Changes in the tonic activity of wrist extensor motor units induced by stimulating antagonistic group I afferents in humans

The question as to whether the firing patterns of low- and high-threshold motor units in the extensor carpi radialis muscles are affected differently by group I afferents from the wrist flexors depending on the motor task being performed was investigated in six subjects. The motor units were voluntarily activated during a task consisting of either selectively contracting the wrist extensor muscles or co-activating the wrist and finger antagonist muscles by clenching the hand around a manipulandum. The motor units (n=40) were identified on the basis of their firing thresholds, their macro-potential areas, and their twitch contraction times. The effects on the motor-unit tonic activity of stimulating the wrist flexor afferents were assessed in terms of the changes in the firing probability, which were analysed after computing peri-stimulus time histograms using the cumulative sum procedure. Median nerve stimulation induced four main changes in the tonic firing pattern of the extensor motor units. An early, short-lasting increase in the firing probability (event E1) was found to occur in the high-threshold motor units, either in both tasks (6/13) or only during hand clenching (2/13). A short-latency decrease in the firing probability (event E2) was found to occur in all the motor units, the amount of which increased from the fast- to slowly contracting motor units, especially during hand clenching. A later decrease (event E3) followed by a large, late increase (event E4) in the tonic activity of the motor units was found to occur in all the motor units, without any task-dependent effects. All these various events were consistently observed in 12 pairs of motor units, each consisting of one slowly and one fast-contracting motor unit, which were tested simultaneously. These findings suggest that median nerve stimulation may selectively alter the tonic firing patterns of identified extensor motor units, depending on their functional characteristics (recruitment threshold, motor unit macro-potential area, contraction time) rather than on the excitatory drive to the motoneurone pool. The possible origins of these various events are discussed, and it is argued that, in the wrist extensor and flexor muscles that act as synergists during manipulatory finger movements and gripping tasks, the spinal pathways which assist the voluntary command may selectively modulate the firing patterns of identified motor units, to fit the requirements of the on-going motor task.     

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.     

Changes in the inhibitory control exerted by the antagonist Ia afferents on human wrist extensor motor units during an attention-demanding motor task

The aim of this study was to determine the extent to which an attention-demanding visuomotor task affects the strength of the inhibitory control exerted by the wrist flexor group Ia afferents on the wrist extensor motoneurons. Effects of median nerve stimulation on the tonic activity of wrist extensor single motor units were analyzed in terms of the interspike interval (ISI) lengthening. Results show that the inhibitory effects exerted by the antagonistic group Ia afferents were significantly enhanced when the wrist extensor motoneurons were involved in an attention-demanding task. Enhanced inhibition from antagonist afferents may reflect task-related changes in the excitability of the di- and/or polysynaptic pathways mediating reciprocal inhibition due to either the action of descending inputs and/or an increase in the efficiency of the Ia inputs to the premotoneuronal inhibitory interneurons. Modulation of the inhibition exerted by proprioceptive antagonist afferents may be one of the processes which contribute to the fine adjustment of the wrist muscle force output required in fine handling tasks.     

Cortical excitability and motor task in man: an investigation of the wrist extensor motor area

Task-related changes in the corticospinal excitation of the right extensor carpi radialis (ECR) muscle were investigated in 16 healthy human subjects. The subjects were asked to perform a tonic isometric wrist extension or to clench their hand around a manipulandum, thereby coactivating the antagonistic wrist muscles. At matched levels of background EMG in the ECR muscle, transcranial magnetic stimulation (TMS) was applied through a figure-of-eight coil at 20-30 sites spaced 1 cm apart over the hand area of the left motor cortex. The cortical maps of the representation of the ECR muscle constructed in this way did not change between the two motor tasks. Nevertheless, for all investigated cortical sites TMS evoked a smaller motor evoked potential (MEP) in the ECR muscles during hand clenching than during wrist extension. A similar decrease in the short-latency peak in the poststimulus time histogram (PSTH) of single ECR motor units to TMS during hand clenching was found in seven subjects (number of motor units = 35). In contrast, short-latency peaks in the PSTH evoked by electrical stimulation of the motor cortex had a similar size during the two tasks (number of motor units = 9; two subjects). Already the initial 0.5-1.0 ms of the short-latency peak evoked by TMS was depressed during hand clenching, which suggests that decreased excitability of corticospinal cells with monosynaptic projections onto ECR motor units was involved. This decreased excitability was not explained by increased intracortical inhibition, which was found to be of a similar size during hand clenching and wrist extension. The task-related changes in the efficiency of the motor cortex output are discussed in relation to the function of the wrist antagonist muscles in handling and gripping tasks.     

Pattern of projections of group I afferents from forearm muscles to motoneurones supplying biceps and triceps muscles in man

Summary (1) Two independent methods were used, in man, to assess the modifications of the excitability of biceps and triceps brachii motoneurone pools following the stimulation of group I afferents coming from muscles acting at the wrist: (a) the modifications of the excitability of a motoneuronal population were studied using a reflex technique, (b) the modifications of the excitability of an isolated motor unit were estimated using a post-stimulus time histogram (p.s.t.h.) method. (2) The activation of group I afferents contained in the median nerve, originating from wrist flexors and pronators, resulted in a strong, short-latency facilitation of the biceps brachii motoneurones. A similar effect was also evoked by stimulation of group I afferents in the radial nerve, distally to the branch supplying the brachio-radialis muscle. The latency of both median and radial-induced facilitations is compatible with a monosynaptic linkage. (3) The stimulation of group I afferents in the median or the radial nerves produced inhibition of triceps motoneurones, with a latency compatible with a disynaptic linkage. (4) The prolonged vibration of the tendon of the flexor carpi radialis (FCR) or of the extensor carpi radialis (ECR) raised the threshold for both the facilitation of biceps and the inhibition of triceps motoneurones. The same pattern of excitatory and inhibitory convergence could also be obtained when the electrical conditioning stimulus to the median or radial nerves was replaced by a tap applied to the tendons of FCR or ECR respectively. Both results suggest that the conditioning fibres were Ia fibres. (5) The pattern of distribution of Ia afferents from muscles acting at the wrist onto motoneurones of muscles acting at the elbow has been compared to that described in the cat and monkey. A comparison has also been made between Ia connections of muscles acting at different joints in the upper and lower limb in man. The differences are discussed in relation to the manipulating capacity of the hand.     

Modulation of H reflex of pretibial muscles and reciprocal Ia inhibition of soleus muscle during voluntary teeth clenching in humans

A previous study has demonstrated that the soleus H reflex is facilitated in association with voluntary teeth clenching in proportion with biting force in humans. The present study tried to elucidate the functional significance of this facilitation of the soleus H reflex, by examining 1) whether the facilitation of the H reflex is reciprocal or nonreciprocal between the ankle extensors and flexors and 2) whether the reciprocal Ia inhibition of crural muscles is facilitated or depressed in association with voluntary teeth clenching. The H reflex of the pretibial muscles was evoked by stimulation of the common peroneal nerve in seven healthy subjects with no oral dysfunction. The pretibial H reflex was facilitated in association with voluntary teeth clenching in a force-dependent manner. The facilitation started preceding the onset of electromyographic activity of the masseter muscle. Stimulation of the common peroneal nerve at low intensities subthreshold for evoking the M wave of the pretibial muscles inhibited the soleus H reflex after a short latency corresponding with a disynaptic inhibition, indicating that the reciprocal Ia inhibition was depressed in association with voluntary teeth clenching. Thus, the present study has shown that voluntary teeth clenching evokes a nonreciprocal facilitation of ankle extensor and flexor muscles and attenuated reciprocal Ia inhibition from the pretibial muscles to the soleus muscle. It is concluded that voluntary teeth clenching contributes to improve stability of stance rather than smoothness of movements.     

Remote effects of self-paced teeth clenching on the excitability of hand motor area

We studied remote effects of teeth clenching on motor cortical and spinal cord excitability using transcranial magnetic stimulation (TMS), brainstem electrical stimulation (BES), and ulnar nerve stimulation (F-wave) in eight normal volunteers. The TMS, BES, and ulnar nerve stimulation at the wrist were given at different intervals (0–200 ms) after the onset of masseter contraction. Surface electromyographic responses were recorded from the first dorsal interosseous muscle. Responses at different intervals were compared with the response elicited when the subject made no teeth clenching (control response). In TMS, conditioned responses (during teeth clenching) were significantly larger than the control at all intervals. In contrast, in BES and F-waves, conditioned responses were not larger than the control at an early phase (intervals shorter than 50 ms), whereas they were larger than the control at later intervals (longer than 50 ms). These results suggest that facilitation occurs in the hand motor area at the early phase of teeth clenching, and spinal facilitation dominates at its late phase. This time course of facilitation may indicate that the motor cortex must regulate hand muscles finely at the early phase of teeth clenching, and spinal cord may stabilize them firmly at the late phase. The excitability changes of the hand motor area may be in parallel with that of the masseter motor area which reflects the pattern of masseter contraction when the subject activates the masseter muscle phasically at the early phase and sustains that contraction at the late phase. Electronic Publication     

Human motor compensations for thixotropy‐dependent changes in resting wrist joint position after large joint movements

Aim: Resting tension of relaxed skeletal muscle fibres held at a given length varies with the immediate previous history of length changes and contractions. The primary aim of this study was to explore the motor control consequences of this history‐dependency in healthy subjects. Methods: Angular position and passive torque were recorded from the intact wrist joint. Integrated surface electromyography (IEMG) was recorded from wrist extensor and flexor muscles. Results: In relaxed subjects, wrist joint position was displaced towards dorsiflexion after a single high‐amplitude dorsiflexion movement combined with a strong flexor/extensor co‐contraction (dorsiflexion conditioning), whereas after volarflexion conditioning there was a shift towards volarflexion. These after‐effects could be abruptly cancelled by short periods (～5 s) of rapid flapping hand movements or forceful isometric co‐contractions, findings indicative of muscle thixotropy. The IEMG‐evaluated motor after‐effects were as follows. A slowly subsiding wrist flexor contraction was needed to restore and maintain the original resting wrist position after dorsiflexion conditioning whereas a slowly subsiding extensor contraction was needed for the same goal after volarflexion conditioning. Furthermore, ongoing wrist extensor IEMG activity required to actively hold the wrist in a moderate dorsiflexed position or to resist a constant volar torque at resting position was temporarily reduced after dorsiflexion conditioning and enhanced (not significantly) after volarflexion conditioning. Conclusion: The results provide evidence that during voluntary maintenance of a desired wrist joint position the motor commands to the position‐holding muscles are unconsciously adjusted to compensate for thixotropy‐dependent variations in the resting tension of the muscles.     

Synaptic connections from large afferents of wrist flexor and extensor muscles to synergistic motoneurones in man

 Short-latency excitatory Ia reflex connections were determined between pairs of human wrist flexor and extensor muscles. Spindle Ia afferents were stimulated by either tendon tap or electrical stimulation. The activity of voluntarily activated single motor units was recorded intramuscularly from pairs of wrist flexor or extensor muscles. Cross-correlation between stimuli and the discharge of the motor units provided a measure of the homonymous or heteronymous excitatory input to a motoneurone. Homonymous motoneurone facilitation was generally stronger than that of the heteronymous motoneurones. The principal wrist flexors, flexor carpi radialis (FCR) and flexor carpi ulnaris (FCU), were tightly connected through a bidirectional short-latency reflex pathway. In contrast, the extensor carpi ulnaris (ECU) and the extensor carpi radialis (ECR) did not have similar connections. ECU motoneurones received no short-latency excitatory Ia input from the ECR. ECR motoneurones did receive excitatory Ia input from ECU Ia afferents; however, its latency was delayed by several milliseconds compared with other heteronymous Ia excitatory effects observed. The wrist and finger extensors were linked through heteronymous Ia excitatory reflexes. The reflex connections observed in humans are largely similar to those observed in the cat, with the exception of heteronymous effects from the ECU to the ECR and from the extensor digitorum communis (EDC) to the ECU, which are present only in humans. The differences in the reflex organization of the wrist flexors versus the extensors probably reflects the importance of grasping. Received: 19 August 1996 / Accepted: 6 March 1997     

A comparison of peripheral and rubrospinal synaptic input to slow and fast twitch motor units of triceps surae

1. Post-synaptic potentials (PSPs) evoked by electrical stimulation of a variety of input systems have been compared in triceps surae motoneurones innervating slow and fast muscle units, the speed of contraction of which was also determined.2. Stimulation of high threshold afferents in both flexor and extensor muscle nerves, and of joint afferents, evoked polysynaptic PSPs which were predominantly hyperpolarizing in both fast and slow twitch motor units.3. Volleys in cutaneous afferents in the sural and saphenous nerves evoked polysynaptic PSPs composed of mixtures of inhibitory and excitatory components. The inhibitory components were predominant in slow twitch motor units, while in fast twitch units there was a trend towards excitatory predominance.4. Repetitive stimulation of the red nucleus caused predominantly inhibitory PSPs in slow twitch units and mixed or predominantly excitatory PSPs in fast twitch units. There was a correlation in the excitatory/inhibitory balance between PSPs of cutaneous and rubrospinal origin in those motoneurones in which both types of PSPs were studied.5. The amplitudes of group Ia disynaptic inhibitory PSPs were found to be correlated with motor unit twitch type: IPSPs in slow twitch units were larger than those in fast twitch units. Rubrospinal conditioning volleys were found to facilitate group Ia IPSPs in both fast and slow twitch motor units.6. The results suggest that there may be several basic patterns of synaptic input organization to motoneurones within a given motor unit pool. In addition to quantitative variation in synaptic distribution, there is evidence that qualitative differences in excitatory to inhibitory balance also exist in the pathways conveying input from cutaneous afferents and rubrospinal systems to triceps surae motoneurones. These qualitative differences are correlated with the motor unit twitch type.     

Human motor compensations for thixotropy-dependent changes in resting wrist joint position after large joint movements

AIM: Resting tension of relaxed skeletal muscle fibres held at a given length varies with the immediate previous history of length changes and contractions. The primary aim of this study was to explore the motor control consequences of this history-dependency in healthy subjects. METHODS: Angular position and passive torque were recorded from the intact wrist joint. Integrated surface electromyography (IEMG) was recorded from wrist extensor and flexor muscles. RESULTS: In relaxed subjects, wrist joint position was displaced towards dorsiflexion after a single high-amplitude dorsiflexion movement combined with a strong flexor/extensor co-contraction (dorsiflexion conditioning), whereas after volarflexion conditioning there was a shift towards volarflexion. These after-effects could be abruptly cancelled by short periods ( approximately 5 s) of rapid flapping hand movements or forceful isometric co-contractions, findings indicative of muscle thixotropy. The IEMG-evaluated motor after-effects were as follows. A slowly subsiding wrist flexor contraction was needed to restore and maintain the original resting wrist position after dorsiflexion conditioning whereas a slowly subsiding extensor contraction was needed for the same goal after volarflexion conditioning. Furthermore, ongoing wrist extensor IEMG activity required to actively hold the wrist in a moderate dorsiflexed position or to resist a constant volar torque at resting position was temporarily reduced after dorsiflexion conditioning and enhanced (not significantly) after volarflexion conditioning. CONCLUSION: The results provide evidence that during voluntary maintenance of a desired wrist joint position the motor commands to the position-holding muscles are unconsciously adjusted to compensate for thixotropy-dependent variations in the resting tension of the muscles.     

Synaptic connections from large muscle afferents to motoneurones in human lower limbs

OBJECTIVES: Our study was undertaken to investigate reciprocal inhibition in humans both from ankle flexors to extensors and from ankle extensors to flexors. METHODES: Changes in the firing probability of single motor units in response to electrical stimulation of muscle nerves (the peristimulus time histogram technique) were used to derive the reciprocal projections of muscle spindle Ia afferents to the motoneurones of ankle muscles. Discharges of units in ankle flexors (the tibialis anterior muscle [TA]) and extensors (soleus [SOL] and medial gastrocnemius [MG] muscles) were investigated respectively after stimulation of the posterior tibial (PTN) and common peroneal (CPN) nerves (predominantly on the deep branch). In eight normal subjects aged 24 to 40 years, one motor unit per each muscle was studied. RESULTS: CPN stimulation produced reciprocal Ia inhibition in the SOL of 5 of 7 of them and in the MG of 3 of 5, whereas PTN stimulation produced reciprocal Ia inhibition in the TA of only 2 of 6 subjects. CONCLUSIONS: These findings suggest that at low level contraction reciprocal Ia inhibition from ankle flexors to extensors may be stronger than that from ankle extensors to flexors.     

Twitch potentiation after voluntary versus electrically induced isometric contractions in human knee extensor muscles

Twitch potentiation in knee extensor (KE) muscles after a 7-s conditioning isometric maximal voluntary contraction (MVC trial), submaximal (25% MVC) voluntary contraction (SVC trial) and submaximal tetanic contraction (25% MVC) induced by percutaneous electrical stimulation at 100 Hz (PES trial) was compared in 12 men aged 19–25 years. Isometric twitch characteristics of KE muscles were measured before conditioning contraction and following 10-min recovery by supramaximal electrical stimulation of the femoral nerve. During MVC trial, twitch peak torque (Pt) potentiated (P < 0.05) immediately after the conditioning contraction with sharp decline during the first and third minute of recovery. No significant potentiation of twitch Pt was observed in SVC trial. During PES trial, twitch Pt was potentiated (P < 0.05) within 3–10 min of recovery. The time-course of isometric twitch was not significantly altered by conditioning contractions. It was concluded that twitch potentiation in the KE muscles differed markedly following the three conditioning contractions.     

Motor-unit responses in human wrist flexor and extensor muscles to transcranial cortical stimuli

1. Transcranial cortical stimuli (TCCS) were used to elicit motor responses in contralateral wrist flexor and extensor muscles of healthy adult subjects. The motor responses were assessed by surface EMG recordings, by needle recordings of single motor-unit discharges, and by measurements of wrist twitch force. Our main aim was to analyze the single-unit events underlying those changes in latency, amplitude, and duration of the compound EMG responses, which could be induced by voluntary preactivation of target muscles and by changes in stimulation strength. 2. Different stimulus strengths were tested with and without background contractions in the flexor or extensor muscles. For each test (consisting of a series of 20 stimuli) the compound EMG responses were averaged and displayed together with the averaged wrist force signals. Responses of individual flexor and extensor motor units were displayed in raster diagrams and peristimulus time histograms. For units exhibiting a background firing, the mean background interdischarge interval was calculated and compared with the subsequent poststimulus intervals. 3. In relaxed muscles, a shortening of onset latency of evoked compound EMG responses was observed when raising stimulation strength. A similar latency reduction was not seen in any of the single-unit recordings. This would be consistent with the size principle of motoneuron recruitment. 4. A shortening of onset latency of evoked EMG potentials was observed also as a result of a voluntary preactivation. Such latency shifts, which were seen also in single-unit recordings, might be attributed to variations in the time required for D and I wave temporal summation at the anterior horn cell. 5. When raising stimulation strength or when adding voluntary background contraction, the evoked compound EMG potential grew not only in amplitude but also in duration, as later peaks of activity were added to the initial ones. Under optimal conditions (strong stimulus + background contraction), the period of excitation (termed E1) had an onset latency of approximately 15 ms and a duration of approximately 35 ms and was similar for wrist flexor and extensor muscles. 6. We never saw the same flexor or extensor unit fire more than once during the E1 period. For units preactivated by a background contraction, the stimulus-triggered impulse exhibited latency shifts, which, to a large extent, depended on the timing of the stimulus in relation to a preceding background discharge and which could be influenced by a change in stimulation strength.(ABSTRACT TRUNCATED AT 400 WORDS)     

Comparison of fluctuations of motor unit recruitment and de-recruitment thresholds in man

Recruitment and de-recruitment thresholds of motor units in the wrist extensor muscles can undergo important random fluctuations, even when they are measured during stereotyped contractions and relaxations. These fluctuations were statistically quantified and compared. The statistical analysis indicated that recruitment and de-recruitment thresholds display the same kind of fluctuations, and that the successive measurements are randomly distributed following a quasi-normal law. We suggest that the notion of force threshold for motor unit recruitment and de-recruitment might be oversimplified and that a motor unit seems to have a range of force in which it can be recruited or de-recruited. Comparison of the mean values of recruitment and de-recruitment thresholds of the motor units in the extensor carpi radialis muscles showed that de-recruitment thresholds were significantly lower than recruitment thresholds. This difference in the thresholds, together with the difference in the motor unit discharge frequency during a contraction and a relaxation, suggests a differential control of the motoneurone activity during contractions and relaxations.     

Studies of some twitch and fatigue properties of different motor unit types in the ankle muscles of the adult cat.

Contractile responses of motor units in the gastrocnemius, soleus and pretibial flexor muscles of adult cats were elicited by intracellular stimulation of motoneurones. The motor units were classified into types FF, FR and S (Burke et al. 1971) and their responses to the same stimulation patterns as those used in a previous investigation of whole muscles (Hammarberg and Kellerth 1975 a) were studied. The duration of motoneurone afterhyperpolarization was short in both the fast twitch FF and FR units; it was longer in the soleus S units than in the S units of the pale muscles. Twitch time-to-peak was less than 30 ms in the FF and FR units, but exceeded 40 ms in the S units. Soleus S units were slower than S units of the pale muscles. Potentiation was observed in the gastrocnemius units, but not in the soleus S units. A short rest allowed fatigued extensor units of the FF and FR types to regain some contractile strength. This was less evident in the S units which, on the other hand, were extremely resistant to fatigue. Differences in response patterns between corresponding motor unit types of the flexor and extensor muscles were observed. A few fast twitch units were identified in the slow soleus muscle.     

Interhemispheric inhibition in human wrist muscles

The interhemispheric interactions between homologous wrist extensor and flexor muscles representations in the right and left primary cortex (M1) were studied using a paired-pulse transcranial magnetic stimulation in healthy subjects. The magnitude of interhemispheric inhibition (IHI) was studied in 9 right-handed subjects at short (10 ms, SIHI) and long (40 ms, LIHI) interstimulus intervals between the magnetic conditioning (CS) and test stimulus in the motor dominant to non-dominant cortex and vice versa, while the right or left hand was at rest or performing a unimanual sustained tonic contraction (holding a pen with the hand contralateral to the CS). A bidirectional powerful interhemispheric inhibition could be elicited at the short and long IHI phases (SIHI and LIHI) in wrist extensor and flexor muscles in most of the subjects at rest. SIHI but not LIHI was significantly bidirectionally reduced during unimanual contraction of the hand contralateral to the CS stimulation in comparison with rest. The amount of IHI after the stimulation of the “non-dominant” right hemisphere was not reduced in comparison with IHI after stimulation of the “dominant” left hemisphere whatever the active or resting condition. IHI directed to the wrist muscles had a similar level than IHI directed to digit muscles (FDI) at rest. Our data indicate that contralateral wrist muscles activity evokes a global, bidirectional reduction in IHI which was more pronounced for SIHI. These results provide additional evidence that changes in interhemispheric interactions between the M1s are involved in the control of unimanual movements including suppression of unwanted motor activity in the opposite limb during unilateral movements.     

Studies of Some Twitch and Fatigue Properties of Different Motor Unit Types in the Ankle Muscles of the Adult Cat

Contractile responses of motor units in the gastrocnemius, soleus and pretibial flexor muscles of adult cats were elicited by intracellular stimulation of motoneurones. The motor units were classified into types FF, FR and S (Burke et al. 1971) and their responses to the same stimulation patterns as those used in a previous investigation of whole muscles (Hammarberg and Kellerth 1975 a) were studied. The duration of motoneurone afterhyperpolarization was short in both the fast twitch FF and FR units; it was longer in the soleus S units than in the S units of the pale muscles. Twitch time-to-peak was less than 30 ms in the FF and FR units, but exceeded 40 ms in the S units. Soleus S units were slower than S units of the pale muscles. Potentiation was observed in the gastrocnemius units, but not in the soleus S units. A short rest allowed fatigued extensor units of the FF and FR types to regain some contractile strength. This was less evident in the S units which, on the other hand, were extremely resistant to fatigue. Differences in response patterns between corresponding motor unit types of the flexor and extensor muscles were observed. A few fast twitch units were identified in the slow soleus muscle.     

Relative contribution from different nerves to recurrent depression of Ia IPSPs in motoneurones

1. The pattern of depression of Ia IPSPs by volleys in recurrent motor axon collaterals was investigated in motoneurones supplying hind-limb muscles in the cat. The test IPSPs were evoked by stimulation of dorsal roots and the conditioning antidromic volleys by stimulation of motor fibres in different peripheral muscle nerves.2. In all motor nuclei investigated the strongest depression of Ia IPSPs is evoked from motor fibres to muscles whose Ia afferents produce the IPSPs. For example, the Ia IPSP from the knee extensor recorded in motoneurones to a knee flexor is most effectively depressed by antidromic stimulation of motor fibres to the knee extensor.3. The origin of recurrent inhibition of alpha-motoneurones and of Ia inhibitory interneurones with the same Ia input display a striking similarity. This suggests that the same population of Renshaw cells mediates effects to motoneurones and to Ia inhibitory interneurones.4. The functional significance of impulses in motor axon collaterals was discussed and it was suggested that they have an important role in the control of the excitatory as well as inhibitory Ia actions to motoneurones. The recurrent inhibition may limit the Ia effects to excitation of homonymous motoneurones, which would provide optimal conditions for control of individual muscles via the gamma-loop.     

Suppression of motor evoked potentials in biceps brachii preceding pronator contraction

Reciprocal control of antagonists is essential for coordinated limb movement. While Ia afferent dependent reciprocal inhibition has been extensively studied, reports of the control of antagonists during preparation for a motor action are limited. It has been demonstrated that corticomotor (CM) excitability of antagonists is suppressed prior to wrist extension/flexion suggesting the existence of a pre-contraction cortical control mechanism for distal upper limb antagonists. It is unknown whether pre-contraction suppression is evident in the control of proximal upper limb antagonists. Here we used transcranial magnetic stimulation and a rhythmic motor task to assess pre-contraction changes in excitability of corticospinal pathways projecting to biceps brachii (BB), when BB was an agonist (forearm supinator) or an antagonist. We found a suppression of motor evoked potential (MEP) amplitude in BB prior to pronator contraction and facilitation prior to BB contracting as a supinator. The extent of modulation was more profound as the agonist contraction approached. In contrast, there was no suppression evident in brachioradialis and triceps brachii under similar conditions indicating that pre-contraction suppression was specific to the antagonist BB. Our data in combination with published data from wrist muscles suggest that pre-contraction suppression of CM excitability may be a centrally induced mechanism to prevent antagonistic activity before Ia afferent dependent reciprocal inhibition is imposed. The importance of assessment of this inhibitory mechanism in neurologically impaired populations is discussed.