Impulses at the artifactual nerve end

Neuronal survival or death after dendrite transection close to the perikaryon: correlation with electrophysiologic, morphologic, and ultrastructural changes

We investigated the probability of survival of mouse spinal neurons in monolayer cultures after transection lesions of dendrites made within 400 microns of the perikarya. Based on a total of 650 lesioned neurons, the following observations were made. First, neuronal survival is a function of lesion distance from the perikaryon and of process diameter at the lesion site. For an average lesion diameter of 3 microns, dendrite transections at 50 microns, 100 microns, and 150 microns were associated with survival probabilities of 30%, 53%, and 70%, respectively. Second, the fate of the injured cells was definitely established 24 hours after injury and very likely was determined as early as 2 hours. Third, early stages of deterioration leading to cell death were associated with cytoplasmic phase brightness on light microscopy, correlating with an appearance of numerous, small, electron-lucent vacuoles and swollen mitochondria on electron microscopy. The cytoplasm of these moribund cells stained darkly and contained no visible microtubules or neurofilaments. Fourth, the magnitude and time course of injury potentials recorded at the somata were a function of the lesion distance and did not return to prelesion levels within 30 minutes after transection. Fifth, at 24 hours after injury, the average membrane potential of lesioned neurons was 8% below that of control neurons. Sixth, at a lesion distance of approximately 300 microns both the injury potential and the probability of cell death approach zero. We conclude that, in the model system used, neuronal survival after dendrite amputation depends on physical parameters of the lesion that determine the magnitude of the injury current reaching the soma. Survival is not assured if the injury is inflicted within 250 microns of the cell body, and cell death is likely for lesions within 50 microns of the soma. The below-normal membrane potentials at 24 hours after injury suggest a possible greater vulnerability of recovering neurons to secondary insults. The characteristic mitochondrial disruption and loss of microtubules implies that the calcium component of the injury current contributes to cell death.

Components of the plasma membrane of growing axons. III. Saxitoxin binding to sodium channels

The density of sodium channels was measured in growing and mature axons of the olfactory nerve of the bullfrog, using as a probe the drug saxitoxin (STX). The toxin binds to control nerves from adult animals in a saturable manner with a dissociation constant of approximately 23 nM at 4 degrees C and a capacity of 72 fmol/mg wet weight, equivalent to about five sites per square micrometer of axolemma. In growing nerves, obtained from adult frogs 4-5 wk following removal of the original nerve, the STX-binding capacity per wet weight of tissue is markedly reduced, to approximately 25% of control values, and appears to decrease in the proximodistal direction. STX-binding data, expressed as STX/mg wet weight, was converted to STX/micron 2 of axolemma using stereologically derived values of membrane area per milligram wet weight of nerve. The axolemmal content (area/mg wet weight) of all regions of growing nerve is substantially decreased compared to controls, but increases in the proximodistal direction by 60%. These changes in axolemmal area result in calculated STX receptor densities (per unit axolemmal area) which, in distal regions, are approximately at the level of the mature nerve and, in proximal regions, are actually increased above controls by 50 to 70%. Upon comparing the axolemmal density of intramembrane particles, reported in the companion paper, with the calculated density of STX receptors in both mature and growing nerves, we find a correlation between STX receptors and intramembrane particles with diameters of 11.5-14.0 nm. The growing axon's gradient of sodium channels and the shift from this gradient to a uniform distribution in the mature axon suggest (a) that sodium channels are inserted into the perikaryal plasmalemma and diffuse from there into the growing axolemma, and (b) that the axolemma undergoes functional maturation during growth.

Chemical and morphological studies on garfish peripheral nerves

Gangliosides were extracted, separated by thin layer chromatography, and quantitated in three cranial nerves of the garfish (Lepisosteus osseus): the completely unmyelinated olfactory nerve (OLF), and two nerves composed of both myelinated and unmyelinated fibers, viz., the main trunk of the maxillary nerve (MAX) and a branch of the maxillary nerve (BR-MAX). Morphological studies on each of these nerves were done to verify that the OLF had been excised free of any contamination from the accompanying myelinated BR-MAX, to aid in the interpretation of the biochemical findings, and to clarify the nature of the OLF supporting cell. The chief chemical findings were (1) documentation of the presence of gangliosides in nerves previously thought not to contain them, (2) demonstration that gangliosides can be associated with unmyelinated nerves, (3) demonstration of a greater proportion of one simple ganglioside (G-6) in the OLF but greater proportions of two complex gangliosides (G-2 and G-0) in the MAX and BR-MAX, and (4) that either GM4 or a variant of the GM3 is present in OLF. The morphological findings with respect to the difficulty of ascribing only peripheral nervous system characteristics to the OLF supporting cell are discussed in relation to the ganglioside band chromatographing slightly ahead of GM4 in the OLF.

Rate of movement and composition of rapidly transported proteins in regenerating olfactory nerve

In a previous study, three successive groups of regenerative fibers, growing initially at 5.8, 2.1, and 0.8 mm/day, were observed in the regenerating garfish olfactory nerve. In the present study, fast axonal transport in the most rapidly regenerating axons (phase I and II) has been examined. Rapid transport in phase I fibers occurs at a velocity of 208 +/- 9 mm/day at 23 degrees, a rate identical to that measured in intact nerves. This first phase of regenerating fibers represents only 3 to 5% of the original axonal population, but each fiber appears to contain 6 to 16 times more transported radioactivity than an axon in an intact nerve. Subcellular distribution of rapidly moving material in phase I and II fibers was closely related to the distribution obtained in intact nerves. Small but significant differences indicate a shift of the transported radioactivity from a heavier to a light axonal membranous fraction. This shift might be characteristic of the immature membrane of a growing axon. The polypeptide distribution of transported radioactivity was also very similar to that of a normal nerve, with most of the radioactivity associated with high-molecular-weight polypeptides.

Fiber analysis of the feline inferior cardiac sympathetic nerve

The left inferior cardiac nerves of nine adult cats were examined with the electron microscope. Electrical recordings were also made from four of these nerves. Most of the nerves contained between 25,000 and 40,000 unmyelinated fibers and 30 to 150 myelinated fibers. The diameters of the unmyelinated fibers ranged from 0.1 mu to 1.8 mu with mean diameters ranging from 0.47 to 0.78 mu in the various nerves. The myelinated fibers ranged in diameter from 1 mu to 9 mu with a mean difference of approximately 3 mu. Electrical recordings showed two peaks in both the C fiber and the Adelta fiber compound action potentials. Conduction velocities of the two C fiber peaks were approximately 0.7 and 0.9 m/sec. The two groups of A delta fibers had conduction velocities of approximately 7 and 16 m/sec.

A quantitative analysis of isotope concentration profiles and rapid transport velocities in the C-fibers of the garfish olfactory nerve

In the olfactory nerve of the long-nosed garfish (Lepisosteus osseus), unusually well-defined isotope concentration distributions can be established with the rapid transport process. Transport velocities of two profile loci can be accurately described and a quantitative profile analysis is possible after profile normalization. Results from such studies indicate that: (1) peak amplitudes decrease exponentially as a function of distance from the olfactory mucosa according to the equation p = 2130 exp (-0.109chi); (2) the wavefront base and the peak apex loci move at rates of 221 +/- 2 and 201 +/- 4 mm/day, respectively (at 23 degrees C), revealing a peak dispersion or broadening during transport; (3) the broadening is asymmetric with material shifting to the rear of the peak; (4) plateau regions are established behind the peak with material deposited by the peak; (5) only 20% of the total radioactivity in a cut nerve reaches the nerve terminals in the rapid transport peak while 80% is deposited along the axon; (6) profile areas from cut nerves decrease and lose 15% of their activity in 20 hr, while intact nerve profiles increase 10% in 16 hr due to continued somal contribution to the profile; (7) the displacement of the wavefront base (WFB) and peak apex (PA) profile loci can be described by the functions s(WFB) = (0.055T - 0.345)t - 1.43 s(PA) = (0.053T - 0.391)t - 2.71 (8) transport velocities are linear functions of temperature between 10 and 25 degrees C and increase 370% in that range. A linear extrapolation of the WFB and PA functions to 37 degrees C yields 410 and 377 mm/day, respectively.

Evidence that tetrodotoxin and saxitoxin act at a metal cation binding site in the sodium channels of nerve membrane

The effects of monovalent, divalent, and trivalent cations on the binding of tetrodotoxin and saxitoxin to intact nerves and to a preparation of solubilized nerve membranes have been examined. All eight divalent and trivalent cations tested, and the monovalent ions Li(+), Tl(+), and H(+) appear to compete reversibly with the toxins for their binding site. The ability of lithium to reduce toxin binding is paralleled by its ability to reduce tetrodotoxin-sensitive ion fluxes through the nerve membrane. We conclude that the toxins act at a metal cation binding site in the sodium channel and suggest that this site is the principal coordination site for cations (normally Na(+) ions) as they pass through the membrane during an action potential. The dissociation constant for Li(+) is 0.1-0.2 M and for Na(+) > 0.6 M, reflecting the weak binding necessary for rapid passage of sodium ions through the channel.

Isolation and characterization of plasma membrane fractions from garfish Lepisosteus osseus olfactory nerve

Garfish Lepisosteus osseus olfactory nerve, because of its large size and the unusually high concentration of axonal membrane, is an excellent source of axonal membrane. A procedure is described for the isolation of two types of plasma membranes from the nerve which are obtained in yields of about 20 mg (fraction I) and 1.5 mg (fraction II) per g of wet nerve. Both membrane fractions consist mostly of rounded membrane vesicles, with a unit membrane thickness of approximately 7.5 nm. The two membrane fractions are different in their lipid to protein ratios, Na-K ATPase activities, polypeptide patterns on sodium dodecyl sulfate (SDS) gel electrophoresis, and fatty acid compositions. They have similar phospholipid composition. On the basis of the relative concentration of axonal and Schwann cell plasma membranes in the nerve, the Na-K ATPase activities of the two membrane fractions and a comparison of the properties of the membrane fractions to those of squid and lobster nerve membrane preparations, fraction I seems to be the axonal membrane and fraction II the Schwann cell plasma membrane. Fraction I has a low protein to lipid ratio. Its polypeptide pattern on SDS gel appears to be much more complex as compared to that of fraction II membrane.

Ion fluxes through the sodium channels of garfish olfactory nerve membranes

1. The efflux of (22)Na from garfish olfactory nerves, after treatment with ouabain and equilibration with (22)Na, occurs in two components of almost equal size. One component represents efflux from the extracellular space and the other, much slower, component represents efflux from the intracellular space through the axonal membrane.2. The rate of efflux of (22)Na through the membrane is increased from 0.038 to 0.055 min(-1) by veratrine, and restored back to 0.036 min(-1) by the additional application of tetrodotoxin or saxitoxin (10(-7)M).3. 50% inhibition of this increased sodium ion flux occurs at tetrodotoxin and saxitoxin concentrations of 12 and 6 nM respectively.4. The tetrodotoxin-sensitive efflux of (22)Na is almost unchanged in nerves equilibrated with hypertonic 0.85 M-NaCl, whereas it is largely eliminated in nerves equilibrated with 0.85 M-LiCl. We interpret this to indicate that there exists within the sodium channel a specific metal cation site which is responsible for the co-ordination of the ions during their passage through the membrane. Lithium ions bind to this site relatively strongly, with a dissociation constant of 0.2-0.3 M, whereas sodium ions bind less strongly, with a dissociation constant greater than 0.75 M. Other evidence indicates that this site is the site at which tetrodotoxin and saxitoxin act.5. Batrachotoxin has a similar action to veratrine but is effective in concentrations at least 100 times lower.

Photoreceptor spike responses in the hardshell clam, Mercenaria mercenaria

Spikes were recorded from single axons in the siphonal nerve of the hardshell clam Mercenaria mercenaria which respond to dimming of light. No axons were found to respond to the onset, or increase, of illumination. In a dark-adapted state there is little or no ongoing spike activity. The responsive area of a single axon is a circle of approximately 85 microm diameter on the inner siphon wall. The number of spikes elicited at the off of constant-duration flashes grows as approximately the 0.4 power of flash intensity. For constant intensity and constant light-time fraction, the off-response increases with increasing duration at least up to 500 s duration. For long durations, the response grows as the logarithm of stimulus duration. Subthreshold light can suppress the off-response from preceding illumination. In a light-adapted state, the off-response is greater and its latency shorter than in the dark-adapted state. The fine structure of groups of cell processes thought to comprise the photoreceptor in Mercenaria is described. On the basis of morphological and physiological findings it is suggested that phototransduction occurs in the fine distal processes of the axons from which we have recorded. Axonal processes were found to contain well organized pentalaminar whorls which may be the site of photo-pigment concentration. The action spectrum obtained from the integrated responses of nerve bundles appears to be that of a single Dartnall pigment having maximal absorption at about 510 nm.

The binding of labelled tetrodotoxin to non-myelinated nerve fibres

1. Tritiated tetrodotoxin has been prepared and purified, and its binding to rabbit, lobster, and garfish non-myelinated nerve fibres examined.2. In each case a component of the binding curve was found that saturated at concentrations of a few nanomolar.3. In addition, non-specific binding, indicated by a linear dependence of the amount bound on concentration, occurred.4. The kinetics of wash-in and wash-out of the radioactive toxin were consistent with a model in which all binding was rapid and reversible and in which diffusion into and out of the nerve bundle was rate-limiting. This was shown by numerical solution of the appropriate non-linear diffusion equation. An extension of the limited biophase model that allows for non-specific binding was shown to give good semiquantitative approximations to the proper diffusion equation; and (unlike the latter) the extension was shown to have a relatively simple solution.5. A number of pharmacological agents were tested for competition with, or perturbation of, tetrodotoxin binding: sodium, calcium and hydrogen ions, lidocaine, batrachotoxin and saxitoxin. Apart from a small calcium effect, only saxitoxin, whose effect on sodium current is similar to that of tetrodotoxin, was found to interfere with binding. Increasing saxitoxin concentrations resulted in reduced amounts of tetrodotoxin binding in a manner consistent with a competition between the two toxins for the same site.

Garfish olfactory nerve: easily accessible source of numerous long, homogeneous, nonmyelinated axons

The olfactory nerve of the garfish, Lepisosteus, is about 1 millimeter in diameter and about 20 centimeters long, depending on the size of the fish; it is easily prepared by breaking off successive scored segments of the rostrum. It consists of a relatively homogeneous population of about 10(7) nonmyelineated nerve fibers, each about 0.24 micrometer in diameter. In most other nerves each fiber is separated from all others by an enfolding Schwann cell, but in the olfactory nerve the fibers are directly in contact with one another in groups of several hundred fibers. The Schwann cell, not directly concerned with propagation of the nerve impulse, forms a thin layer at the periphery of the group and makes up a small proportion of the total cellular material. The volume of axon cytoplasm is about five times greater than that of Schwann cell cytoplasm, and the axon surface is about 30 times the Schwann cell surface. The ratio of surface to volume for axons of a typical olfactory nerve is about 5400 times that for the squid axon of the same diameter. The large proportion of axonal membrane recommends this nerve for use in chemical and physical studies of properties of axon membranes.