Serial activation of distinct cytoarchitectonic areas of the human S1 cortex after posterior tibial nerve stimulation.

MEG recordings visualized non-invasively a dynamic anterior-posterior activation in the pyramidal cell population of the human primary somatosensory cortex (S1) after posterior tibial nerve stimulation. Somatosensory evoked fields (SEFs) were recorded over the foot area in response to right posterior tibial nerve stimulation at the ankle in six normal subjects. A newly developed MEG vector beamformer technique applied to the SEFs revealed two distinct sources in the mesial wall of the left hemisphere around the primary P37m response typically separated by 1.3 cm. The first source was located in area 3b and oriented toward the contralateral hemisphere. The second source was assumed to be in an area near the marginal sulcus and the source orientation was directed posteriorly. The first source began to be active during the initial slope of the P37m. The second source was active after the P37m peak and the signal intensities of the first and second sources were equal at a mean latency of 2.6 ms after the peak of P37m. Then the first source became inactive and the second source was dominant after about 5 ms post-P37m peak. These findings suggest that a single peaked posterior tibial nerve P37m consists of partially overlapping two subcomponents generated in area 3b and an area near the marginal sulcus.     

Soleus muscle reflexes evoked by stimulation of the posterior tibial nerve compared to tendon reflexes in man (author's transl)]

The characteristics of the reflex response evoked in the soleus muscle after stimulation of the posterior tibial nerve at the ankle were contrasted with the achilleus tendon reflex in 14 healthy volunteers and in a group of selected neurological patients. The following features were examined: conditions of stimulation including stimulation frequency; appearance of reflex activity in muscles other than soleus, effect of the vibratory stimulation, interactions with voluntary contraction. Moreover, a reflex response is described in the soleus following stimulation of the sural nerve. The results have shown a marked jitter in latencies of the responses, a pattern of coactivation of antagonistic muscles, a clear increase of amplitude under vibration or voluntary contraction, normal responses both in spasticity and in S1 radiculopathy with achilleus areflexia. All these data differ from those observed with the tendon jerk of the same amplitude and differentiate the two responses. It is concluded that the reflex evoked by stimulation of the tibial nerve at the ankle is a polysynaptic response of cutaneous origin.     

Comparison of somatosensory evoked high-frequency oscillations after posterior tibial and median nerve stimulation.

OBJECTIVE: We compared the high-frequency oscillations (HFOs) evoked by posterior tibial nerve (PTN) and median nerve (MN) stimulation. METHODS: Somatosensory evoked potentials (SEPs) were recorded with a filter set at 10-2000 Hz to right PTN and to right MN stimulation in 10 healthy subjects. The HFOs were obtained by digitally filtering the wide-band SEPs with a band-pass of 300-900 Hz. RESULTS: HFOs were recorded in 8 of the 10 subjects for PTN, and in all subjects for MN stimulation. The HFOs after both PTN and MN stimulation started approximately at or after the onset of the primary cortical response (P37 and N20) and ended around the middle of the second slope. HFO amplitudes and area after PTN stimulation were significantly smaller than those after MN stimulation. HFO duration after PTN stimulation was markedly longer than that after MN stimulation. However, HFO interpeak latencies did not differ between the two nerves. CONCLUSIONS: The present findings suggest that the HFOs after PTN and MN stimulation reflect a neural mechanism common to the hand and foot somatosensory cortex.     

Somatosensory evoked potentials after posterior tibial nerve stimulation in focal spinal cord diseases.

We investigated the somatosensory evoked potentials (SEPs) produced by posterior tibial nerve (PTN) stimulation in 8 infants and children with focal spinal cord disorders. The spinal responses of the PTN-SEPs were considered to assist in the localization of spinal lesions because their abnormalities were consistent with the neurologic and/or radiologic findings in all 6 examinations that revealed abnormal spinal SEPs. The cortical responses correlated significantly with proprioception in the lower limbs because proprioception was only disturbed when the cortical SEPs were absent (3 examinations). When both the spinal and cortical responses were abnormal, the spinal lesion probably involved the dorsal column so extensively that it completely interrupted the afferent impulses. In contrast, when SEP studies demonstrated abnormal spinal and normal cortical SEPs (3 examinations), the dorsal column involvement was probably less severe; therefore, both the spinal and cortical responses provided useful information regarding afferent conduction in the dorsal column. PTN-SEPs appear to have the potential to be of value in the diagnosis of focal spinal disease, especially in infants and young children who cannot cooperate with detailed neurologic examinations.     

Ontogenesis of lumbar spinal somatosensory evoked potentials after posterior tibial nerve stimulation in the rat.

Although data are available regarding lumbar spinal somatosensory evoked potentials (SSEPs) after posterior tibial nerve stimulation in the mature rat, age-related changes spanning the period of early development have not been defined. We obtained lumbar spinal SEPs after posterior tibial nerve stimulation in 6 cohorts of animals (N = 36) ranging in age from weanling (15 days) to early adulthood (110 days) by recording from needle electrodes placed in the L1-2 and L5-6 interspinous ligaments. Absolute latencies of the major wave form components at the two recording sites declined rapidly until the mid-juvenile period (36 days) and more slowly thereafter. Mean peripheral conduction velocities (+/- S.D.) increased from 11.06 (+/- 0.02) to 33.22 (+/- 3.55) m/sec and mean central conduction velocities (+/- S.D.) increased from 6.27 (+/- 1.07) to 23.64 (+/- 3.84) m/sec from 15 to 110 days respectively. The linear relationships of central and peripheral conduction velocities to both age and weight as defined by standard regression were highly significant. No sex differences were noted for peripheral velocities at all ages studied. Central velocities revealed significant sex differences at 110 days but not earlier. This study demonstrates that lumbar spinal SEPs after posterior tibial nerve stimulation undergo a predictable evolution which can be represented by a simple cable model of a lengthening myelinated pathway.     

Evidence for a monosynaptic mechanism in the tonic vibration reflex of the human masseter muscle.

Vibration of the masseter and temporalis muscles in normal human adult subjects elicits a tonic vibration reflex with unexpected features. The electromyographic response is not asynchronous as in the limb muscles, but involves well-defined spikes with a one-to-one temporal relation to the vibration cycles. The effect of various parameters such as muscle stretch, vibration frequency or amplitude, etc, has been investigated. The small latency fluctuation of the vibration-induced spikes is compatible with a monosynaptic reflex mechanism which is considered to be assisted by a polysynaptic facilitatory background of proprioceptive origin.     

Analysis of the interference electromyogram of human soleus muscle after exposure to vibration

The properties of m. soleus surface EMG recorded under conditions of voluntary contraction against vibrational stimulation were studied using vibration-triggered averaging and spectral estimates. The averaging procedure indicated EMG bursts locked to the vibration cycle. Narrow peaks appeared in the EMG spectrum at vibration frequency and harmonics. These effects were more pronounced in rectified EMG at low vibration frequencies (30-70 Hz) and in EMG at high frequencies (70-120 Hz). The disappearance of the peak after ischemic blockade preceded that of the tendon reflex. The peak normalized to the EMG power decreased when the force was enlarged. The peak augmented with prolonged contraction under vibration. The results are suggested to reflect alterations of the relative weight of the excitatory inflow through short spindle-motoneuron connections in the overall motoneuron inflow.     

Magnetoneurographic registration of propagating magnetic fields in the lumbar spine after stimulation of the posterior tibial nerve

Stimulation of the posterior tibial nerve has been associated with different somatosensory evoked potentials (SEP) recorded along the spine and thorax. The aim of this study was to register and describe the magnetic fields corresponding to different components of spinal SEP after stimulation of tibial nerves. In nine healthy subjects, right and left posterior tibial nerves were transcutaneously electrostimulated at the ankles. Neuromagnetic fields were registered over a circular 800 cm(2) area of the lumbosacral spine using a 61-channel biomagnetometer. Magnetic field maps were constructed and examined visually for dipolar patterns. Equivalent current dipoles (ECD) were calculated for each somatosensory evoked field (SEF) using a least-squares fit in a spherical model. In seven subjects dipolar SEF were detected over the lower back at two separate latencies and locations and propagating ECD could be localized. Both the first and second components found agreed anatomically and functionally with respect to propagation in the underlying nerve fibers. It was possible to record and identify SEF which correspond to the SEP described in the literature. Dipole localization based on an equivalent current dipole model allowed a basic evaluation of the plausibility of the measurements with respect to the processes being examined.     

Maturational study of short-latency somatosensory evoked potentials after posterior tibial nerve stimulation in infants and children

SSEPs produced in response to PTN stimulation were studied in 41 normal infants and children from 4 months to 16 years in age. SSEPs were recorded on the scalp with reference electrodes attached to the contralateral knee, shoulder and earlobe. Four positive SSEPs, PI, PII, PIII and PIV, named in order of appearance, and one negative SSEP, N0, were recorded as FFPs on the scalp with the cKn reference. Following these FFPs, the cortical component P1 which corresponded to P37 in adults was recorded. Preceding P1, another negative wave, N1, could be recognized solely at Cz' mainly at the onset of P1. P1 and N1 could be identified in all children with derivations with noncephalic references, although they could not be identified in 5 of 41 children with a Cz' - Fpz derivation. PI, bilobed in configuration, was considered to originate at the sacral plexus or entry to the spinal canal. PII was the least reproducible potential and was considered to originate at the dorsal root, dorsal horn or conus medullaris. PIII, PIV and N0 were considered to originate at the cervical cord, brain stem and thalamus, respectively. With the peak latencies of PI, PII, PIII, PIV, N0, N1 and P1, the RV was calculated in order to eliminate the influence of body height. The RV of the later appearing components leveled off in the older age categories. The RV of P1 reached a steady level at 3 years of age. RVs of PII and PIII appeared to level off by the age of 6 years. The RV of PIV leveled off by the age of 9 years. RVs of N0 and N1 leveled off by the age of 12 years, and that of P1 decreased until over 12 years of age. Furthermore, to eliminate the influence of naturation in the peripheral nerves, the RV was obtained from PI-PIV, PI-P1, PIV-P1 and N1-P1 interpeak latencies. The RVs of these 4 interpeak latencies all decreased until over 12 years of age. Accordingly, the maturation of afferent conduction in the central nervous system after PTN stimulation appeared to be complete after 12 years of age.     

Subcortical somatosensory evoked potentials after median nerve and posterior tibial nerve stimulation in high cervical cord compression of achondroplasia

Children with achondroplasia may have high cervical myelopathy from stenosis of the cranio-cervical junction resulting in neurological disability and an increased rate of sudden death. To detect myelopathy we recorded somatosensory evoked potentials after median nerve (MN) and posterior tibial nerve (PTN) stimulation in 77 patients with achondroplasia aged 0.3–17.8 years (mean 2.7 years). In addition to the conventional technique of recording the cortical components and the central conduction time (CCT) we employed non-cephalic and mastoid reference electrodes to record the subcortical waveforms N13b and P13 (MN-SEP) as well as P30 (PTN-SEP), respectively, which are generated near the cranio-cervical junction. The findings were related to the MRI results. Thirty-four patients had abnormal MRI findings including spinal cord compression (n = 28) and/or myelomalacia (n = 24) at or below the cranio-cervical junction. The sensitivity of the MN-SEPs was 0.74 including all abnormal upper cervical cord MRI findings (specificity 0.98), and the sensitivity was 0.79 (specificity 0.92) for cervical cord compression, respectively. The sensitivity of the PTN-SEPs was 0.52 (specificity 0.93) for all abnormal MRI findings and 0.59 (specificity 0.92) for cervical cord compression. The subcortical SEPs N13b and P13 as well as P30 were more sensitive than the conventional recordings. The MN-SEPs, notably the subcortical tracings, are useful for the detection of cervical myelopathy in children with achondroplasia. The PTN-SEPs are less sensitive. However, the tibial nerve SEPs might contribute additional information from the lumbar or thoracic spinal cord, which was, however, not tested in this study.     

Conditioning of the H reflex by stimulation of the posterior tibial nerve in Parkinson''s disease.

The excitability curve of the H reflex conditioned by stimulation of a mixed nerve was studied in eight Parkinsonism patients, before and after L-dopa therapy. There was no significant variation between the two curves. However, there was a reduction of the normal early inhibition of the H reflex conditioned by exteroceptive stimulation. This indicates the presence of alterations in the organisation of the reflex pathways at a spinal level in this disease.     

Influence of concurrent tactile stimulation on somatosensory evoked potentials following posterior tibial nerve stimulation in man

A topographical study was made of SEPs following stimulation of the right posterior tibial nerve at the ankle, with and without concurrent tactile stimulation of the soles of either foot or the palm of the right hand. Effects of the interfering stimulus were best demonstrated by subtracting the wave forms to derive "difference' potentials. The majority of SEP components were significantly attenuated by tactile stimulation of the ipsilateral foot, and the difference wave form was of similar morphology to the control response. Components of opposite polarity peaking at 39 msec were consistent with the field of a cortical generator with dipolar properties, situated in the contralateral hemisphere just posterior to the vertex with the positive poles oriented towards the ipsilateral side. By analogy with median SEP findings, these potentials were believed to originate in the foot region of area 3b where neurones are mainly concerned with cutaneous sensory processing. When the tactile stimulus was applied to the contralateral foot, difference potentials maximally recorded just posterior to the vertex were of smaller amplitude but similar morphology to ipsilateral foot difference components. This suggested the possibility that input from the two lower extremities may converge at cortical or subcortical level, the effect being manifested in the response of certain neurones in area 3b. With both contralateral foot and ipsilateral hand stimulation, other difference potentials were present which suggested that there may be cortical regions responding to combinations of sensory stimuli applied to various parts of the body surface.     

Middle-latency somatosensory evoked potentials following median and posterior tibial nerve stimulation in Down's syndrome.

Middle-latency somatosensory evoked potentials (SEPs) following median and posterior tibial nerve stimulation were studied in 40 patients with Down's syndrome and in age- and gender-matched healthy controls as well as in middle-aged and aged healthy subjects. In median nerve SEPs, latencies of the initial cortical potentials, N18 and P18, showed no significant difference, but the following potentials N22, P25, N32, P41 and P46 were relatively or significantly shorter in latency in Down's patients than in the controls. Amplitudes of all components in Down's patients were significantly larger than those of age- and gender-matched controls as well as of those of middle-aged healthy subjects, but there was only a small difference in their amplitudes from aged healthy subjects. Results of posterior tibial nerve SEPs were generally consistent with those of median nerve SEPs. Therefore, 'short latency with large amplitude' is the main characteristic of middle-latency SEPs in Down's syndrome, possibly related to accelerated physiological aging of the central nervous system.     

Bit-mapped imaging of somatosensory evoked potentials after stimulation of the posterior tibial nerves and dorsal nerve of the penis/clitoris.

Somatosensory evoked potentials (SEPs) to unilateral or bilateral posterior tibial nerve (PTN) stimulation and to stimulation of the dorsal nerve (DN) of the penis/clitoris were recorded on 32 channels in 10 volunteers. SEPs to unilateral PTN stimulation consisted of the classic 'W' complex P38-N45-P56-N75 maximal on the ipsilateral central and parietal leads, and two negative waves, N33 and N37, maximal on the contralateral post- and prerolandic areas, respectively. A lemniscal P30 was also recorded. Bilateral PTN stimulation caused, by algebraic summation, the disappearance of both N33 and N37; the W complex was symmetrical and the amplitude of P30 increased. The SEPs to DN stimulation were also symmetrical, and N33 and N37 were absent. These features can be explained by the bilateral character of DN stimulation. They also differed from bilateral PTN SEPs in 3 respects; the absence of P30, the small amplitude and the weaker gradients of field distribution of the 'W' complex, and the somewhat different distribution of penile from clitoral or bilateral PTN, N45 and P56. These differences can be explained both by physiological (the different fiber composition of the DN) and anatomical (the deeper localization of the DN cortical receiving area) mechanisms.     

Somatosensory evoked potentials after median and tibial nerve stimulation in healthy newborns.

Cortical and spinal somatosensory evoked potentials (SEPs) have been recorded after median and tibial nerve stimulation in healthy newborns. Spinal SEPs were readily obtained and recorded in all but one neonates after stimulation of both nerves. Cortical SEPs were more frequently recorded after median nerve (87%) than after tibial nerve stimulation (73%) but the shape of cortical SEPs obtained after tibial nerve stimulation was less variable. The mean feature of cortical SEPs was a negative wave (N27) for median nerve and a positive wave (P32) for tibial nerve. The present results demonstrate the feasibility of obtaining in the same baby, spinal and cortical SEPs after stimulation of median and tibial nerve, giving information on the functional integrity of central and peripheral somatosensory pathways which supply upper and lower limbs.     

Single fiber EMG and repetitive nerve stimulation of the same extensor digitorum communis muscle in myasthenia gravis.

OBJECTIVE: To compare voluntary single fiber electromyography (v-SFEMG) and repetitive nerve stimulation (RNS) at the same extensor digitorum communis (EDC) muscle in myasthenia gravis (MG). METHODS: We examined v-SFEMG and RNS successively on the same day in the same EDC muscle. We studied 45 examinations of both v-SFEMG and RNS in 29 patients suffering from MG, together with examinations of RNS in 30 control subjects. RESULTS: Forty-one of 45 (91%) v-SFEMGs showed abnormal results, whereas only 18/45 (40%) RNSs showed an abnormal decrement. The percentage of decrement showed similar correlations with 3 v-SFEMG parameters: percentage of abnormal pairs, percentage of blocking pairs, and the mean MCD value. Examinations showing a significant decrement in RNS had at least 60%, and usually no less than 90%, abnormal pairs, and 10-80% blocking pairs. Some muscles without a decrement had up to 50% blocking pairs. CONCLUSIONS: These results suggest that the blocking phenomenon observed in v-SFEMG is not a direct counterpart of the decrement in RNS. This must be partly because fibers contributing to the decrement are continuously blocked during voluntary contraction, and partly, because smaller motor units explored by v-SFEMG are probably more abnormal in MG than larger motor units mainly contributing to a decrement. Both factors make v-SFEMG much more sensitive than RNS.     

Color imaging of parietal and frontal somatosensory potential fields evoked by stimulation of median or posterior tibial nerve in man

Somatosensory evoked potentials (SEPs) to median or fingers or posterior tibial nerve stimulation were recorded with earlobe reference in normal young adults. A system of 16 electrodes on the scalp served to create bit-mapped images of the potential fields at 1 msec intervals. The P14 (median SEP) or P30 (tibial SEP) far fields thought to reflect the afferent volley in the medial lemniscus produced widespread positivity over the scalp. Subsequent components had a characteristic focal distribution suggesting that they reflected one or more generators in cortical areas. For the median SEP, the parietal N20 and the prerolandic P22 showed differences in onset and offset times as well as distribution that precluded their being related to the same generator. While N20 was contralateral, P22 extended ipsilaterally. P22 may be generated in the motor area 4 and the supplementary motor area. P22 was also distinct from the P27 field restricted to the contralateral parietal region. The frontal N30 had a bilateral distribution and the P45 presented variable features. For the tibial SEP, no phase reversal was confirmed between the parietal P38 (midline-ipsilateral focus) and N33 (contralateral focus). N37 over the contralateral prerolandic region might reflect a generator in the motor region. P58 was more symmetrically distributed than P38, possibly because it reflected generators more posteriorly on the parietal convexity. N75 had a widespread field with focus on the ipsilateral side of midline.     

Current source density mapping of somatosensory evoked responses following median and tibial nerve stimulation.

Potential recording of brain activities always encounters the problem resulting from the activation of reference electrodes. Current source density (CSD) computation does not take reference sites into account and consequently may better localize the generator sources. In the past, several attempts have been made to record CSDs of the somatosensory evoked responses (SERs) following median nerve stimulation. In order to compare the generating mechanisms of SERs following median nerve and tibial nerve stimulation, the scalp CSD distributions of the median nerve SER and the tibial nerve SER were compared in 5 normal subjects. In the median nerve SER, far-field potentials such as P14 and N16 were abolished in the CSD records. N20, P25 and N35 showed almost identical CSD distributions, albeit P25 had a reversed polarity. By contrast, the tibial nerve SER showed similar distributions for P40 and P60 CSDs, but N50 had a different distribution from the others. In the potential records, P40 and P60 were distributed predominantly ipsilateral to the stimulus (paradoxical lateralization), whereas the P40 and P60 CSDs formed a dipole localized over the contralateral foot somatosensory area. N50 disclosed the same tendency, although it had a slightly different CSD pattern from that of P40 and P60. These findings suggest that the median nerve and tibial nerve SER components are not necessarily comparable and that under certain circumstances CSDs are better indicators of local electrical events than the corresponding potentials.