RELATIONSHIP BETWEEN MYELIN SHEATH DIAMETER AND INTERNODAL LENGTH IN AXONS OF THE ANTERIOR MEDULLARY VELUM OF THE ADULT-RAT

Relations between myelin sheath diameters and internodal lengths were measured in whole mounts of osmium stained intact anterior medullary v elum (AMV) from glutaraldehyde perfused adult rats. The AMV is a sheet of CNS tissue which roofs the IVth ventricle and contains fascicles o f myelinated fibres which arise mainly from the nucleus of the IVth cr anial nerve. These fibres displayed a broad range of myelin sheath ext ernal diameters and internodal lengths, from < 1-12 mu m and 50-750 mu m, respectively. Myelin sheath external diameter was a measurement of the axonal diameter plus the thickness of its myelin sheath, while in ternodal length was measured as the distance between consecutive nodes . There was a broadly linear relationship between myelin sheath diamet ers and internodal lengths, with the smaller diameter sheaths tending to have shorter internodes than the larger. However, the correlation w as weak and for any given diameter myelin sheaths displayed considerab le variation in their internodal lengths. The smallest diameter myelin sheaths, < 4 mu m; consistently had shorter internodes than predicted by a linear regression and, in an analysis of consecutive internodes in single fibres, the slope was flattened in fibres with a diameter > 4 mu m. Our results indicated that small and large calibre fibres may have different myelin sheath diameter-internodal length interrelations .