Conduction velocities and fiber diameters in a cutaneous nerve of the pigeon

Summary The characteristics of fibers of a cutaneous nerve supplying the wing skin of the pigeon have been investigated with electrophysiological and electron microscopic techniques.Recordings of the compound action potential showed four distinct peaks with conduction velocities of about 30 m/s, 12 m/s, 4 m/s and 0.5 m/s.From electron micrographs both fiber diameters and thickness of myelin sheath were assessed and used as criteria for segregating various fiber populations. Altogether four groups could be discerned: large thickly myelinated fibers, small thickly myelinated fibers, small thinly myelinated fibers, and unmyelinated or C-fibers. The subdivision of the thickly myelinated fibers into two populations is evidenced mainly by corresponding peaks in the compound action potential. The thinly myelinated fibers with a mean diameter of 2 m contributed about 90% of all myelinated fibers in this nerve.When comparing fiber dimensions and conduction velocities of this avian nerve with those of mammalian cutaneous nerves, the lower CV's of avian nerve fibers can be explained by smaller diameters and thinner myelin sheaths.The results of this investigation are a prerequisite for latency considerations in central somatosensory pathways in birds.Abbreviations CAP compound action potential - CV conduction velocity - D fiber diameter - d axon diameter - g ratio d/D - m thickness of myelin sheath     

Conduction in ulnar nerve bundles that innervate the proximal and distal muscles: a clinical trial

Background  

This study aims to investigate and compare the conduction parameters of nerve bundles in the ulnar nerve that innervates the forearm muscles and hand muscles; routine electromyography study merely evaluates the nerve segment of distal (hand) muscles.     

Conduction along myelinated and demyelinated nerve fibres during the recovery cycle: Model investigations

The membrane excitability changes as well as the underlying mechanisms of these changes in a normal and in a systematically paranodally demyelinated nerve fibre have been investigated by paired stimulation during the first 30 ms of the recovery cycle. The ionic current kinetics determining the observed changes in the action potential parameters are presented also. The simulation of the conduction in the normal fibre is based on the Frankenhaeuser and Huxley (1964) and Goldman and Albus (1968) equations, while in the case of a demyelinated fibre according to the same equations modified by Stephanova (1988a). It has been shown for the demyelinated membrane that increased demyelination increases both the threshold current for the second potential as well as the absolute refractory period. With increasing interpulse interval, the subnormality of the membrane excitability is followed by supernormality in the case of the demyelinated membrane. For the recovery cycle of 30 ms under consideration no supernormality of the normal membrane excitability is obtained. With interpulse interval from 8.8 to 10.9 ms, the highest degree of demyelination (l=30 m) is accompanied by a refractory period of transmission. The membrane properties of the normal and demyelinated fibres recover 20 ms after the first pulse. For short interpulse intervals, the amplitude of the second action potential is decreased, and a slower propagation velocity is obtained. The most sensitive phenomenon is the excitability of the demyelinated membrane, which remains unrecovered 30 ms after the first pulses has been applied.     

The lateral root of the ulnar nerve

In 158 brachial plexuses the origin of the fibers of the ulnar nerve-whether only from the medial or also from the lateral fascicle-was investigated. A lateral root was found in 56%. This lateral root may either be accompanied by fibers of the median nerve (type 1) or may run separately (type 2). Where this root crosses the medial root of the median nerve, either a small minority of fibers of the latter nerve may run behind the ulnar fibers (type a), or all median fibers are in front of them (type b). Considering the relation 56:44% between ulnar nerves with and without a lateral root both possibilities have to be considered as normal variations, none as a variety. In analogy to the term 'median loop' the term 'ulnaris loop' is suggested for specimens with a lateral root.     

Conduction of a nerve impulse along a myelineated fiber while varying the membrane properties of nodes of Ranvier

The mathematical model of a frog myelinated axon [1, 3] has been used to study the dependence of the conduction velocity (theta) on the parameters of the nodal membrane: its capacity (CN), leakage conductance (gl), sodium and potassium maximum conductances (gNa, gK). Calculations have shown that theta practically does not depend on gK:theta raises only by 3% when gK is diminished to zero. The increase of theta with reducing of gl or CN can be described by formulae: theta (m/s) = 19-300.gl (muS) or theta = 16-1.87.CN(pF) (Fig 1,2). Theta depends strongly on gNa:theta approximately equal to (gNa)7/8 (Fig 3). Due to the capacity of Na channels (determined by the gating charge movement) there is a maximum in the relation between the number of Na channels per node and the theta (Fig 4). A clear-cut maximum does exist also in the curve relating theta to the nodal membrane area (Fig. 5a). The position of the maximum and the shape of this curve depend on gl and CN but not on gNa (Fig. 5b).     

Conduction along myelinated and demyelinated nerve fibres with a reorganized axonal membrane during the recovery cycle: model investigations

The changes in the excitability of the reorganized axonal membrane in myelinated and demyelinated nerve fibres as well as the causes conditioning such changes have been investigated by paired stimulation during the first 30 ms of the recovery cycle. The variations of the action potential parameters (amplitude and velocity) are traced also. The simulation of the conduction along the normal fiber is based on the Frankenhaeuser and Huxley (1964) and Goldman and Albus (1968) equations, while the demyelination is considered to be an elongation of the nodes of Ranvier. The axonal membrane reorganization is achieved by means of potassium channel blocking and increase of the sodium-channel permeability. It is shown that potassium channels block decreases membrane excitability for the myelinated and demyelinated fibres in the cases of initial and paired stimulation. With increasing sodium-channel permeability on the background of the blocked potassium channels, the membrane excitability is increased. For the fibres with a reorganized membrane, a supernormality of the membrane excitability is obtained, the latter remaining unrecovered during the 30 ms cycle under investigation. The supernormality of the excitability grows from the demyelinated fibre without reorganized membrane to the demyelinated fibre with reorganized one. For short interstimulus intervals, the second action potential propagates along the fibres with a reduced velocity and a decreased amplitude. No supernormality of the potential parameters (amplitude, velocity) is observed during the cycle up to 30 ms. The membrane properties of the myelinated and demyelinated fibres with blocked potassium channels recover in the interval from 15 to 20 ms depending on whether the sodium channels' increase of the permeability is added on the background of the blocked potassium channel or not. In the recovery cycle, the axonal membrane reorganization leads to an improvement of the conduction along most severely demyelinated fibres.     

Conduction velocity of the ulnar nerve, length of the forearm and body weight as correlates of gestational age in the infant pigtail macaque.

The conduction velocity of peripheral nerves is an indication of their growth. Ulnar nerve conduction velocity, forearm length, postpartum age and weight were measured in 25 monkeys (Macaca nemestrina) to determine the best way to estimate gestational age. Correlation analysis showed that while conduction velocity is strongly correlated with gestational age (r = 0.68), both forearm length (r = 0.91) and weight (r = 0.94) are more strongly correlated. Of the three simple measurements made, weight provides the best estimate of gestational age.     

Motor and sensory nerve conduction velocities in Yucatan minipigs

Motor and/or sensory conduction velocities are used to assess peripheral nervous system disorders. Although the miniature pig represents a model of choice for long-term pharmacological experimentation, no study has so far been reported on this model in relation to the measurement of nerve conduction velocities. We developed the present technique and applied it to 34 3-18-month-old Yucatan minipigs. Motor and sensory conduction velocities were measured using the anterior tibial nerve and the internal plantar nerve, a branch of the posterior tibial nerve, respectively. The nerve conduction velocity data of motor (MNCV) and sensory (SNCV) nerves, together with the amplitude of the sensory nerve signal, were logarithmically dependent on the age of the tested animals (r(2)=0.92, 0.81 and 0.76, respectively). The mean values of MNCV and SNCV were 70.9 +/- 1.1 and 67.9 +/- 0.2 m/s, respectively, at the age of 16 months for these miniature pigs. In order to validate this model, we compared it with other known models when the velocities reached a plateau at the end of the study. These values were found to be higher than those in humans or rats, but are comparable to those of the baboon, one of the best large animal models for human pathologies. Because the physiology and metabolism of the minipig resemble those of humans, and due to its long lifetime, this animal represents a good model for studying the development of neuropathology.     

Conduction of hyperpolarization along hamster feed arteries: augmentation by acetylcholine

The conduction of vasodilation along resistance vessels has been presumed to reflect the electrotonic spread of hyperpolarization from cell to cell along the vessel wall through gap junction channels. However, the vasomotor response to acetylcholine (ACh) encompasses greater distances than can be explained by passive decay. To investigate the underlying mechanism for this behavior, we tested the hypothesis that ACh augments the conduction of hyperpolarization. Feed arteries (n = 23; diameter, 58 +/- 4 microm; segment length, 2-8 mm) were isolated from the hamster retractor muscle, cannulated at each end, and pressurized to 75 mmHg (at 37 degrees C). Vessels were impaled with one or two dye-containing microelectrodes simultaneously (separation distance, 50 microm to 3.5 mm). Membrane potential (E(m)) (rest, approximately -30 mV) and electrical responses were similar between endothelium and smooth muscle, as predicted for robust myoendothelial coupling. Current injection (-0.8 nA, 1.5 s) evoked hyperpolarization (-10 +/- 1 mV; membrane time constant, 240 ms) that conducted along the vessel with a length constant (lambda) = 1.2 +/- 0.1 mm; spontaneous E(m) oscillations (approximately 1 Hz) decayed with lambda = 1.2 + 0.1 mm. In contrast, ACh microiontophoresis (500 nA, 500 ms, 1 microm tip) evoked hyperpolarization (-14 +/- 2 mV) that conducted with lambda = 1.9 +/- 0.1 mm, 60% further (P < 0.05) than responses evoked by purely electrical stimuli. These findings indicate that ACh augments the conduction of hyperpolarization along the vessel wall.     

Conduction velocity of the human phrenic nerve in the neck

PurposeTo measure phrenic nerve conduction velocity in the neck in humans.ScopeWe studied 15 healthy subjects (9 men, 32.4 ± 6.7). We performed bipolar electrical phrenic stimulation in the neck, from a distal and a proximal stimulation site, and recorded diaphragm electromyographic responses on the surface of the chest. The ratio of the between-site distance to the latency difference provided phrenic velocities. Ulnar motor velocity was assessed similarly. In addition, five homogeneous patients with Charcot-Marie-Tooth disease type 1A (CMT1A) were studied for validation purposes. We obtained diaphragmatic responses from the two stimulation sites in all cases. The distal latencies (anterior axillary line recording) were 6.51 ± 0.63 ms (right) and 6.13 ± 0.64 ms (left). The minimal between site distance was 39 mm. Phrenic motor velocity was 55.2 ± 6.3 m s^?1 (right) and 56.3 ± 7.2 m s^?1 (left). In CMT1A, phrenic velocities were 17.1 ± 8.1 m s^?1 (from 7 to 32 m s^?1) and were similar to ulnar and median velocities.ConclusionsPhrenic nerve velocities can be estimated in humans and compare with upper limb motor conduction velocities. This should refine the investigation of phrenic function in peripheral neuropathies.     

Conduction in bundles of demyelinated nerve fibers: computer simulation

This study presents a model of action potential propagation in bundles of myelinated nerve fibers. The model combines the single-cable formulation of Goldman and Albus (1967) with a basic representation of the ephaptic interaction among the fibers. We analyze first the behavior of the conduction velocity (CV) under the change of the various conductance parameters and temperature. The main parameter influencing the CV is the fast sodium conductance, and the dependence of CV on the temperature is linear up to 30 degrees C. The increase of myelin thickness above its normal value (5 microm) gives a slight increase in CV. The CV of the single fiber decreases monotonically with the disruption of myelin, but the breakdown is abrupt. There is always conduction until the thickness is larger than 2% of its original value, at which with at this point a sharp transition of CV to zero occurs. Also, the increase of temperature can block conduction. At 5% of the original thickness there is still spike propagation, but an increase of 2 degrees C causes conduction block. These results are consistent with clinical observations. Computer simulations are performed to show how the CV is affected by local damage to the myelin sheath, temperature alterations, and increased ephaptic coupling (i.e., coupling of electrical origin due to the electric neutrality of all the nerve) in the case of fiber bundles. The ephaptic interaction is included in the model. Synchronous impulse transmission and the formation of "condensed" pulse states are found. Electric impulses with a delay of 0.5 ms are presented to the system, and the numerical results show that, for increasing coupling, the impulses tend to adjust their speed and become synchronized. Other interesting phenomena are that spurious spikes are likely to be generated when ephaptic interaction is raised and that damaged axons suffering conduction block can be brought into conduction by the normal functioning fibers surrounding them. This is seen also in the case of a large number of fibers (N=500). When all the fibers are stimulated simultaneously, the conduction velocity is found to be strongly dependent on the level of ephaptic coupling and a sensible reduction is observed with respect to the propagation along an isolated axon even for low coupling level. As in the case of three fibers, spikes tend to lock and form collective impulses that propagate slowly in the nerve. On the other hand, if only 10% of fibers are stimulated by an external input, the conduction velocity is only 2% less than that along a single axon. We found a threshold value for the ephaptic coupling such that for lower values it is impossible to recruit the damaged fibers into conduction, for values of the coupling equal to this threshold only one fiber can be restored by the nondamaged fibers, and for values larger than the threshold an increasing number of fibers can return to normal functioning. We get values of the ephaptic coupling such that 25% of axons can be damaged without change of the collective conduction.     

A surgical approach to the motor branch of the ulnar nerve

Isolated injury to the motor branch of the ulnar nerve is a relatively rare injury, often initially misdiagnosed. If repair is attempted through the original laceration without complete motor branch exposure, results can be less than satisfactory. A recent case illustrates this injury and provides us with an opportunity to review the surgical anatomy of the motor branch of the ulnar nerve. The surgical approach to the motor branch has been detailed and specifically emphasizes complete motor branch exposure from the main ulnar nerve trunk to the most distal motor branch entry into the adductor pollicis muscle. This approach permits definition of the exact level of the nerve injury, preservation of any intact proximal fine motor branches, and facilitates the mechanics of nerve repair.     

Recurrent anterior dislocation of the ulnar nerve at the cubital tunnel

Recurrent anterior dislocation of the ulnar nerve at the cubital tunnel is reported in two patients. This was due to traumatic attenuation of the flexor carpi ulnaris retinaculum. The mechanism of injury in both patients was a fall with the shoulder abducted and the elbow acutely flexed. Both patients had relief of their neurologic symptoms following anterior submuscular transposition of the ulnar nerve.