123I-MIBG myocardial scintigraphy in diabetic patients: relationship with 201Tl uptake and cardiac autonomic function

The purpose of this study is to investigate the effects of aging on the human stretch reflexes. The EMG and torque responses of the stretch reflex of the wrist flexors were evoked by ramp-and-hold mechanical perturbations. The stretch reflexes were recorded at seven test conditions with different stretch velocity and muscle preload. The test results from young and older healthy adult subjects were compared. In average, the absolute amplitude of the short-latency (20-40 ms) EMG (recorded from flexor carpi radialis) reflex response was significantly lower in the older group. If the data were normalized and expressed in percentage of the maximal voluntary EMG activity, however, this group difference was not significant. There was no change in the reflex gain of the short-latency reflex with aging. For the long-latency (50-90 ms) EMG reflex response, both the normalized amplitude and the reflex gain were significantly enhanced with aging, probably through supraspinal mechanisms. There was no significant difference in the threshold velocity for the evoked EMG reflexive activities between age groups. There were also no changes in the reflexive wrist flexion torque with aging. These results suggested that the number of motor units recruited during the stretch reflex activity declined with aging although the percentage of motor units recruited was not affected by aging. It is concluded that the central regulating mechanisms of the spinal motoneuron excitability are not compromised by aging. The automatic gain compensation phenomenon is also preserved with aging.     

The activation of back muscles during locomotion in the developing rat

The development of posture during locomotion was studied in rats from the 11th day until adulthood. The EMGs were recorded and analyzed of the left and right longissimus muscles at caudal, intermediate and rostral levels as well as of the gastrocnemius, the tibialis and the vastus medialis muscles and movements were simultaneously recorded on videotape. Results indicate that from the 12th day of life, burst activity occurs in the longissimus muscles which is phase-related to the stepcycle. Until the 21st day these muscles are most strongly activated during burst activity in the gastrocnemius muscle in the contralateral hindleg but thereafter this activation coincides with bursts in the ipsilateral gastrocnemius muscle. At adult age such activation in the LL is restricted to fast walking or to accelerations. Latencies between bursts in the longissimus muscles and the gastrocnemius muscles vary around 100 ms until the 25th day, but thereafter they decrease to adult values of less than 10 ms. The large variations in these phase-relations at all ages suggest that supraspinal influences and afferent input are important factors in this coupling. The shift from a contra- to an ipsilateral coupling between bursts in the longissimus and in the gastrocnemius muscles might indicate that an ontogenetically older pattern of locomotion with the trunk muscles playing a major role in propulsion, is replaced by a newer pattern, mainly effected by extremity movements.     

Combinations of short oligonucleotides with increase duplex formation stability as combined primers for sequencing

The effects of yohimbine, on inhibition of C-fibre evoked responses of spinal lamina X neurones by electrical stimulation of the locus coeruleus (LC) were studied in rats. LC stimulation inhibited by 67.33 +/- 25.14%, the C-fibre evoked responses in 24 out of 30 WDR neurones and the C-component of the flexion reflex by 77.62 +/- 18.77%. The A-fibre evoked responses with wide dynamic range (WDR) and low threshold (LT) neurones were not significantly affected by LC stimulation. After administration of yohimbine, LC-induced inhibition of lamina X neurones was attenuated by 77.43 +/- 23.22% without significant changes in C-fibre evoked responses. The results suggest that alpha 2-adrenoceptors may be involved in mediation of LC-induced inhibition on lamina X neurones.     

Effects of experimental muscle pain on muscle activity and co-ordination during static and dynamic motor function

The relation between muscle pain, muscle activity, and muscle co-ordination is still controversial. The present human study investigates the influence of experimental muscle pain on resting, static, and dynamic muscle activity. In the resting and static experiments, the electromyography (EMG) activity and the contraction force of m. tibialis anterior were assessed before and after injection of 0.5 ml hypertonic saline (5%) into the same muscle. In the dynamic experiment, injections of 0.5 ml hypertonic saline (5%) were performed into either m. tibialis anterior (TA) or m. gastrocnemius (GA) and the muscle activity and co-ordination were investigated during gait on a treadmill by EMG recordings from m. TA and m. GA. At rest no evidence of EMG hyperactivity was found during muscle pain. The maximal voluntary contraction (MVC) during muscle pain was significantly lower than the control condition (P < 0.05). During a static contraction at 80% of the pre-pain MVC muscle pain caused a significant reduction in endurance time (P < 0.043). During dynamic contractions, muscle pain resulted in a significant decrease of the EMG activity in the muscle, agonistic to the painful muscle (P < 0.05), and a significant increase of the EMG activity of the muscle, antagonistic to the painful muscle (P < 0.05). Muscle pain seems to cause a general protection of painful muscles during both static and dynamic contractions. The increased EMG activity of the muscle antagonistic to the painful muscle is probably a functional adaptation of muscle co-ordination in order to limit movements. Modulation of muscle activity by muscle pain could be controlled via inhibition of muscles agonistic to the movement and/or excitation of muscles antagonistic to the movement. The present results are in accordance with the pain-adaptation model (Lund, J.P., Stohler, C.S. and Widmer, C.G. In: H. Vaer?y and H. Merskey (Eds.), Progress in Fibromyalgia and Myofascial Pain. Elsevier, Amsterdam, 1993, pp. 311-327.) which predicts increased activity of antagonistic muscle and decreased activity of agonistic muscle during experimental and clinical muscle pain.     

Changes in the excitability of soleus muscle short latency stretch reflexes during human hopping after 4 weeks of hopping training

Changes in the excitability of the human triceps surae muscle short latency stretch reflexes were investigated in six male subjects before and after 4 weeks of progressive two-legged hopping training. During the measurements the subjects performed 2-Hz hopping with: preferred contact time (PCT) and short contact time. The following reflex parameters were examined before and after the training period: the soleus muscle (SOL) Hoffmann-reflex (H-reflex) at rest and during hopping, the short latency electromyogram (EMG) components of the movement induced stretch reflex (MSR) in SOL and medial gastrocnemius muscle (MG), and the EMG amplitude of the SOL and MG tendon reflexes (T-reflexes) elicited at rest. The main results can be summarized as follows: the SOL T-reflex had increased by about 28% (P < 0.05) after training while the MG T-reflex was unchanged; the SOL MSR (always evident) and the MG MSR (when observable) did not change in amplitude with training, and before training the SOL H-reflex in both hopping situations was significantly depressed to about 40% of the reference value at standing rest (P < 0.05). After training the H-reflex during PCT hopping was no longer depressed. As the value of the measured mechanical parameters (the total work rate, joint angular velocity and the ankle joint work rate) was unchanged after training in both hopping situations, the reflex changes observed could not be ascribed to changes in the movement pattern. To explain the observed changes, hypotheses of changes in the excitability of the stretch reflex caused by the training were taken into consideration and discussed.     

Selective activation by bryostatin-1 demonstrates unique roles for PKC epsilon in neurite extension and tau phosphorylation

The purpose of this study was to contrast the discharge patterns of the same motor units during movements and during isometric contractions that were produced with comparable torque-time characteristics. Subjects performed elbow flexion and extension movements with predetermined acceleration characteristics. The average acceleration and deceleration profiles for the movements were reproduced in the isometric setting by presenting the kinematic profiles as templates for torque production. Trained subjects were able to match the first agonist (AG1) and antagonist (ANT) electromyographic (EMG) bursts, but tended to produce a smaller second agonist burst (AG2) in the isometric contraction. Twenty-five motor units from triceps brachii were studied. The same motor units (with one exception) were recruited and subsequently discharged in a similar fashion in both the isometric and movement tasks in the AG1 and ANT EMG bursts, with fewer motor unit discharges in the AG2 burst in the isometric contraction. The central control mechanisms appear to be the same for the acceleration phase of movement and isometric contraction, but differ during the deceleration phase.     

Experimental muscle pain increases the human stretch reflex

In this study we investigated the effect of human experimental muscle pain on H- and stretch reflexes as indicators of changes in muscle spindle sensitivity. Fourteen healthy, male volunteers participated in the study. Muscle pain was produced by infusion of 5% hypertonic saline over a period of 10-15 min in m. soleus and in m. tibialis anterior. Reflexes were elicited in the relaxed and active soleus muscle (10-15 Nm ankle torque) before, during and after muscle pain. Control measurements were made with infusions of 0.9% isotonic saline. Surface electromyograms (EMG) were measured from the soleus muscle, and torque was measured from the ankle joint. With pain in the soleus muscle the mechanical stretch reflex response (ankle torque) increased significantly (P = 0.0007) as compared to before pain. With pain in the tibialis anterior muscle both the mechanical and EMG responses increased significantly (P = 0.001; P = 0.0003) as compared to before pain. The H-reflex showed no significant changes during the infusions in either muscles. This study has demonstrated a muscle pain-related increase in the amplitude of the stretch reflex without a corresponding increase in the H-reflex amplitude. One explanation could be an increased dynamic sensitivity of the muscle spindles during muscle pain caused by an increased firing rate in the dynamic gamma-motoneurones. However, the data could not support the vicious cycle model because the excitability of the alpha-motoneurone pool was unchanged.     

Recovery of locomotion after ventral and ventrolateral spinal lesions in the cat. I. Deficits and adaptive mechanisms

The recovery of treadmill locomotion of eight adult cats, subjected to chronic ventral and ventrolateral spinal lesions at low thoracic levels (T11 or T13), preserving at least one dorsolateral funiculus and the dorsal columns, was documented daily using electromyographic (EMG) and kinematic methods. The data show that all cats eventually recovered quadrupedal voluntary locomotion despite extensive damage to important pathways (such as the reticulospinal and the vestibulospinal) as verified by injection of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) caudal to the site of lesion. Initially (in the early period after the spinal lesion), all the cats suffered from pronounced locomotor and postural deficits, and they could not support their hindquarters or walk with their hindlimbs. Gradually, during the recovery period, they regained quadrupedal walking, although their locomotion was wobbly and inconsistent, and they suffered from poor lateral stability. EMG and kinematic data analyses showed a tendency for an increase in the variability of the step cycle duration but no major changes in the step cycle structure or in the intralimb coupling of the joints. However, the homolateral fore- and hindlimb coupling was highly perturbed in cats with the largest lesions. Although the general alternating pattern of extensor and flexors was maintained, there were various changes in the duration and amplitude of the EMG bursts as well as a lack of amplitude modulation during walking uphill or downhill on the treadmill. In cats with larger lesions, the forelimbs also seem to take a greater propulsive role than usual as revealed by a consistent increase of the activity of the triceps. In cats with smaller lesions, these deficits were transient, but, for the most extensively lesioned cats, they were pronounced and lasted long term postlesion even after reaching a more or less stable locomotor behavior (plateau period). It is concluded that recovery of quadrupedal locomotion is possible even after a massive lesion to ventral and ventrolateral quadrants, severing the vestibulospinal pathway and causing severe, although incomplete, damage to the reticulospinal tract. The quick recovery in the less lesioned cats can be attributed to remaining pathways normally implicated in locomotor function. However, in the most extensively lesioned cats, the long period of recovery and the pronounced deficits during the plateau period may indicate that the compensation, attributed to remaining reticulospinal pathways, is not sufficient and that other pathways in the dorsolateral funiculi, such as the corticospinal, can sustain and adapt, up to a certain extent, the voluntary quadrupedal walking.     

Spontaneous electromyographic activity in adult rat soleus muscle

Single-motor-unit and gross electromyograms (EMG) were recorded from the soleus muscle in six unrestrained rats. The median firing frequencies of nine motor units were in the 16-25 Hz range, in agreement with previous studies. One additional motor unit had a median firing frequency of 47 Hz. This unit and one of the lower-frequency units regularly fired doublets. Motor-unit firing frequency was well correlated to whole-muscle EMG during locomotion. Integrated rectified gross EMG revealed periods of continuous modulation, phasic high-amplitude events, and tonic low-amplitude segments. The tonic segments typically were caused by a small number of motor units firing at stable high frequencies (20-30 Hz) for extended periods of time without detectable activity in other units. This long-lasting firing in single motor units typically was initiated by transient mass activity, which recruited many units. However, only one or a few units continued firing at a stable high frequency. The tonic firing terminated spontaneously or in conjunction with an episode of mass activity. Different units were active in different tonic segments. Thus there was an apparent dissociation between activity in different single motor units and consequently between single-motor-unit activity and whole-muscle EMG. It is proposed that the maintained tonic motor-unit activity is caused by intrinsic motoneuron properties in the form of depolarizing plateau potentials.     

Functional properties of postganglionic sympathetic neurones supplying the submandibular gland in the anaesthetized rat

Sympathetic neurones supplying the submandibular salivary gland innervate blood vessels, secretory and myoepithelial cells. Here we examined whether these functionally different sympathetic neurones show distinct reflex response patterns. In anaesthetized rats, single unit activity was recorded from postganglionic axons projecting to the gland. Neurones were tested for their responses to stimulation of baroreceptors, cutaneous nociceptors and cold receptors and to gustatory stimuli applied to the tongue. Respiratory modulation was also analysed. Only a few postganglionic neurones identified electrically (5-10%) were spontaneously active. They were excited by noxious and cold stimuli, inhibited by baroreceptor stimulation and exhibited respiratory modulation. None of the units responded to gustatory stimuli. Thus, in anaesthetized rats spontaneously active sympathetic neurones supplying the submandibular gland behave like vasoconstrictor neurones. Sympathetic neurones with other functions are probably silent.     

Impulse initiation in the mammalian muscle spindle during combined fusimotor stimulation and succinyl choline infusion

1. This is a report of observations on the responses of the primary and secondary endings of soleus muscle spindles of the anesthetized cat to the combined effects of the depolarizing neuromuscular blocker succinyl choline (SCh), given intravenously, and fusimotor stimulation. The findings were interpreted in terms of a dual pacemaker model for activity generated in the bag1 intrafusal fiber interacting with activity coming from bag2 and chain fibers. 2. In preliminary experiments it was found, using whole ventral root stimulation at fusimotor strength, that spindle responses to fusimotor stimulation were not blocked by SCh, whereas extrafusal junctions blocked rapidly. In the presence of SCh, fusimotor responses of spindle secondary endings were, on average, slightly larger than their control values before SCh was given, whereas fusimotor responses of primary endings were slightly smaller. 3. A study of the responses of spindle primary endings to stimulation of single dynamic (gamma D) and static (gamma S) axons in the presence of SCh revealed a fundamental difference in behavior. None of the responses to stimulation of gamma D axons (9 gamma D axons with 8 primary endings) showed significant summation with the responses to SCh. By contrast, the 20 gamma S axons studied showed varying degrees of summation with the responses to SCh. The responses of secondary endings to gamma S stimulation in the presence of SCh resembled those of primary endings and gamma S stimulation. 4. To explain these differences it is proposed that the primary ending has two separate sites of impulse initiation, one close to terminals on the bag1 intrafusal fiber (innervated by gamma D axons) and a second close to terminals on the bag2 and chain fibers (innervated by gamma S axons). It is proposed that the maintained increase in spindle firing observed during SCh infusion is the result of a bag2 contracture. The response to gamma S stimulation, contracting bag2 and chain fibers, adds to the SCh response. The degree of summation varies depending on whether the gamma S activates bag2 fibers, chain fibers, or both. The bag1 contracture, together with the effects of gamma D stimulation, acts through a separate pacemaker and therefore does not sum with the steady increase in spindle firing in the presence of SCh. There may be pacemaker switching between the bag1 generator and the bag2 and chain generator. 5. If the model is representative of most spindles containing the three kinds of intrafusal fibers, and the contractions of bag2 and chain fibers generate activity through a common impulse generator, then this bears on the question of the functional independence of the bag2 and chain fiber systems.     

"Novelty" neurons in the frog auditory system

Some neurones in the torus semicircularis of paralyzed lake frogs (Rana ridibunda) exhibited strong response habituation to tonal bursts following with interstimuli intervals of 1 to 5 sec. The habituation became less evident with a devrease of the repetition rate or increase of the stimulus intensity. These units revealed the phenomenon of "postzero" habituation. The frequency selectivity of the units exhibited by their responses to the first stimulus presentation was very poor. Usually the habituation occurred within a wide frequency band, although there were some exceptions. Afther the habituation was completed, the unit's response could be elicited by tones of a different frequency. The more distant on the frequency scale the tones were, the more vigorous was the reaction. Some properties of these "novelty" units can be explained by assuming that they are multipolar neurones located in the nucleus magnocellularis of the torus.     

Plaster corset--an important part in the surgical treatment of scolioses (author''s transl)

Single postganglionic neurones to hairy skin and hairless skin of the hindleg were investigated on spinal cord heating and spinal cord cooling in chloralose anesthetized cats. 1. Spontaneously active postganglionic neurones which were classified as vasoconstrictor neurones were depressed by spinal cord heating and excited by spinal cord cooling. The overall response to spinal cord cooling was smaller than that to spinal cord heating. 2. Postganglionic neurones to the hairless skin, which had most likely sudomotor function, responded initially to spinal cord heating with a few impulses or not at all. As judged by the skin potentials recorded from the hairless skin the sweat glands were also only weakly activated at the beginning of the heat stimuli. 3. Six silent postganglionic neurones, 3 each to the hairy skin and to the hairless skin, were excited during spinal cord heating. The response of these neurones consisted of a dynamic and a static component and started at the beginning of the heating stimuli with latencies of less than 10S. The neurones could not be excited by any other stimuli and were classified as cutaneous vasodilator neurones. 4. Quantitative analysis of 4 spontaneously active postganglionic (vasoconstrictor) neurones and 3 silent postganglionic (vasodilator) neurones revealed that the threshold of the responses of these neurones to spinal cord heating was 40-42 degrees C (on the dorsal spinal cord) and that the response increase was maximal at the highest temperatures tested (43-44 degrees C).?     

Voluntary inhibition of the myotatic reflex and premotor response to joint angle displacement

EMGs from the deltoid muscle of the nonpreferred arm were obtained from 18 subjects (9 males, 9 females). The EMGs resulted from a stretch reflex (reflex latency) from the fall of the relaxed arm from an electromagnet and from the resultant EMGs of a voluntary response to stop the preceived falling arm (premotor response). Ten normal trials were followed by 10 inhibition trials. By conscious relaxation of the arm and inhibition of the deltoid muscle, all subjects on most trials were able to completely eliminate the voluntary premotor responses. Elimination of the myotatic reflex proved much more difficult with two subjects completely inhibiting it on all trials and ten subjects on at least one trial; however when the reflex was not inhibited, the inhibited condition resulted in significantly reduced reflex duration and an increased reflex latency. No sex difference was found in the ability to inhibit the EMG responses.     

The central neural mechanism of the gastric motility in the dog's medulla oblongata on the vagal inhibitory and excitatory reflexes (author's transl)

The influences of the vagal inhibitory and excitatory reflexes on the gastric motor centers in the dog's medulla oblongata were investigated. Dogs were anaesthetized with Nembutal, and supplemented Gallamine at need. The brain stem was transected on the level of inferior colliculi of midbrain, the spinal cord transected on the level of between C1 and C2. Bilateral splanchnic nerves were also severed. Electrical activities from the inhibitory and excitatory areas in medulla oblongata were recorded by using a concentric circle electrode which was inserted into medulla oblongata from the dorsal surface of it. Following results were obtaind. 1) Electrical activities of the gastric inhibitory areas were classifed into three types (see Table 1). Type I: The augmentation of electrical activities of the gastric inhibitory areas during theinhibitory reflex were associated with the diminution of them during the exictatory reflex. Type II: Although electrical activities of the gastric inhibitory areas were augmented during the inhibitory reflex, any changes of them were not obtained during the excitatory reflex. Type III: Any changes of electrical activities of the gastric inhibitory areas were not obtained during both inhibitory and excitatory reflexes. 2) Electrical activities of the gastric excitatory areas were classified into two types (see Table 2). Type I: The diminution of electrical activites of the gastric excitatory areas during the inhibitory reflex were associated with the augmentation of them during the excitatory reflex. Type II: Although any changes of electrical activities of the gastric excitatory areas were not observed during the inhibitory reflex, they were augmented during the excitatory reflex.     

Characteristics of locomotor control in children with cerebral palsy

The typical features of electromyographical (EMG) recordings from children with cerebral palsy (CP) consist of a coactivation of antagonistic leg muscles during the stance phase, a low and tonic activation of extensor EMG, and enhanced stretch reflex excitability with short latency. This characteristic reflex pattern is suggested to reflect an arrested normal maturation. The strong similarity between the walking pattern of CP children (8-16 years of age) and the reflex pattern during the process of learning to walk (7-10 months of age) lets us draw the following conclusion. During normal maturation a close parallelism exists between the control of group I afferent inhibition with the suppression of mono/oligosynaptic stretch reflexes and group II afferent facilitation with the increase of polysynaptic (mainly extensor) EMG responses. This maturation depends on supraspinal control, and does not occur in CP children. In adult patients with a supraspinal lesion, a regression to this early reflex pattern takes place.     

Command neurons in the unconditioned reflex arc of the grape snail

Independent intracellular activation of any one of the five identified neurones elicits a behavioral act of pneumostome closing. Similar closing is evoked by adequate stimulation of different modalities. Comparison of unconditioned reflex properties and characteristics of responses of the studied cells leads to the conclusion that neurones under discussion are command elements for the unconditioned reflex. Command neurones respond to all sensory stimuli which can evoke reflex closing. There is a lot of synchronous EPSPs in their spontaneous activity, but no action potentials. A spike discharge of command neurones evoked by an adequate stimulus always precedes the effector movements but there is no action potentials during the pneumostome closing. Conclusion is made that the functional role of command neurones in the unconditioned reflex arc consists in evaluating the sensory input and in triggering motor programmes.     

Vestibular-evoked electromyographic responses in soleus: a comparison between click and galvanic stimulation

The aim of this study was to demonstrate, if possible, vestibulospinal reflex responses in soleus using a stimulus known to be capable of exciting vestibular afferents, namely 100-dB (NHL) clicks. We were able to show short-latency electromyographic (EMG) responses after clicks in five of eight normal subjects, and then we compared these responses with those after transmastoid galvanic stimulation (12 normal subjects). Stimulation of the side towards which the head was rotated (i.e. the side facing backwards) with either clicks or the cathode (anode applied to the opposite side) gave an initial excitatory response in soleus, while click or cathodal stimulation of the opposite side (i.e. the side facing forwards) gave an initial inhibitory response. Onset latencies and modulation with changes in postural task were identical for both click- and galvanic-evoked responses. In addition, there was a significant correlation between the amplitudes of the responses in soleus after click and galvanic stimulation (R2=0.72). These similarities suggest that the earliest reflex responses in soleus after clicks and galvanic stimulation may be mediated by a common central pathway. In contrast, there was no correlation between the amplitudes of responses evoked by 100-dB clicks in soleus and those evoked by the same stimulus in the sternocleidomastoid. We conclude that vestibular activation by clicks can evoke reflex responses in lower-limb muscles and these responses have similar characteristics to the earliest responses evoked by galvanic vestibular stimulation.     

Effects of intrathecal alpha1- and alpha2-noradrenergic agonists and norepinephrine on locomotion in chronic spinal cats

Noradrenergic drugs, acting on alpha adrenoceptors, have been found to play an important role in the initiation and modulation of locomotor pattern in adult cats after spinal cord transection. There are at least two subtypes of alpha adrenoceptors, alpha1 and alpha2 adrenoceptors. The aim of this study was to investigate the effects of selective alpha1 and alpha2 agonists in the initiation and modulation of locomotion in adult chronic cats in the early and late stages after complete transection at T13. Five cats, chronically implanted with an intrathecal cannula and electromyographic (EMG) electrodes were used in this study. Noradrenergic drugs including alpha2 agonists (clonidine, tizanidine, and oxymetazoline) and an antagonist, yohimbine, one alpha1 agonist (methoxamine), and a blocker, prazosin, as well as norepinephrine were injected intrathecally. EMG activity synchronized to video images of the hindlimbs were recorded before and after each drug injection. The results show differential effects of alpha1 and alpha2 agonists in the initiation of locomotion in early spinal cats (i.e., in the first week or so when there is no spontaneous locomotion) and in the modulation of locomotion and cutaneous reflexes in the late-spinal cats (i.e., when cats have recovered spontaneous locomotion). In early spinal cats, all three alpha2 agonists were found to initiate locomotion, although their action had a different time course. The alpha1 agonist methoxamine induced bouts of nice locomotor activity in three spinal cats some hours after injection but only induced sustained locomotion in one cat in which the effects were blocked by the alpha1 antagonist prazosin. In late spinal cats, although alpha2 agonists markedly increased the cycle duration and flexor muscle burst duration and decreased the weight support or extensor activity (effects blocked by an alpha2 antagonist, yohimbine), alpha1 agonist increased the weight support and primarily the extensor activity of the hindlimbs without markedly changing the timing of the step cycle. Although alpha2 agonists, especially clonidine, markedly reduced the cutaneous excitability and augmented the foot drag, the alpha1 agonist was found to increase the cutaneous reflex excitability. This is in line with previously reported differential effects of activation of the two receptors on motoneuron excitability and reflex transmission. Noradrenaline, the neurotransmitter itself, increased the cycle duration and at the same time retained the cutaneous excitability, thus exerting both alpha1 and alpha2 effects. This work therefore suggests that different subclasses of noradrenergic drugs could be used to more specifically target aspects of locomotor deficits in patients after spinal injury or diseases.     

Discharge pattern of single motor units in the tonic vibration reflex of human triceps surae

Using a single fibre EMG electrode the firing pattern of 46 motor units in the triceps surae has been studied during vibration of the Achilles tendon at frequencies of 25--200 Hz. Potentials activated in the tonic vibration reflex (TVR) were phase-locked to the vibration cycle but tended to become somewhat less so with continued vibration. The firing pattern of voluntarily activated motor units became locked to the waveform by the application of the vibrator. The discharges of 21 motor units were studied during low threshold (sub-M wave) tetanic stimulation of the tibial nerve at 25--100 Hz. No evidence was found of synchronization of potentials activated in the resulting tonic contraction. During weak voluntary contractions, stimulation also failed to regularize voluntarily activated motor units. The findings can be reconciled by postulating that, in normal man, vibration activates monosynaptic and polysynaptic pathways, the latter circuit being adequate to generate reflex contraction, while the former merely affects the temporal patterning of the motor outflow.