In vivo magnetic and electric recordings from nerve bundles and single motor units in mammalian skeletal muscle. Correlations with muscle force.

Abstract

Recent advances in the technology of recording magnetic fields associated with electric current flow in biological tissues have provided a means of examining action currents that is more direct and possibly more accurate than conventional electrical recording. Magnetic recordings are relatively insensitive to muscle movement, and, because the recording probes are not directly connected to the tissue, distortions of the data due to changes in the electrochemical interface between the probes and the tissue are eliminated. In vivo magnetic recordings of action currents of rat common peroneal nerve and extensor digitorum longus (EDL) muscle were obtained by a new magnetic probe and amplifier system that operates within the physiological temperature range. The magnetically recorded waveforms were compared with those obtained simultaneously by conventional, extracellular recording techniques. We used the amplitude of EDL twitch force (an index of stimulus strength) generated in response to graded stimulation of the common peroneal nerve to enable us to compare the amplitudes of magnetically recorded nerve and muscle compound action currents (NCACs and MCACs, respectively) with the amplitudes of electrically recorded nerve compound action potentials (NCAPs). High, positive correlations to stimulus strength were found for NCACs (r --0.998), MCACs (r = 0.974), and NCAPs (r = 0.998). We also computed the correlations of EDL single motor unit twitch force with magnetically recorded single motor unit compound action currents (SMUCACs) and electrically recorded single motor unit compound action potentials (SMUCAPs) obtained with both a ring electrode and a straight wire serving as a point electrode. Only the SMUCACs had a relatively strong positive correlation (r = 0.768) with EDL twitch force. Correlations for ring and wire electrode-recorded SMUCAPs were 0.565 and -0.366, respectively. This study adds a relatively direct examination of action currents to the characterization Address reprint requests to Dr.

Detection of Peripheral Nerve and Skeletal Muscle Action Currents Using Magnetic Resonance Imaging

“…Gielen et al (8) measured I = 3 µ A in a rat EDL muscle. However, the radius of this muscle, r , is large compared with the above mentioned nerves and is about 1.5 mm.…”

“…Action currents in nerves and muscles have been directly measured using toroidal pickup probes by us and our former colleagues (8, 22, 23, 25). From these measurements of the current, I , and the radius of the fiber, r , we can calculate the magnetic field created by this current at the surface of the fiber using the Ampere’s law, $$B=\frac{{\mu }_{0}I}{2\pi r},$$ where µ 0 is the magnetic permeability of free space.…”

“…However, a large body of research exists in the biomagnetic literature in which magnetic fields of nerves, muscles, and even single axons were calculated numerically (19, 26) or directly measured using ferrite-core, wire-wound toroids (7, 8, 18, 21, 22, 23, 24, 25). Our goal in this paper is to use some of these experimental results to estimate the MRI signal caused by action currents.…”

Effect of peripheral nerve action currents on magnetic resonance imaging

“…Action currents in nerves have been directly measured using toroidal pickup probes by us and our former colleagues [7,19,20,22]. From these measurements of the current, I, and the radius of the fiber, r, we can calculate the magnetic field created by this current at the surface of the fiber using the Ampere's law,…”

Is it possible to detect dendrite currents using presently available magnetic resonance imaging techniques?

“…Also, there are many papers in the biomagnetic literature in which magnetic fields of nerves, muscles, and even single axons were calculated numerically [25, 28, 35] or measured using ferrite-core, wire-wound toroids [9, 10, 23, 27, 29, 30, 32–34]. Many investigations have shown results obtained from water phantoms, humans, bloodless turtle brains, rat brains, and theoretical calculations [2, 4, 6, 17].…”