A study of conduction velocity in nonmyelinated nerve fibers.

By treating a nonmyelinated nerve fiber as a continuous cable consisting of three distinct zones (Resting, transitional, and excited), the following mathematical expression was derived: (formula: see text) where v is the conduction velocity, d the diameter of the fiber, R the resistance of the membrane of unit area at the peak of excitation, rho the resistivity of the medium inside the fiber, and C the capacity of membrane per unit area. The validity of this expression was demonstrated by using squid giant nerve fibers intracellularly perfused with dilute salt solutions. The relationship between these results and previous theories and experiments on conduction velocity is discussed.     

Estimation of the conduction velocity distribution of human sensory nerve fibers.

A new method for estimating the distribution of conduction velocities (DCV) of peripheral nerve fibers has been developed. It also enables estimation of single nerve fiber action potential (SFAP), which agrees with the physiological knowledge. Two compound nerve action potentials (CAPs) elicited by electrical stimulation of a nerve bundle were recorded at different conduction distances. The distances between the stimulation and recording electrodes were measured on the skin surface along the nerve bundle. Starting with an arbitrary SFAP, the first estimated DCV was calculated from a CAP by the regularized non-negative least squares method. The next SFAP was then calculated by deconvolution of the other CAP and the estimated DCV. A lowpass filter with an appropriate cutoff frequency was used to obtain better conversion. The process was iterated until the CAP error defined as /CAP(calculated)-CAP/(2) was small enough. The conduction distances contained errors in measurement, especially in the distal segment, that distorted the estimated results. The Fibonacci search, therefore, was adopted to optimize the distance according to the CAP error. The accuracy of this method was demonstrated by a simulation study performed with two CAPs calculated from an arbitrary bimodal DCV and a biphasic SFAP to which a Gaussian white noise was added. The reliability of this method was checked in normal subjects by recording a pair of CAPs elicited by stimulation of the median nerve at the wrist and the elbow.     

Fenestration in the myelin sheath of nerve fibers of the shrimp: A novel node of excitation for saltatory conduction

Giant nerve fibers of the shrimp family Penaeidae conduct impulses at the velocity highest among all animal species (∼210 m/s; highest in mammals = 120 m/s). We examined these giant and other small nerve fibers morphologically using a differential interference contrast microscope as well as an electron microscope, and found a very specialized form of excitable membrane that functions as a node for saltatory conduction of the impulse. This node appeared under the light microscope as a characteristic pattern of concentrically aligned rings in a very small spot of the myelin sheath. The diameter of the innermost ring of the node was about 5 μm, and the distance between these nodes was as long as 12 mm. Via an electron microscope, these nodes were characterized by a complete lack of the myelin sheath, forming a fenestration that has a tight junction with an axonal membrane. Voltage clamp measurements by a sucrose gap technique demonstrated that the axonal membrane at these fenestration nodes is exclusively excitable and that the large submyelinic space is a unique conductive pathway for loop currents for saltatory conduction through such fenestration nodes. © 1996 John Wiley & Sons, Inc.     

Ionic currents of the nodal membrane underlying the fastest saltatory conduction in myelinated giant nerve fibers of the shrimp Penaeus japonicus.

The myelinated giant nerve fiber of the shrimp, Penaeus japonicus, is known to have the fastest velocity of saltatory impulse conduction among all nerve fibers so far studied, owing to its long distances between nodal regions and large diameter. For a better understanding of the basis of this fast conduction, a medial giant fiber of the ventral nerve cord of the shrimp was isolated, and ionic currents of its presynaptic membrane (a functional node) were examined using the sucrose-gap voltage-clamp method. Inward currents induced by depolarizing voltage pulses had a maximum value of 0.5 microA and a reversal potential of 120 mV. These currents were completely suppressed by tetrodotoxin and greatly prolonged by scorpion toxin, suggesting that they are the Na current. Both activation and inactivation kinetics of the Na current were unusually rapid in comparison with those of vertebrate nodes. According to a rough estimation of the excitable area, the density of Na current reached 500 mA/cm2. In many cases, the late outward currents were induced only by depolarizing pulses larger than 50 mV in amplitude. The slope conductance measured from late currents were mostly smaller than that measured from the Na current, suggesting a low density of K channels in the synaptic membrane. These characteristics are in good harmony with the fact that the presynaptic membrane plays a role as functional node in the fastest impulse conduction of this nerve fiber.     

On the relation between fibre diameter and conduction velocity in myelinated nerve fibres

Myelinated fibres less than 1 micrometer in diameter are rare in the peripheral nervous system; but fibres down to 0.2 micrometer in diameter exist in the central nervous system. These observations are consistent with Rushton's theory on the effects of fibre size on conduction in myelinated nerve when the different processes of myelination in the peripheral and central nervous systems are taken into account.     

Continuous conduction of impulses in peripheral myelinated nerve fibers

1. Conduction of impulses in peripheral myelinated fibers of a nerve trunk is a continuous process, since with uninjured nerve fibers: (a) within each internodal segment the conduction time increases continuously and linearly with increasing conduction distance; (b) the presence of nodes of Ranvier does not result in any detectable discontinuity in the conduction of the impulse; (c) the ascending phase of the spike always has an S shape and never presents signs of fractionation; (d) the shape and magnitude of the spike are constant at all points of each internodal segment. 2. Records have been presented of the external logitudinal current that flows during propagation of an impulse in undissected single nerve fiber (Fig. 6). 3. Propagation of impulses across a conduction block occurs with a readily demonstrable discontinuity.     

The effect of temperature on conduction velocity in human muscle fibers

The effects of variation of intramuscular temperature (T) on conduction velocity (CV) of the action potential along single human muscle fibers of the biceps brachii was studied in situ in 15 normal volunteers (mean age 39 years, range 21–62 years). Cooling was obtained by direct application of ice over a rectangular skin region including the stimulating and recording area. The intramuscular T was monitored by a needle thermocouple (copperconstantane). In all the 24 muscle fibers studied, a linear relationship was observed between CV and T. The slopes of the regression lines, ranging between 0.190 and 0.079 m/s, were positively correlated with the starting CV at 36°C ranging between 2.2 and 5.2 m/s. If conduction changes are expressed as a percentage of the basal CV at 36°C, the CV/T coefficient is the same for all the fibers and independent of the individual CV: 3.4% of CV/°C.     

Some physiological characteristics of myelinated and unmyelinated fibers with very low conduction velocity

The compound action potential arising in response to supramaximal stimulation of Aδ- or C-fibers of a cat cutaneous nerve (the saphenous nerve) was investigated by methods improving the signal/noise ratio in the record of the unit evoked response. By the use of optical and computer (BÉSM-3M) methods of coherent signal accumulation followed by averaging, potentials of nerve fibers ranging in amplitude from 20 to 0.05 µV and in duration from 10 to 0.4 msec were distinguished from the apparatus noise. A continuous distribution of nerve fibers by conduction velocity was found over the range from 80 to 0.15 m/sec. The conditions of appearance of low-amplitude action potentials of nerve fibers with a low conduction velocity are discussed.     

On the formal theory of nerve conduction

A general solution of the formal nerve conduction problem is given. As illustrations of the general method, the capacitative single-factor and the non-capacitative Lapicque problems are solved. Comparisons between velocity formulae for capacitative and non-capacitative models indicate that previously determined non-capacitative velocities are considerably too high.     

On the cable theory of nerve conduction

Conduction of an impulse in the nonmyelinated nerve fiber is treated quantitatively by considering it as a direct consequence of the coexistence of two structurally distinct regions, resting and active, in the fiber. The profile of the electrical potential change induced in the vicinity of the boundary between the two regions is analyzed by using the cable equations. Simple mathematical formulae relating the conduction velocity to the electrical parameters of the fiber are derived from the symmetry of the potential profile at the boundary. The factors that determine the conduction velocity in the myelinated nerve fiber are reexamined.