Electrophysiology and morphology of myelinated nerve fibers. V. Intramembranous particle distribution in nerve fiber membranes

Ultrastructure of afferent axon endings in a neuroma

Injured sensory axons with endings trapped in a nerve-end neuroma become a source of abnormal impulse discharge and neuropathic pain. We have examined the ultrastructure of such endings anterogradely transported WGA-HRP and freeze-fracture replication, with emphasis on the postinjury period during which the abnormal neural discharge is maximal. Most axons ended in a terminal swelling, depleted of myelin but surrounded by Schwann cell processes. These 'neuroma endbulbs' were richly packed with membrane-bound organelles, and had a smoothly undulating surface with (in neuromas of several weeks standing) a moderate number of short filopodia. Massive sprouting did not occur until several months postinjury. Both p- and e-faces of endbulb axolemma had larger intramembranous particles, on average, than corresponding internodal membrane of control axons. This change, interpreted as indicating remodelling of axolemmal channel (and perhaps receptor) content, may be related to the abnormal electrical behavior of neuroma afferents.

Structural abnormalities in freeze-fractured sciatic nerve fibres of diabetic mice

Nodal and paranodal regions of myelinated sciatic nerve fibres from diabetic (db/db) mice were examined in freeze fracture replicas. In some fibres, the axolemma was found to display abnormalities in the paranodal region. These include shallow, undifferentiated junctional indentations, thinning of the indentations with widening of the non-junctional grooves between them, particle clusters within the non-junctional grooves, and patches in which axolemmal E-face particles are distributed randomly rather than in the form of linear strings within grooves. Nodal structure, in contrast, is hardly affected. Nodal E-face and P-face particle densities in db/db axons are not significantly different from those in age-matched controls, although we found a few examples in which the E-face density fell slightly below the normal range. Occasional fibres showing evidence of paranodal or segmental demyelination were also seen. The results support paranodal pathology as a potential basis for reduced nerve conduction velocity in diabetic nerves but provide no evidence for significant changes in nodal structure or in nodal Na channel density in sciatic nerve fibres of the db/db mouse.

Focal axonal injury: the early axonal response to stretch

The development of a model for axonal injury in the optic nerve of the guinea pig has allowed analysis of early morphological changes within damaged axons. We provide evidence that the initial site of damage after stretch is the nodes of Ranvier, some of which develop 'nodal blebs'. The development of nodel blebs is correlated with the loss of subaxolemmal density, disruption of the neurofilament cytoskeleton and aggregation of membranous profiles of smooth endoplasmic reticulum. Nodal blebs are numerous 15 min after injury but less so at later survivals. The glial-axonal junction is intact at early survivals in damaged nodes. Marked accumulation of membranous organelles occurs in the paranodal and internodal regions adjacent to damaged nodes between two and six hours and is correlated with disruption of the myelin sheath. Axotomy and the formation of degeneration bulbs occurs between 24 and 72 h. The area of axonal injury is invaded by phagocytic cells by 72 h and large numbers of myelin figures occur within the neuropil until 14 days. The results are compared with those of other studies of diffuse axonal injury and other neuropathies. The time course of axonal changes is more rapid than during Wallerian degeneration. Our data from longer surviving animals is exactly comparable with published data. We are confident that the principal site of axonal injury is the node of Ranvier. We suggest that damage at the node results in disruption of axonal transport, which in turn leads to a cascade of events, culminating in axotomy between 24 and 72 h after the initial insult.

The effects of myoinositol on the autonomic neuropathy in the streptozotocin diabetic rat--a freeze fracture study

This report describes a freeze fracture study of rat dorsal sympathetic chain in normal, streptozotocin diabetic on normal diet, and a group fed a 1% myoinositol normal diet. In nonmyelinated fibres, the diabetic group had significant loss of particles on the P face of juxta-axonal Schwann cell membranes, whilst myelinated nerves showed a profound loss of particles on P and E faces. In the group with adjuvant myoinositol the particle numbers were normal in both types of nerve. Axonal plasma membranes showed increased numbers of particles in the P face of the nonmyelinated membranes in the diabetic, but were normal in the myoinositol group. Myelinated axonal membranes showed no significant difference between the diabetic and the normal. Rats on a myoinositol diet showed a significant decrease in the E face particles. These results are consistent with those described in the sciatic myelinated nerves by earlier authors and reinforce the view that myoinositol is an important membrane constituent as phosphatidylinositol and is essential for the expression of normal protein particle numbers.

Cytochemical characteristics of the axon membrane at nodes of Ranvier in resting, tetrodotoxin-blocked, and electrically stimulated peripheral nerves of the rat

To test whether intraaxonal ferric ion-ferrocyanide staining at nodes of Ranvier is influenced by functional state of the nodal membrane, the tibial nerves of Sprague-Dawley rats were subjected to one of the following experimental procedures prior to fixation and staining: General anesthesia to induce nerves at rest, tetrodotoxin blocking of nerve activity, and high-frequency (100 Hz) electrical stimulation. In all cases most but not all nodes were stained. No statistically significant differences were found either in the percentage of total stained nodes or in the percentage of stained nodes from large- and small-diameter fibers among the three conditions tested. An explanation is offered to account for the apparent discrepancy between these results and those from other studies involving related cytochemical markers of the nodal apparatus.

Distribution of particle aggregates in the internodal axolemma and adaxonal Schwann cell membrane of rodent peripheral nerve

Freeze-fracture studies on myelinated fibres from the internodal regions of rat and mouse sciatic nerve show symmetrical particle aggregates within the adaxonal Schwann cell plasmalemma and particle clusters in the axolemma. These are mainly confined to the vicinity of the internal mesaxon and the Schmidt-Lanterman incisures. The Schwann cell particle aggregates are concentrated as bands over the cytoplasmic pockets of Schmidt-Lanterman incisures and the paramesaxonal pockets. In the axolemma there are linear rows of particle aggregates along the groove related to the inner mesaxon and in bands to either side of it. The morphological features suggest the possibility of metabolic coupling between the axoplasm and the Schwann cell cytoplasm via the periaxonal space.

[Sodium channels in electrically excitable membranes]

Properties of sodium channels in electrically excitable membranes can be obtained from sodium currents, gating currents, sodium-current fluctuations and currents through single channels. Gating processes in sodium channels are related to intramembranous charge displacements and are modified by toxins, local anaesthetics and chemical agents. From the results a schematic diagram of a sodium channel can be derived.