Ultrastructural Morphology and Morphometry of Phrenic Nerve in Rats

Aging affects the number and morphological heterogeneity of rat phrenic motor neurons and phrenic motor axons

Diaphragm muscle (DIAm) motor units comprise a phrenic motor neuron (PhMN), the phrenic nerve and the muscle fibers innervated, with the size of PhMNs and axons characteristic of motor unit type. Smaller PhMNs and their axons comprise slow (type S) and fatigue-resistant (type FR) DIAm motor units, while larger PhMNs and their axons comprise more fatigable (type FF) motor units. With aging, we have shown a loss of larger PhMNs, consistent with selective atrophy of type IIx/IIb DIAm fibers and reduced maximum DIAm force. In the present study, we hypothesized that with aging there is a loss of larger myelinated phrenic α motor axons. Female and male young (6 months) and old (24 months) Fischer 344 rats were studied. PhMNs were retrogradely labeled by intrapleural injection of 488-conjugated CTB. The phrenic nerves were excised ~1 cm from the DIAm insertion and mounted in resin, and phrenic α motor axons were delineated based on size (i.e., >4 μm diameters). In older rats, the number of larger PhMNs and larger phrenic α motor axons were reduced. There were no differences in non-α axons. In addition, there was evidence of demyelination of larger phrenic α motor axons in older rats. Together, these findings are consistent with the selective age-related vulnerability of larger PhMNs and denervation of type FF motor units, which may underlie DIAm sarcopenia.

Modeling the Impact of the Variation in Peripheral Nerve Anatomy on Stimulation

Introduction

The peripheral nervous system has a complex anatomical structure. Stimulation of nerve fibers in the peripheral nervous system depends on the fiber diameter and myelination as well as its location within the nerve, packing fraction and fascicle distribution within the nerve bundle. This paper analyzes the impact of the variation in peripheral nervous system anatomy and the distance of the stimulating electrodes on the probability of generating an action potential.

Methods

A mathematical model for effective fascicle conductivity has been developed to capture the variation in the packing fraction and fiber diameter. A linear activating function is utilized to analyze the impact of this effective conductivity and fascicle distribution as an indicator of generating an action potential.

Results

Finite element simulations are performed for the nerve-electrode configuration to evaluate the electric field. The simulation results are used to analyze the activating function for different packing fractions and type of nerve fibers. The effect of electrode distance on activating function and the total current through a nerve bundle has also been studied.

Discussion

The simulation results indicate that the peripheral nerve anatomy and electrode distance have a significant effect on the action potential generation.

Bands of Fontana are caused exclusively by the sinusoidal path of axons in peripheral nerves and predict axon path; evidence from rodent nerves and physical models

The precise cause of the bands of Fontana, striations on peripheral nerves visible to the naked eye, has been the subject of debate for hundreds of years. Some researchers have described them as reflecting the sinuous course of nerve fibres passing through nerves, and others have proposed that endoneurial collagen and sheaths surrounding nerves play a role in their appearance. We hypothesised that the bands are caused exclusively by reflection of light from the surfaces of nerve fibres travelling in phase in sinusoidal waveforms through peripheral nerves. We aligned images of obliquely illuminated nerves with confocal images of axons in those nerves, and the numbers and positions of the bands precisely matched the axonal waves. We also developed three-dimensional models of nerves with representations of the sinusoidal path of axons at their surface. We observed patterns resembling the bands of Fontana when these models were obliquely illuminated. This provides evidence that the bands of Fontana can be caused by light reflected sinusoidal path of axons alone. We subsequently describe a mechanism of band production based on our observations of both nerves and models. We report that smaller diameter nerves such as phrenic nerves and distal branches of sciatic nerves have shorter band intervals than larger nerves, such as proximal trunks of sciatic nerves, and that shorter band intervals correlate with longer axons per unit length of nerve, which suggests a greater tolerance to stretch. Inspection of banding patterns on peripheral nerves may permit prediction of axon length within nerves, and assist in the interpretation of nerve conduction data, especially in diseases where axon path has become altered.

Substance P in Dorsal Root Ganglion Neurons in Young and Adult Rats, after Nociceptive Stimulation during the Neonatal Period

The nervous system is highly plastic during the neonatal period, being sensitive to noxious stimuli, which may cause short- and long-term pain responsivity changes. Understanding plasticity in peripheral pain pathways is crucial, particularly when the nervous system is still under development and remodeling process. Substance P (SP) is widely used as a marker for peripheral neurons with unmyelinated and small myelinated fibers. We investigated the number of SP immunoreactive neurons in the dorsal root ganglion (DRG) of male and female Wistar rats, 15 and 180 days after nociceptive stimulation during the neonatal period. Right and left 5th lumbar (L5) DRG were incubated in rabbit polyclonal anti-substance P primary followed by biotinylated donkey anti-rabbit secondary antibodies. Reaction was revealed with a nickel-diaminobenzidine solution. Labeled neurons were counted and compared between ages, genders and groups. Gender differences were present in both ages, with the number of SP-positive DRG neurons being larger in 15-days-old males on both sides. After 180 days, males showed a larger number of SP-positive neurons than females only on the nociceptive stimulated side. An increased number of SP-positive neurons in the DRG on the stimulated side was present in females, immediately after nociceptive stimulation, but not after 180 days. In conclusion, neonatal noxious stimulation caused a permanent increase in SP-positive DRG neurons in males that was not observed in females, suggesting that differences in pain processing/responsivity between genders could be related to morphological alterations of the nervous system. Anat Rec, 301:849-861, 2018. © 2017 Wiley Periodicals, Inc.

Stimulation-induced Ca(2+) influx at nodes of Ranvier in mouse peripheral motor axons

KEY POINTS

In peripheral myelinated axons of mammalian spinal motor neurons, Ca(2+) influx was thought to occur only in pathological conditions such as ischaemia. Using Ca(2+) imaging in mouse large motor axons, we find that physiological stimulation with trains of action potentials transiently elevates axoplasmic [C(2+)] around nodes of Ranvier. These stimulation-induced [Ca(2+)] elevations require Ca(2+) influx, and are partially reduced by blocking T-type Ca(2+) channels (e.g. mibefradil) and by blocking the Na(+)/Ca(2+) exchanger (NCX), suggesting an important contribution of Ca(2+) influx via reverse-mode NCX activity. Acute disruption of paranodal myelin dramatically increases stimulation-induced [Ca(2+)] elevations around nodes by allowing activation of sub-myelin L-type (nimodipine-sensitive) Ca(2+) channels. The Ca(2+) that enters myelinated motor axons during normal activity is likely to contribute to several signalling pathways; the larger Ca(2+) influx that occurs following demyelination may contribute to the axonal degeneration that occurs in peripheral demyelinating diseases. Activity-dependent Ca(2+) signalling is well established for somata and terminals of mammalian spinal motor neurons, but not for their axons. Imaging of an intra-axonally injected fluorescent [Ca(2+)] indicator revealed that during repetitive action potential stimulation, [Ca(2+)] elevations localized to nodal regions occurred in mouse motor axons from ventral roots, phrenic nerve and intramuscular branches. These [Ca(2+)] elevations (∼ 0.1 μm with stimulation at 50 Hz, 10 s) were blocked by removal of Ca(2+) from the extracellular solution. Effects of pharmacological blockers indicated contributions from both T-type Ca(2+) channels and reverse mode Na(+)/Ca(2+) exchange (NCX). Acute disruption of paranodal myelin (by stretch or lysophosphatidylcholine) increased the stimulation-induced [Ca(2+)] elevations, which now included a prominent contribution from L-type Ca(2+) channels. These results suggest that the peri-nodal axolemma of motor axons includes multiple pathways for stimulation-induced Ca(2+) influx, some active in normally-myelinated axons (T-type channels, NCX), others active only when exposed by myelin disruption (L-type channels). The modest axoplasmic peri-nodal [Ca(2+)] elevations measured in intact motor axons might mediate local responses to axonal activation. The larger [Ca(2+) ] elevations measured after myelin disruption might, over time, contribute to the axonal degeneration observed in peripheral demyelinating neuropathies.

Endurance training induces structural and morphoquantitative changes in rat vagus nerve

ABSTRACT Introduction: Many nervous system tissues and cells suffers positive changes when faced to exercise training. However, data on vagus nerve adaptation from exercise-induced study is absent. Objective: To analyze the effect of an endurance training on the vagus nerve morphology of rats. Methods: Wistar rats (6 months of age) were divided into two groups: control group (CG, n=8), and aerobic trained group (AT, n=8). AT was submitted to a treadmill training program of five times per week during 12 weeks. The maximum speed stipulated in the training protocol corresponded to 60% of the mean maximum intensity achieved by the group in the test of maximum effort. Results: Twelve weeks of treadmill training resulted in left ventricular hypertrophy in the AT group com-pared to CG. There was a significant increase in the area of both the myelinated and unmyelinated axons, and in the area of myelin sheath with training. The number of neurotubules and neurofilaments in myelinated fibers of aerobic trained group was significantly greater than CG (p≤0.05). Conclusion: Endurance training promoted significant increase in morphometric parameters of the vagus nerve in the same way it affect somatic nerves.

Phrenic nerve diabetic neuropathy in rats: unmyelinated fibers morphometry

We have demonstrated that phrenic nerves' large myelinated fibers in streptozotocin (STZ)-induced diabetic rats show axonal atrophy, which is reversed by insulin treatment. However, studies on structural abnormalities of the small myelinated and the unmyelinated fibers in the STZ-model of neuropathy are limited. Also, structural changes in the endoneural vasculature are not clearly described in this model and require detailed study. We have undertaken morphometric studies of the phrenic nerve in insulin-treated and untreated STZ-diabetic rats and non-diabetic control animals over a 12-week period. The presence of neuropathy was assessed by means of transmission electron microscopy, and morphometry of the unmyelinated fibers was performed. The most striking finding was the morphological evidence of small myelinated fiber neuropathy due to the STZ injection, which was not protected or reversed by conventional insulin treatment. This neuropathy was clearly associated with severe damage of the endoneural vessels present on both STZ groups, besides the insulin treatment. The STZ-diabetes model is widely used to investigate experimental diabetic neuropathies, but few studies have performed a detailed assessment of either unmyelinated fibers or capillary morphology in this animal model. The present study adds useful information for further investigations on the ultrastructural basis of nerve function in diabetes.