The origin and nature of beading: a reversible transformation of the shape of nerve fibers.

Nerve fibers which appear beaded (varicose, spindle-shaped, etc.) are often considered the result of pathology, or a preparation artifact. However, beading can be promptly elicited in fresh normal nerve by a mild stretch and revealed by fast-freezing and freeze-substitution, or by aldehyde fixating at a temperature near 0 degree C (cold-fixation). The key change in beading are the constrictions, wherein the axon is much reduced in diameter. Axoplasmic fluid and soluble components are shifted from the constrictions into the expansions leaving behind compacted microtubules and neurofilaments. Labeled cytoskeletal proteins carried down by slow axonal transport are seen to move with the soluble components and not to have been incorporated into and remain with, the cytoskeletal organelles on beading the fibers. Lipids and other components of the myelin sheath are also shifted from the constrictions into the expansions, with preservation of its fine structure and thickness. Additionally, myelin intrusions into the axons are produced and a localized bulging into the axon termed "leafing". The beading constrictions do not arise from the myelin sheath: beading occurs in the axons of unmyelinated fibers. It does not depend on the axonal cytoskeleton: exposure of nerves in vitro to beta, beta'-iminodipropionitrile (IDPN) disaggregates the cytoskeletal organelles and even augments beading. The hypothesis advanced was that the beading constrictions are due to the membrane skeleton; the subaxolemmal network comprised of spectrin/fodrin, actin, ankyrin, integrins and other transmembrane proteins. The mechanism can be activated directly by neurotoxins, metabolic changes, and by an interruption of axoplasmic transport producing Wallerian degeneration.

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

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 of the normal biophysical properties of peripheral nerve, muscle, and muscle single motor units.

Tryptamine enhancement of neurotransmitter release under conditions that normally depress calcium influx.

Neurotransmitter release, resulting in excitatory and inhibitory junction potentials (E- and IJPs) is normally mediated by an influx of calcium ions into nerve terminals following depolarization. At a lobster neuromuscular junction, tryptamine is shown to greatly enhance the amplitude and duration of evoked E- and IJPs in low Ca2+-high Mg2+ media that depress Ca2+ influx. This suggests that in the presence of tryptamine, intracellular Ca2+ sources may support evoked, phasic neurotransmitter release.

The effects of tryptamine on transmitter release at a lobster neuromuscular junction.

Electrophysiological techniques were employed to characterize the effects of tryptamine at excitatory and inhibitory neuromuscular junctions of the stretcher muscle in the carpopodite of lobster walking limbs. Tryptamine was found to have a concentration dependent, biphasic effect on excitatory junction potential (EJP) amplitude. At concentrations of 0.01-0.5 mM tryptamine increased the amplitude of evoked EJPs, but at higher concentrations (greater than 0.5 mM) the amplitude was decreased by this amine. The high concentrations also decreased the amplitude of inhibitory junction potentials (IJPs) and reduced the frequency of miniature excitatory junction potentials (MEJPs). When a preparation in which the EJPs had been depressed by tryptamine was washed with the control, artificial sea water solution, the EJPs were increased to an amplitude greater than that of the pre-tryptamine control. Current-voltage relationship studies showed that tryptamine did not affect the effective resistance of the muscle fiber membrane. Tryptamine had no effect on the amplitude of excitatory responses evoked by the iontophoretic application of glutamate. Concentrations of tryptamine ranging to 10.0 mM affected neither the conduction velocity in meropodite or that of giant central nervous system (CNS) axons. We conclude that tryptamine affects synaptic transmission pre-synaptically by influencing the transmitter release process. Mass spectrometric analysis showed tryptamine to be an endogenous substance in the lobster; but the concentrations were low in the tissues analyzed. The highest concentration (approximately 0.3 nmol/g wet tissue) was in the subesophageal ganglion. Tryptamine was not detected in the meropodite nerves or carpopodite muscles.

Diagnosing intestinal ischemia using a noncontact superconducting quantum interference device.

Intestinal ischemia is associated with changes of the basic electric rhythm (BER) of the small intestine. We hypothesized that these changes can be measured noninvasively using a superconducting quantum interference device (SQUID). After general anesthesia, a laparotomy was performed on 10 animals and the jejunum was placed in a nonmagnetic recording chamber containing Krebs' solution at 38 degrees C. Five animals had electrodes placed while five others were placed under the SQUID. Injection of thrombin into the mesenteric artery decreased blood flow (measured with a laser doppler flow-meter) 95% within 5 minutes. SQUID measurements showed significant decreases (P < 0.01 for all changes) in the frequency (15.5 +/- 0.3 to 8.9 +/- 0.2 cycles/min) and the propagation velocity of slow waves (3.5 +/- 0.2 to 1.9 +/- 0.3 sec). The changes in intestinal biomagnetic activity after ischemia were similar to the changes in electrical activity. The SQUID magnetometer is a reliable noncontact device that can detect early intestinal ischemia in animal models. We have recently recorded human small bowel biomagnetic activity using a SQUID magnetometer and believe further technical developments will permit the noninvasive diagnosis of mesenteric ischemia.

Preliminary electrophysiological characterization of functionally vestigial muscles of the head: potential for command signaling.

In devastating neurological disorders, such as quadriplegia resulting from high-level spinal cord injury, it is essential to focus on functions that have been spared and optimally exploit them to enhance the individual's quality of life. It follows that certain muscles, which prior to the paralysis of much of the rest of the body seemed to have no useful function, might be used to provide unique signals to control assistive devices. This report presents preliminary electrophysiological data demonstrating potentially useful myoelectrical signals from 3 functionally vestigial muscles in humans; the posterior, anterior, and superior auricular muscles. In phylogenetically lower species, these muscles serve to position the ear to enhance hearing. The auricular muscles receive their major innervation from cranial nerve VII and should not be compromised by even high-level spinal cord lesions. In this study, it was found that the muscles could be voluntarily activated and, by standard surface-electrode recording, had potentials ranging to 680 microV in amplitude. Posterior auricular muscle potentials were used to command a paddle in a computer ping-pong task that employed a CyberLink interface. The t values for accuracy scores and ball hits were both significant at the p = .0001 level. These facts indicate that the auricular muscles may be useful for controlling assistive devices.

Initial characterization of the effects of Aloe vera at a crayfish neuromuscular junction.

This study examines the effects of Aloe vera on neurotransmission processes in a well-established invertebrate neuromuscular junction preparation. We studied concentration-response relationships of an Aloe vera extract on excitatory junctional potentials (EJPs) at the opener muscle of the dactyl in the first and second walking limbs of crayfish (Procambarus clarkii and simulans). We observed concentration-dependent depolarizations of the muscle fibre membrane resting potential, depression of EJP amplitudes and an increase in latency to onset of the EJP following electrical stimulation of the isolated excitatory axon in the meropodite. These effects occurred with Aloe concentrations within the 1%-10% (wt-vol) range. Effects of lower concentrations, ranging to a minimum of 0.01% were equivocal. The effects of Aloe were at least partially, and in a majority of cases totally, reversible. EJPs reduced by Aloe could be restored by increasing the nerve stimulation amplitude. This, along with the latency increase, suggests a depression of action potential generation and conduction. The results provide a preliminary characterization of the effects of Aloe vera on the neurotransmission process and suggest that these effects may at least partially account for Aloe's analgesic and antiinflammatory effects. This study shows that the crayfish NMJ preparation should be useful for further elucidating the location(s) and mechanism(s) of action of Aloe on the nervous system.

Stretch of mammalian nerve in vitro: effect on compound action potentials.

Stretch of nerve has been reported to decrease the amplitude of the compound action potential (CAP) with a complete block appearing in approximately 30 minutes. But for the most part, those experiments were carried out in vivo, and it is generally accepted that the failure of responses was due to a closure of vessels supplying the nerve with a resulting ischemia and anoxia. These studies were undertaken to determine if stretch of nerve has effects that are independent of interference with its vascular supply. In the studies, lengths of rat sciatic and dog peroneal nerves were removed and placed in a chamber supplied with oxygen in which their CAPs were continuously elicited and recorded. This in vitro preparation obviated interference with the nerve's metabolism on stretching. We have previously shown that the form change termed 'beading,' appearing within 10 seconds and reversing as quickly on relaxation, can be elicited with tensions of only several grams. We wished to determine if stretch adequate to produce beading could alter CAPs with the same rapidity. Tensions below 2 g had little effect. On applying tensions of 10-100 g, levels well above those needed to bead the fibers, both increases and decreases of CAP amplitude were seen. The changes occurred within 10 seconds of stretch application, the time at which beading arises with stretch. Although the decreases of CAP amplitudes could be accounted for by beading, the degree of CAP change did not correspond to the amount of tension applied. We hypothesize that the constrictions in the beaded fibers increase axial resistivity and diminish local currents so as to block conduction. The lack of an increasing degree of decreased CAP amplitude with increases in tension is ascribed to the inhibition of elongation offered by the collagen fibrils present in nerve. Collagenase applied to nerves allowed a further increase in length, producing a 'hyperbeading,' showing much longer lengths of beading constrictions on stretch. This would further increase axial resistance and is taken to account for the greater decreases of CAP amplitudes seen following collagenase treatment. To account for those cases where increases of CAP amplitude were seen on stretch, we hypothesize that stretch can also cause an increase in the excitability of the nodes. The outcome of stretch in any given nerve would be the resultant of two opposing actions; beading of the internodes causes a decrease of local currents leading to block of CAPs, while an increased excitability of the nodes acts to augment the responses.

Electrophysiological and behavioral effects of ethanol on crayfish.

The effects of EtOH on the crayfish Procambarus clarkii and P. simulans were examined behaviorally in vivo and electrophysiologically in vitro on pre- and postjunctional mechanisms of synaptic plasticity at opener excitor nerve-muscle junctions. Addition of 75 mM EtOH to the bath water of holding tanks produced 47 to 54 mM EtOH levels in the hemolymph (blood) within 24 hr. These hemolymph EtOH levels were maintained for weeks by daily changes of the bath water containing 75 mM EtOH. After 24 hr of exposure to 75 to 150 mM EtOH in vivo, crayfish showed behavioral signs of intoxication as measured by a significant increase in righting reflex times and a significant decrease in tail-flip escape behavior. After 2 weeks of chronic exposure to 75 mM EtOH, crayfish showed behavioral tolerance as measured by a decrease in righting time and an increase in tail-flip escape behavior to control levels. EtOH applied acutely to opener nerve-muscle preparations in vitro at 10 to 100 mM concentrations produced an increased probability of transmitter release as measured by an increased frequency of spontaneous release of transmitter quanta and an increased amplitude of facilitated synaptic potentials evoked by 10 to 40 Hz stimulation of the excitor axon. Acute application of 300 to 600 mM EtOH resulted in a decreased amplitude of facilitated synaptic potentials due primarily to a decrease in postsynaptic input resistance. These data suggest that EtOH has a concentration-dependent biphasic effect on synaptic transmission.

Magnetic field of a single muscle fiber. First measurements and a core conductor model.

We present the first measurements of the magnetic field from a single muscle fiber of the frog gastrocnemius, obtained by using a toroidal pickup coil coupled to a room-temperature, low-noise amplifier. The axial currents associated with the magnetic fields of single fibers were biphasic and had peak-to-peak amplitudes ranging between 50 and 100 nA, depending primarily on the fiber radius. With an intracellular microelectrode, we measured the action potential of the same fiber, which allowed us to determine that the intracellular conductivity of the muscle fiber in the core conductor approximation was 0.20 +/- 0.09 S/m. Similarly, we found that the effective membrane capacitance was 0.030 +/- 0.011 F/m2. These results were not significantly affected by the anisotropic conductivity of the muscle bundle. We demonstrate how our magnetic technique can be used to determine the transmembrane action potential without penetrating the membrane with a microelectrode, thereby offering a reliable, stable, and atraumatic method for studying contracting muscle fibers.

The biomagnetic signature of a crushed axon. A comparison of theory and experiment.

The response of a crayfish medial giant axon to a nerve crush is examined with a biomagnetic current probe. The experimental data is interpreted with a theoretical model that incorporates both radial and axial ionic transport and membrane kinetics similar to those in the Hodgkin/Huxley model. Our experiments show that the effects of the crush are manifested statically as an elevation of the resting potential and dynamically as a reduction in the amplitude of the action current and potential, and are observable up to 10 mm from the crush. In addition, the normally biphasic action current becomes monophasic near the crush. The model reflects these observations accurately, and based on the experimental data, it predicts that the crush seals with a time constant of 45 s. The injury current density entering the axon through the crush is calculated to be initially on the order of 0.1 mA/mm2 and may last until the crush seals or until the concentration gradients between the intra- and extracellular spaces equilibrate.