjusted to the task requirements.

Facilitation of motor evoked potentials and H-reflexes of flexor carpi radialis muscle induced by voluntary teeth clenching

In order to examine the effects of remote facilitation on cortical and spinal sites, we recorded motor evoked potentials (MEPs) and H-reflexes from the flexor carpi radialis muscle of 13 healthy subjects. The H-reflex was used to assess excitability changes at the spinal level, while the MEP following transcranial magnetic stimulation was used to study excitability changes at the cortical level. We induced remote facilitation by means of voluntary teeth clenching (VTC), the so-called Jendrassik maneuver, because this procedure is known to be effective and reliable. Although the facilitation induced by VTC was observed in both evoked potentials (i.e., H-reflex and MEP), which is consistent with previous reports, MEP onset latencies were shortened by VTC in proportion to an increased MEP amplitude, whereas the latencies of the H-reflex were not. Furthermore, statistically significant relationships between MEP latencies and amplitudes were observed in all subjects, whereas no such relationships were observed for the H-reflex. On the basis of these results, two neural pathways are presumed: one involving a release of pre-synaptic inhibition at the spinal level and the other involving an unmasking of lateral excitatory projections at the cortical level.

Increased muscle spindle sensitivity to movement during reinforcement manoeuvres in relaxed human subjects

1. The effects of reinforcement manoeuvres, such as mental computation and the Jendrassik manoeuvre, on muscle spindle sensitivity to passively imposed sinusoidal stretching (1.5 deg, 2 Hz) in relaxed subjects were analysed. 2. The unitary activity of 26 muscle spindle afferents (23 Ia, 3 II) originating from ankle muscles was recorded using the microneurographic method. Particular care was paid to the subjects' state of physical and mental relaxation. 3. The results showed that the activity of 54 % of the Ia afferents was modified during mental computation. The modifications took the form of either an increase in the number of spikes (mean, 26 % among 11 Ia fibres) or a shortening in the latency of the response to sinusoidal stretching (mean, 13 ms among 3 Ia fibres), or both. They were sometimes accompanied by an enhanced variability in the instantaneous discharge frequency. The three secondary endings tested exhibited no change in their sensitivity to stretch during mental computation. 4. The increased sensitivity to passive movements sometimes began as soon as the instructions were given to the subjects and sometimes increased during mental computation. In addition, the increased sensitivity either stopped after the subjects gave the right answer or continued for several minutes. 5. During the performance of a Jendrassik manoeuvre, the Ia units underwent changes similar to those described above for mental computation. 6. It was concluded that muscle spindle sensitivity to movement can be modified in relaxed human subjects. The results reinforce the idea that the fusimotor system plays a role in arousal and expectancy, and contribute to narrowing the gap between human and behaving animal data.

Spinal motoneuronal excitability in hyperkinesis: effects of the Jendrassik manoeuvre

H-reflex recovery functions and homosynaptic depression curves were studied in unmedicated hyperkinetic and control children in the presence and absence of the Jendrassik manoeuvre. The Jendrassik manoeuvre failed to affect recovery function reflex amplitude across or within groups, but reflex amplitude was augmented in the homosynaptic depression procedure for hyperkinetic children who failed to show recovery function facilitation. The absence of Jendrassik-associated reflex facilitation in children during the recovery function and during selective homosynaptic depression rates may reflect developmental influences on processes underlying this phenomenon. However, reflex augmentation in non-facilitating hyperkinetic subjects suggests that these processes are present in children and can be revealed under conditions of decreased motoneuronal excitability. These findings suggest that reduced gamma-efferent drive may contribute to decreased motoneuronal excitability observed in hyperkinetic children, but the possible role of other Jendrassik-activated influences, such as coactivation of alpha and gamma motoneurons or selective activation of alpha motoneurons by processes independent of increased gamma-efferent drive, cannot be excluded.

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.

Physiological tremor enhanced by manoeuvres affecting the segmental stretch reflex

In view of recent evidence that physiological tremor can be enhanced by positive feedback via the segmental stretch reflex, several manoeuvres and procedures were employed to enhance the finger and hand tremor of healthy subjects--the purpose being to determine if tremorogenic effects, at least in part, are due to increase efficacy of the stretch reflex servo. Mechanical events during tremor (and during voluntary or electrically induced muscle twitches) were recorded together with EMG activity from wrist and finger flexor muscles and discharges from primary spindle endings in these muscles. Physiological tremor can be enhanced not only by manoeuvres which increase the gain of segmental stretch reflexes (Jendrassik manoeuvre) but also by manoeuvres which increase the contrast in spindle firing during stretch versus shortening phases of tremor, thus enhancing reflex modulation. Effects of the latter type can be achieved by procedures which alter mechanical twitch properties of extrafusal fibres (isoproterenol infusions and fatigue) and by procedures which involve application of spindle stimuli acting preferentially during stretch phases of tremor movements (muscle vibrations). Physiological tremor, which can be temporarily enhanced by an externally applied muscle perturbation, also becomes accentuated by those small "pseudo-myoclonic" jerks which occur in all normal subjects attempting to perform slow, smooth movements.

Human muscle afferent responses to tendon taps. I. Characteristics of the waveform recorded with transcutaneous electrodes

Responses of muscle afferent nerve fibres to tendon taps of digital muscles in the human can be recorded with surface electrodes attached to the skin over the nerve at the wrist. Using an effectively monopolar recording method, a considerable improvement in signal amplitude is achieved with simultaneous reduction in mechanical artefacts using differential amplification techniques. The characteristics of the afferent waveform (latency and duration) are discussed in relation to the applied stimulus. The contribution to the afferent response from receptors other than in the muscle have been shown to be minimal. Afferent fibres from primary muscle spindle endings are thought to be the major contributors to the afferent waveforms recorded by this technique.

Percutaneous recordings from peripheral nerves (preliminary communication)

The recently developed technique provides a precise assessment of neutral activity via intrafascicular axonal recordings from human peripheral nerves. With this technique it is possible to record from cutaneous afferent fibers. Receptive fields belonging to cutaneous afferents are readily perceived by patients due to orthodromic conduction of activity induced by mechanical activation of appropriate fibers. Temporal synchrony between peripherally applied stimuli and the proximally placed recording electrode assure continuity of the nerve. By sampling axonal activity from cutaneous fasciles at regular intervals following trauma of undefined etiology, a definite appraisal can be made concerning the integrity of a nerve and, hence, the need for surgical intervention. This is a preliminary report of the first such applications of this technique.

Afferent input to movement-related precentral neurones in conscious monkeys

Monkeys were trained to perform a stereotyped movement task, and to accept passive manipulation and natural stimulation of the limbs while remaining relaxed and quiet. All training, both of movement and for relaxation, was with food rewards. The effects of natural stimuli on 257 precentral neurones showing consistent modulations in discharge frequency during the performance of the movement task were investigated. Most precentral neurones had small, stable input zones located on the contralateral arm: 197 facilitatory and 17 inhibitory responses were obtained, while the remaining 43 cells were unaffected by the natural stimuli used. The most common natural stimulus capable of influencing precentral neurones was joint movement: 152 cells responded to joint movement, including 98 which only responded to movement at a single joint. Joint movement rather than joint position was the effective stimulus and none of these cells was influenced by palpation of muscles acting at the joint. The next most common natural stimulus capable of influencing precentral neurones was muscle palpation: 35 cells responded to a tap applied to a localized portion of the muscle belly, including 26 cells which also responded to movement of the joint at which the muscle acted. The direction of joint movement which influenced the cell was usually such as to stretch the muscle containing the receptors for the effective afferent input set up by tapping. The natural stimulus which influenced the smallest number of precentral neurones was tactile stimulation of the skin: 27 cells had cutaneous receptive fields, most of which were small ( < 5 cm 2 ) and confined to the hand. Included in the total sample were 51 pyramidal tract neurones. The behaviour of these was found to be similar to the unidentified neurones examined in the same animals with respect to their afferent input. However, there was a tendency for pyramidal tract neurones to be in receipt of a more convergent input than unidentified neurones in their vicinity. The majority of neurones recorded in close proximity to one another (within 500 μm or less) usually received their afferent input from the same peripheral region, but a significant proportion of such cells received inputs from different and remote peripheral zones. Hence the afferent input to the precentral motor cortex is not organized to provide independent and spatially segregated projections from particular peripheral sites only to limited and localized radial aggregations of neurones.

Effects of the Jendrassik manoeuvre on muscle spindle activity in man

Twenty-eight mechanoreceptive units identified as primary or secondary spindle afferents were sampled from muscle nerve fascicles in the median, peroneal, and tibial nerves of healthy adult subjects. The responses of these units to sustained passive muscle stretch, to passive stretching movements, to tendon taps, and electrically-induced muscle twitches were studied while the subject performed repeated Jendrassik manoeuvres involving strong voluntary contractions in distant muscle groups. The manoeuvres had no effect upon the afferent spindle discharges as long as there were no EMG signs of unintentional contractions occurring in the receptor-bearing muscle and no mechanotransducer signs of unintentional positional changes altering the load on that muscle. Unintentional contractions in the receptor-bearing muscle frequently occurred during the manoeuvres, however, and then coactivation of the spindle afferents was observed. Multiunit afferent responses to Achilles tendon taps, led off from tibial nerve fascicles, were in a similar way uninfluenced by the Jendrassik manoeuvres, even when these resulted in marked reinforcement of the calf muscle tendon jerk. The results provide no evidence for fusimotor sensitization of spindles in muscles remaining relaxed during the Jendrassik manoeuvre, and reflex reinforcement occurring without concomitant signs of active tension rise in the muscles tested is presumed to depend upon altered processing of the afferent volleys within the cord.

Muscle spindle activity in man during voluntary fast alternating movements

Single unit activity in primary spindle afferent nerve fibres from finger and foot flexors was recorded with tungsten microelectrodes inserted into the median and peroneal nerves of healthy subjects. During voluntary fast alternating finger and foot movements, simulating the tremor of Parkinsonism, two types of discharges were seen in the Ia afferent fibres: (1) stretch responses occurring during the flexor relaxation phases, and (2) discharges occurring during the flexor contraction phases. Contrary to the stretch responses the spindle contraction discharges could be eliminated by a partial lidocaine block of the muscle nerve proximal to the recording site, indicating that they resulted from fusimotor activation of intrafusal fibres. On the basis of the temporal relations between the beginning and end of individual EMG-bursts, the start of the spindle contraction discharges and the latency of the stretch reflex in the muscles concerned, the following conclusions were drawn: the recurrent extrafusal contractions in movements of this type are initiated by the fast direct alpha route, but individual contraction phases generally last long enough to be influenced subsequently by the coactivated fusimotor loop through the spindles. It is postulated that this gamma loop influence during alternating movements helps to keep flexor and extensor muscles working in a regular reciprocal fashion with contractions adjusted in strength to the external loads.