Cerebral magnetic responses to stimulation of tibial and sural nerves

We report somatosensory evoked magnetic fields (SEFs) to stimulation of the mixed posterior tibial nerve (PTN) and the sensory sural nerve (SN) in 6 healthy subjects. The first peak of the responses occurred at 39-50 ms (P40m), with a 2-3 ms longer latency for SN than PTN stimulation. Within 200 ms several other deflections followed, with interindividually varying waveforms and latencies. Magnetic field mappings indicated that the source of P40m for PTN can be modelled by a single equivalent current dipole at the primary sensorimotor cortex; for the smaller responses to SN stimulation the single dipole model was less applicable. Field patterns for later deflections differed from those of P40m, indicating that several current sources within or near the primary foot projection area are sequentially activated after stimulation of both a mixed and a sensory lower limb nerve. These late deflections could not always be satisfactorily explained by single current dipoles, suggesting more complex geometries for the underlying neural activity.     

Multiple point electrical stimulation of ulnar and median nerves.

A computer-assisted method of isolating single motor units (MUs) by multiple point stimulation (MPS) of peripheral nerves is described. MPS was used to isolate 10-30 single MUs from thenar and hypothenar muscles of normal subjects and patients with entrapment neuropathies, with the original purpose of obtaining a more representative mean motor unit potential for estimating the number of MUs in a muscle. The two important results that evolved from MPS however, were: (1) in the absence of 'alternation' MUs were recruited in an orderly pattern from small to large, and from longer to shorter latencies by graded electrical stimulation in both normal and pathological cases, (2) a comparison of the sizes of MUs recruited by stimulation proximal and distal to the elbow suggested that axonal branching can occur in the forearm 200 mm or more proximal to the motor point in intrinsic hand muscles.     

Effects of prolonged repetitive stimulation of median,ulnar and peroneal nerves

It is important to know the effects of prolonged repetitive nerve stimulation (RNS) when it is used in neurophysiological studies. RNS with up to 100 supramaximal stimuli was given to the median, ulnar, and peroneal nerves of normal subjects and the ulnar nerves of subjects with early amyotrophic lateral sclerosis (ALS), recording evoked compound muscle action potentials (CMAPs). In all nerves, there was a decline in the CMAP area and a decrease in CMAP duration. For the peroneal nerve there was a decline in the CMAP amplitude, but a similar decline was not seen in the median or ulnar nerves. Cooling of the muscles resulted in decrement of both the amplitude and area with RNS. In ALS subjects, CMAP amplitude and area both declined after RNS of the ulnar nerve. In this study we describe the changes in CMAP with prolonged RNS among commonly tested normal nerves. Our findings have important implications with regard to RNS. Muscle Nerve, 2010     

Multichannel detection of magnetic compound action fields of median and ulnar nerves

We have recorded magnetic compound action fields, evoked by median and ulnar nerve stimulation at the wrist, with a 7-channel 1st-order SQUID gradiometer. Responses at cubita, elbow and brachial plexus were usually monophasic, with polarities and field patterns corresponding to the intracellular current flow in the leading edge of the action potential volley.     

Early deflections of cerebral magnetic responses to median nerve stimulation

We have recorded early components of somatosensory evoked magnetic fields with a sensitive 7-channel first-order gradiometer using a wide recording passband (0.05-2000 Hz) and high sampling frequency (8000 Hz). The left median nerve was stimulated at the wrist and responses were recorded over the right hemisphere. The responses typically consisted of a N20m peaking at 18-20 msec, a small P22m peaking at 21-23 msec and a P27m peaking at 29-31 msec. The topography of N20m could be explained by a tangential current dipole in the posterior wall of the central sulcus (probably in area 3b). The equivalent dipoles of P27m were located on average 10 mm antero-medially to the sources of N20m. This suggests that P27m may get a contribution from the anterior wall of the central sulcus. An increase of stimulus repetition rate from 2 to 5 Hz decreased the amplitude of P27m more than that of N20m, which implies that these two deflections are generated by different neural networks.     

Use of collision tests to identify physiological differences between the median and ulnar nerves

Introduction: Median and ulnar nerves are used in comparative electrophysiological studies. We analyzed the conduction values in these hand nerves in healthy volunteers to find any physiological differences between them. Methods: We performed standard conduction studies and conduction velocity distribution (CVD) tests with estimation of 3 quartiles in 31 healthy right-handed volunteers (17 women, 14 men) with a mean age of 44.8 ± 15.5 years. Results: The conduction velocities in all quartiles of CVD tests were statistically faster in the ulnar nerve (P < 0.00001), with no differences in the spread of conduction values and no differences between sides. In the ulnar nerve, CVD velocities in all quartiles were faster in the female group (P < 0.05). Discussion: The ulnar nerve has more fibers conducting with high velocities than does the median nerve. Electrophysiological comparisons between hand nerves must be performed carefully. Muscle Nerve 59:470–474, 2019     

Recovery of peripheral and central responses to median nerve stimulation

We recorded the responses to paired stimuli delivered to the median nerve at the wrist in 8 healthy adult volunteers, in order to characterize the recovery of function after a single conditioning stimulus. Responses were recorded over the nerve at the ipsilateral elbow and in the Erb's point region, over the second cervical spinous process, and over the contralateral 'hand area' of the scalp. The data from 1 subject were discarded because of possible artifactual contamination. In the others, the peripheral responses recovered both in latency and amplitude over a time period that accorded with previously published studies. We found, however, that the recovery periods for latency and amplitude of the responses recorded over the spine and scalp were prolonged compared with the corresponding values for the peripheral responses. Except for the responses recorded over the scalp, the recovery of amplitude either preceded or occurred at the same time as latency. By contrast, for the responses recorded over the scalp, there was a delay in the recovery of amplitude compared with latency. The differences in recovery period that we found at different levels of the nervous system are presumably related to structural and electrophysiological differences in afferent pathways, the presence of interposed synapses, and the intrinsic refractory properties of central neuronal populations.     

The H-reflex to magnetic stimulation of lower-limb nerves.

We elicited H-reflexes by magnetic and electrical stimulation of several different nerves in 10 healthy subjects and two patients with S-1 radiculopathy. The posterior tibial nerve at the popliteal fossa and the femoral nerve at the inguinal ligament were tested with both electrical and magnetic stimulation; the proximal sciatic nerve was tested only with magnetic stimulation. Muscle activity was recorded from the soleus muscle for posterior tibial and sciatic nerve stimulation and from the vastus medialis muscle for femoral nerve stimulation. No significant difference was found between the latency of H-reflexes evoked by magnetic or electrical stimulation. With magnetic stimulation, the mean (+/- SD) Ia sensory fiber conduction velocity in the proximal segment of the sciatic nerve was 72.4 +/- 3.3 m/s, while the motor nerve fiber conduction velocity in the same portion of the nerve was significantly slower, at 60.6 +/- 2.0 m/s. In two patients with unilateral S-1 radiculopathy, the latency of the H-reflex from the soleus muscle to both magnetic and electrical stimulation of the posterior tibial nerve was absent or prolonged on the affected side. Magnetic stimulation can be used to study the H-reflex and Ia fiber conduction velocity and is particularly advantageous when testing deeply located nerve trunks.     

Short latency somatosensory evoked responses to median nerve stimulation in healthy humans: electric and magnetic recordings

Somatosensory Evoked Potentials (SEPs) and Somatosensory Evoked magnetic Fields (SEFs) to median nerve stimulation at wrist were recorded in 5 healthy subjects and the components between 15 and 30 ms after the stimulus were evaluated on the hemiscalp contralateral to the stimulated wrist. SEPs were measured by means of a 32-channel recorder and compared with SEFs obtained via multiple measurements with a 4-channel sensor. Equivalent dipole localization was carried out for the magnetic components peaking at about 15, 20 and 24 ms. The scalp distribution of SEPs, illustrated by bit mapped color images, were qualitatively explained by three separate sources. The first is described as a tangentially oriented dipole placed behind the Central Sulcus and responsible for the parietal N20-"late P25" waves and for the frontal P20-N30 ones. The second is represented by a radieal dipole placed just in front of the Central Sulcus and pointing towards the motor strip, responsible for the rolandic P22 component. The third is just behind the Central Sulcus and is radieally oriented towards the surface of the postcentral sensory area for the "early P25" parietal wave. The SEFs distributions, illustrated by color isofield contour maps, were quantitatively explained by a unique tangential dipole localized, with good resolution, well behind the Sulcus for the 15 ms waves and slightly frontal to this site for the waves peaking at around 20 and 24 ms. The equivalent dipole has been localized at a depth of about 5 cm (15 ms component), 2 cm (20 ms components) and 4 cm (24 ms component), across the studied subjects. It is stressed that the dipole responsible for the magnetic pattern is likely to be the same tangential dipole responsible for a part of the electric pattern. Due to their radieal orientation, the other two dipoles, proposed for the SEPs maps, would be mostly undetectable by a magnetic investigation.     

Comparison of cervical SEPs on median, radial and ulnar nerve stimulation

Cervical responses (SEPs) to stimulation of the median, radial and ulnar nerves were studied in 9 healthy subjects. In recordings from e Cv₇ electrode referenced to a scalp electrode P11 presented a bilobed (P11a+P11b) profile for all three nerves whereas from Cv₂ only P11b appeared as a rule. P11a and P11b were more distinct in the ulnar than in the median and radial nerves. The P11_onset-P13_onset interval was virtually the same for the radial and median nerves and approximately 0.4 msec longer for the ulnar nerve. This difference probably represents the Cv₈ to Cv₆ intramedullary conduction time. An exact evaluation of P11_onset is possible only in low cervical recordings, though the P11_peak may be a useful landmark when recording from Cv₂. P11a would appear to originate at (or near) spinal entry, P11b high in the cervical cord and P13 at supraspinal level.

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Sommario In nove soggetti, sono state studiate le risposte cervicali da stimolazione dei nervi mediano, radiale ed ulnare. Utilizzando un riferimento cefalico, P11 appare bilobata, registrando da Cv₇, per tutti e tre i nervi, mentre solo la seconda delle due subcomponenti (P11a e P11b, rispettivamente) viene registrata generalmente da Cv₂. P11a e P11b sono meglio distinte fra di loro nel caso del nervo ulnare, che non del radiale o mediano. L'intervallo P11_onset-P13_onset è sovrapponibile fra mediano e radiale e circa 0.45 msec più lungo per il nervo ulnare; questa differenza dovrebbe essere dovuta ad un tempo di conduzione intramidollare fra Cv₈ e Cv₆. Una esatta valutazione di P11_onset è possibile solo a livello cervicale inferiore, mentre il picco P11b può essere utilizzato nel computo delle latenze solo da Cv₂. P11a dovrebbe originare all'ingresso spinale, P11b nel tratto cervicale superiore, P13 a livello sopraspinale.

     

Cerebral magnetic fields to lingual stimulation.

We recorded cerebral magnetic fields to electric stimulation of the tongue in 7 healthy adults. The two main deflections of the response peaked around 55 msec (P55m) and 140 msec (N140m). During both of them the magnetic field pattern, determined with a 7- or 24-channel SQUID magnetometer, suggested a dipolar current source. The topography of P55m can be explained by a tangential dipole at the first somatosensory cortex (SI) in the posterior wall of the central sulcus. The equivalent source of N140m is, on average, about 1 cm lateral to the source of P55m. The reported method allows non-invasive determination of the cortical tongue representation area.     

Significant CMAP decrement by repetitive nerve stimulation is more frequent in median than ulnar nerves of patients with amyotrophic lateral sclerosis

Introduction: Several studies have shown a significant amplitude decrement in compound muscle action potentials (CMAPs) on repetitive nerve stimulation (RNS) of muscles involved in amyotrophic lateral sclerosis (ALS).In ALS, muscle wasting preferentially affects the thenar muscles (APB) rather than the hypothenar muscles (ADM). Methods: We performed RNS studies in the APB and ADM muscles of 32 ALS patients to determine whether the effect of RNS differs between the median and ulnar nerves. Results: The decremental responses to RNS were greater in the APB than in the ADM. Reduced CMAP amplitude was negatively correlated with CMAP decrement in median but not in ulnar nerves. Conclusions: The greater CMAP decrement in median nerve is attributable to preferential involvement of the APB in the pathophysiology of ALS or some underlying difference in the biology of the two muscles/nerves. Further investigations will better our understanding of the pathophysiology of ALS. Muscle Nerve, 2012     

NC-stat sensory nerve conduction studies in the median and ulnar nerves of symptomatic patients.

OBJECTIVE: This study evaluated validity and reliability of automated median and ulnar sensory nerve conduction study (NCS) measurements by the NC-stat. METHODS: Median and ulnar distal sensory latencies (DSL) and amplitudes (SNAP) were measured in sixty subjects with the NC-stat and by a neurologist (reference) using traditional instrumentation. The median-ulnar DSL differences (MUD) was calculated. Validity was quantified by the Pearson correlation. Reliability was evaluated by the intraclass correlation coefficient (ICC), Bland-Altman analysis, and inter-rater agreement of MUD abnormalities. RESULTS: As a result of differences in electrode placement, NC-stat and reference mean values had systematic differences. The correlation ranged from 0.70 (ulnar DSL) to 0.91 (median DSL). The ICC ranged from 0.69 (ulnar DSL) to 0.91 (median DSL). In Bland-Altman analysis of DSLs, NC-stat measurements had a bias of 0.56 ms (median) and 0.31 ms (ulnar) and precision of 0.31 and 0.30 ms. Inter-rater agreement for MUD abnormalities was 93.8% (raw) and 0.80 (Kappa). CONCLUSIONS: NC-stat validity and reliability metrics were similar to traditional NCS. Use of the NC-stat would require applicable reference ranges. SIGNIFICANCE: NC-stat median and ulnar NCS are valid and reliable. This device may be useful for increasing availability of NCS when clinically appropriate.     

Normal sensory nerve conduction velocities of the median and ulnar nerves in children from birth to 15 years

Conduction velocity of the fastest fibres in sensory nerves (SCV), amplitude and shape of the sensory evoked potentials (SEPs) were studied in 94 normal children aged 5 days up to 15 years. Surface electrodes were used both for nerve stimulation and sensory evoked potential recording. Normal minimal value in young adults is reached between 2 and 4 years of age, earlier in the proximal segment of the nerve (wrist-elbow) than in the distal segment (digit-wrist). Amplitude of the median SEP at the wrist increases about 155% during the first 2 years of life; this increase becomes more progressive and moderate from 2 to 15 years of age. At the elbow, the SEP has a smaller amplitude and is made of two separate peaks in 80-100% of the children between 6 months and 10 years of age. The presence of those two separate components lends substance to the view that there are two groups of fibres with different degrees of maturation. Methodology and results are discussed along with previous data from the literature.     

Sympathetic responses to repetitive trans-spinal magnetic stimulation

Purpose  

Electromagnetic fields have been administered, with mixed success, in order to treat a variety of ailments. Transcranial magnetic stimulation (TMS) elicits brief changes in peripheral sympathetic nervous system (SNS) activity. The purpose of this study was to explore the utility of repetitive trans-spinal magnetic stimulation (rTSMS) for acute and prolonged modulation of SNS in adult humans.     

Surface recording of orthodromic sensory nerve action potentials in median and ulnar nerves in normal subjects

Reference values of sensory nerve compound action potentials (CAP) were established in 50 healthy subjects by examination with surface electrodes placed in a transverse and a longitudinal position at the wrist. Longitudinal electrodes record potentials of larger amplitude and shorter duration than transversally recorded CAPs. Both amplitude and duration were increased by an increase of interelectrode distance. The use of amplitude and duration as variables is encouraged by the well-defined biologic variation obtained when fixed interelectrode distance is used and when values are corrected for age and temperature. The strong correlation between amplitude and duration might indicate that the decrease of amplitude with age is caused by the reduction of conduction velocity rather than axonal loss. The ulnar nerve CAPs differ from those of the median nerve mostly with regard to influence of age and temperature. The reasons for this are discussed.