Michael Rasminsky

Responses to light of retinal neurons regenerating axons into peripheral nerve grafts in the rat

We have recorded unitary activity from axons regenerated into peripheral nerve grafts inserted into the retina of adult rats. Some retinal ganglion cells regenerating axons into these grafts had responses to light similar to those of intact retinal ganglion cells. The number of units that responded to light in these blind-ended grafts declined between 9 and 48 weeks after graft insertion. Axotomized retinal ganglion cells regenerating axons into peripheral nerve grafts thus appear, at least temporarily, to maintain or resume normal function.

Conduction of nervous impulses in spinal roots and peripheral nerves of dystrophic mice.

Conduction was studied in the sacral ventral roots and ventral tail nerves of dystrophic mice (dy/dy) and phenotypically normal littermates. In myelinated ventral root fibers of normal mice, conduction velocity was uniform with internodal conduction time 45 +/- 5 musec (26 degrees C). In ventral root fibers of dystrophic mice, conduction velocity was decreased and strikingly non-uniform; both saltatory and continuous conduction were observed in different portions of the same nerve fiber. Continuous conduction with velocity less than 2 m/sec (26 degrees C) was characteristically observed in mid-root where the axons are bare; conduction was saltatory close to the exit from the spinal canal and near the spinal cord where the axons are myelinated. Maximum conduction velocity in ventral tail nerves was 21 +/- 3 m/sec for dystrophic mice and 31 +/- 4 m/sec for littermate controls (37 degrees C). Internodal lengths were somewhat decreased in the dystrophic peripheral nerves but there was no significant difference in maximum fiber diameters, myelin thickness or nodal morphology between dystrophic and normal nerves.

Functional Activity of Rat Brainstem Neurons Regenerating Axons Along Peripheral Nerve Grafts

To investigate activation and discharge patterns of central nervous system neurons that regenerate lengthy axons along peripheral nerve grafts we inserted a 4 cm long autologous segment of sciatic nerve into the dorsolateral medulla oblongata of adult rats. Two to 6 months after grafting, the distribution of the cells of origin of the regenerating axons in many nuclei of the brainstem was documented by retrograde horseradish peroxidase labelling from the cut end of the grafts. Functional properties of neurons regenerating axons into the grafts were studied by recording from single regenerated fibers teased from the grafts. Conduction velocities of graft fibers ranged from less than 1 m/s to 25 m/s (30 degrees C). Spontaneous centrifugal impulse traffic in the grafts included units firing in bursts synchronously with the respiratory cycle. Activity in other units was either elicited or inhibited by natural or electrical stimulation of the periphery. Most units recorded in the grafts were neither spontaneously active nor responsive to stimulation of primary afferents. We conclude that: there are central nervous system neurons projecting into the grafts that respond to both excitatory and inhibitory transsynaptic influences; at least some of the spontaneous and induced activity recorded from axons in the grafts resembles that known for normal nerve cells in the regions of the brainstem from which axonal growth arises; and it is possible that many central neurons regenerating axons into peripheral nerve grafts have significantly reduced or altered synaptic inputs.

Activity of medullary respiratory neurons regenerating axons into peripheral nerve grafts in the adult rat

Autologous segments of peroneal nerve were implanted into the medulla oblongata of young adult rats. To investigate activity of medullary respiratory neurons regenerating axons into these grafts, unitary recording from single fibers was performed on small strands teased from the grafts. Spontaneous activity was observed in teased fibers in 7 of 9 grafts recorded 2-5 months after graft implantation. Respiratory-related activity was found in 5 of these grafts and could in most cases be characterized as emanating from medullary respiratory neurons other than cranial motoneurons. The integrity of the input connections to the neurons that had regenerated axons was manifested by normal patterns of unitary respiratory-related activity and by the responsiveness of firing patterns of these neurons to lung hyperinflation and to the inspiratory off-switch effect induced by vagal stimulation. No spontaneous respiratory activity was found in fibers teased from any of the 10 grafts studied 9-11 months after implantation. Five of these grafts were blind-ended as were the 2-5-month grafts; the other 5 grafts formed bridges between the medulla and C4 ventral horn. No physiologic evidence of functional connections with phrenic motoneurons was found in these bridge grafts. These experiments indicate that physiologic function is maintained or regained in some respiratory neurons regenerating axons into peripheral nerve grafts but that this function is not indefinitely preserved in the absence of functional reconnection with an appropriate target.

Receptive-field properties of adult cat’s retinal ganglion cells with regenerated axons

Abstract Receptive-field properties of retinal ganglion cells (RGCs) that had regenerated their axons were studied by recording single-unit activity from strands teased from peripheral nerve (PN) grafts apposed to the cut optic nerve in adult cats. Of the 286 visually responsive units recorded from PN grafts in 20 cats, 49.7% were classified, according to their receptive-field properties, as Y-cells, 39.5% as X-cells, 6.6% as W-cells, and 4.2% were unclassified. The predominant representation of Y-cells is consistent with a corresponding morphological study (Watanabe et al. 1993a), which identified α-cells as the RGC type with the largest proportion of regenerating axons. Among the X-cells, we only found ON-center types, whereas both ON-center and OFF-center Y-cells were found. As in intact retinas, the receptive-field center sizes of Y-cells and W-cells were larger than those of X-cells at corresponding displacements from the area centralis. Within the 10° surrounding the area centralis, the receptive fields of X-cells with regenerated axons were larger than those in intact retinas, suggesting that some rearrangement of retinal circuitry occurred as a consequence of degeneration and regeneration. Receptive-field center responses of Y-, X-, and W-type units with regenerated axons were similar to those found in intact retinas, but the level of spontaneous activity of Y- and X-type units was, in general, less than that of intact RGCs. Receptive-field surrounds were weak or not detected in more than half of the visually responsive RGCs with regenerated axons.

Goodbye to all that: a short history of CCSVI

The current issue of the MS Journal contains carefully performed studies by the group from the University of Texas1 and by the Italian CoSMo Collaborative Study Group2 that together with the previous study by the Texas group3 will hopefully finally definitively put paid to the hypothesis that multiple sclerosis (MS) is caused by chronic cerebrospinal venous insufficiency (CCSVI). It could be argued that such studies would have been unnecessary in a rational society, but they were unfortunately absolutely required in the hyper-connected society in which we live. In the last 4 years the CCSVI hypothesis has turned the MS world upside down. The initial report of venous abnormalities in MS purported to demonstrate that 100% of individuals with MS had significant constrictions of veins draining the brain.4 This quickly and logically led to attempts to correct the abnormality by vein dilation and/or stenting.5 Elements of the media, ever anxious for a heartwarming story of a new and simple solution to a tragic problem that has played havoc with many lives, quickly trumpeted the story of the intrepid Italian investigator who had invented an enticingly labeled “liberation procedure” for the treatment of MS.6,7 From the outset there was healthy skepticism in the neurologic community. Not only did the contention that a venous anomaly was present in 100% of individuals with MS send up red flags, but the theory that a venous anomaly, even if indeed present, could cause MS seemed biologically implausible for many reasons: the fact that the redundancy of the cerebral venous drainage system would be expected easily to accommodate obstruction in single veins; the lack of similarity of the pathology in MS to the brain pathology known to occur with severe cerebral venous obstruction; the failure of the hypothesis to account for the relapsing– remitting character of MS; and the implicit dismissal of the well-established consensus8 that MS is an inflammatory disorder reflecting some as yet incompletely understood derangement of the immunological system. Nonetheless, the enthusiasm within a substantial part of the MS patient community for the “liberation procedure,” fanned by social networking sites and the blogosphere, generated enormous pressures on granting agencies such as the MS Societies and government funding agencies in North America, Europe and Australia to pursue the possibility that CCSVI was a viable explanation for MS. The discussion became particularly animated in Canada where the prevalence of MS is high and the media-generated interest in CCSVI was much greater than elsewhere.9,10 Although the Canadian MS Society expressed no enthusiasm for the concept of CCSVI, it adopted a publically agnostic position about the hypothesis, no doubt in welljustified fear of antagonizing the large constituency within the MS community whose hopes of a miraculous cure had been aroused by the fevered publicity. The evolution of the discussion in Canada offers a particularly instructive, and at times disheartening, example of a complex interaction among patient advocacy groups, scientists, non-governmental and governmental funding agencies, politicians, the press and the blogosphere. In 2010 the Canadian Institutes of Health Research (CIHR) together with the Canadian MS Society created an expert panel with a watching brief re CCSVI. This panel initially advocated funding only studies that would examine the hypothesis that there was indeed a venous anomaly in MS, and has carefully monitored those studies which have been published. A similar stance was taken by the US and other MS Societies, without the involvement of government funding agencies. An initial CIHR-sponsored meta-analysis of the initially published studies of CCSVI ostensibly indicated an increased level of CCSVI in patients with MS;11,12 however, the reviewing panel was careful to note that any conclusions were precluded by the heterogeneity of the studies and the lack of reporting of the success of blinding in most of the studies. The authors of the papers published in this issue of the journal have offered detailed penetrating critiques2,3 of the methodological difficulties inherent in many of the early papers that purported to offer evidence for the existence of CCSVI at high frequency in MS patients, papers that were included in the CIHR meta-analysis. As the public discussion concerning the wisdom of further clinical trials of vein unblocking progressed, the Canadian paper of record, the Globe and Mail, editorialized that “Canada should fund medical trials of a revolutionary treatment for multiple sclerosis, and not act as if the rejection of those trials by an expert panel must be obeyed. This is a political question, not a purely scientific one.”13 Similarly, the lead medical reporter for CTV followed her initial “documentary”5 about Dr Paolo Zamboni with numerous stories Goodbye to all that: a short history of CCSVI 502400 MSJ191110.1177/1352458513502400Multiple Sclerosis JournalRasminsky and terBrugge 2013

Conduction properties of single nerve fibers in developing rat spinal nerve roots

We have examined conduction properties and distribution of membrane currents in ventral spinal root axons of rats 12-47 days of age. Internodal length of the largest myelinated fibers increases at a steady 17-19 micron/day during this period as internodal conduction time decreases. Despite the rapid remodelling of fiber dimensions during the first few weeks of postnatal life, potassium channels are continually excluded from participation in action potential generation at the axon membrane of most nodes of Ranvier.