Correlation between the size of the compound muscle and sensory nerve action potentials recorded from the foot in distal axonopathy

Nerve conduction study was performed on 71 diabetic patients with distal sensorimotor axonopathy. Of 76 lower limbs studied, 46.1% showed no recordable sural compound sensory nerve action potential (CSNAP), and 55.3% no superficial peroneal CSNAP. Only 2.6% revealed no recordable compound muscle action potential (CMAP) from the abductor hallucis (AH) muscle, and 9.2% showed no obtainable CMAP from the extensor digitorum brevis (EDB) muscle. There were fairly good positive correlations between the amplitudes of the sural CSNAPs and AH CMAPs (r = 0.66), and between the superficial peroneal CSNAP and EDB CMAP amplitudes (r = 0.63). There were no instances in which a CSNAP could be obtained from the sural or superficial peroneal sensory nerve, but a CMAP could not be recorded from the AH or EDB muscle. If the CMAP amplitudes of the AH and EDB muscles were reduced to less than 0.3 mV, usually a CSNAP could longer be recorded from the sural and superficial peroneal sensory nerves. The size of the CSNAP is a more sensitive measure compared to the CMAP in revealing the presence of distal sensorimotor axonopathy.     

Carpal tunnel syndrome: comparison of the compound muscle action potentials recorded at the thenar region from ulnar and median nerve stimulation

Thenar muscles are primarily innervated by the median nerve. However, compound muscle action potentials (CMAPs) evoked by ulnar nerve stimulation can be recorded at the thenar region due to proximity of some ulnar-innervated muscles, and from volume conduction events. This study was to determine if loss of thenar muscle mass from carpal tunnel syndrome (CTS) could alter the size of ulnar CMAPs obtained at the thenar region, because of changes in the physical surroundings and electrical conductivity. Supramaximal CMAPs were recorded over the thenar eminence to electrical stimulation of the ulnar nerve at the wrist and median nerve at the palm in 102 hands with CTS. Needle EMG was done in the thenar muscles. Severity of needle EMG abnormality was negatively correlated with median-evoked CMAP amplitude (r = -0.76), but not with ulnar-evoked CMAP amplitude (r = -0.12). There was no correlation between the absolute amplitudes of the median and ulnar CMAPs (r = -0.13). Needle EMG abnormality had modest negative correlation (r = -0.43) with median/ulnar CMAP amplitude ratio. Mean median/ulnar CMAP amplitude ratios for normal EMG and for mild, moderate, and severe needle EMG abnormalities were 3.72, 3.31, 1.56, and 0.37, respectively. The absolute amplitude of the ulnar CMAP recorded at the thenar area does not seem to be influenced significantly by the degree of thenar muscle loss (atrophy) from median nerve pathology. However, if the median/ulnar CMAP amplitude ratio falls below 0.5, the study suggests severe loss of motor units in the thenar muscles.     

Intestinal smooth muscle electrical potentials recorded from surface electrodes

Electrical recordings have been made from cutaneous electrodes placed on the trunk with a view to establishing the source of the slow potential changes which can be recorded. Cutaneous recordings have been made on 16 normal subjects in both the fasted and fed states, and in some cases simultaneous recordings have been made from electrodes attached to the end of a naso-gastric tube and sucked onto the stomach wall. Frequency and correlation analyses have been used to help interpret the potential changes. A significant frequency component at approximately 3 cycles/min (average 3·02±0·21/min) was found in 88% of the stretches of record analysed. It is shown that this is the basic electrical rhythm of the stomach. In a smaller number of recordings (28%), a significant frequency component at 10–12 cycles/min was found. The source of these rhythms has not been proven, but is likely to be from either the small or large bowel.     

Morphology of action potentials recorded from human nerves using microneurography

This study investigated the morphology of action potentials and the frequency of occurrence of the various waveforms encountered when using microneurography to record single-unit muscle afferent activity in humans. With 75% of the afferents recorded in this study (55 of 73 afferents), action potentials had a doublepeaked morphology. For action potentials with an initial, positive double peaked morphology, the relevant afferent conducts impulses past the microelectrode, with the second peak representing current fluctuations at the node of Ranvier proximal to the electrode. Accordingly, in the majority of recordings, the afferent is capable of conducting impulses to the spinal cord. The mean interpeak interval for these double-peaked units was 168 s (range 90–310 s). This represents marked prolongation of conduction time across the impaled internode. When the interpeak interval was relatively short (90–120 s), the double peaked morphology could be recognized only if the low pass filter was high (10 kHz). The probability of recording a double peaked unit was the same whether the recording was acquired early or late in a 3-h experiment. Conduction block developed in 6 of 73 single units during the recordings. These findings indicate that the majority of isolated single afferents and, indeed, the majority of afferents within the relevant fascicle are capable of transmitting impulses across the recording site, even though conduction across the impaled internode is slow. Conduction block due to direct injury or pressure is relatively uncommon.     

Facilitation of motor evoked potentials in the anterior tibial muscle by repetitive subthreshold electrical stimulation

Subthreshold electrical stimulation with an intensity less than the threshold for evoking M-waves is applied repetitively to the common peroneal nerve via surface electrodes. The stimulation intensity is varied by adjusting the pulse width from 0 to 240 μs, while the pulse interval (40 ms) and current amplitude are kept constant. Single magnetic stimuli are applied to the motor cortex using a circular coil. Motor evoked potentials are recorded from the anterior tibial muscle in six normal subjects for various subthreshold stimulation intensities. Signal processing (filtering in the time and frequency domains) removes the artifact caused by the subthreshold electrical stimulation from the motor evoked potential. Statistically significant motor evoked potential facilitation (p<0.05) is observed for pulse widths ranging from 72 to 240 μs in all the tested subjects. A pulse width corresponding to 90% of the electrical threshold facilitated the motor evoked potential in five of the six subjects.     

Far-field recording of compound muscle action potentials produced during brachial plexus stimulation.

In 15 normal subjects, compound muscle action potentials (CMAPs) produced during electrical stimulation of the brachial plexus were obtained at a distance (far field) from their sources (upper-limb muscles). These far-field CMAPs were recorded at the forehead, neck, and tip of the middle finger and were referred to a relatively inactive region (contralateral forearm or knee). At the proximal recording site (neck or forehead), the far-field CMAPs were generally triphasic in appearance when all of the muscles of the upper limb were activated during brachial plexus stimulation. The initial component was a negative wave; this was followed by a positive component which terminated in a negative potential. At the distal recording site (tip of middle finger), the far-field CMAPs usually had a biphasic appearance, with an initial positivity followed by a period of negativity. The contours of the major components of the far-field CMAPs were frequently interrupted by a series of small amplitude negative and positive peaks or inflections.     

The characteristics of spontaneous and evoked action potentials recorded from the rabbit''s uterine nerves

1. A method is described for recording in vivo the action potentials of afferent and efferent fibres in whole nerves supplying the rabbit's uterus.

2. Examination of these nerves under the light microscope and the electron microscope showed them to be composed almost entirely of non-myelinated fibres.

3. Two types of spontaneous action potential were observed; one travelled at about 4 m/sec and probably came from the myelinated fibres, the other travelled at 0·4-1·4 m/sec and certainly came from non-myelinated fibres.

4. The efferent fibre spikes were shown to be faster and higher than the spikes from uterine afferent fibres, but slower and smaller than spikes from broad ligament afferent fibres.

5. Apart from differences in conduction velocity and height, all spikes were basically similar, lasting about 1·5 msec. The height was related to the square of the velocity. Some more complex spikes were also observed.

6. The compound action potential evoked by stimulation of the uterine nerve had three peaks, conducted at 1·3, 0·8 and 0·6 m/sec, respectively, and thought to correspond to the fast afferent fibres, the efferent fibres and the slow afferent fibres, respectively. There were also some late peaks due to reflexion of the antidromic action potentials from the ganglion cells.

7. Stimulation of the hypogastric nerve also evoked a compound action potential in the uterine nerves. Stimulation of the pelvic nerves had no effect.

8. By means of ganglion blocking agents, the uterine ganglia were shown to lie in the pelvic plexus, peripheral to the hypogastric nerve, but central to the uterine nerves.

9. It is argued that the spontaneous action potentials came from individual fibres rather than Remak bundles, and that the recording technique used detected the activity of all but the smallest fibres.

     

Relationship between the size of the recording electrodes and morphology of the compound muscle action potentials

To determine the effect of recording electrode size on the compound muscle action potentials (CMAPs), maximally evoked CMAPs were obtained from the abductor digiti minimi muscle in ten normal subjects with circular-shaped surface recording electrodes of different diameters (4, 6, 8, 10, 12, and 14 mm). The electrodes were concentrically located on the same recording point on the muscle at its endplate zones when obtaining the CMAPs. The CMAP onset latency remained unchanged for all recording electrode sizes. The CMAP amplitude decreased with increasing electrode size. The mean absolute amplitude difference between CMAPs recorded with a 4-mm diameter electrode and those with a 14-mm diameter electrode was 1.22 mV, representing an amplitude decrement of 10% with this increase in electrode size. The general shape or morphology of the CMAPs was relatively well preserved.     

Volume-conducted or "far-field" compound action potentials originating from the intrinsic-hand muscles

When a recording electrode is situated at a relatively far and roughly equal distance from multiple muscle action potential (MAP) sources such as the intrinsic-hand muscles (far-field recording), the individual MAPs generated by muscle fibers may contribute to the resultant compound potential with nearly equal weights depending on the orientation of the individual MAP fields. This is in contrast to recording MAPs directly over the muscle (near-field recording), in which the resultant potential primarily reflects the MAPs that were near the recording electrode. Far-field recorded compound MAPs(CMAPs) thus may provide another dimension or perspective into viewing the overall spatio-temporal relationship among MAPs generated by a large number of muscle fibers. In this study, we described CMAPs from intrinsic-hand muscles that were recorded at a distance from their potential sources.     

Incremental responses of compound muscle action potentials in normal rats produced by repeated electrical stimulation

The purpose was to clarify the mechanism of normal incremental amplitude responses in compound muscle action potentials (CMAPs) and single fiber action potentials (SFAPs) produced by repeated high frequency electrical stimulation. Whereas the amplitude response of CMAPs and SFAPs increased during stimulation at 20 c.p.s. of the right flexor carpi ulnaris muscles of 50 rats, their durations gradually decreased. Incremental responses of the CMAPs and SFAPs evoked by repeated stimulation were suppressed in 25 rats by 30 mg/kg of magnesium sulfate and in 25 rats by 120 mg/kg of dantrolene sodium. We speculate that the incremental responses of CMAPs and SFAPs evoked by repeated stimulation were suppressed by the unchanged shape of the muscle after the injection of those chemicals.     

Post-exercise facilitation of compound muscle action potentials evoked by transcranial magnetic stimulation in healthy subjects

Post-exercise facilitation (PEF) of motor evoked potentials (MEPs) was studied by transcranial magnetic stimulation in 15 healthy subjects following standardized and controlled isometric contraction of the biceps brachii muscle. PEF was highly dependent on the time delay (TD) from muscle relaxation to delivery of the magnetic stimulus and only to a minor degree on the duration of the maintained muscular contraction of 2, 4, and 6 s. In addition, PEF was unaffected by the contraction levels of 25%, 50%, and 100% of maximal voluntary contraction (MVC). There was a linear relationship between the log amplitude of the post-exercise MEPs and the TD. The time point at which PEF had vanished was calculated to be 15.2 s. In order to challenge the question whether segmental and/or suprasegmental mechanisms are primarily responsible for PEF, MEPs and H-reflexes were recorded from the soleus muscle following a sustained plantar flexion at the ankle joint in three healthy subjects. PEF of MEPs was present at a TD of 1000 ms following a sustained contraction of 6 s at a level of 50% of MVC. It was accompanied by a pronounced decrease in the soleus H-reflex amplitude at a TD of 1000 ms.     

Influence of a self-etching primer on compound nerve action potentials

The aim of this study was to evaluate the effect of self-etching primers on nerve conductance. A self-etching primer (One Up Bond F) which combines etching and bonding in one step, and a fifth-generation bonding agent (Prime&Bond NT ) were tested. Isolated rat sciatic nerves were placed between two platinum electrodes in a bath containing Tyrode solution. The bonding agents were brought into contact with the nerves and the evoked compound action potentials (CAP) were recorded before and after contact with the materials. One Up Bond F caused total inhibition of the CAP within an average time of 7 min. All CAPs in this group were blocked irreversibly. As with Prime&Bond NT, the reduction in CAP was 45.9% after an application time of 15 min, after which readings were terminated. Recovery of the CAP in this group were maintained after rinsing with fresh tyrode solution. One Up Bond F elicited faster blocking of nerve conductance under the conditions of this model. In the context of dentin desensitization with bonding agents, the self-etching primer may be more effective, clinically.     

Models and analysis of compound nerve action potentials

Upon electrical stimulation of a peripheral nerve, a compound action potential (CAP) can be recorded, a procedure that is widely used to study the functional condition of a nerve. The CAP provides relevant information about such parameters as the number of active myelinated fibers and their propagation velocities. This paper reviews methods of model-based CAP analysis that have emerged during the last decade. First, the basic model formulation for the CAP as resulting from a linear summation of the underlying single fiber action potentials (SFAPs) is discussed. Subsequently, a survey is given of volume conductor model approaches for the prediction of the SFAPs. Attention is then concentrated on the "inverse" problem of extracting relevant nerve parameters from experimentally recorded CAPs. Finally, the applicability of such methods is discussed and a direction for future developments is outlined.