Reduction of the muscle activity associated with self-imposed electrical stimulation of mixed nerves supplying lower limb muscles in man.

Similarly to brief hammer taps self-imposed on the limb segments, self-triggered electrical stimulation delivered to the tibial, femoral or peroneal nerves are associated with anticipatory reduction in the muscle activity (RMA) of the target leg muscles. The anticipatory RMA starts shortly before the expected perturbation and lasts until the onset of the impact. The purpose of the present work is to study to what extent the selective activation of the different homonymous and heteronymous afferents could modify the central programs and the associated time and space distribution of the anticipatory RMA pattern in the target leg muscles. Our results show that the anticipatory RMA pattern is a result mainly of a pre-programmed non-specific central command, rather than consequence of the specific composition of the afferent volleys originating from different homonymous and heteronymous nerves. The commands for the voluntary movement triggering the stimulation and the associated anticipatory RMA are closely linked but independently controlled. By their synchronisation and co-ordination the central nervous system accounts the differences in length of the particular motor pathways. It is suggested, that in contrast to the classic anticipatory postural adjustments, the anticipatory RMA is presumably an alternative mode of feed-forward control diminishing the undesired effects of the afferent inputs.

Anticipatory reduction of the muscle activity associated with self-triggered electrical stimulation of mixed nerves and mecanical taps on muscle tendons.

Brief hammer taps self-imposed on the Achilles tendon or self-triggered electrical single-pulse stimulation delivered to the tibial nerve in the middle of the popliteal fossa are associated with an anticipatory reduction in the muscle activity (RMA) of the reflex-bearing soleus. The anticipatory RMA starts shortly prior tothe onset of the perturbation and lasts until the consequent T- or H-responses. When the amplitudes of the control T- and H-reflexes in a resting state are the same and equal to 50% of Hmax, then the anticipatory RMA patterns revealed with mechanical and electrical stimulation are similar. By selective skin stimulation in the medial region of the popliteal fossa (1.5-2.0 cm apart from the tibial nerve) with the same stimulus strength, the anticipatory RMA is significantly reduced. It is apparent that the main afferent sources for the elaboration of anticipatory RMA are the I afferents (presumably Ia afferents) and partly low threshold fast conducting cutaneous afferents.

Contribution of a Ia muscle afferent activation to the rise of H reflexes and somatosensory evoked potentials in man.

Electrical stimuli were applied to the tibial nerve in the popliteal fossa in man in order to investigate how information is transferred from group I muscle afferents to motoneurons and to the somatosensory cortex. For control purposes, identical stimuli were applied to the skin beside the electrode above the nerve. The somatosensory evoked potential (SEP) to skin stimulation alone had a peak latency which was 5 ms longer than the SEP to transcutaneous nerve stimulation. Influences by stimulation of the skin above the nerve could thus be excluded. The threshold intensity to evoke a liminal H reflex was at least two times higher than the threshold for a SEP. In most of the subjects, there was a correlation between the H reflex and the SEP size. If two identical stimuli were applied to the posterior tibial nerve with an interval of 1 s, the second H reflex was 30% smaller than the first one (postactivation depression). The corresponding SEPs were, however, only slightly reduced. Postactivation depression was probably caused by general intrinsic properties of synapses of group I muscle afferents. The results of this investigation indicate that: (1) a large volley in group I muscle afferents is necessary to evoke a liminal H reflex, whereas transmission from muscle afferents to the somatosensory cortex is very efficient; (2) these feedback signals to motoneurons and the somatosensory cortex are used independently.

Association between muscle architecture and quadriceps femoris H-reflex.

The purpose of the present study was to establish the relationship between muscle architecture and H-reflex recordings in quadriceps femoris muscle. H-reflexes were elicited in human quadriceps femoris muscle over a broad area of skin to document the shape and amplitude of the H-potentials. This, in combination with recording monopolar and bipolar H-potentials, was performed to determine the location and method for measuring maximum-amplitude H-reflexes. The influence of neural and peripheral factors on the H-potential during passive length changes was studied by comparing the amplitude of H-potentials to motor unit action potentials. Monopolar recordings of the H-potential were found to be preferable to bipolar recordings because of the reproducibility of shape and easier distinction between the M- and H-potentials. The location for recording maximum H-potentials was in the distal one third of the quadriceps femoris muscle, over the border between vastus lateralis and rectus femoris. The inferred relationship between H-potential amplitude and reflex excitability must be made with caution in quadriceps femoris muscle because the amplitude of both the motor unit potential and H-potential change as a function of muscle length.

Self-generated rapid taps directed to the opposite forearm in man: anticipatory reduction in the muscle activity of the target arm.

A brief hammer tap applied passively (by the experimenter) to the forearm elicits a short-latency reflex response in the forearm flexors and extensors. When the same tap is performed actively (by the subject) using the opposite forearm, the reflex response is preceded by a short-lasting anticipatory reduction in the muscle activity appearing around the impact. This anticipatory reduction is interpreted as an alternative mode of feedforward motor control associated with damping of kinetic impulses generated within the bimanual system.

Similarity in shape, timing and amplitude of H- and T-reflex potentials concurrently recorded along the broad skin area over soleus muscle.

Variations in shape, timing and amplitude of both mono- and bipolarly measured H- and T-reflex potentials can be influenced to a great extent by the muscle architecture and the peculiarities of the extracellular potential field. The "best point" for bipolar measurements, where the amplitude of the bipolar H- and T-potentials is maximal, occurred for the various subjects at a distance of 3.0 to 5.0 cm below the insertion of the gastrocnemii on the Achilles tendon. In contrast, the corresponding "best point" for monopolar H- and T-potentials is located 5.0 to 9.0 cm below the gastrocnemii insertion. The shape, total duration and timing of H- and T-potentials, concurrently measured at the various points along soleus muscle are similar. When the amplitude of the monopolar H- and T-potentials are levelled at the "best point" for monopolar measurements, the changes in the amplitude of both sets of potentials, monopolarly and bipolarly measured along soleus muscle, are identical. These results imply similar efferent outputs for both H- and T-reflexes, i.e. recruitment of motoneurons of comparable size.

Activity of wrist muscles elicited during imposed or voluntary movements about the elbow joint.

To examine the coordination of muscles during multijoint movement, we compared the response of wrist muscles to perturbations about the elbow joint with their activation during a volitional elbow movement. The purpose was to test the following two predictions: (a) Responses can occur in muscles not stretched by the perturbation, as has been reported for other multijoint systems; and (b) the motor pattern in response to a perturbation mimics an opposing volitional motor pattern across the two joints. We recorded the electromyographic (EMG) activity of elbow and wrist muscles as well as the flexion/extension motions at the elbow and wrist joints during individual trials that either involved a response to a torque perturbation that extended the elbow or required volitional elbow flexion. The results of this study confirmed that responses were elicited in the nonstretched wrist muscles when the elbow joint was perturbed. The same motor sequence of elbow and wrist flexors was present for both the volitional and perturbation task (with the forearm supinated), regardless of whether the wrist joint was immobilized or freely moving. The findings suggest that the nervous system relies on the purposeful coupling of elbow and wrist flexors to counter the inertial effects during the unrestricted voluntary movement, even though the coupling does not appear to be purposeful during the perturbation or with the wrist immobilized. The coupling of elbow and wrist flexors, however, was not rigidly fixed, as evidenced by muscle onsets that adapted over repeated perturbation trials and a reversal of the wrist muscle activated (wrist extensor) when the forearm was pronated. Hence, the coupling of muscle activities can be modified quantitatively when not beneficial and can be altered qualitatively with different initial configurations of the arm.

Hypoxia reduces the conduction velocity of the excitation along the striated muscles in man.

The conduction velocity of the excitation along the striated muscles in man, hereafter refered to as muscle fiber conduction velocity (MFCV) was used to estimate the muscle functional state under hypoxaemic hypoxia. The averaged MFCV was counted by the method based on the surface measurements (using bipolar "branched" multielectrodes) of the electrically elicited compound action potentials of the brachial biceps muscle. The arterial and venous blood-gas and acid-base parameters were simultaneously measured and the body temperature was controlled intramuscularly. With afebrile patients under normoxaemia or hypoxaemia following brain stroke, MFCV decreased proportionnally to the arterial partial oxygen pressure (PaO2) rate; a strong positive correlation (r = 0.882) between MFCV and PaO2 was found up to PaO2 of 12.0 kPa and MFCV 3.5 m/s. In the case of over PaO2 = 12.0 kPa, the relationship entered into saturation; the further increase PaO2 was not followed by concomitant changes in MFCV. The results show that MFCV-testing is a valuable tool to evaluate the functional state of the striated muscle under hypoxia.

Discharge pattern of tonically activated motor units during unloading.

In order to analyse the EMG pattern during unloading of brachial biceps muscle, the interference EMG and single motor unit activity were investigated. The measurements were done on seven healthy subjects with two types of unloading techniques: a) active unloading, when the subjects resisted against an external load (10, 20, 30 and 40 N) which is suddenly released, and b) passive unloading, performed by low inertia torque motors with independently adjustable background extension and suddenly applied flexion torques. Following active unloading the silent period duration, the amplitude of the rebound and its segmentation into consecutive bursts is changing with initial load, whereas the silent period latency remains constant. Following passive unloading the acceleration influences predominantly the amplitude of the rebound, without changing its latency and silent period duration. The initial voluntary activity influences both silent period duration and rebound parameters (latency, amplitude and duration).

Peripheral effects on the amplitude of monopolar and bipolar H-reflex potentials from the soleus muscle.

Variations in the amplitude of mono- and bipolarly measured H-reflex potentials can be influenced by muscle architecture and changes in muscle length. In the passive soleus muscle with the ankle joint fixed at 90 degree, the maximal-amplitude bipolar H-potentials were obtained along the midline of soleus at a distance of 2.0-4.0 cm below the insertion of the gastrocnemii on the Achilles tendon. In contrast, the optimal location of monopolar H-potentials was 5.0-8.0 cm below the gastrocnemii insertion. Stepwise passive shortening of soleus resulted in an increase in the amplitude of both H- and motor-unit potentials. This correspondence implicates peripheral factors, such as changes in muscle fibre diameter and inclination to the skin surface, as mechanisms mediating the changes in the amplitude of the potentials. Such effects necessitate caution in interpretation of the association between H-potential amplitude and monosynaptic reflex excitability.

Dependence between intra- and extracellular action potentials of isolated frog muscle fibres at different temperatures.

The intra- and extracellular action potentials of isolated frog muscle fibre immersed in a volume conductor at different temperatures are described. It was found that upon increasing the temperature the amplitude of the first time derivative of the intracellular action potentials increased linearly, whereas the amplitude of the second time derivative increased nonlinearly. The duration of the separate phases of the time derivatives shortened upon heating, as the velocity of spreading of the excitation increased. The length of the separate phases of the space derivatives of the action potential shortened when increasing the temperature. The amplitudes of the space derivatives were calculated. The changes in the derivatives of the action potential were explained by the influence of the temperature on the peak inward and outward transmembrane current. The changes in the extracellular action potentials produced by the temperature near the membrane and at longer radial distance at points far and near the end of the fibre are described. They were explained by the changes in the space derivatives of the intracellular action potential as well as by the features of the distribution of the extracellular potential field in the volume conductor around the finite-in-length excitable fibre.

Extracellular potential field of unmyelinated active axons.

Experimental and model investigations were carried out on the extracellular action potentials and potential fields of some unmyelinated axons: the medial and the lateral giant axons of Lumbriscus terrestris and small unmyelinated axons (nerve fibres of group C). Close to the membrane the extracellular action potentials are similar in shape to the second space derivative of the intracellular action potentials--they are triphasic in case of intracellular action potential with depolarization after-potential, having four phases in case of intracellular action potentials with hyperpolarization after-potential. Upon increasing the radial distance, the amplitudes of the different phases of the extracellular action potentials decrease at a different rate, while their maxima are shifted, which results in considerable changes in their shape.

Extracellular potential field of excited isolated frog muscle fibres immersed in a volume conductor.

The extracellular potential field of isolated frog muscle fibres immersed in a volume conductor was studied at radial distances up to 3 mm during excitation. The shape of the field distant from both the point of the origin of the excitation and the end of the fibre as well as changes in the field when depolarization wave approached the fibre end were described. Different amplitude decrease rates in individual phases of the extracellular potential and the peak-to-peak amplitude at different temperatures were found. Extracellular potentials at long radial distances were recorded using an averaging technique. The shape of the extracellular potentials at long radial distances over the fibre and beyond its end were very similar to the shape of extraterritorial potentials of a single motor unit.

Effect of short interstimulus intervals on the intra- and extracellular action potentials of isolated frog muscle fibres.

The changes in the time and space derivatives of the intracellular action potentials of isolated frog muscle fibers immersed in hypertonic sucrose-Ringer's solution were studied using paired stimulation with different interstimulus intervals. The peak-to-peak amplitude and the amplitudes of the separate phases of delta V/delta t and delta 2V/delta t2 decreased at intervals shorter than 10.25 ms as the duration of the phases lengthened. The velocity of spreading decreased at intervals below 7-10 ms and increased at longer intervals up to 40-100 ms. The peak-to-peak amplitude and the amplitudes of the separate phases of delta V/delta x and delta 2V/delta x2 decreased at intervals below 40-100 ms. The length of the separate phases of the space derivatives was almost constant. The extracellular action potentials near the membrane were similar to delta 2V/delta x2. When changing the interstimulus intervals the amplitude of the potentials recorded at different radial distances changed almost proportionally. In normal Ringer's solution the extracellular action potentials at different radial distances also changed almost proportionally depending on the intervals. They decreased at the shortest intervals and increased above the level measured at single stimuli for intervals longer than 7-10 ms, but shorter than 40-100 ms.

Influence of the muscle fibre end geometry on the extracellular potentials.

Intra- and extracellular action potentials of isolated frog muscle fibres were recorded at different distances to the end of the fibre. The first and second time derivatives of the intracellular action potentials were also recorded. The intracellular action potentials and their first and second time derivatives were almost the same regardless of the place of recording. With the decrease in the axial distance to the end the extracellular action potentials changed gradually in a complicated manner from a shape similar to the second time derivative into a shape similar to the first time derivative. Extracellular potentials, having two negative maxima, were recorded over the terminal taper part of the fibres. These alterations were simulated by a mathematical model. It was shown that the changes in the shape of the extracellular action potentials around the end of the fibres were mainly due to the existence of the fibre end though a better correspondence of the experimentally recorded and the calculated extracellular action potentials was obtained when the morphology of the fibre end was taken into consideration.

Changes in the muscle fibre extracellular action potentials in long-lasting (fatiguing) activity.

The dependence of extracellular action potentials (ECAPs) of single frog muscle fibres on intracellular action potentials (ICAPs) was studied during long-lasting (fatiguing) activity. The conduction velocity, peak-to-peak amplitude and amplitudes of the separate phases of the first and second ICAP time derivatives decreased during long-lasting activity. The phases of the first and second ICAP space derivatives also decreased in amplitude and lengthened. ECAPs near the membrane were similar in shape and proportional in amplitude to (formula; see text) when recording at a distance from both the end of the fibre and the point of stimulation. At long radial distances, the amplitudes of the separate ECAP phases depended on the amplitude and length of the corresponding phases of (formula; see text). Thus the decrease in ECAP amplitude during long-lasting activity at long radial distances was less than at points close to the muscle fibre membrane. The consequences of these findings for the changes in electromyograms recorded by needle or superficial electrodes during long-lasting (fatiguing) activity are discussed.

Is the Mmax/Hmax ratio a reliable index of the monosynaptic excitability in man? Recruitment curves for H- and M-responses using monopolar and bipolar recording techniques.

The amplitude changes of M- and H-potentials, recorded monopolarly and bipolarly from different leading-off areas on the surface of m. triceps surae upon varying the stimulus intensity from threshold to supramaximal values, in a state of rest and fixed muscle length, are investigated. It is shown that: a) the Mmax/Hmax amplitude ratio cannot serve as a reliable index for a quantitative evaluation of the monosynaptic reflex excitability of the motoneurone pool and b) recording along the medial line of m. soleus in the region from 1.5 to 8.0 cm under the distal insertion of the two gastrocnemii into the Achilles tendon in a state of rest and submaximal values of the stimulus intensity, both mono- and bipolar recording techniques can equivalently be used in studying the changes in the monosynaptic reflex excitability.

Spindle activity and monosynaptic reflex excitability during foreperiod.

Healthy volunteers were instructed to perform an isometric plantar foot flexion as quickly as possible after a foreperiod (FP) of 1000 msec defined by two clicks (warning signal (WS) and response signal (RS). In 6 volunteers the H reflex was evoked in triceps surae muscle and recorded by surface electrodes (stimulus intensity 30% of maximum). The H reflex was elicited at WS and RS as well as during FP at intervals of 100 msec. H reflex amplitudes were taken as a sign of monosynaptic reflex excitability (MSRE). Amplitudes during FP were compared with the average control values at rest. Relaxation of lower limb muscles before and during FP was controlled by EMG. MSRE was increased in the first part of FP with a maximum at 300 msec after WS and decreased in the second part, with a minimum at 800 msec after WS. In a second series of experiments, in 10 volunteers, single fiber activity from primary muscle spindle afferents was recorded with tungsten electrodes from deep peroneal nerve (6 records) and from tibial nerve (3 records). The activity of primary spindle afferents before and during the FP was calculated by instantaneous discharge frequency and histograms of spike distribution. The EMG was taken from sural triceps and anterior tibial muscles with needle electrodes; a mechanogram of tendon deflection was taken by an appropriate strain gauge. In 5 primary afferents without spontaneous activity at rest and during FP, discharge started with a delay of 10-15 msec after the onset of EMG activity during the motor reaction.(ABSTRACT TRUNCATED AT 250 WORDS)