A new mechanism controlling conduction in stretched myelinated nerves and a comprehensive nerve conduction model

Analysing published experimental findings this paper revealed that for myelinated nerves the conduction velocity (CV) increases on stretching out of the nerve, which has not been pointed out by anyone before. This apparently contradicts existing concepts since stretching out of a nerve fibre reduces its diameter which is expected to reduce the CV. Besides, the change is reversible and immediate, which cannot be explained with existing knowledge either. In order to explain this anomaly, the present work invoked a new resistance to ion flow between the nerve axon and the extracellular fluid created by interdigitated fingerlike processes of myelin sheaths coming from two sides of a node of Ranvier, analyzing published electron microscopic images. When stretched out, the gaps between the processes increase, decreasing the resistance to ion flow and thereby hastening depolarization, increasing CV in turn. The gaps close immediately on the release of the stretching force because of the pull of the elastic endoneurium, thus retrieving the original CV. To represent this new mechanism, a new resistive element has been added to the existing electrical model of a myelinated nerve, which is being claimed to be the dominant component that determines the conduction delay. Stretching also affects other nerve parameters and this paper developed a mathematical formulation involving all these parameters to show satisfactorily that CV indeed increases with stretching, in which the contribution of the proposed resistance dominates. The paper also proposed an appropriate modification of the representative schematic model commonly used to depict propagation of action potential in a myelinated nerve fibre. The suggested new mechanism and the resistance model is a breakthrough in the explanation of neural conduction and opens up the door for new study as well as for reviewing all previous experiments on myelinated nerves afresh. Bangladesh Journal of Medical Physics Vol.11 No.1 2018 P 38-56

A Pioneering Study of Retinal Pigment Epithelial Photocoagulation to Seal Retinal Breaks and Prevent Hypotony during Pars Plana Vitrectomy

Background: During vitrectomy, a relatively high level of accuracy is required when using retinal laser energy, which is difficult to control in the presence of subretinal fluid and other conditions. We explored the clinical effects of retinal pigment epithelium (RPE)photocoagulation in closing retinal breaks and preventing hypotony during vitrectomy. We describe the possible indications of RPE photocoagulation. Methods: This was a prospective, non-random, uncontrolled case series study. We recruited 20 patients, with retinal detachment in 20 eyes. In this study, RPE photocoagulation was performed under the detached retina and retinal holes. RPE laser photocoagulation was performed in 3-4 rows at the edge of the periphery around the hole and the tear, replacing the traditional retinal photocoagulation and closing part or all of the retinal break. The energy of RPE photocoagulation was 100-150mJ for 120-200ms to seal the breaks, and the same energy but little gaps for the RPE without covering of retina. Results: After vitrectomy, in 19 eyes, there was a visible pigmentation reaction around the hole and tear, except for 1 eye the retinal hole located in the myelinated nerve fibre area. The mean best-corrected visual acuity (BCVA) at 6 months after surgery was significantly higher than that before surgery (p=0.002). At the last follow-up, the mean BCVA remained significantly higher than that before surgery (p=0.001).There was no significant difference in BCVA between the 6th month and the last follow-up (p=0.806).The thickness of the neurosensory layer in RPE photocoagulation and retinal photocoagulation area at 1 month after surgery was 216.33±54.42μm and 87.67±34.65μm, respectively. By the end of the follow-up, there were no serious complications, and the retinas of all 20 eyes were reattached. No hypotony occurred after surgery. Conclusions: The effects of RPE photocoagulation and retinal photocoagulation are similar in closing retinal breaks. The RPE laser sealed the retinal breaks and did not damage the neurosensory layer, but still caused pigmentation formation on the RPE, resulting in adhesion between the neurosensory layer and the RPE layer. Laser photocoagulation could be considered as comparable to glue instead of welding when closing the retinal breaks. Trial registration ChiCTR1900021504,2019-02-24.

Prevalence of the optic disc anomalies in the adult South Indian population

PurposeTo determine the prevalence of congenitally abnormal disc (all anomalies) in an adult population in southern India.MethodsSubjects aged ≥40 years (n=6013) underwent a complete ophthalmic examination. Optic disc anomalies were diagnosed according to the definitions given in the article.ResultsOptic disc anomalies were found in 81 eyes of 66 (1.1%, 95% CIs 0.00834 to 0.01361) patients. The prevalence of each anomaly in the descending order was peripapillary myelinated nerve fibre (0.28%), epipapillary glial tissue on the optic disc (0.28%), peripapillary vascular loops (0.16%), tilted disc (0.09%), optic disc coloboma (0.08%), optic nerve hypoplasia (0.04%), optic disc pit (0.04%), optic disc pigmentation (0.03%), optic nerve head drusen (0.03%), Bergmeister’s papilla (0.03%), optic disc pit and coloboma (0.01%).ConclusionsThe prevalence of optic disc anomalies is 1.1% in the adult South Indian population.

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.