Myelin movements in mature mammalian peripheral nerve fibers

Excitation block in a nerve fibre model owing to potassium-dependent changes in myelin resistance

The myelinated nerve fibre is formed by an axon and Schwann cells or oligodendrocytes that sheath the axon by winding around it in tight myelin layers. Repetitive stimulation of a fibre is known to result in accumulation of extracellular potassium ions, especially between the axon and the myelin. Uptake of potassium leads to Schwann cell swelling and myelin restructuring that impacts the electrical properties of the myelin. In order to further understand the dynamic interaction that takes place between the myelin and the axon, we have modelled submyelin potassium accumulation and related changes in myelin resistance during prolonged high-frequency stimulation. We predict that potassium-mediated decrease in myelin resistance leads to a functional excitation block with various patterns of altered spike trains. The patterns are found to depend on stimulation frequency and amplitude and to range from no block (less than 100 Hz) to a complete block (greater than 500 Hz). The transitional patterns include intermittent periodic block with interleaved spiking and non-spiking intervals of different relative duration as well as an unstable regime with chaotic switching between the spiking and non-spiking states. Intermittent conduction blocks are accompanied by oscillations of extracellular potassium. The mechanism of conductance block based on myelin restructuring complements the already known and modelled block via hyperpolarization mediated by the axonal sodium pump and potassium depolarization.

Cytokines, signal transduction, and inflammatory demyelination: review and hypothesis

The mechanism of focal demyelination in multiple sclerosis has been a long-standing enigma of this disorder. Cytokines, a diverse family of signalling molecules, are viewed as potential mediators of the process based on clinical observations and studies with animal models and tissue/cell culture systems. Myelin and oligodendrocyte (OL) destruction occur in cultured preparations subjected to cytokines such as tumor necrosis factor-alpha (TNF alpha) and lymphotoxin (LT). Many studies have shown these and other cytokines to be elevated at lesion sites and in the CSF of multiple sclerosis (MS) patients, with similar findings in animal models. Some variability in the nature of MS lesion formation has been reported, both OLs and myelin being primary targets. To account for myelin destruction in the presence of apparently functional OLs we hypothesize that cytokines such as TNF alpha and LT alpha contribute to myelin damage through triggering of specific reactions within the myelin sheath. We further propose that neutral sphingomyelinase (SMase) is one such enzyme, two forms of which have been detected in purified myelin. An additional event is accumulation of cholesterol ester, apparently a downstream consequence of cytokine-induced SMase. The resulting lipid changes are viewed as potentially destabilizing to myelin, which may render it more vulnerable to attack by invading and resident phagocytes.

[Changes in the femoral nerve and medial articular nerve after medial arthrotomy and replacement of the anterior cruciate ligament]

The effect of combined medial arthrotomy (Payr's approach) and replacement of the anterior cruciate ligament upon the femoral nerve and the medial articular nerve has been studied. Resection and prosthetic replacement of the anterior cruciate ligament was performed on the right hindlimb of twelve Merino sheep. Three unoperated animals served as controls. Eight to 48 weeks after the operation, myelinated axons of the femoral nerve and the medial articular nerve were examined using morphological and morphometric methods. Compared with the un-operated control side, only discrete morphological alterations were detectable, such as an increase in non-circular axons. In contrast, considerable morphometric changes were observed: A decrease in large axons and an increase in small axons were found in the femoral nerve of the operated side resulting in a significant reduction in axon diameter (p < or = 0.05). This decrease in axon diameter was associated with a significant increase in the number of axons. Both changes persisted throughout the period examined (48 weeks). Similar changes, i.e. increase in small axons, decrease in large axons and reduction in mean axon diameter were also observed in the medial articular nerve. The results show that medial arthrotomy with prosthetic replacement of the anterior cruciate ligament results in irreversible changes of the femoral nerve and the medial articular nerve. These findings indicate that the medial articular nerve is injured during medial arthrotomy. Mechanisms are discussed which may explain the alterations observed in the femoral nerve a long distance (30 to 35 cm) away from the operation site.

Axonal neuropathy in a patient with monoclonal IgM kappa reactive with Schmidt-Lantermann incisures

We report a patient with a progressive, predominantly sensory neuropathy and a IgM kappa M-protein that binds to Schmidt-Lantermann incisures. A sural nerve biopsy showed primary axonal damage and IgM deposits at Schmidt-Lantermann incisures were seen by direct immunoperoxidase. Serum from the patient injected into rat sciatic nerve reacts with the incisures as with those in the patient's nerve. The IgM kappa M-protein reacts with chondroitin sulfate C and binds to a broad nerve protein band with a mobility of between 170 and 118 kDa. Peripheral neuropathy may be related to the M-protein, which had immunocytochemical reactivity not previously described for patients with polyneuropathy and IgM monoclonal gammopathy.

Functions and distribution of voltage-gated sodium and potassium channels in mammalian Schwann cells

Recent patch-clamp studies on freshly isolated mammalian Schwann cells suggest that voltage-gated sodium and potassium channels, first demonstrated in cells under culture conditions, are present in vivo. The expression of these channels, at least at the cell body region, appears to be dependent on the myelinogenic and proliferative states of the Schwann cell. Specifically, myelin elaboration is accompanied by a down regulation of functional potassium channel density at the cell body. One possibility to account for this is a progressive regionalization of ion channels on a Schwann cell during myelin formation. In adult myelinating Schwann cells, voltage-gated potassium channels appear to be localized at the paranodal region. Theoretical calculations have been made of activity-dependent potassium accumulations in various compartments of a mature myelinated nerve fibre; the largest potassium accumulation occurs not at the nodal gap but rather at the adjacent 2-4 microns length of periaxonal space at the paranodal junction. Schwann cell potassium channels at the paranode may contribute to ionic regulation during nerve activities.

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.

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)

Slow pulsatile movements of Schwann cells in vitro: a time-lapse cinemicrographic study

Slow pulsatile movements of Schwann cells in vitro were studied quantitatively by using time-lapse cinemicrography. Schwann cells from peripheral nerves of 3-day-old rats were cultured in serum-free medium. Most Schwann cells showed intermittent episodes of pulsatile movement; each episode consisted of one or several contractile pulses. About half of the episodes consisted of a single pulse, and episodes with more than four pulses were rare. The average episode of activity lasted 2.6 min, while the average duration of a single pulse was 1.5 min. The mean quiescent interval between episodes of activity was 3.7 min. Some cells showed no pulsatile activity. Active cells averaged 6.6 episodes/h. The fraction of time which a Schwann cell spent in pulsatile activity varied widely, with an average of 28%. Behavior of Schwann cells in HEPES-buffered Hanks saline was generally similar to that in the complete medium. Raising K+ to 40 mM or Ca++ to 10 mM did not markedly affect the time course of the pulsatile motility, although the contractions were more vigorous in the high Ca++. Pulsatile movement was reversibly inhibited by cytochalasin B and appeared to be potentiated by drugs that disrupt microtubules.

Postnatal differentiation of the rat trochlear nerve

A morphometric analysis of postnatal differentiation in the rat trochlear nerve was studied by light and electron microscopy as an initial basis for understanding motor unit heterogeneity in the extraocular muscles (EOM). A total of 35 animals were examined 7--90 days postnatal (dpn). The mean number of fibers increased from 222 to 7 dpn to 274 in the adult and the size distribution became bimodal at 21 dpn. In the adult 17% of the myelinated fibers had a mean diameter of 2.5 micrometer and 83% were 7.3 micrometer. The estimated number of unmyelinated axons decreased from about 40% at 7 dpn to 20% at 14 dpn and 16% in the adult. The myelinated fiber diameter was more highly correlated with age and body weight than was fiber number. Certain organelles characteristic of active membrane growth were present in the Schwann cell cytoplasm at the paranode region. Redundant loops were prominent at 10 dpn, when many axons were still in Schwann cell bundles. During the third postnatal week a number of alterations were noted which may reflect a loss of polyneuronal innervation. These included thicker myelin sheaths and ultrastructural evidence of axonal degeneration. Branching of myelinated fibers was limited to the intramuscular portions of the nerve at 18 dpn. The g-ratio of the largest fibers at selected ages was nearly constant at .71 and was correlated with fiber diameters (r = 0.40), except at 14 dpn. The periodicity of the myelin sheath had either an inverse or constant relationship to the number of lamellae. The significance of the results is discussed in relation to postnatal development, the size principle and heterogeneity in the EOM motor units.

Schmidt-Lanterman incisures. II. A light and electron microscope study of remyelinating peripheral nerve fibres

A mechanism has been proposed previously for the formation of Schmidt-Lanterman incisures during ontogeny as myelinated nerve fibres increase in diameter. In this study, the mode of formation of incisures and their development were examined by light and electron microscopy in remyelinating fibres (16-200 d) of the sural nerve of rats following lysophosphatidyl choline (LPC)-induced segmental demyelination. In remyelination, a basic number of incisures is incorporated into the myelin sheath ab initio. Apparently these incisures which are at first circumferentially incomplete, being localised to the region of the mesaxons, progressively extend around the whole circumference of the myelin sheath. As fibre diameter increases, additional incisures are added, seemingly, by the intrusion of Schwann cell cytoplasm between the compact myelin lamellae, from the abaxonal side of the myelin sheath. The progressive differentiation of incisures is described in terms of their complement of organelles. The results are discussed with reference to the possible functions of incisures.

Membrane specializations and cytoplasmic channels of Schwann cells in mammalian peripheral nerve as seen in freeze-fracture replicas

Mammalian Schwann cells in rat, rabbit and human fetal nerves were studied using several cryoprotective agents for electron microscopic study of freeze-fracture replicas. The findings in fixed and unfixed tissue reveal surface plasmalemma caveolar specializations and the outer layer membrane junctional complexes found in non-mammalian species. The plasmalemma also reveals a complex arrangement of contours outlining cytoplasmic channel networks distinct from the long-recognized Schmidt-Lanterman incisures and paranodal cytoplasmic loops. A specialized interconnected channel system in the outer "loose" myelin layer displays relatively uniform dimensions comparable in diameter to nodal microvilli, paranodal loops and some incisures. An adaxonal tubular channel system constituting the "axon-Schwann network" is found in the internodal region in addition to other variants of the adaxonal Schwann plasmalemma. The several forms of sequestration of Schwann cell cytoplasm presumably underlie the specialized needs of cytoplasmic continuity in a dynamic functional entity in which large domains of cytoplasm have been displaced by the formation of compact myelin.

Freeze-fracture properties of central myelin in the bullfrog

The freeze-fractured membrane of the central myelin sheath has three classes of particulate components: (i) Particles inherent to the compact myelin lamellae. These are distributed at random and cleave predominantly with the P (protoplasmic) face. (ii) Particles which comprise the intramyelinic tight junctions. These are arranged in strands and are located at the inner and outer mesaxon, the paranodal loops, the cytoplasmic incisures, and occasionally within the compact regions of the myelin sheath. (iii) Particles localized exclusively at the portion of the paranodal loop membrane involved in the septate-like junction with the axolemma. These are regularly spaced and are organized in parallel rows. In the central myelin sheaths of bullfrogs fixed by perfusion with aldehydes and cryoprotected in 30% glycerol, the randomly distributed particles differ in size and shape from those of the axolemma. They possess a reasonably well defined bimodal distribution with respect to particle shape--most can be described either as globules or as ellipsoids. The globular particles range in diameter from 60 to 150 A. The ellipsoidal particles are 100-200 A long and 15-50 A wide. The total number of particles per square micron on the P face is approximately 1500. About half of these are of the globular type and half of the ellipsoidal type. In poorly fixed specimens, loss of interlamellar adhesion and loss of randomly distributed particles seem to coincide. Evidence is presented against the hypothesis that the tight junctions between compact myelin lamellae represent the radial component of the myelin. The possible relation between the types of particulate components seen in freeze-fracture and the classes of protein isolated from central myelin fractions is briefly discussed.

On some aspects of schwann-sheath development and the possible role of their desmosome-like structures

An investigation was made of ventral and dorsal spinal roots (L7 and S1) in the postnatal life of the rabbit. The development of the Schwann sheath of the nerve fibers was studied and correlated with the distribution and morphological characteristics of the desmosome-like structures connected with them. The association of these structures with constituents of the endoplasmic granular and smooth-surfaced reticulum of the Schwann cell is pointed out. The possibility is discussed of a direct relationship between the inner mesaxons, as well as along the length of the peripheral myelin lamellae. In addition, it is suggested that in some cases already existing membrane formations of the adjacent cytoplasm become attached to the developing Schwann sheaths, which results in branching mesaxons or "poli-mesaxonal" nerve fibers.