Axon Counts Yield Multiple Options for Triceps Fascicular Nerve to Axillary Nerve Transfer.

PURPOSE: To evaluate the relative axonal match between potential donor and recipient nerves, so that maximal reinnervation potential may be reached with the least chance of donor site morbidity. METHODS: In 10 fresh-frozen cadaveric specimens, the main trunk and anterior, posterior, sensory and teres minor branches of the axillary nerve were identified, as were the radial nerve branches to the long, medial, and lateral heads of the triceps. The swing distances of the triceps fascicular nerve branches and the axillary nerve branches relative to the inferior border of the teres major muscle were recorded. Histomorphological analysis and axon counts were performed on sections of each branch. RESULTS: The median number of axons in the main axillary trunk was 7,887, with 4,052, 1,242, and 1,161 axons in the anterior, posterior, and teres minor branches, respectively. All specimens had a single long head triceps branch (median, 2,302 axons), a range of 1 to 3 branches to the medial head of the triceps (composite axon count, 2,198 axons), and 1 to 3 branches to the lateral head of the triceps (composite average, 1,462 axons). The medial and lateral head branches had sufficient swing distance to reach the anterior branch of the axillary nerve in all 10 specimens, with only 4 specimens having adequate long head branch swing distances. CONCLUSIONS: It is anatomically feasible to transfer multiple branches of the radial nerve supplying the medial, lateral, and sometimes, long head of the triceps to all branches of the axillary nerve in an attempt to reinnervate the deltoid and teres minor muscles. CLINICAL RELEVANCE: Understanding the axon counts of the different possible transfer combinations will improve operative flexibility and enable peripheral nerve surgeons to reinnervate for both abduction and external rotation with the highest donor/recipient axon count ratios.

Not GABA but glycine mediates segmental, propriospinal, and bulbospinal postsynaptic inhibition in adult mouse spinal forelimb motor neurons.

The general view is that both glycine (Eccles, 1964) and GABA (Curtis and Felix, 1971) evoke postsynaptic inhibition in spinal motor neurons. In newborn or juvenile animals, there are conflicting results showing postsynaptic inhibition in motor neurons by corelease of GABA and glycine (Jonas et al., 1998) or by glycine alone (Bhumbra et al., 2012). To resolve the relative contributions of GABA and glycine to postsynaptic inhibition, we performed in vivo intracellular recordings from forelimb motor neurons in adult mice. Postsynaptic potentials evoked from segmental, propriospinal, and bulbospinal systems in motor neurons were compared across four different conditions: control, after gabazine, gabazine followed by strychnine, and strychnine alone. No significant differences were observed in the proportion of IPSPs and EPSPs between control and gabazine conditions. In contrast, EPSPs but not IPSPs were recorded after adding strychnine with gabazine or administering strychnine alone, suggesting an exclusive role for glycine in postsynaptic inhibition. To test whether the injected (intraperitoneal) dose of gabazine blocked GABAergic inhibitory transmission, we evoked GABAA receptor-mediated monosynaptic IPSPs in deep cerebellar nuclei neurons by stimulation of Purkinje cell fibers. No monosynaptic IPSPs could be recorded in the presence of gabazine, showing the efficacy of gabazine treatment. Our results demonstrate that, in the intact adult mouse, the postsynaptic inhibitory effects in spinal motor neurons exerted by three different systems, intrasegmental and intersegmental as well as supraspinal, are exclusively glycinergic. These findings emphasize the importance of glycinergic postsynaptic inhibition in motor neurons and challenge the view that GABA also contributes.

Human muscle spindle sensitivity reflects the balance of activity between antagonistic muscles.

Muscle spindles are commonly considered as stretch receptors encoding movement, but the functional consequence of their efferent control has remained unclear. The "α-γ coactivation" hypothesis states that activity in a muscle is positively related to the output of its spindle afferents. However, in addition to the above, possible reciprocal inhibition of spindle controllers entails a negative relationship between contractile activity in one muscle and spindle afferent output from its antagonist. By recording spindle afferent responses from alert humans using microneurography, I show that spindle output does reflect antagonistic muscle balance. Specifically, regardless of identical kinematic profiles across active finger movements, stretch of the loaded antagonist muscle (i.e., extensor) was accompanied by increased afferent firing rates from this muscle compared with the baseline case of no constant external load. In contrast, spindle firing rates from the stretching antagonist were lowest when the agonist muscle powering movement (i.e., flexor) acted against an additional resistive load. Stepwise regressions confirmed that instantaneous velocity, extensor, and flexor muscle activity had a significant effect on spindle afferent responses, with flexor activity having a negative effect. Therefore, the results indicate that, as consequence of their efferent control, spindle sensitivity (gain) to muscle stretch reflects the balance of activity between antagonistic muscles rather than only the activity of the spindle-bearing muscle.

Constituent ratio of motor fibers from the C5-C7 spinal nerves in the radial nerve is greater in pup rats than in adult rats.

Clinically, injuries of C5-C7 of the brachial plexus cause falling of the wrist and fingers in infants but not in adults unless 4 consecutive spinal nerves are injured. The purpose of this study was to compare the constituent difference of spinal nerves in the radial nerve between pup and adult rats.A group of 16 pup rats and a group of 16 adult rats were each divided into 2 groups of 8 (P1 and A1 groups, C5-C6 were divided; P2 and A2 groups, C5-C7 were divided]). A nerve conduction study and histological examination were performed to evaluate radial nerve innervation to the extensor digitorum communis muscle after dividing the spinal nerves. Retrograde tracing with 5% cholera toxin B for anterior horn motoneurons of the spinal cord innervating the radial nerve was performed in 8 pup rats and 8 adult rats. Results showed that the division of C5-C7 caused more significant damage to radial nerve innervation to the extensor digitorum communis in pups than in adults, although the division of C5-C6 did not. In pups, the percentages (median with interquartile) of anterior horn motoneurons of the spinal cord innervating the radial nerve were 36.4 (28.3-38.5) in C5-C6, 28.1 (24.5-32.5) in C7, and 37.5 (36.5-39.3) in C8-T1. In adults, they were 24.2 (23.6-27.8) in C5-C6, 21.8 (19.5-26.3) in C7, and 50.7 (48.7-55.5) C8-T1.This study implies that C7 innervation in the radial nerve in humans may be more critical to the function of this nerve in infants than in adults.

Regeneration of the adult zebrafish brain from neurogenic radial glia-type progenitors.

Severe traumatic injury to the adult mammalian CNS leads to life-long loss of function. By contrast, several non-mammalian vertebrate species, including adult zebrafish, have a remarkable ability to regenerate injured organs, including the CNS. However, the cellular and molecular mechanisms that enable or prevent CNS regeneration are largely unknown. To study brain regeneration mechanisms in adult zebrafish, we developed a traumatic lesion assay, analyzed cellular reactions to injury and show that adult zebrafish can efficiently regenerate brain lesions and lack permanent glial scarring. Using Cre-loxP-based genetic lineage-tracing, we demonstrate that her4.1-positive ventricular radial glia progenitor cells react to injury, proliferate and generate neuroblasts that migrate to the lesion site. The newly generated neurons survive for more than 3 months, are decorated with synaptic contacts and express mature neuronal markers. Thus, regeneration after traumatic lesion of the adult zebrafish brain occurs efficiently from radial glia-type stem/progenitor cells.

Microanatomical and immunohistochemical study of the human radial nerve at the antecubital fossa.

BACKGROUND: Poor prognosis of radial nerve repair in elderly patients may be due to changes in intraneural anatomy with age. Also, chances of Complex Regional Pain Syndrome-Type I (CRPS-I) following radial nerve injury are comparatively high. The present study is to find the fascicular pattern of radial nerve (at antecubital fossa), microanatomic morphometric characteristics of its connective tissue components and changes with age and study of intraneural sympathetic fiber content. METHODS: Twenty human (21-87 years) cadaveric radial nerves have been collected from antecubital fossa and the study has been performed at magnifications (10x, 20x and 40x objective) after routine histological (hematoxylin & eosin stain) processing was done for morphometric analysis (total cross-sectional, fascicular and non-fascicular area) and immunohistochemical (tyrosine hydroxylase) processing for sympathetic fibers. RESULTS: The radial nerve is of a polyfascicular type with a grouped pattern of nerve fascicular distribution. The number of fascicles range from 9 to 17, whereas the number of fascicles per square millimeter of a cross-sectional area is 1.95. In elderly cases, there is significant increase in total radial nerve cross-sectional area due to an increase in its non-fascicular connective tissue area and excessive adipose tissue deposition in interfascicular domains. The average sympathetic fiber area is 0.046 mm(2) without definite relationship to age. CONCLUSION: There is an increase in total nerve cross-sectional area of the radial nerve in elderly cases. There is no relationship of sympathetic content to age. Our study makes an attempt to build a normal data base for radial nerve which might be helpful during the application of diagnostic procedures.

Analgesic and antinociceptive effects of peripheral nerve neurostimulation in an advanced human experimental model.

Electrical peripheral nerve neurostimulation (PNS) is reported to be an effective pain treatment. An objective proof of antinociceptive effect is lacking. The human experimental study addressed PNS effects on nociception and pain by electrophysiology and psychophysics. In 23 healthy volunteers, 39 sessions were conducted. Three experiments (PNS ipsilateral, PNS contralateral, Control) consisted of 13 sessions each. Conditioning PNS (100 Hz) of left (PNS ipsilateral) or right (PNS contralateral) superficial radial nerve trunk evoked non-painful, tingling sensations on the hand dorsum. Local cutaneous anesthesia at PNS site provided for preferential nerve trunk stimulation. Cortical laser-evoked potentials (LEP) after painful stimulation at left hand dorsum were recorded together with mechanical and thermal perception thresholds at the same site before (T1), during (T2), and after (T3) PNS or a no stimulation period (Control). Mechanical and thermal perception decreased in the anesthetized area. Late LEP amplitude decreased independently of PNS site. Exclusively under ipsilateral PNS, N2 latency increased and laser ratings decreased. Mechanical detection threshold transiently increased during ipsilateral PNS at hand dorsum. PNS induced strong reduction of mechanical perception due to peripheral collision of orthodromic (test stimulus) and antidromic (PNS) selective Abeta fiber excitation. Delay of N2 component and reduction of laser pain were specific to ipsilateral PNS. Divergent and common effects of ipsilateral and contralateral PNS suggest a combination of peripheral and central antinociceptive mechanisms. The study in man documents inhibition of nociception and pain by PNS and provides with an experimental model for future objectives in neuromodulation.

Chondroitin sulfate glycosaminoglycans control proliferation, radial glia cell differentiation and neurogenesis in neural stem/progenitor cells.

Although the local environment is known to regulate neural stem cell (NSC) maintenance in the central nervous system, little is known about the molecular identity of the signals involved. Chondroitin sulfate proteoglycans (CSPGs) are enriched in the growth environment of NSCs both during development and in the adult NSC niche. In order to gather insight into potential biological roles of CSPGs for NSCs, the enzyme chondroitinase ABC (ChABC) was used to selectively degrade the CSPG glycosaminoglycans. When NSCs from mouse E13 telencephalon were cultivated as neurospheres, treatment with ChABC resulted in diminished cell proliferation and impaired neuronal differentiation, with a converse increase in astrocytes. The intrauterine injection of ChABC into the telencephalic ventricle at midneurogenesis caused a reduction in cell proliferation in the ventricular zone and a diminution of self-renewing radial glia, as revealed by the neurosphere-formation assay, and a reduction in neurogenesis. These observations suggest that CSPGs regulate neural stem/progenitor cell proliferation and intervene in fate decisions between the neuronal and glial lineage.

Tests for presynaptic modulation of corticospinal terminals from peripheral afferents and pyramidal tract in the macaque.

The efficacy of sensory input to the spinal cord can be modulated presynaptically during voluntary movement by mechanisms that depolarize afferent terminals and reduce transmitter release. It remains unclear whether similar influences are exerted on the terminals of descending fibres in the corticospinal pathway of Old World primates and man. We investigated two signatures of presynaptic inhibition of the macaque corticospinal pathway following stimulation of the peripheral nerves of the arm (median, radial and ulnar) and the pyramidal tract: (1) increased excitability of corticospinal axon terminals as revealed by changes in antidromically evoked cortical potentials, and (2) changes in the size of the corticospinal monosynaptic field potential in the spinal cord. Conditioning stimulation of the pyramidal tract increased both the terminal excitability and monosynaptic fields with similar time courses. Excitability was maximal between 7.5 and 10 ms following stimulation and returned to baseline within 40 ms. Conditioning stimulation of peripheral nerves produced no statistically significant effect in either measure. We conclude that peripheral afferents do not exert a presynaptic influence on the corticospinal pathway, and that descending volleys may produce autogenic terminal depolarization that is correlated with enhanced transmitter release. Presynaptic inhibition of afferent terminals by descending pathways and the absence of a reciprocal influence of peripheral input on corticospinal efficacy would help to preserve the fidelity of motor commands during centrally initiated movement.

The effect of infrared laser on sensory radial nerve electrophysiological parameters.

OBJECTS: It has been claimed that laser may have bio-stimulation effect on the nerve tissues. This study has been designed to investigate the effect of different doses of infra-red (IR) laser exposure on the electrophysiological parameters of sensory nerves. METHODS AND SUBJECTS: Forty healthy subjects (20-35 years old) with no history of neurological conditions participated in this study. IR laser (780 nm, 20 mw) was applied over five blocks (1 cm2 each and 0.5 J/cm2) of 5 cm length of the left and right superficial radial nerve. The IR laser radiation was started from proximal to distal in the right hand and vise versa in the left hand. Antidromic sensory nerve conduction velocity was evaluated before and after first (0.5 J), third (1.5 J) and fifth (2.5 J) exposures. During the test, we measured the onset and peak latency, amplitude and duration of sensory action potentials. RESULTS: Paired t-test was used to assess the difference between pre- test and post- test data. After IR laser exposure with all doses, significant increased in latencies was observed (P < 0.001), while significant decreased in amplitude and duration was found only in the group who received the doses of 1.5 and 2.5 J of exposure (P < 0.001). There was no difference between right and left hands. CONCLUSION: Our results showed that the minimal dose of IR laser may not produce enough effects on the sensory nerves, while the higher doses such as 1.5 and 2.5 J may activate the mechanism of nerve blockage.

Transmission security for single, hair follicle-related tactile afferent fibers and their target cuneate neurons in cat.

Transmission from single, identified hair follicle afferent (HFA) nerve fibers to their target neurons of the cuneate nucleus was examined in anesthetized cats by means of paired recording from individual cuneate neurons and from fine, intact fascicles of the lateral branch of the superficial radial nerve in which it is possible to identify and monitor the activity of each group II fiber. Selective activation of individual HFA fibers was achieved by means of focal vibrotactile skin stimulation. Forearm denervation precluded inputs from sources other than the monitored HFA sensory fiber. Transmission characteristics were analyzed for 21 HFA fiber-cuneate neuron pairs in which activity in the single HFA fiber of each pair reliably evoked spike output from the target neuron at a fixed latency. As the cuneate responses to each HFA impulse often consisted of 2 or 3 spikes, in particular at HFA input rates up to approximately 20 imp/s, the synaptic linkage displayed potent amplification and high-gain transmission, characteristics that were confirmed quantitatively in measures of transmission security and cuneate spike output measures. In response to vibrotactile stimuli, the tight phase locking in the responses of single HFA fibers was well retained in the cuneate responses for vibration frequencies up to approximately 200 Hz. On measures of vector strength, the phase locking declined across the synaptic linkage by no more than approximately 10% at frequencies up to 100 Hz. However, limitations on the impulse rates generated in both the HFA fibers their associated cuneate neurons meant that the impulse patterns could not directly signal information about the vibration frequency above 50-100 Hz. Although single HFA fibers are also known to have secure synaptic linkages with spinocervical tract neurons, it is probable that this linkage lacks the capacity of the HFA-cuneate synapse for conveying precise temporal information, in an impulse pattern code, about the frequency parameter of vibrotactile stimuli.

Roles of capsaicin-insensitive nociceptors in cutaneous pain and secondary hyperalgesia.

Polymodal nociceptors respond to mechanical, thermal and chemical stimuli. Whereas sensitivities to heat and to the irritant substance capsaicin have recently been linked via the properties of the vanilloid receptor type 1 receptor ion channel, sensitivity to noxious mechanical stimuli such as the pinpricks used in clinical neurology seems to be unrelated. We investigated the peripheral neural basis of pinprick pain using quantitative psychophysical techniques combined with selective conduction block by nerve compression and selective desensitization by topical capsaicin treatment. Complete A-fibre block by compression of the superficial radial nerve (criterion: loss of first pain sensation) lowered the stimulus-response function for pinprick pain (-82 +/- 6% versus baseline). Topical pretreatment of the skin with a 10% capsaicin cream also lowered the pinprick stimulus-response function (-32 +/- 10%), whereas laser-evoked heat pain was eliminated completely (-96 +/- 2%). Under combined capsaicin desensitization and A-fibre blockade, pinprick pain was eliminated completely (-98 +/- 1%). Intradermal injection of 40 microg capsaicin into normal skin between two skin areas that had been pretreated with either capsaicin cream or vehicle produced secondary hyperalgesia with a 260% enhancement of the stimulus-response function for pinprick pain in both areas. In contrast, axon reflexive flare spread only into the vehicle-treated area. These results suggest that capsaicin-sensitive afferents, including polymodal A-fibre and C-fibre nociceptors, make a small contribution to pinprick pain and that capsaicin-insensitive C-fibres do not contribute significantly to either mechanical or heat pain. Pinprick pain is mediated primarily by capsaicin-insensitive A-fibre nociceptors, which include high-threshold mechanoreceptors and type I mechano-heat nociceptors. In addition, central sensitization to input from these A-fibre nociceptors is the primary mechanism that accounts for the enhanced pain in response to punctate mechanical stimuli in the zone of secondary hyperalgesia. These capsaicin-insensitive A-fibre nociceptors may also mediate hyperalgesia in neuropathic pain.

Distribution of median, ulnar and radial motoneurons in the monkey spinal cord: a retrograde triple-labeling study.

Topographic distribution of motoneurons innervating hand muscles through the median (Mn), ulnar (Ul), or radial (Rd) nerves was examined using a retrograde multiple-labeling technique in the macaque monkey. The Mn and Ul motoneurons, i.e. flexor motoneurons, were distributed from C6 to T2 and from C7 to T2 segments of the spinal cord, respectively, while the Rd motoneurons, i.e. extensor motoneurons, were distributed from C4 to T2. The present study further revealed partial intermingling of the cell bodies and partial overlap of the dendritic fields among the motoneurons projecting through different nerves, indicating that subregions of motoneuronal pool participate in coordination between the flexor and extensor, or among the flexor muscles. It was suggested that there exists a control mechanism for precise hand movements in the spinal cord.

Resetting of sympathetic rhythm by somatic afferents causes post-reflex coordination of sympathetic activity in rat.

1. We have proposed previously that graded synchronous activity is produced by periodic inputs acting on weakly coupled or uncoupled oscillators influencing the discharges of a population of cutaneous vasoconstrictor sympathetic postganglionic neurones (PGNs) in anaesthetized rats. 2. Here we investigated the effects of somatic afferent (superficial radial nerve, RaN) stimulation, on the rhythmic discharges of this population. We recorded (1) at the population level from the ventral collector nerve and (2) from single PGNs focally from the caudal ventral artery of the tail. 3. Following RaN stimulation we observed an excitatory response followed by a period of reduced discharge and subsequent rhythmical discharges seemingly phase-locked to the stimulus. 4. We suggest that the rhythmical discharges following the initial excitatory response (conventional reflex) result from a resetting of sympathetic rhythm generators such that rhythmic PGN activity is synchronized transiently. We also demonstrate that a natural mechanical stimulus can produce a similar pattern of response. 5. Our results support the idea that in sympathetic control, resetting of multiple oscillators driving the rhythmic discharges of a population of PGNs may provide a mechanism for producing a sustained and coordinated response to somatic input.

[Polyclonal antibodies, labeling radial nerve cells of the starfish Asterias amurensis]. / Poliklonal'nye antitela, markiruiushchie kletki radial'nogo nerva morskoi zvezdy Asterias amurensis.

The results of preliminary studies suggest that the cytoskeletal fraction of the radial nerve of the starfish Asterias amurensis contained a 32 kDa protein, which is tissue specific. This protein was isolated from the radial nerve by preparative electrophoresis and used as an antigen for raising polyclonal antibodies. When testing these antibodies on sections of the starfish tissues, it was shown that they interact only with the proteins present in the radial nerve cells. A conclusion was drawn that the raised antibodies may be used as a cell marker when studying regeneration of the nervous system in starfish.

Variation in diagnostic strategy of the EMG examination--a multicentre study.

OBJECTIVES: In order to improve the universal quality of the EMG examination, knowledge about the variation among physicians is needed. METHODS: The variation among physicians in diagnostic strategy or criteria for diagnosing was analysed from a multicentre database with 940 EMG examinations sampled by seven physicians from six laboratories in Europe. RESULTS: For the whole group of patients as well as for the subgroup of patients with polyneuropathy, variation among physicians in examination techniques, number of examined structures per patient and number of abnormal structures per patient required for a diagnosis was found. Some of the variation may be explained by use of different techniques, which showed differences in sensitivity, while some of the variation may be due to differences in diagnostic strategy and criteria for diagnosing. CONCLUSIONS: The study indicates a need for development and revision of international guidelines for EMG practice although implementation of standards requires caution.

Proprioceptive control of wrist extensor motor units in humans: dependence on handedness.

The effectiveness of the monosynaptic proprioceptive assistance to the wrist extensor motoneurone activity was investigated during voluntary contraction in relation to the subjects' handedness. The reflex responses of 411 single motor units to homonymous tendon taps were recorded in the wrist extensor carpi radialis muscles in both arms of five right-handed and five left-handed subjects. In the right-handed subjects, the motor unit reflex responses were clearly lateralized in favour of their right arm, whereas no side-related differences were observed in the left-handed subjects, whatever the motor units' mechanical properties and firing rates. When the muscle spindle sensitivity was by-passed by electrically stimulating the primary afferents in both arms of three right-handed and three left-handed subjects, no side-related differences were observed in the Hoffmann reflex (H-reflex) amplitude in either of the two lateralization groups. The effectiveness of the primary afferent synapses on to the motoneurones therefore does not seem to depend on the subject's handedness. Without excluding the possibility of structural changes being involved at the periphery, the comparisons carried out on the data obtained using electrical vs mechanical stimulation suggest that the asymmetrical effectiveness of the proprioceptive assistance observed in favour of the right arm in the right-handed subjects might result from either the gamma or beta drive being more efficient. This asymmetry might result from the preferential use of the right hand in skilled movements. In a predominantly right-handed world, however, left-handed people might tend to develop the ability to use their right arm almost as skillfully as their preferred left arm, which could explain the symmetrical effectiveness of the proprioceptive assistance observed here in the left-handers' wrist extensor muscles.

Microsurgical nervous reconstruction using autografts: a two-year follow-up.

BACKGROUND: To analyze factors influencing surgical results after microsurgical reconstruction of injured nerves. METHODS: We report a series of 32 cases of microsurgical nerve reconstruction after traumatizing lesions using nervous autografts. The series comprises 8 reconstructions of radial nerve, 8 of median nerve, 6 of ulnar, and 10 of sciatic nerve. Motor and sensitive deficits have been evaluated preoperatively using a standard grading. RESULTS: A useful sensory recovery has been obtained in radial nerve, as well as in median and ulnar nerve reconstruction. Good results regarding the motor recovery have been obtained in superior limb nerve reconstruction. Sciatic nerve reconstruction showed the worse results. CONCLUSIONS: There is a big discrepance between superior limb vs inferior limb results. A more than 10 cm gap has a negative influence on the progression of regenerating axons, especially in median and ulnar nerves.

Body mass index effect on common nerve conduction study measurements.

This study was performed to determine whether there is a difference in nerve conduction study (NCS) measures based on body fat (body mass index; BMI). Two hundred fifty-three subjects had the following NCS tests performed on them: median, ulnar, peroneal, and tibial motor studies; median, ulnar, radial, and sural sensory studies; median and ulnar mixed nerve studies; and H-reflex studies. BMI was calculated as weight (kg) divided by height (m) squared. A repeated measures analysis of variance was run adjusting for age, sex, and height and using BMI as both a continuous variable and by dividing BMI into upper, middle, and lower thirds. The sensory and mixed nerve amplitudes correlated significantly (P < or = 0.01) with BMI for all nerves tested, with means being approximately 20-40% lower in the obese than in the thin subjects. No correlation was noted between BMI and nerve conduction velocity, H-reflex latency, or most of the other motor/sensory/mixed measures. The correlation between increased BMI and lower sensory/mixed nerve amplitudes should be taken into account in clinical practice.

Inhibitory action of forearm flexor muscle afferents on corticospinal outputs to antagonist muscles in humans.

1. To find out whether muscle afferents influence the excitability of corticospinal projections to antagonist muscles, we studied sixteen healthy subjects and one patient with a focal brain lesion. 2. Using transcranial magnetic and electrical brain stimulation we tested the excitability of corticomotoneuronal connections to right forearm muscles at rest after conditioning stimulation of the median nerve at the elbow. Somatosensory potentials evoked by median nerve stimulation were also recorded in each subject. 3. Test stimuli delivered at 13-19 ms after median nerve stimulation significantly inhibited EMG responses elicited in forearm extensor muscles by transcranial magnetic stimulation, but did not inhibit responses to electrical stimulation. In contrast, magnetically and electrically elicited responses in forearm flexor muscles were suppressed to the same extent. 4. The higher the intensity of the test shocks, the smaller was the amount of median nerve-elicited inhibition. Inhibition in extensor muscles was also smaller during tonic wrist extension, or if the induced electrical stimulating current in the brain flowed from posterior to anterior over the motor strip rather than vice versa. Test responses evoked by magnetic transcranial stimulation in the first dorsal interosseous and in brachioradialis muscles were not inhibited after median nerve stimulation at the elbow. Stimulation of digital nerves failed to inhibit motor potentials in extensor muscles. 5. Test stimuli delivered at 15 and 17 ms after radial nerve stimulation significantly inhibited EMG responses elicited in forearm flexor muscles by magnetic transcranial stimulation. 6. In the patient with a focal thalamic lesion, who had dystonic postures and an absent N20 component of the somatosensory-evoked potentials but normal strength, median nerve stimulation failed to inhibit magnetically evoked responses in forearm extensor muscles. 7. We propose that activation of median nerve muscle afferents can suppress the excitability of cortical areas controlling the antagonist forearm extensor muscles acting on the hand. The inhibitory effect occurs at short latency and might assist spinal pathways mediating reciprocal inhibition by contrasting the co-activation of antagonistic pools of corticospinal cells.