L. Eberle

Magnetic coil stimulation of straight and bent amphibian and mammalian peripheral nerve in vitro: locus of excitation.

1. According to classical cable theory, a magnetic coil (MC) should excite a linear nerve fibre in a homogeneous medium at the negative‐going first spatial derivative of the induced electric field. This prediction was tested by MC stimulation of mammalian phrenic and amphibian sciatic nerve and branches in vitro, immersed in Ringer solution within a trough, and identifying the sites of excitation by recording responses of similar latency to local electrical stimulation. Subsequently, the identified sites of excitation were compared with measurements of the induced electric field and its calculated first spatial derivative. A special hardware device was used to selectively reverse MC current direction and to generate predominantly monophasic‐ or polyphasic‐induced pulse profiles whose initial phases were identical in polarity, shape and amplitude. When using the amphibian nerve preparation, a complication was excitation at low threshold points related to cut branches. 2. Reversal of monophasic current resulted in latency shifts corresponding approximately to the distance between induced cathode and anode. The location of each site of excitation was at, or very near, the negative‐going first spatial derivative peaks of the induced electric field measured parallel to the straight nerve. Significantly, excitation of the nerve did not occur at the peak of the induced electric field above the centre of the ‘figure of eight’ MC junction. 3. A polyphasic pulse excited the nerve at both sites, by the negative‐going first phase at one location, and approximately 150 microseconds later, by the reversed negative‐going second phase at the other location. Polyphasic and monophasic pulses elicited responses with similar latency when the induced current flowed towards the recording electrode. 4. Straddling a nerve with non‐coding solid lucite cylinders created a localized spatial narrowing and increase in the induced electric field, resulting in a lowered threshold of excitation. The corresponding closer spacing between first spatial derivative peaks was exhibited by a significant reduction in latency shift when MC current direction was reversed. 5. When a nerve is bent and the induced current is directed along the nerve towards the bend, the threshold of excitation is reduced there. Increasing the angle of the bend from 0 deg to more than 90 deg graded the decrease in threshold. 6. In a straight nerve the threshold was lowest when current was directed towards the cut end.(ABSTRACT TRUNCATED AT 400 WORDS)

Influence of pulse sequence, polarity and amplitude on magnetic stimulation of human and porcine peripheral nerve

1 Mammalian phrenic nerve, in a trough filled with saline, was excited by magnetic coil (MC)‐induced stimuli at defined stimulation sites, including the negative‐going first spatial derivative of the induced electric field along a straight nerve, at a bend in the nerve, and at a cut nerve ending. At all such sites, the largest amplitude response for a given stimulator output setting was elicited by an induced damped polyphasic pulse consisting of an initial quarter‐cycle hyperpolarization followed by a half‐cycle depolarization compared with a predominantly ‘monophasic’ quarter‐cycle depolarization. 2 Simulation studies demonstrated that the increased efficacy of the induced quarter‐cycle hyperpolarizing‐half‐cycle depolarizing polyphasic pulse was mainly attributed to the greater duration of the outward membrane current phase, resulting in a greater outward charge transfer afforded by the half‐cycle (i.e. quarter‐cycles 2 and 3). The advantage of a fast rising initial quarter‐cycle depolarization was more than offset by the slower rising, but longer duration depolarizing half‐cycle. 3 Simulation further revealed that the quarter‐cycle hyperpolarization‐half‐cycle depolarization showed only a 2.6 % lowering of peak outward current and a 3.5 % lowering of outward charge transfer at threshold, compared with a half‐cycle depolarization alone. Presumably, this slight increase in efficacy reflects modest reversal of Na+ inactivation by the very brief initial hyperpolarization. 4 In vitro, at low bath temperature, the nerve response to an initial quarter‐cycle depolarization declined in amplitude as the second hyperpolarizing phase progressively increased in amplitude and duration. This ‘pull‐down’ phenomenon nearly disappeared as the bath temperature approached 37 °C. Possibly, at the reduced temperature, delay in generation of the action potential permitted the hyperpolarization phase to reduce excitation. 5 Pull‐down was not observed in the thenar muscle responses to median nerve stimulation in a normal human at normal temperature. However, pull‐down emerged when the median nerve was cooled by placing ice over the forearm. 6 In a nerve at subnormal temperature straddled with non‐conducting inhomogeneities, polyphasic pulses of either polarity elicited the largest responses. This was also seen when stimulating distal median nerve at normal temperature. These results imply excitation by hyperpolarizing‐depolarizing pulse sequences at two separate sites. Similarly, polyphasic pulses elicited the largest responses from nerve roots and motor cortex. 7 The pull‐down phenomenon has a possible clinical application in detecting pathologically slowed activation of Na+ channels. The current direction of the polyphasic waveform may become a significant factor with the increasing use of repetitive magnetic stimulators which, for technical reasons, induce a cosine‐shaped half‐cycle, preceded and followed by quarter‐cycles of opposite polarity.

In vitro evaluation of a 4-leaf coil design for magnetic stimulation of peripheral nerve

The performance of a 4-leaf magnetic coil was evaluated during magnetic stimulation of a peripheral nerve in vitro. The site of stimulation was below the coil center, and a 90 degrees rotation of the coil was equivalent to a change in current polarity. A hyperpolarizing magnetic stimulus failed to slow or block a propagating action potential.

Age-related changes in power spectra of efferent phrenic activity in the piglet

Power spectral analysis of phrenic nerve discharge in neonatal swine revealed the presence of both high-frequency oscillations (HFO) (95-150 Hz) and medium-frequency oscillations (MFO) (15-35 Hz). The HFO was shown to be age-related; the MFO was not. The data indicated that at least one manifestation of maturation of the respiratory rhythm generator is the increase with age of the frequency of the HFO.

Recognition potential: sensitivity to visual field stimulated

The recognition potential (RP) was distinguished from P3 and eye blink responses by its sensitivity to visual area stimulated. Images were flashed in upper and lower hemifields. Current source density profiles were computed, using 16 midline scalp electrodes. For P3 and eye blink profiles, the hemifield stimulated was not a significant factor. For the recognition potential, upper and lower field stimulation produced radically different profiles. An improved recognition potential signal was obtained by a new mathematical procedure. It used the difference in sensitivity to visual area stimulated to reject P3 and eye blink responses.

Power spectral analysis of the baroreflex in neonatal swine.

The baroreflex was observed in neonatal swine as young as 4 h of age. Bolus injections of Na nitroprusside (NP) and phenylephrine (PE), induced changes in blood pressure and elicited changes in both heart rate and in cervical sympathetic and splanchnic discharge; changes in sympathetic discharge were reflected in altered power spectral magnitude. Measures of heart rate showed that the magnitude of the PE-induced decreases was positively correlated with increasing postnatal age. The results indicate that the baroreflex, as indicated by changes in sympathetic discharge and heart rate, is present in early neonatal swine.

Studies of 3-dimensional voltage distributions induced in homogeneous media volume conductors by round and butterfly magnetic coils

Magnetic coils (MCs) for physiological stimulation of the human nervous system are discussed. Using a butterfly MC at threshold intensity over the motor cortex, focal movements can be elicited in contralateral digits. These observations can in part be explained by physical models of homogeneous volume conductors which allow measurement of induced voltage gradients in 3-D space. In an infinite-volume conductor, a round MC induces the greatest voltage gradient under the windings. The butterfly MC induces a sharp, central peak separated from smaller lateral peaks.<<ETX>>