Surprising thermal transition in fish myelin.
A new structural transition in nerve myelin has been discovered by means of X-ray diffraction of excised teleost nerves in physiological saline. The reversible transition is between two structures, designated AS and AL, with repeating distances (d spacings) differing by 25-35 A. When the temperature of bream spinal cord is lowered from room temperature to 4 degrees C, much but not all of the AS (short spacing) myelin changes into AL (long spacing) myelin. The change is reversed when the temperature is raised back to 22 degrees C, and it occurs a second time when the temperature is lowered again to 4 degrees C. The myelin in bream optic nerve undergoes a similar thermal transition, but the myelin in brachial plexus does not. The thermal transition does not involve the liquid crystal-to-gel transition observed in lipids and natural membranes. When a specimen is kept at constant temperature, there is a gradual conversion from AS to AL myelin which is not thermally reversible, suggesting the existence of two distinct subclasses of AL. Similarly, two subclasses are indicated for AS myelin since part of it does not transform thermally. The observations reported here may have significance for the evolutionary development of myelin.
D000818:Animals; D004594:Electrophysiology; D005399:Fishes; D009186:Myelin Sheath / Q000502:physiology*; D009413:Nerve Fibers, Myelinated / Q000502:physiology; D009900:Optic Nerve / Q000502:physiology; D013116:Spinal Cord / Q000502:physiology; D013696:Temperature; D014961:X-Ray Diffraction