Median and tibial nerve somatosensory evoked potentials: middle-latency components from the vicinity of the secondary somatosensory cortex in humans

The topography of the middle-latency N110 after radial nerve stimulation suggested a generator in SII. To support this hypothesis, we have tried to identify a homologous component in the tibial nerve SEP (somatosensory evoked potential). Evoked potentials following tibial nerve stimulation (motor+sensory threshold) were recorded with 29 electrodes (bandpass 0.5–500 Hz, sampling rate 1000 Hz). For comparison, the median nerve was stimulated at the wrist. Components were identified as peaks in the global field power (GFP). Map series were generated around GFP peaks and amplitudes were measured from electrodes near map maxima. With median nerve stimulation, we recorded a negativity with a maximum in temporal electrode positions and 106±12 ms peak latency (mean±SD), comparable to the N110 following radial nerve stimulation. After tibial nerve stimulation the latency of a component with the same topography was 131±11 ms (N130). Both N110 and N130 were present ipsi- as well as contralaterally. Amplitudes were significantly higher on the contralateral than the ipsilateral scalp for both median (3.1±2.4 μV vs. 1.7±1.6 μV) and tibial nerve (1.9±1.2 μV vs. 0.6+1 μV). The topography of the N130 can be explained by a generator in the vicinity of SII. The latency difference between median and tibial nerve stimulation is related to the longer conduction distance (cf. N20 and P40). The smaller ipsilateral N130 is consistent with the bilateral body representation in SII.     

Auditory brain stem responses to monoaural stimulation registered in symmetrical areas of cat's brain cortex

In five anaesthetized cats (Nembutal 35 mg/kg) with 14 chronically implanted recording epidural electrodes the auditory brain stem responses (ABR) to monoaural stimulation (click) in symmetrical areas of the brain cortex were recorded. Each ABR to acoustic stimulus of sufficient intensity is formed by a complex of alternating five positive (P1-P5) and four negative (N1-N4) peaks; two further small peaks often follow on this complex. The amplitude of ABR peaks N3, P4, N4 and P5 to monoaural stimulation in symmetrical areas of cat's cortex was always higher in records from the hemisphere contralateral to the stimulated ear than in records from the ipsilateral one. The amplitude of P3 ABR peak behaved to the contrary--it was higher on ipsilateral hemisphere. On the other hand the amplitude of ABR peaks P1, N1, P2 and N2 to monoaural stimulation in symmetrical areas of the brain cortex showed no degree of lateralization in our experimental animals. The present findings support indirectly the presumption that each peak of the ABR is generated by a particular acoustic brain stem structure.     

The development of the somatosensory evoked potential in the unanaesthetized fetal sheep

Somatosensory evoked potentials were recorded in utero from 13 chronically instrumented fetal lambs (97 to 148 days of gestation) following electrical stimulation of the upper lip or upper limb. Several clear and reproducible peaks were observed. Following upper lip stimulation, peaks were seen with mean peak latencies of 9, 13.2, 17.8, 21.3, 33.8 and 206 ms at a gestational age of 125 days. Similar peaks, but of slightly later mean latencies, were seen following limb stimulation. These peaks demonstrated significant gestational age related falls in peak latencies (P less than 0.05). Several of the mid to late latency peaks, notably those occurring at 21.3, 33.8 and 206 ms, demonstrated changes (P less than 0.05) in both latency (longer in low voltage) and amplitude (reduced in low voltage) dependent on electrocorticographic state. Rate of stimulus presentation also had a significant effect on both amplitude and latency of several peaks (P less than 0.05) with this effect lessening with advancing gestational age. Evoked potentials can thus be successfully obtained from chronically instrumented fetal lambs and provide a useful indice for studies of neural maturation.     

Functional aspects of evoked alpha and theta responses in humans and cats

A systems theoretical approach was used to compare possible functional roles of theta (4--7 Hz) and alpha (8--15 Hz) response components of brain evoked potentials. These response components were described earlier by Ba\c sar (1980). We recorded EEG and evoked potentials (EPs) from occipital scalp locations in 11 subjects. We used auditory and visual stimuli as inadequate and adequate stimuli, respectively (``cross-modality' measurements). The combined EEG-EP epochs were analysed in frequency domain with fast Fourier transform and adaptive digital filters. Alpha (8--15 Hz) response components turned out to be dependent on whether the stimulus was adequate or not (median amplitude with inadequate vs. adequate stimulation: vs. ). Theta (4--7 Hz) response components were less dependent on stimulus modality (inadequate vs. adequate stimulation: vs. ). In EP recordings the occipital alpha response almost disappeared in the first 250 ms following auditory stimulation. Comparable behaviour was observed in similar experiments with recordings from the cat visual cortex (area 17) and with occipital magnetoencephalographic recordings. Taking into account the above-mentioned previous reports on intracranial recordings in primary sensory areas of the cat brain and preliminary results of magnetoencephalographic measurements, we propose the following hypothesis: alpha responses in a time window of about 250 ms after stimulation might predominantly reflect primary sensory processing whereas the theta responses in the first 250 ms after stimulation might be more involved in supra-modality -- or cross-modality -- associative-cognitive processing. Received: 25 February 1994 / Accepted in revised form: 5 August 1994     

Responses of the uterine circulation to sympathetic nerve stimulation

Pregnant and non-pregnant sheep uteri were perfused in situ with arterial blood at a constant flow rate. Unilateral stimulation (1-2 ma, u ms pulse) of the distal end of the severed sympathetic chain (L3-L4) at frequencies between 5 and 25 Hz produced a graded increase in uterine artery pressure in both horns. At 25 Hz, pressure in the horn ipsilateral to the stimulated sympathetic chain increased by 28 +/ 2% in four pregnant animals and 32 +/ 5% in six non-pregnant ewes. The response of the contralateral horn was significantly smaller than that of the ipsilateral horn (P less than or equal to 0.05). The response was alpha-mediated since it was abolished by local injection of dibenzyline into the middle uterine artery. The responses of the pregnant and non-pregnant animals were similar, indicating that pregnancy did not alter the alpha-adrenergic responses of the uterine vasculature.     

Response properties of auditory activated cells in the occipital cortex of the blind mole rat: an electrophysiological study

Previous studies have demonstrated that despite its blindness, the subterranean blind mole rat (Spalax ehrenbergi) possesses a noticeable lateral geniculate nucleus and a typical cyto-architectural occipital cortex that are reciprocally connected. These two areas, as revealed by the metabolic tracer 2-deoxyglucose, are activated by auditory stimuli. Using single unit recordings, we show that about 57% of 325 cells located within the occipital cortex of anesthetized mole rats responded to at least one of the following auditory stimuli — white noise, pure tones, clicks, and amplitude modulated tones — with the latter two being the most effective. About 85% of cells driven by either contralateral or ipsilateral stimulation also responded to binaural stimulation; about 13% responded only to binaural stimulation; and 2% were driven exclusively by contralateral stimulation. Comparing responsiveness and response strength to these three modes of stimulation revealed a contralateral predominance. Mean latency (±SD) of ipsilateral and contralateral responses were 48.5±32.6 ms and 33.5±9.4 ms, respectively. Characteristic frequencies could be divided into two distinct subgroups ranging between 80 and 125 Hz and between 2,500 and 4,400 Hz, corresponding to the most intensive spectral components of the vibratory intraspecific communication signals and airborne vocalizations.Abbreviations BMF best modulation frequency - CF characteristic frequency - 2-DG 2-deoxyglucose - dLGN dorsal lateral geniculate nucleus - IC inferior colliculus - LGN lateral geniculate nucleus - OC occipital cortex - MTF modulation transfer function - SAM sinusoidally amplitude modulation - SC superior colliculus     

Vibrissal inputs into the motor cortex of adult and developing rats

Field potentials (FP) and responses of single neurones to electrical stimulation of vibrissal pads have been recorded in motor cortex in the albino mature and developing rats. The FPs were characterized by 3-phasic shape and high stability in mature rats. The FPs evoked by contralateral stimuli have a range of onset latency of 4 to 24 ms (peak of distribution 8-11 ms); those to ipsilateral stimuli have a latency of 4 to 23 ms (peak of distribution 12-16 ms). Responses of single neurones were evoked with a latency of 9 to 20 ms. Usually, the FPs were evoked by both contralateral and ipsilateral stimulation, and in some tracks were effective only ipsilateral stimuli in the developing rats beginning from the 11th day of life. The FPs in such animals were less stable and more fatigable. During 2-4 weeks of life, FPs evoked by contralateral stimulation appeared with a latency of 15 to 46 ms; during the same period, a latency of single unit responses ranged between 20 to 33 ms. The FPs to ipsilateral stimuli appeared with a latency of 18 to 47 ms, a latency of single unit responses of 27 to 47 ms. The results indicate functional immaturity of vibrissal system up to the end of the first month of rat life.     

Scalp-recorded short and middle latency peroneal somatosensory evoked potentials in normals: comparison with peroneal and median nerve SEPs in patients with unilateral hemispheric lesions

Peroneal somatosensory evoked potentials (SEPs) were performed on 23 normal subjects and 9 selected patients with unilateral hemispheric lesions involving somatosensory pathways.Recording obtained from right and left peroneal nerve (PN) stimulations were compared in all subjects, using open and restricted frequency bandpass filters. Restricted filter (100–3000 Hz) and linked ear reference (A1–A2) enhanced the detection of short latency potentials (P1, P2, N1 with mean peak latency of 17.72, 21.07, 24.09) recorded from scalp electrodes over primary sensory cortex regions. Patients with lesions in the parietal cortex and adjacent subcortical areas demonstrated low amplitude and poorly formed short latency peroneal potentials, and absence of components beyond P3 peak with mean latency of 28.06 msec. In these patients, recordings to right and left median nerve (MN) stimulation showed absence or distorted components subsequent to N1 (N18) potential.These observations suggest that components subsequent to P3 potential in response to PN stimulation, and subsequent to N18 potential in response to MN stimulation, are generated in the parietal cortical regions.     

Comparison in man of short latency averaged evoked potentials recorded in thalamic and scalp hand zones of representation

Recordings were performed in the thalamus of 13 patients suffering from either abnormal movements or intractable pain, with the aim of delimiting the region to be destroyed or stimulated in order to diminish the syndrome. In 11 of these patients averaged evoked potentials were recorded simultaneously from the scalp and specific thalamus (VP) hand area levels following median nerve stimulation. These recordings were done during the operation or afterwards when an electrode was left in place for a program of stimulation.The latencies of onsets and peaks on the scalp ‘P15’ were compared with those of the VP wave; a clear correspondence was found. Moreover, when increased stimulation was used, both waves began to develop in parallel. Thus in the contralateral ‘P15’ a component exists due to the field produced by the thalamic response. To explain the presence of an ipsilateral scalp ‘P15’ wave, we propose that a second wave having the same latency and a slightly shorter peak exists on the scalp due to a field produced by a brain-stem response. This double origin of ‘P15’ is also shown by the different changes which the ipsilateral and contralateral waves present during changes in alertness.The scalp ‘N18–N20’ is also composed of at least 2 components. The first peak appears on the scalp with a latency shorter than that of the negativity which develops in the thalamus. The N wave, moreover, increases in latency with rapid stimulus repetition. We propose with others that ‘N18’ is a cortical event reflecting the arrival of the thalamo-cortical volley. The second component, ‘N20,’ has a peak latency closely correlated to that of the thalamic negativity. This component was present alone in ‘N’ when rapid stimulation (> 4/sec) was used, which did not change the thalamic response. It must be a field produced by the thalamic negativity.     

Topographic analyses of somatosensory evoked potentials following stimulation of tibial,sural and lateral femoral cutaneous nerves

Using topographic maps, we studied the scalp field distribution of somatosensory evoked potentials (SEPs) in response to the stimulation of the tibial (TN), sural (SN) and lateral femoral cutaneous (LFCN) nerves in 24 normal volunteers. Cortical peaks, i.e., N35, P40, N50 and P60 were generally dominant in the contralateral hemisphere for the LFCN-SEP, whereas all peaks except N35 had dominance in the ipsilateral hemisphere for TN- and SN-SEPs. The findings imply that ipsilateral or contralateral peak dominance for the lower extremity SEP is determined by where the cortical leg representation occurs. As a result, mesial hemisphere representation results in peak dominance projected to the hemisphere ipsilateral to stimulation. Representations at the superior lip of the interhemispheric fissure or lateral convexity lead to midline or contralateral peak dominance. These findings also suggest that the paradoxically lateralized P40 is not the result of a positive field dipole shadow generated by the primary negative wave in the mesial hemisphere, but is the primary positive wave, analogous to P26 of the median nerve SEP. Accordingly, contralaterally dominant N35 is likely equivalent to the first cortical potential of N20 in the median nerve SEP. The difference in vector directions of potential fields between N35 and P40 may account for the opposite hemispheric dominance for these peaks in TN- and SN-SEPs.     

新生大鼠离体脊髓薄片侧角中间外侧核细胞的电生理特性

在新生大鼠离体脊髓薄片的中间外侧核作细胞内记录,研究细胞膜的静态与动态电生理特性。细胞的静息电位(RP)变动于-46—-70mV,膜的输入阻抗为108.3±67.9MΩ(X±SD,下同),时间常数9.9±5.6ms,膜电容138.6±124.2pF。用去极化电流进行细胞内刺激时,大部份细胞(85.4％)能产生高频率连续发放,其余细胞(15.6％)仅产生初始单个发放。胞内直接刺激引起的动作电位(AP)幅度为63.4±9.0mV,时程2.4±0.6ms,阈电位水平在RP基础上去极18.7±6.2mV。大部份细胞的锋电位后存在明显的超极化后电位,其幅度为5.1±2.7mV、持续90±31.8ms。刺激背根可在记录细胞引起EPSP或顺向AP,少数细胞尚出现IPSP。而刺激腹根则可引起逆向AP。     

Frontal cortical afferents facilitate striatal nitric oxide transmission in vivo via a NMDA receptor and neuronal NOS-dependent mechanism

Striatal nitric oxide (NO) signaling plays a critical role in modulating neural processing and motor behavior. Nitrergic interneurons receive synaptic inputs from corticostriatal neurons and are activated via ionotropic glutamate receptor stimulation. However, the afferent regulation of NO signaling is poorly characterized. The role of frontal cortical afferents in regulating NO transmission was assessed in anesthetized rats using amperometric microsensor measurements of NO efflux and local field potential recordings. Low frequency (3 Hz) electrical stimulation of the ipsilateral cortex did not consistently evoke detectable changes in striatal NO efflux. In contrast, train stimulation (30 Hz) of frontal cortical afferents facilitated NO efflux in a stimulus intensity-dependent manner. Nitric oxide efflux evoked by train stimulation was transient, reproducible over time, and attenuated by systemic administration of either the NMDA receptor antagonist MK-801 or the neuronal NO synthase inhibitors 7-nitroindazole and NG-propyl-L-arginine. The interaction between NO efflux evoked via train stimulation and local striatal neuron activity was assessed using dual microsensor and local field potential recordings carried out concurrently in the contralateral and ipsilateral striatum, respectively. Systemic administration of the non-specific NO synthase inhibitor methylene blue attenuated both evoked NO efflux and the peak oscillation frequency (within the delta band) of local field potentials recorded immediately after train stimulation. Taken together, these observations indicate that feed-forward activation of neuronal NO signaling by phasic activation of frontal cortical afferents facilitates the synchronization of glutamate driven oscillations in striatal neurons. Thus, NO signaling may act to amplify coherent corticostriatal transmission and synchronize striatal output.     

Somatosensory evoked potentials by tibial nerve stimulation in the rabbit

Somatosensory evoked potentials by tibial nerve stimulation were obtained in ten New Zealand rabbits. The subcortical or cortical source of the three negative and three positive peaks present in the first 55 ms is discussed viewing the results obtained by different surface electrode locations and by stereotaxic recordings. The authors report interanimal, interhemispheric and test-retest variability of latencies and amplitudes of subcortical and cortical components.     

Attenuation of phrenic motor discharge by phrenic nerve afferents

Short latency phrenic motor responses to phrenic nerve stimulation were studied in anesthetized, paralyzed cats. Electrical stimulation (0.2 ms, 0.01-10 mA, 2 Hz) of the right C5 phrenic rootlet during inspiration consistently elicited a transient reduction in the phrenic motor discharge. This attenuation occurred bilaterally with an onset latency of 8-12 ms and a duration of 8-30 ms. Section of the ipsilateral C4-C6 dorsal roots abolished the response to stimulation, thereby confirming the involvement of phrenic nerve afferent activity. Stimulation of the left C5 phrenic rootlet or the right thoracic phrenic nerve usually elicited similar inhibitory responses. The difference in onset latency of responses to cervical vs. thoracic phrenic nerve stimulation indicates activation of group III afferents with a peripheral conduction velocity of approximately 10 m/s. A much shorter latency response (5 ms) was evoked ipsilaterally by thoracic phrenic nerve stimulation. Section of either the C5 or C6 dorsal root altered the ipsilateral response so that it resembled the longer latency contralateral response. The low-stimulus threshold and short latency for the ipsilateral response to thoracic phrenic nerve stimulation suggest that it involves larger diameter fibers. Decerebration, decerebellation, and transection of the dorsal columns at C2 do not abolish the inhibitory phrenic-to-phrenic reflex.     

Excitatory convergence of periaqueductal gray and somatic afferents in the solitary tract nucleus: role for neurokinin 1 receptors

Our previous studies (Boscan P, Kasparov S, and Paton JF. Eur J Neurosci 16: 907-920, 2002) showed that activation of somatic afferents attenuated the baroreceptor reflex via neurokinin type 1 (NK(1)) and GABA(A) receptors within the nucleus of the solitary tract (NTS). The periaqueductal gray matter (PAG) can also depress baroreceptor reflex function and project to the NTS. In the present study, we have tested the possibility that the dorsolateral (dl)-PAG projects to the NTS neurons that also respond to somatic afferent input. In an in situ, arterially perfused, unanesthetized decerebrate rat preparation, somatic afferents (brachial plexus), cervical spinal cord, and dl-PAG were stimulated electrically, whereas NTS neurons were recorded extracellularly. From 45 NTS neurons excited by either brachial plexus or dl-PAG stimulation, 41 received convergence excitatory inputs from both afferents. Onset latency and evoked peak discharge frequency from brachial plexus afferents were 39.4 +/- 4.7 ms and 10.7 +/- 1.1 Hz, whereas this was 43.9 +/- 6.4 ms and 7.9 +/- 1 Hz, respectively, following dl-PAG stimulation. As revealed by using a paired pulse stimulation protocol, monosynaptic connections were found in 9 of 36 neurons tested from both spinal cord and dl-PAG. We tested NK(1)-receptor sensitivity in 38 neurons that received convergent inputs from brachial plexus/PAG. Fifteen neurons were sensitive to selective antagonism of NK(1) receptors. CP-99994, the NK(1) antagonist, failed to alter ongoing firing activity but reduced the evoked peak discharge frequency following stimulation of both brachial plexus (from 12.3 +/- 1.8 to 7.2 +/- 1.3 Hz; P < 0.01) and PAG (from 7.8 +/- 1.5 to 4.5 +/- 1 Hz; P < 0.01). We conclude that 1) somatic brachial and PAG afferents can converge onto single NTS neurons; 2) this convergence occurs via either direct or indirect pathways; and 3) NK(1) receptors are activated by some of these inputs.     

Sound production in two species of damselfishes (Pomacentridae): Plectroglyphidodon lacrymatus and Dascyllus aruanus

Agonistic sounds of two pomacentrid species, Plectroglyphidodon lacrymatus and Dascyllus aruanus , were recorded in captivity. Plectroglyphidodon lacrymatus produced trains of 2–5 pops, each composed of 18–25 cycles, with an average duration of 56 ms; most energy ranged from c. 100 to 1000 Hz. Dascyllus aruanus produced pops and chirps. Pops were generally composed of a single pulse, with 2–14 peaks and an average duration of 6·7 ms. Pops contained energy >4 kHz, and peak frequency ranged from 680 to 1300 Hz. Chirps consisted of trains of 12–42 short pulses of three to six cycles, with durations varying from 0·6 to 1·27 ms; peak frequency varied from 3400 to 4100 Hz. Sound production in P. lacrymatus suggested that pomacentrids are derived from an ancestral taxon capable of sound production and that this capacity is a synapomorphy for the family. Although in the Pomacentridae, pops are typically composed of a single pulse, which is longer and higher pitched than chirps composed of a series of shorter pulses, D. aruanus chirps were higher pitched than its pops. Thus, acoustic variation in the genus Dascyllus is probably not more restricted than in the Pomacentridae.     

Calcium sparks in human ventricular cardiomyocytes from patients with terminal heart failure

Cardiomyocytes from terminally failing hearts display significant abnormalities in e-c-coupling, contractility and intracellular Ca(2+) handling. This study is the first to demonstrate the influence of end-stage heart failure on specific properties of Ca(2+) sparks in human ventricular cardiomyocytes. We investigated the frequency and characteristics of spontaneously arising Ca(2+) sparks in single isolated human myocytes from terminally failing (HF) and non-failing (NF) control myocardium by using the Ca(2+) indicator Fluo-3. The Ca(2+) sparks were recorded by line-scan images along the longitudinal axis of the myocytes at a frequency of 250Hz. After loading the sarcoplasmic reticulum (SR) with Ca(2+) by repetitive field stimulation (10 pulses at 1Hz) the frequency of the Ca(2+) sparks immediately after stimulation (t = 0s) was reduced significantly in HF compared to NF (4.15 +/- 0.42 for NF vs. 2.81 +/- 0.20 for HF sparks s(-1), P = 0.05). This difference was present constantly in line-scan recordings up to 15s duration (t = 15s: 2.75 +/- 0.65 for NF vs. 1.36 +/- 0.34 for HF sparks s(-1), P = 0.05). The relative amplitude (F/F(0)) of Ca(2+) sparks was also significantly lower in HF cardiomyocytes (1.33 +/- 0.015 NF vs. 1.19 +/- 0.003 HF, t = 0s) and during subsequent recordings of 15s. Significant differences between HF and NF were also present in calculations of specific spark properties. The time to peak was estimated at 25.75 +/-0.88ms in HF and 18.68 +/- 0.45ms in NF cardiomyocytes (P = 0.05). Half-time of decay was 66.48 +/- 1.89ms (HF) vs. 44.15 +/- 1.65ms (NF, P < 0.05), and the full width at half-maximum (FWHM) was 3.99 +/- 0.06 microm (HF) vs. 3.5 +/- 0.07 microm (NF, P < 0.05). These data support the hypothesis that even in the absence of cardiac disease, Ca(2+) sparks from human cardiomyocytes differ from previous results of animal studies with respect to the time-to-peak, half-time of decay and FWHM. The role of elevated external Ca(2+) in HF was studied by recording Ca(2+) sparks in HF cardiomyocytes with 10mmol external Ca(2+) concentration. Under these conditions, the average spark amplitude was increased from 1.19 +/- 0.003 (F/F(0), 2mmol Ca(2+)) to 1.26 +/- 0.01 (F/F(0), 10mmol Ca(2+)). We conclude that human heart failure causes distinct changes in Ca(2+) spark frequency and characteristics comparable to results established in animal models of heart failure. A reduced Ca(2+) load of the SR alone is unlikely to account for the observed differences between HF and NF and additional alterations in intracellular Ca(2+) release mechanisms must be postulated.     

家兔主动脉神经高阈压力感受反射性速重调中枢过程的机能特征

Experiments were performed on 37 urethane-anesthetized rabbits. The aortic nerves, carotid sinus nerves and vagus nerves were cut, MAP and renal sympathetic nerve activity (RSNA) were recorded. The conditional stimulation CSc (0.5 ms, 10 Hz, 4-6V, 5 min) was used to mimic the information of baroreflex non-medullated afferent fibers responding to acute increase of BP. Test stimulation TSa (0.02 ms, 0-80 Hz/30 s, 4-6V) and TSc (0.5 ms, 0-20 Hz/30s, 4-6V) was used to examine the responses of baroreflex A- and C-fibers. After CSc at 1 min the reflex MAP and RSNA of TSc was attenuated at 45.5% (P less than 0.01) and 10.6% (P less than 0.05), the MAP response of TSa was attenuated at 32.1% (P less than 0.05), but the RSNA response was not. From the further investigation it is concluded that the characteristics of central acute resetting are dependent on the components of baroreflex afferent fibers. The reflex responses are attenuated mainly by correspondent afferent components.     

Activation phase ensures kinematic efficacy in flight-steering muscles of Drosophila melanogaster

During tethered flight in Drosophila melanogaster, spike activity of the second basalar flight-control muscle (M.b2) is correlated with an increase in both the ipsilateral wing beat amplitude and the ipsilateral flight force. The frequency of muscle spikes within a burst is about 100 Hz, or 1 spike for every two wing beat cycles. When M.b2 is active, its spikes tend to occur within a comparatively narrow phase band of the wing beat cycle. To understand the functional role of this phase-lock of firing in the control of flight forces, we stimulated M.b2 in selected phases of the wing beat cycle and recorded the effect on the ipsilateral wing beat amplitude. Varying the phase timing of the stimulus had a significant effect on the wing beat amplitude. A maximum increase of wing beat amplitude was obtained by stimulating M.b2 at the beginning of the upstroke or about 1 ms prior to the narrow phase band in which the muscle spikes typically occur during flight. Assuming a delay of 1 ms between the stimulation of the motor nerve and muscle activation, these results indicate that M.b2 is activated at an instant of the stroke cycle that produces the greatest effect on wing beat amplitude.     

Direct recording of somatosensory evoked potentials in the vicinity of the dorsal column nuclei in man: their generator mechanisms and contribution to the scalp far-field potentials

Somatosensory evoked potentials (SEPs) in the vicinity of the dorsal column nuclei in response to electrical stimulation of the median nerve (MN) and posterior tibial nerve (PTN) were studied by analyzing the wave forms, topographical distribution, effects of higher rates of stimulation and correlation with components of the scalp-recorded SEPs. Recordings were done on 4 patients with spasmodic torticollis during neurosurgical operations for microvascular decompression of the eleventh nerve. The dorsal column SEPs to MN stimulation (MN-SEPs) were characterized by a major negative wave (N1; 13 msec in mean latency), preceded by a small positivity (P1) and followed by a large positive wave (P2). Similar wave forms (P1′-N1′-P2′) were obtained with stimulation of PTN (PTN-SEPs), with a mean latency of N1′ being 28 msec. Maximal potentials of MN-SEPs and PTN-SEPs were located in the vicinity of the ipsilateral cuneate and gracile nuclei, respectively, at a level slightly caudal to the nuclei. The latencies of P1 and N1 increased progressively at more rostral cervical cord segments and medulla, but that of P2 did not. A higher rate of stimulation (16 Hz) caused no effects on P1 and N1, while it markedly attenuated the P2 component. These findings suggest that P1 and N1 of MN-SEPs, as well as P1′ and N1′ of PTN-SEPs, are generated by the dorsal column fibers, and P2 and P2′ are possibly of postsynaptic origin in the respective dorsal column nuclei.The peak latency of N1 recorded on the cuneate nucleus was identical with the scalp-recorded far-field potential of P13–14 in all patients, while no scalp components were found which corresponded to P2. These findings support the previous assumption that the scalp-recorded P13–14 is generated by the presynaptic activities of the dorsal column fibers at their terminals in the cuneate nucleus.