The range of velocities of axoplasmic flow. A new approach, and its application to mice with genetically inherited spinal muscular atrophy

On the appearance of acetylcholine receptors in denervated rat diaphragm, and its dependence on nerve stump length

Acetylcholine (ACh) sensitivity and extrajunctional receptor distribution of the rat diaphragm were closely monitored during the early period following denervation. Both contracture in response to 10 microgram/ml of ACh and extrajunctional binding of [125I]alpha-bungarotoxin ([125I]alpha-BTX)) were first detectable 30 h after cutting the phrenic nerve in the thorax. If the nerve were cut more proximally, leaving a 3.5 cm distal nerve stump, the same level of ACh contracture and [125I]alpha-BTX binding did not appear until 40 h after operation. This 10-h delay was far longer than the 3-h delay in transmission failure reportedly dependent on stump length. The earliest detectable extrajunctional [125I]alpha-BTX binding appeared throughout the entire muscle fiber, and was not localized to the endplate region as would be expected if degeneration in the nerve terminal induced new receptors. However, later significant increases in [125I]alpha-BTX binding at the endplate region could have resulted from such degeneration. All these results are consistent with neurotrophic regulation of muscle ACh receptors, working via a mechanism involving axonal transport.

Early course of Wallerian degeneration in myelinated fibres of the rat phrenic nerve

A quantitative study of Wallerian degeneration was carried out on teased fibres. The breakdown into ovoids was used as the criterion of degeneration. The fragmentation of fibres begins near the point of nerve interrruption and spreads along the unbranched parts of axons at velocities correlated with fibre diameter and internodal length. The latent period, preceding the onset of fragmentation, lasts from 25.6 h in thin fibres to 45.0 h in the thickest fibres. The speed of the subsequent advance along the nerve varies correspondingly from 250 to 46 mm/day. In each internode the first ovoids appear in the middle, the ends of the internodal segment being initially spared. The spatiotemporal pattern of degeneration is consistent with the hypothesis that a neuronal trophic substance, normally ensuring the integrity of the axon, exerts transcellularly an inhibitory influence on the Schwann cell. This influence disappears when the amount or concentration of migrating trophic substances falls below a critical level in a stretch of axon. The overlying Schwann cells become mobile and exhibit intense metabolic activity, leading eventually to axonal disruption.

Axoplasmic flow in myelinated and unmyelinated nerves

[14C]Leucine was injected into the floor of the IVth ventricle of rabbits in order to label the axoplasmic flow of proteins in the efferent fibres of the vagus nerve. At intervals from 6 h to 180 days after injection, the rabbits were sacrificed and the cervical and abdominal vagus and recurrent laryngeal nerves removed. The radioactivity in the protein, lipid and low molecular weight fractions of the nerve segments was estimated by liquid scintillation counting. Two rates of axoplasmic flow were thus demonstrated in the abdominal vagus nerve; a fast rate of about 450 mm/day and a slow rate of 30 mm/day. This agrees very well with previous work. Peaks of activity in the recurrent laryngeal nerve corresponded to rates of 9 and 270 mm/day, but their level was low, being not significantly above sciatic nerve background levels. The number of efferent fibres in the two nerves and the corresponding axonal areas were calculated in order to estimate the amount of labelled material in the flow. The abdominal vagus was shown to contain 4-5 times as much label per unit area of axoplasm as the recurrent laryngeal. A difference was therefore demonstrated in the same anatomical nerve between myelinated fibres in the recurrent laryngeal and unmyelinated fibres in the abdominal vagus nerve, both in the rates of flow and in the amounts of material contained in the flow.