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Biomedical subjects

S Ochs

Publications and source records attributed to S Ochs.

At least 19 recordsLinked to original sources
PubMed2000

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.

Action Potentials↗
PubMed1999

Biomechanics of stretch-induced beading.

To account for the beading of myelinated fibers, and axons of unmyelinated nerve fibers as well of neurites of cultured dorsal root ganglia caused by mild stretching, a model is presented. In this model, membrane tension and hydrostatic pressure are the basic factors responsible for axonal constriction, which causes the movement of axonal fluid from the constricted regions into the adjoining axon, there giving rise to the beading expansions. Beading ranges from a mild undulation, with the smallest degree of stretch, to more globular expansions and narrow intervening constrictions as stretch is increased: the degree of constriction is physically limited by the compaction of the cytoskeleton within the axons. The model is a general one, encompassing the possibility that the membrane skeleton, composed mainly of spectrin and actin associated with the inner face of the axolemma, could be involved in bringing about the constrictions and beading.

Animals↗
PubMed1997

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.

Animals↗
PubMed1996

Origin of beading constrictions at the axolemma: presence in unmyelinated axons and after beta,beta'-iminodipropionitrile degradation of the cytoskeleton.

Myelinated nerve fibres become beaded when nerves are subjected to a mild stretch; the beading is seen as varicosities, a series of alternating constrictions and enlargements, when using freeze-substitution or cold-fixation to hold this labile form change in place during fixation. One possibility for how this form change comes about is that the myelin sheath or its Schwann cell initiates beading. We now report, however, that a similar beading is seen in the axons of unmyelinated fibres. In electron micrographs, longitudinal sections of axons show the series of constrictions and expansions typical of beading. In cross-sections, axons with unusually small diameter, corresponding to the constrictions, are seen to contain closely packed microtubules and neurofilaments while neighbouring swollen axons with widely dispersed microtubules correspond to the beading expansions. Another possibility for the form change is that the cytoskeleton is responsible for beading. We discovered that direct exposure of nerves to beta, beta'-iminodipropionitrile in vitro for 1-6 h causes both axonal microtubules and neurofilaments to become degraded and replaced by an amorphous residue. Nevertheless, beta,beta'-iminodipropionitrile-treated nerves show constrictions in myelinated fibres when stretched. An even greater degree of beading with narrower and longer constrictions appears in some fibres, with the expanded regions having oblate ends giving the appearance of a string of sausages. In cross-sections taken through the constrictions, a greater than usual reduction of axonal area was seen, this was due to the loss of cytoskeletal organelles which would act to limit the degree of constriction. With longer exposure to beta, beta'-iminodipropinitrile more fibres show complete degeneration of the cytoskeleton and form ovoids typical of Wallerian degeneration. Unmyelinated axons of beta, beta'-iminodipropionitrile-treated nerves which showed degeneration of their cytoskeleton with its replacement by amorphous material still demonstrated beading. As neither the myelin sheath nor the intact cytoskeleton within the axon is necessary for beading, by exclusion, we consider beading constrictions to be initiated at the level of the axolemma. In our hypothesis the membrane skeleton is responsible; namely, the spectrin, actin and other molecular species lining the inside of the axolemma and binding to transmembrane proteins. The membrane skeleton may be activated by stretch via transmembrane proteins (e.g. beta 1-integrins). The membrane skeleton mechanism may also be directly engaged in the production of Wallerian degeneration or be induced by neurotoxic agents.

Animals↗
PubMed1994

The beaded form of myelinated nerve fibers.

When the nerves are lightly stretched and fixed by freeze-substitution, their fibers show the form-change termed "beading" which consists of a series of undulating constrictions and swellings in the internodes. This form change has not ordinarily been seen in chemically fixed nerves, or when it has, it has been ascribed to a pathological change or an artifact. We now report that beading is also retained in normal nerves when, following a light maintained stretch, they are fixed with aldehydes at a temperature close to 0 degrees C. The degree of beading in single fibers teased from the aldehyde fixed nerves was graded and found to be maximal at 0 degrees C, falling off with increased temperature until, at temperatures above 16 degrees C, most fibers showed no beading or a very mild beading. The fibers of nerves cold-fixed at 0 degrees C displayed the characteristics as freeze-substituted fibers, but with a somewhat smaller number of maximally beaded fibers and an 18% reduction in microtubule numbers in the axons. Desheathing or slitting the sheaths of the nerves before cold-fixation increased the probability of retaining beading. Exposure of stretched nerves to the aldehyde fixative at room temperatures for times as short as 3-5 min before they were cold-fixed showed a diminished degree of beading, indicating that aldehydes can have a deleterious effect on the beading mechanism which we hypothesize to be present in the fiber. This action is distinct from the general cross-linking action of aldehydes.

Animals↗
PubMed1994

The relation of the beading of myelinated nerve fibers to the bands of Fontana.

The bands of Fontana, appearing as spirals or irregular light and dark strips crossing the surface of unstretched nerves, are due to the wavy disposition of nerve fibers within the epineural-perineural sheaths. A mean tension of 2.7 +/- 0.23 (S.E.M.) g applied to segments of rat tibial nerves straightens the fibers and unbands the nerves causing them to lengthen by 9.35 +/- 0.89%. The nerves cold-fixed in situ at that point showed the myelinated fibers to be beaded. On relaxation the nerves rebanded and the fibers were no longer beaded. The tension at which unbanding occurred was better determined when the epineural-perineural sheaths were slit longitudinally. Under these conditions, unbanding occurred at a mean tension of 0.59 +/- 0.08 g and the nerves lengthened by 8.56 +/- 0.58%. The lengthening was not statistically different from that seen in sheathed nerves. In preparations with the epineural-perineural sheaths removed, banding was lost with tensions of 0.20 +/- 0.03 g and the nerves lengthened by 12.1 +/- 1.04%. The tensions needed were significantly lower than that for the sheathed and slit-sheath nerve groups. When cold-fixed, when banding was lost, the fibers were seen to be beaded. Banding of the desheathed nerves returned on relaxation of the nerves. However, after tensions of 8 g they showed plasticity in which the ends of the nerves needed to be pushed together to initiate rebanding in comparison to sheathed or sheath-slit nerves which rebanded spontaneously following relaxation after even higher tensions of 40 g. At the highest tensions the nerves remained extended and could not be forcibly rebanded.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗
PubMed1992

Effects of alcohol feeding on synthesis and secretion of apolipoproteins by regenerating rat sciatic nerve.

The amounts of apolipoprotein (apo) E and A1 released into the culture medium were examined in the regenerating nerves distal to a crush site following chronic alcohol feeding. Cultured minced segments of regenerating nerves taken from rats fed an alcohol-containing liquid diet for 5 weeks released only 50% of apoE but nearly 200% of apoA1 when compared with rats pair-fed with a control diet. The extent of decrease in medium apoE corresponded to the decrease of apoE mRNA in the nerve. Thus, chronic alcohol ingestion affects apoE synthesis of regenerating nerves by changing its mRNA level. On the other hand, apoA1 mRNA remained undetectable in regenerating and intact nerves whether the rats were fed alcohol or not. Furthermore, the amount of apoA1 released by the regenerating nerve into the culture medium was not significantly larger than that present in the nerve tissue prior to incubation. Therefore, it is most likely that apoA1 released by the injured nerve originated from the bloodstream and the increase in apoA1 content seen in the crushed nerve of alcohol-fed rats is due to an enhanced permeability of the nerve-blood barrier. Since the burst of apolipoproteins in the injured nerve is likely to play a role in nerve regeneration, the perturbation of apolipoprotein contents in regenerating nerves by chronic alcohol consumption may contribute to the pathogenesis of alcoholic neuropathy.

Alcoholism↗
PubMed1990

Myelin intrusions in beaded nerve fibers.

Small intrusions form in the internodes in or near the constrictions of beaded fibers prepared by fast-freezing and freeze-substituting mildly stretched nerves in the cat and rat. They appear as inwardly directed folds of the inner lamellae of the myelin sheath, or regularly formed spheres composed of lamellae with major dense and interperiod lines like those of the myelin sheath. A splitting of the lamellae and separation of the major dense lines may occur with an accumulation of Schwann cell cytoplasm between them, the result of an influx of cytoplasmic fluid from nearby constrictions. Longitudinally oriented microtubules have been observed in the intrusions, in the adaxonal Schwann cell cytoplasm, and in the innermost lamellae of the myelin sheath. The paranodes contain a number of larger intrusions in the form of spurs and globules along with shelve-like folds of the myelin sheath oriented in the longitudinal direction. Axoplasmic fluid driven from the constrictions during beading can enter the paranodes to smooth out their folds leaving the globular and spur-shaped myelin intrusions in isolation. Their wall thickness, measured from the central opening to the surface of the intrusion, is the same as that of the myelin sheath or, in some cases, double, the result of the folding of a spur-like intrusion upon itself. Intrusions unconnected to the sheath are seen in unbeaded fibers with regular, compact lamellae surrounded by axolemma. Others lack a covering axolemma and consist of variably disorganized and irregularly shaped lamellae suggesting that they are undergoing fragmentation and dissolution within the axon. The hypothesis is advanced that the intrusions in the internodes arise from an excess of lipid and other myelin components when the diameter of the sheath is reduced in the beading constrictions. In the paranodes, excess myelin components moved into these regions form the shelf-like folds which may fuse to form intrusions. These, separated from the myelin sheath, undergo fragmentation and dissolution and are carried by retrograde transport to the cell bodies where their constituent components can be reutilized.

Animals↗
PubMed1989

Slow transport of freely movable cytoskeletal components shown by beading partition of nerve fibers in the cat.

To account for the transport in nerve fibers of tubulin and neurofilament proteins in slow component a, the Structural Hypothesis holds that these proteins are assembled into microtubules and neurofilaments in the cell bodies and the cytoskeletal organelles then moved down in the fibers as part of an interconnected matrix at a uniform rate of about 1 mm/day. The Unitary Hypothesis, on the other hand, considers these proteins to be carried down within the fibers as soluble components or as freely movable small polymers or subunits turning over locally in the stationary cytoskeleton. To differentiate between the two hypotheses, cat L7 dorsal roots were taken at times from 7 to 25 days after their L7 dorsal root ganglia were injected with [3H]leucine to assess the labeling of the cytoskeleton by the use of beading and autoradiography. Beading was induced by a mild stretch and after fast-freezing and freeze-substitution of the roots for histological preparation, the beads were seen in the fibers as a series of expanded regions alternating with constrictions. In the constrictions the cytoskeleton was compacted into an area as small as 5% that of the normal axon, with the axoplasmic fluid and displaceable (freely movable) components squeezed from the constrictions into the adjoining expansions. Roots taken after 7 and 14 days, times consistent with slow component a downflow, were assessed with sodium dodecyl sulfate-polyacrylamide gel electrophoresis and their content of tubulin and neurofilament proteins shown to constitute 40-50% of all the labeled proteins present. In autoradiographs of dorsal roots taken at those times, numerous grains due to radioactivity were located over the non-constricted regions of the fibers. Few or no grains were present over the constrictions after 7 days. The findings are in accord with the labeled tubulins and neurofilament proteins being present in soluble form in the fibers and expressed from the constrictions into the expansions of the beaded fibers. In contrast, a number of fibers in roots taken at 14-20 days after injection showed somewhat higher grain densities over the constrictions, and more so after 25 days, indicating uptake of labeled subunits into the cytoskeletal organelles at later times. The results are consistent with the downflow of tubulin and neurofilament proteins as soluble components which drop off in the axon to turn over locally in their respective cytoskeletal organelles.

Animals↗
PubMed1987

Cytoskeletal organelles and myelin structure of beaded nerve fibers.

Freshly removed cat peripheral nerves and lumbar spinal cord roots were prepared by freeze-substitution to study the form changes, cytoskeletal alterations and myelin structure in beaded nerve fibers. Fibers of unstretched nerves so prepared were close to cylindrical. When lightly stretched with tensions of 2-10 g before being rapidly frozen, beading appeared as a series of constrictions between the more normally expanded regions of the internodes with the paranodal regions spared. Beading also was seen in the fibers of sciatic and radial nerves fast-frozen in situ with the limbs placed in full extension to cause stretching. The cross-sectional area of the axon in the constrictions of beaded fibers was reduced by as much as 95%. The compaction of the microtubules and neurofilaments in the constructions was accounted for by the movement of axoplasmic fluid from the constrictions axially into the nearby regions where the axon and fiber diameters are close to normal. The electron-lucid area approximately 5 nm thick around the microtubules appeared to hinder their close approach in the constrictions although some microtubules touch. The neurofilaments are generally separated at a mean distance of 8-10 nm and approach to a mean distance of 4 nm in the constrictions. Neither the beading nor the reversal of beading, which occurs on relaxation from stretch, was blocked by periods of anoxia lasting several hours. Deletion of calcium from the incubating medium initiated some small amount of beading and additionally greatly augmented the beading on stretch. Beading also was present in some of the myelinated fibers of the dorsal columns of the spinal cord where stretch would not be present. These findings suggest that beading is due to a contractile process in the axon initiated by stretch and by other changed states of the fiber. Concomitantly with the contraction of the axon in the beading constrictions, the myelin sheath in that region was greatly reduced in circumference, to as much as 1/3 to 1/5 of normal. The decrease of the sheath diameter was not accompanied by a change in its thickness or in its lamellar fine structure. A repeat distance of the dense lines of 14 nm was measured in both the constricted and nonconstricted regions. To account for these findings lipid, and most likely other components of the myelin lamellar membranes, must move longitudinally from the constrictions in the plane of the lamellar membranes, and do this within 5-10 s.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1984

Localization of calcium in nerve fibers.

Using the desheathed nerve preparation, a pyroantimonate precipitation method was used to examine the distribution of electron-dense particles seen in various organelles of the nerve fibers following exposure of nerve to various levels of Ca2+ in vitro. The presence of Ca2+ in the electron-dense particles was indicated by their extraction with EGTA and by the use of energy-dispersive X-ray microanalysis. In normal Ringer or in a Ca2+ -free medium, electron-dense particles were seen associated with the outer membrane of the mitochondria, with the smooth endoplasmic reticulum (SER), along the axolemma and yet others scattered throughout the axoplasm. When nerves were incubated in media containing higher than normal concentrations of 20-60 mM Ca2+, an increase in the number of such electron-dense particles was seen in the axoplasm and within the mitochondrial matrix. Nerves loaded with a high concentration of 60mM Ca2+ could be depleted of these particles after transfer to a Ca2+ -free or low Ca2+ Ringer medium. The sequestration of Ca2+ in axonal organelles is discussed with respect to Ca2+-regulatory mechanisms in the axon needed to maintain a low level of Ca2+ which is optimal for the support of axoplasmic transport.

Animals↗
PubMed1984

Depression of fast axonal transport produced by tullidora.

The fast axoplasmic transport of labeled proteins was studied in cats showing hindlimb paralysis 4-7 weeks after a single oral dose of tullidora (Karwinskia humboldtiana) toxins. The isotope (3H-leucine) was injected into the spinal ganglion and the contralateral spinal cord of the seventh lumbar segment in order to study transport in sensory and motor fibers. The axoplasmic transport in motor fibers of the sciatic nerve was clearly altered in tullidora-treated cats. The majority of these animals showed a gradual decline of radioactivity from the cord to the periphery instead of the clear-cut wave front always seen in normal cats. An apparent wave was seen in three treated cats but the wave peak was behind the normal position and the slope of the wave front was reduced. While the rate of transport indicated by the farthest extent of the foot of the slope was not in all cases significantly changed, the results all indicated a hindered transport by the reduced slope front in the distal segments of the motor axons. In contrast, the axoplasmic transport appeared normal in the sensory fibers of all but one tullidora-treated cat. Light and electron microscopy of medial gastrocnemius and sural (cutaneous) nerves revealed axonal constrictions and axolemal irregularities associated with organelle retention after tullidora treatment. Also, some mitochondria appeared swollen. These changes were more frequent and intense in the motor nerve fibers than in the cutaneous nerve fibers.

Animals↗