Corticospinal projections to lower limb motoneurons in man

Summary The projections of cortical neurons activated by transcranial magnetic stimulation to single lower limb spinal motoneurons were examined in 34 normal subjects. Peristimulus time histograms of the discharge times of single, voluntarily activated motor units were used to derive information about postsynaptic potentials in single spinal motoneurons produced by magnetic stimuli applied over the contralateral scalp. All tibialis anterior motor units and the majority of motoneurons innervating the small muscles of the foot showed strong short latency facilitation. About half of the motoneurons of proximal lower limb muscles showed this facilitation. Short latency facilitation of the motoneurons of soleus and medial gastrocnemius was only rarely observed and when present was weak. The short latency facilitation is attributed to the projections of the fast corticospinal pathway with monosynaptic projections to motoneurons. The relative strength of the facilitation in different motoneuron pools is considered to reflect the density of corticospinal projections to that motoneuron pool. The observed pattern of projections in man shows some differences from the pattern of projections in subhuman primates that might reflect the different use of the limb.     

Comparison of human motor cortical projections to abdominal muscles and intrinsic muscles of the hand

Summary Percutaneous electrical stimulation of the motor cortex was used to activate rapidly conducting corticofugal pathways to human abdominal muscles. Following cortical stimulation the response latencies for the abdominal muscles were similar to those for limb muscles which are a similar distance from the motor cortex. Cortically evoked responses recorded from the abdominal muscles had the same latency and similar amplitude during several voluntary tasks including expiration, expulsive manoeuvres and trunk flexion. Responses could also be evoked when the chemical drive to breathe was increased by rebreathing. In addition, the properties of the cortical projection to muscles of the abdominal wall were directly compared with those of the projection to the intrinsic muscles of the hand. The latencies of responses in abdominal muscles and intrinsic muscles of the hand were measured during static contractions over a range of strengths in the same subjects (0–100% maximal voluntary contraction, MVC). For both muscle groups, cortically evoked muscle responses of minimal latency occurred when background contractions reached 10–20% MVC with responses of maximal amplitude at 60% MVC. The variability in latency of fifty consecutive responses were similar for the two muscle groups. Furthermore, post-stimulus time histograms for 4 rectus abdominis motoneurones revealed a brief initial excitatory peak of 1.15ms duration (range 0.96–1.34ms) following cortical stimulation. The characteristics of this peak are the same as reported for motoneurones of intrinsic hand muscles. These findings demonstrate a powerful rapidly conducting pathway from the motor cortex to the human abdominal muscles. This pathway has many of the same properties as the monosynaptic corticospinal projection to the distal muscles of the upper limb.     

Patterns of activity of perioral facial muscles during mastication in man

Summary The activity of buccinator, orbicularis oris superior and inferior, quadratus and triangularis muscles was recorded by surface and needle electrodes during spontaneous unilateral mastication of small or large boluses. Their activity was compared with that of the masticatory muscles masseter and digastric, and with the vertical movement of the jaw. The facial muscles were active during mastication, displaying both tonic and phasic activity. Although the cyclic activity was broadly linked to lowering of the jaw, there was no strict time correspondence between the activity of the various facial muscles themselves, or between the activity of the facial muscles and the digastric. The timing and amplitude of facial muscle activity were affected by the duration of the masticatory cycle, the side on which chewing took place, the size of the bolus, and whether or not lip to lip contact was made.     

Pattern of projections of group I afferents from elbow muscles to motoneurones supplying wrist muscles in man

Summary The pattern of projections of low threshold afferents from triceps and biceps brachii muscles onto motoneurones innervating muscles acting at the wrist was assessed by a reflex and a poststimulus time histogram (psth) technique. Activation of low-threshold afferents originating from elbow flexors or extensors resulted in an early, short-lasting inhibition of wrist flexor motoneurones (flexor carpi radialis, flexor carpi ulnaris). An inhibition was also found in the extensor carpi radialis (ECR) motoneurones after stimulation of low-threshold afferents from triceps. Evidence is presented that Ia fibres contribute to these effects. The inhibitory effects were found in all subjects, but they were constant in only 57% of the reflex experimental sessions and in 25% of the explored motor units. Stimulation of biceps low-threshold afferents was always ineffective on ECR motoneurones. No early facilitation was ever seen in motor nuclei innervating wrist muscles following stimulation of low threshold afferents from biceps and triceps. The pattern of transjoint projections of group I afferents from proximal to distal muscles and from distal to proximal ones (Cavallari and Katz 1989) is discussed in relation to that described in the cat forelimb.     

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     

Presence of homonymous recurrent inhibition in motoneurones supplying different lower limb muscles in humans

Summary The pattern of recurrent inhibition to motoneurones (MNs) innervating different lower limb muscles was investigated in ten healthy subjects. Three complementary experimental designs, all based on the electrophysiological method introduced by Bussel and Pierrot-Deseilligny in 1977, were used in Quadriceps (Qu), Pretibial (Pt), Soleus (Sol) and Abductor hallucis (Abh) motornuclei: 1) measurement of the relationship between the amplitude of conditioning H (H1) and test (H) reflexes; 2) measurement of the effect of the intravenous administration of L-Acetylcarnitine (L-Ac) on the amplitude of the test H reflex after a constant H1 conditioning reflex; 3) measurement of the modifications of the test H reflex in relation to a reference H (Ref H) reflex during a weak tonic voluntary contraction of the homonymous muscle. A complete agreement among results obtained with the different experimental paradigms was observed. Similarly to the Sol, both Pt and Qu MNs were found to be recurrently inhibited: a) the test H reflex exhibited a progressive and consistent depression with increasing amplitude of the H1 conditioning reflex beyond a specific value; b) an additional decrement of the test H reflex was obtained after intravenous administration of L-Ac; c) a decrease in the size of the test H reflex, with respect to its value at rest, was observed during a weak tonic voluntary contraction, in spite of the enhanced MN excitability (as shown by the increase in the Ref H). By contrast, no evidence of recurrent inhibition to the Abh MNs was found. The amplitude of the test H reflex showed no further depression with increasing conditioning reflex discharge, L-Ac administration or during weak voluntary contraction. It is concluded that in the lower limb, MNs acting on the knee and ankle muscles receive recurrent inhibition, but that this is lacking in motornuclei innervating the more distal muscles, such as the intrinsic foot muscles. Some methodological and physiological implications are discussed in relation to the present findings.     

Cutaneous depression of Ib reflex pathways to motoneurones in man

Summary Variations in the H-reflex of soleus (Sol), quadriceps (Q) and short head of biceps femoris (Bi) muscles in normal man were used to investigate the effect of volleys in low threshold cutaneous afferents from the ipsilateral limb on transmission of Ib effects from ankle and Q muscles to these different motoneurone (MN) pools. Stimulation of cutaneous afferents from the foot sole and the toes (but not from the thigh, knee or calf), which did not modify the size of the test reflexes when applied alone, strongly depressed Ib reflex pathways to MNs supplying muscles operating at the knee. The very brief central latency of this depression suggests that tactile cutaneous afferents from the foot have oligosynaptic spinal connexions with the interneurones intercalated in the Ib pathways to MNs. The same cutaneous stimuli did not at all modify Ib inhibition of Sol MNs from triceps surae. These findings are discussed with regard to the role of the different muscles in human locomotion. It is suggested that during the stance phase of heel bipedal locomotion, the cutaneous depression of Ib reflex pathways to MNs supplying muscles operating at the knee might operate in association with the strong Ia connexions from ankle to knee muscles described in the previous paper.This work was supported by grants from Pierre et Marie Curie University (Paris VI) and from the Institut National de la Santé et de la Recherche Médicale (INSERM, 78.1.269.6)Attachée de recherches à l'INSERM     

Functional MRI determination of a dose-response relationship to lower extremity neuromuscular electrical stimulation in healthy subjects

Although empirical evidence supports the use of neuromuscular electrical stimulation (NMES) to treat physical impairments associated with stroke, the mechanisms underlying the efficacy of this modality are poorly understood. Recent studies have employed functional imaging to investigations of brain responses to median nerve stimulation. These studies suggest a dose-response relationship may exist between selected stimulation parameters and hemodynamic responses in sensorimotor regions. However, substantial gaps exist in this literature. The present study was designed to address these deficiencies. Ten healthy subjects participated. In phase one, four stimulus intensity levels were established: (1) sensory threshold [Th], (2) (MM−Th)×0.333+Th [low-intermediate level, LI], (3) (MM−Th)×0.666+Th [high-intermediate level, HI], and (4) maximal motor (MM). In phase two, subjects were scanned using a spiral-echoplanar imaging technique at each stimulus level. Image sets were analyzed to determine hemodynamic responses at the highest Pearson correlation level (r) ascertained for each of five areas of interest (AOI): (1) primary sensory, (2) primary motor, (3) cingulate gyrus, (4) thalamus, and (5) cerebellum. ANOVA demonstrated significant main effects for BOLD signal amplitude (p<0.05) changes in all AOI. Similarly, ANOVA showed significant differences in the volume of activation (p<0.05) with increasing stimulus intensity in four AOI. Secondary analyses of pooled data showed increasing probabilities of activation at higher stimulus intensities within each AOI. Collectively, these data indicate a dose-response relationship exists between lower extremity NMES and brain activation in specific neural regions. The current results, while limited in their generalizability, are foundational for future studies of interventions using NMES. Electronic Publication     

Pattern of reflex responses in lower limb muscles to a resistance in walking man

Summary Reflex responses in the lower limbs were investigated using electromyographic and kinematic techniques in man walking on a treadmill. A momentary resistance was applied to one leg at three selected points in the step cycle. The responses to such stimuli, as well as the locomotor activity, were picked up electromyographically and displayed on a four channel oscilloscope. Four superficial muscles viz: gluteus medius, vastus lateralis, rectus femoris and tibialis anterior were studied in both ipsilateral and contralateral legs. In general it was found that the ipsilateral leg muscles produced a response throughout the step cycle regardless of whether the muscle was active or silent at the time the reflex occurred. In contrast, contralateral leg muscles showed a different pattern of response which depended on where the resistance was applied in the step cycle. The long reflex latency, of the order of 80 ms, was a consistant feature of the responses and suggests the possible involvement of supra-spinal pathways. The latencies for a particular muscle were identical on the ipsi- and contralateral sides. The durations of the swing and stance phases of the step cycle were also recorded but showed no change due to application of the resistance. In general, the results indicate that the body has the inherent ability to reinforce the ongoing locomotor muscle activity in response to external stimuli in order to maintain upright balanced walking.     

The contribution of transcortical pathways to long-latency stretch and tactile reflexes in human hand muscles

Long-latency electromyographic (EMG) responses can be evoked in the first dorsal interosseous muscle (FDI) by unexpected slips of an object (skin stretch) held between the index and thumb, or by forcible adduction of the metacarpophalangeal joint (muscle stretch). The former type of response is due to stimulation of tactile afferents in the skin of the digits, whereas the latter also activates muscle receptors. Previous studies have provided good evidence that long-latency reflex responses to stretch of distal muscles involve activity in a transcortical reflex pathway. The present experiments examined whether cutaneous reflexes also utilise a transcortical route. Transcranial magnetic or electrical stimuli were given over the motor cortex to evoke EMG activity during the period of the long-latency reflex response. When evoked by muscle stretch the responses to magnetic stimulation were facilitated more than those to electric stimulation. In contrast, facilitation was equal during the long-latency reflex elicited by cutaneous stimulation. Because of the different ways in which electrical and magnetic stimuli are believed to activate the motor cortex, we interpret these results to mean that the long-latency response to skin stretch is not mediated by a transcortical mechanism in the majority of subjects, whereas that following muscle stretch is. However, these are average data. In a few individual subjects, the opposite results were obtained. We suggest that there may be differences between subjects in the transcortical contribution to long-latency reflex responses. The implication is that, under normal circumstances, several pathways may contribute to these responses. If so, the relative roles of the pathways may change during different tasks, and in pathological states lesions in one system may well be accompanied by compensatory changes in other systems.     

Different activations of the soleus and gastrocnemii muscles in response to various types of stance perturbation in man

Summary The soleus (Sol) and medial and lateral gastrocnemii (GM and GL) behave differently during various movements, but no attempt has been made to disclose any distinct activation of these muscles during perturbations of upright stance. Therefore the pattern of activation of the three triceps surae (TS) muscles and of the tibialis anterior (TA) was studied in normal subjects following rotational and linear displacements of a movable platform. The effect of postural set on the responses was also studied while holding onto a frame. In free-standing subjects, TS stretches (upward tilt, UT and backward translation, BT) evoked a large short latency response (SLR) in Sol; smaller SLRs were sometimes induced in GM or GL. A medium latency response (MLR) was consistently present in both or only one gastrocnemii. On the average, the amplitude and the frequency of occurrence of the responses were distributed as follows: SLR, Sol>GL>GM; MLR, GM>GL>Sol. The type of perturbation did not affect the latency of all TS muscle responses, but the duration, amplitude and frequency of MLRs were larger during BT than UT. MLRs were followed by an antagonistic reaction (AR) in the TA, larger and more frequent during UT than BT. TA stretches (downward tilt, DT and forward translation, FT) induced a TA MLR, with duration and area larger in FT than DT. ARs occurred in one or more muscles of TS, being larger and more frequent in Sol. Under the conditions of stabilized stance, SLRs were not affected, but all the MLRs and ARs were much reduced in amplitude. The analogies between TS and TA MLRs (frequency of occurrence, latency and suppression under stabilized condition) suggest a common underlying mechanism and a similar postural role. On the other hand, all the TS responses are unequally distributed in the individual muscles and in the various subjects. This recommends caution in drawing conclusions in their absence or from their susceptibility to postural set in patients, when only one muscle of TS is being recorded.     

Input-output properties and gain changes in the human corticospinal pathway

 Experiments were done to determine the form of the input-output relation (i.e. stimulus intensity vs response amplitude) of the corticospinal pathway of the first dorsal interosseous and the tibialis anterior, respectively. Our purpose was to determine from these quantitative relations which input-output parameters would be useful measures in studies dealing with motor cortical task dependence. The motor cortex was excited by focal transcranial magnetic stimuli and the evoked motor response were recorded with surface electromyographic electrodes. In some experiments the discharge probability of single motor units in response to magnetic stimuli of increasing intensity was determined from intramuscular recordings. For both muscles the form of the input-output relation was sigmoidal. The steepness of the relation increased, up to 4–7 times the value at rest, with increasing tonic background activity. The threshold decreased, but only slightly, with increasing tonic background activity. The minimum value of the threshold was reached at activation levels of about 10–20% of the maximum tonic effort, whereas the steepness of the relation reached its maximum at higher activation levels, typically about 30–40% of the maximum tonic effort. These observations imply that these two input-output parameters of the corticospinal pathway – one reflecting the bias level (threshold) and the other the gain (slope) – are determined by different neural mechanisms. The plateau level of the sigmoidal input-output relation was not influenced by the background activation level, except that in some subjects (4/9) it could not be reached when no background motor activity was present. This was probably due, for the most part, to limitation of the maximum stimulator output. Additionally, this finding may reflect a change in the intrinsic excitability of the motor cortex in going from rest to activity, or that convergent inputs from different descending and afferent systems are required for maximal activation of motoneuron pools. Thus, the threshold, steepness and plateau level characterize the input-output parameters of the human corticospinal pathway for a given level of motor activity. In contrast to the nonlinear input-output relation of the corticospinal pathway as whole, which includes the motoneuron pool and any spinal interneuronal relays, the discharge probability of all single motor units was a linearly increasing function of the stimulus strength (r≥0.9, P<0.01). Thus, the sigmoidal input-output relation of the corticospinal pathway, as a whole, is not due to the input-output properties of single motoneurons. The possible neural mechanisms which underlie the shape and parameters of the input-output relation as well as the methodological implications of the results are considered. Received: 2 July 1996 / Accepted: 9 October 1996     

A propriospinal-like contribution to electromyographic responses evoked in wrist extensor muscles by transcranial stimulation of the motor cortex in man

We tested the hypothesis that some of the electromyographic (EMG) responses elicited in preactivated forearm muscles by transcranial stimulation of the human motor cortex are produced by activity in a disynaptic corticospinal linkage involving propriospinal-like interneurones with cell bodies in the spinal C3–4 segments. The experimental design incorporated a previous observation that stimulation of afferents in the superficial radial nerve inhibits propriospinal-like neurones projecting to the extensor carpi radialis (ECR) muscle. Surface EMG responses were recorded from the active ECR muscle after transcranial electrical or magnetic stimulation over the motor cortex. In random trials, single conditioning stimuli at twice perceptual threshold were given to the superficial radial nerve at the wrist at different times before a cortical shock. When the cortex was stimulated electrically, the conditioning stimulus suppressed the EMG responses when the interval between the shocks was 11 ms or more. This was about 3.5 ms longer than the minimum time calculated for a possible direct cutaneous effect on spinal motoneurones. The time course of suppression began earlier and was more complex during magnetic stimulation of the cortex. It is argued that this difference is due to the repetitive I waves generated by the magnetic shock. Whether electrical or magnetic stimulation was used, the first 1–3 ms of the EMG response was relatively unaffected by superficial radial nerve stimulation at any interstimulus interval, whereas clear suppression was seen in the later portion of the response. In contrast, if the EMG response in ECR was suppressed by a conditioning stimulus to the median nerve at the elbow, then all portions of the EMG response were inhibited including the first 1–3 ms. The median nerve effect is thought to be due to direct reciprocal inhibition of the extensor motoneurones. Thus sparing of the initial part of the cortically evoked response with superficial radial stimulation suggests that the latter type of inhibition occurs at a premotoneuronal level. The timing of the effect is compatible with the explanation that corticospinal excitation is produced in ECR motoneurones through both monosynaptic and disynaptic (including propriospinal premotoneuronal) pathways, with superficial radial nerve inhibition being exerted at the propriospinal level.     

Physiological properties of the motor units of the wrist extensor muscles in man

Summary The physiological properties of 355 motor units (MUs) recorded in the extensor carpi radialis muscles were studied in 34 healthy human subjects during isometric contractions. MU selective twitches were educed from the whole muscle force using the spike-triggered averaging method. The twitch contraction times and twitch forces were measured. From these data it was attempted to estimate the distribution of fast and slow MUs in the muscles studied. MU recruitment thresholds were systematically measured during stereotyped slow ramp contractions (force increase=0.25 N·s^-1). Degrees of correlation between contraction times, twitch forces and recruitment thresholds were pair analysed by computing simple regression curves and correlation coefficients. The degrees of correlation were compared between 245 MUs recorded in 34 subjects and 66 MUs recorded in a single subject. Analysis of the instantaneous discharge frequency of 132 MUs showed the existence of a remarkable degree of correlation (correlation coefficient, r=-0.75) between the frequency rise times (discharge onset to maximal frequency) and the MU twitch contraction times; i.e., the frequency rise times increase when the twitch contraction times decrease. The possibility that muscle contraction may be differentially modulated on the basis of this discharge property of the MUs is discussed. The results are compared to previous data and the limitations of the spike-triggered averaging method applied to long muscles in man are extensively discussed.     

Reflex pathways connect receptors in the human lower leg to the erector spinae muscles of the lower back

Reflex pathways connect all four limbs in humans. Presently, we tested the hypothesis that reflexes also link sensory receptors in the lower leg with muscles of the lower back (erector spinae; ES). Taps were applied to the right Achilles’ tendon and electromyographic activity was recorded from the right soleus and bilaterally from ES. Reflexes were compared between sitting and standing and between standing with the eyes open versus closed. Reflexes were evoked bilaterally in ES and consisted of an early latency excitation, a medium latency inhibition, and a longer latency excitation. During sitting but not standing, the early excitation was larger in the ES muscle ipsilateral to the stimulation (iES) than in the contralateral ES (cES). During standing but not sitting, the longer latency excitation in cES was larger than in iES. This response in cES was also larger during standing compared to sitting. Responses were not significantly different between the eyes open and eyes closed conditions. Taps applied to the lateral calcaneus (heel taps) evoked responses in ES that were not significantly different in amplitude or latency than those evoked by tendon taps, despite a 75–94% reduction in the amplitude of the soleus stretch reflex evoked by the heel taps. Electrical stimulation of the sural nerve, a purely cutaneous nerve at the ankle, evoked ES reflexes that were not significantly different in amplitude but had significantly longer latencies than those evoked by the tendon and heel taps. These results support the hypothesis that reflex pathways connect receptors in the lower leg with muscles of the lower back and show that that the amplitude of these reflexes is modulated by task. Responses evoked by stimulation of the sural nerve establish that reflex pathways connect the ES muscles with cutaneous receptors of the foot. In contrast, the large volley in muscle spindle afferents induced by the tendon taps compared to the heel taps did not alter the ES responses, suggesting that the reflex connection between triceps surae muscle spindles and the ES muscles may be relatively weak. These heteronymous reflexes may play a role in stabilizing the trunk for maintaining posture and balance.     

Anatomical study of the zygomatic and buccal branches of the facial nerve: Application to facial reanimation procedures

The facial nerve is responsible for any facial expression channeling human emotions. Facial paralysis causes asymmetry, lagophthalmus, oral incontinence, and social limitations. Facial dynamics may be re‐established with cross‐face‐nerve‐grafts (CFNG). Our aim was to reappraise the zygomaticobuccal branch system relevant for facial reanimation surgery with respect to anastomoses and crossings. Dissection was performed on 106 facial halves of 53 fresh frozen cadavers. Study endpoints were quantity and relative thickness of branches, correlation to “Zuker's point”, interconnection patterns and crossings. Level I and level II branches were classified as relevant for CFNG. Anastomoses and fusion patterns were assessed in both levels. The zygomatic branch showed 2.98 ± 0.86 (range 2–5) twigs at level II and the buccal branch 3.45 ± 0.96 (range 2–5), respectively. In the zygomatic system a single dominant branch was present in 50%, two co‐dominant branches in 9% and three in 1%. In 66% of cases a single dominant buccal twig, two co‐dominant in 12.6%, and three in 1% of cases were detected. The most inferior zygomatic branch was the most dominant branch (P = 0.003). Using Zuker's point, a facial nerve branch was found within 5 mm in all facial halves. Fusions were detected in 80% of specimens. Two different types of fusion patterns could be identified. Undercrossing of branches was found in 24% at levels I and II. Our study describes facial nerve branch systems relevant for facial reanimation surgery in a three‐dimensional relationship of branches to each other. Clin. Anat. 32:480–488, 2019. © 2019 Wiley Periodicals, Inc.     

Neural beta-adrenergic dilatation of the facial vein in man Possible mechanism in emotional blushing

Ring preparations of the superficial buccal segment of the human facial vein, taken from extirpated tissue in 12 patients during neck surgery, were studied in vitro. The vein developed a maintained intrinsic myogenic tone in response to passive stretch and was supplied with α- as well as β-adrenoceptors, both of which could be influenced by transmural nerve stimulation (TNS) and noradrenaline. These unusual characteristics for a vein are basically similar to the ones described for the rabbit facial vein by Pegram, Bevan & Bevan (1976). In man there seemed to be an inter-individual difference with regard to the abundance of ‘innervated’α- and β-adrenoceptors. Facial vein specimens from some subjects thus responded with prompt and pronounced net dilatation to TNS with maximum at 4 Hz and those from others with net constriction with maximum at 16 Hz. The latter showed a reversal into neural β-adrenergic dilatation after α-adrenergic blockade. The human external jugular vein was devoid of intrinsic tone and β-adrenoceptors. It is tentatively proposed that a β-adrenergic neuro-effector mechanism in superficial ramifications of the facial vein in man might be involved in the emotional blushing reaction.     

Bilateral cortical control of the human anterior digastric muscles

Transcranial magnetic stimulation (TCMS) was used to determine the organisation of cortical motor projections to the anterior digastric muscles in 12 normal human subjects. Two distinct types of potentials were evoked in anterior digastric with a figure-of-eight coil. A short-latency (3 ms) response appeared bilaterally on the surface electromyogram (EMG), but only ipsilaterally on intramuscular recordings: this was the result of direct stimulation of the ipsilateral trigeminal motor root. Motor evoked potentials (MEPs) were elicited in the anterior digastric muscles at variable onset latencies of around 10 ms by stimulation of scalp areas antero-lateral to the area for the first dorsal interosseous muscle of the hand. These were evoked bilaterally in relaxed anterior digastric muscles in six of the seven subjects. In the other subject, the responses in the relaxed muscle were exclusively ipsilateral. However, when the anterior digastric muscles were contracted, the responses were bilateral in all subjects. TCMS and spike-triggered averaging revealed that the bilateral responses were not due to the branching of axons from individual digastric motoneurones to muscles on each side. Because the digastric motor nucleus may contain separate populations of ipsi- and contralateral projecting motoneurones, it was necessary to study single motor-unit responses to TCMS to demonstrate a bilateral corticobulbar projection. The responses of 17 single motor units in the anterior digastric muscle to TCMS were recorded. All were activated by contralateral stimulation. Approximately 80% were also activated by ipsilateral TCMS, although one well-characterised motor unit was inhibited by ipsilateral TCMS. When bilateral activation was present, the ipsilateral responses were more secure than the contralateral responses, which may indicate an additional interneurone in the pathway to the contralateral motoneurone. The major conclusions from this study are that (1) the cortical representation of the anterior digastric muscle is antero-lateral to hand muscles; (2) the cortical projection to the anterior digastric muscles is bilateral; (3) the corticobulbar projection is stronger contralaterally than ipsilaterally but may involve at least one additional synapse; and (4) anterior digastric motoneurones do not branch to innervate the muscles bilaterally. Received: 8 March 1999 / Accepted: 16 June 1999     

Visual evoked responses to the upper and lower half-field stimulation in a dark-adapted man

The averaged visual evoked responses (VERs) to flashed blank and checkerboard-patterned (32 checkers) stimulation of the upper and lower half-field of 6° angular subtense were recorded referentially (O_z–A₁₊₂) in seven dark adapted subjects using a luminance range of 3.69 log units (maximum luminance=2220 cd/m²).With the logarithmical rise of stimulation luminance the peak latencies of the maximum positive-negative deflection of the VERs to the upper and lower half-field blank and to the upper half-field patterned stimulation display a monotonick shortening. These three response types do not differ in waveform, polarity and amplitude. Their amplitudes show no significant luminance-dependent changes.The peak latencies of the VERs to the lower half-field patterned stimulation exhibit an U shaped course with the increase of the luminance. At lower luminances they are by about 30 ms shorter in comparison with the VERs to the other three stimulation types, at higher luminances a gradual lengthening is observable. Consequently, at low luminances these VERs have a reversed polarity, at higher luminances the same polarity as the VERs to the other three stimulation types. The amplitude values of the lower half-field patterned responses are the highest and show a triphasic luminance-dependent course.From these and further differences between the VERs to the upper and lower half-field patterned stimulation in connection with the reaction to defocusing and to the subtraction of the luminance-related part of the response it is concluded that the VERs to the lower half-field patterned stimulation only contain a pattern-related component.