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Closely spaced nodes of Ranvier in the mammalian brain
SHORT COMMUNICATIONS
445
Closely spaced nodes of Ranvier in the mammalian brain The distinction between white and gray matter in the central nervous system is based on the presence of tracts of myelinated fibers in the former and relative absence of these in the latter. Nevertheless, neuropil and nuclear regions of the brain and spinal cord contain some myelinated fibers. Because of their complex geometry, there have been few attempts to study patterns of myelination of fibers in neuropil and nuclear regions of the CNS (cf. ref. 4). The one systematic study of fibers from preoptic area, hypothalamus, hypoglossal root, and pyramidal tract of the adult opossum, showed a roughly linear relationship between axon diameter and internode distance, and noted that central fibers and peripheral fibers of comparable diameter (fixed and stained in a similar manner) showed comparable internode distances3. Recent studies 10-1z of preterminal fibers in teleost brain and spinal cord have demonstrated, however, that internode distances in neuropil and nuclear regions can be significantly shorter than previously reported for teleost peripheral nerve s or mammalian CNS3, 6. Internode distances are also relatively short in the skin plexuses of the fish and frog, where there is a progressive decrease in length of myelin segments as the fiber approaches its termination 14, and in teleost neurogenic electric organs 1. The present study was undertaken in order to determine whether there are fibers in mammalian gray matter which deviate in their pattern of myelination from peripheral nerve fibers or fibers in main tracts of the CNS. On the basis of a preliminary survey of fibers in the mammalian brain, we present evidence here which suggests that internode distances in mammalian gray matter can be relatively short. Furthermore, our evidence suggests that shcrt internode distances are characteristic of certain fibers and do not represent variation in internode length along a given fiber. Fifteen #m sections were prepared from the brains of kangaroo rats (Diplodomys merriami), perfused with 10~ formalin and stained by a modified Bodian protargol technique 2, and from the brains cf cats fixed by intravital perfusion with solutions of 2.5 ~o glutaraldehyde in phosphate buffer, followed by immersion in 1 ~o OsO4 in buffer. These were examined by light microscopy. In the case of protargol preparations, Bodian's criteria 3, i.e. the presence of constriction at nodes, were used for identification of nodes and determination of internode distances. Since the protargol method stains only axon cylinders and not myelin, axon diameters (i.e. diameter of the axon without the myelin sheath) were determined with this method. In glutaraldehyde-osmium preparations, nodes could be identified as distinct interruptions of the myelin sheath, associated with constriction of the axon. In these preparations, both axon diameters and fiber diameters (diameter including the sheath) could be measured. The majority of fibers examined, exhibited internode distances comparable to those reported previously for fibers of similar diameter in opossum brain 8. However, careful examination of reticular formation, oculomotor nucleus, and striatum revealed some fibers with internodes considerably shorter than reported previously. The ratio of internode distance to fiber diameter (including the myelin sheath) was as low as 18 for some fibers. One fiber with a short internode distance is shown in Fig. 1.
Brain Research, 32 (1971) 445-448
446
SHOR 1" ('OMMUNICAI I~)NS
Fig. 1. Myelinated fiber from tile mesencephalic reticular f o r m a t i o n of the cat. Two nodes of Ranvier (indicated by arrows) are separated by a distance of approximately 350 Mm; the diameter of the fiber is greater than 12 Mm. The nodes are s h o w n at higher magnification in the insets. Glutaraldehyde o s m i u m , x 440; Insets, ;,~ 1000. Fig. 2. Myelinated fiber from the o c u l o m o t o r nucleus of the k a n g a r o o rat, Diplodomys merriami. N o d e s appear as constricted regions in the a x o n in this protargol preparation. T w o relatively short internodes are s h o w n (nodes indicated by arrows); a third consecutive short internode on the same fiber is n o t included in the figure. ~ 360.
SHORT COMMUNICATIONS
447
Fiber diameter (average of 10 equidistant measurements) is 12.6/~m; the distance between nodes of Ranvier is approximately 350/~m, Since earlier reports have shown that an occasional short internode can be intercalated between longer internodes in both peripheral nerve 7 and main fiber tracts of the mammalian spinal cord 6, observations of successive internodal segments were necessary to distinguish between fibers with an occasional short internode and fibers characterized by uniformly short internodes. In some cases it was possible to follow a fiber along several internodal segments, all of which were relatively short. Fig. 2 illustrates two internodes from a fiber in which 3 consecutive short internodes were observed. Axon diameter along the internodes (average of 9 equidistant measurements) is 3.4/~m, and the internode distances vary between 205 and 215 #m (a variation of about 5 ~). The consistency of the internode spacing in fibers of the type illustrated in Fig. 2 suggests that the relatively short internode distances we have observed can represent a characteristic pattern of myelination of certain central fibers, and do not necessarily represent variation of internode distance along single fibers. Our results suggest that there may be a population of fibers in nuclear regions and neuropil of the mammalian brain which are characterized by a relatively short distance between nodes of Ranvier. The possible effects of variation of internode distance on conduction velocity have been discussed in a previous reporO 1. Conduction velocity le and safety factorS, 9 in preterminal fibers are functions of geometrical parameters, including internode distance and nodal area. Studies on neurogenic electrocytes in sternarchid fishes have demonstrated a functionally significant structural differentiation of nodes of Ranvier along single nerve fibers1,13; thus, the pattern of myelination along a given fiber needs not be invariant. Observations of nodal spacing in teleost neuropil, which are such as to increase conduction velocity less than maximally, suggest that variations in patterns of myelination may modulate spatiotemporal patterning of impulses 11. The occurrence of closely spaced nodes in mammalian brain, and particularly in reticular formation, suggests that variations in the geometry of the central myelin sheath may provide a mechanism for 'velocity matching' or more complex transformations of neural information in the axons of mammalian integrative neurons. This work was supported in part by Grants NB-07512, 5T5-GM-1674 and T01-GM-0102 frcrn the U.S. National Institutes of Health. We wish to thank Dr. G. D. Pappas for helpful advice and for providing laboratory facilities. We also benefited from discussions with Drs. M.V.L. Bennett and D. P. Purpura. Department of Anatomy, Albert Einstein College of Medicine, Bronx, N.Y. 10461 (U.S.A.)
STEPHEN G. WAXMAN RICHARD J. MELKER
1 BENNETT, M. V. L., Comparative physiology: electric organs, Ann. Rev. Physiol., 32 (1970)
471-528. 2 BODIAN,D., A new method for staining nerve fibers and nerve endings in mounted paraffin sections, Anat. Rec., 65 (1936) 89-97. Brain Research, 32 (1971) 445--448
448
SHORT COMMUNICA I IONS
3 BOD1AN, D., A note on nodes of Ranvier in the central nervous system, J. comp. Neurol., 94 (1951) 475-484. 4 BUNGE, R. P., Glial cells and the central myelin sheath, Physiol. Rev., 48 (1968) 197-251. 5 CHUNG, S. H., RAYMOND, S. A., Arid LETTV1N, J. Y., Multiple meaning in single visual units, Brain Behav. Evol., 3 (1970) 72-101. 6 HESS, A., AND YOUr~G, J. Z., The nodes of Ranvier, Proc. roy. Soc. B, 140 (1952) 301-320. 7 LtmifiSKA, L., Short internodes intercalated in nerve fibers, Acta Biol. exp. (Warszawa), 18 (1958) 117-136. 8 THOMAS, P. K., AND YOUNCJ, J. Z., Internode lengths in the nerves of fishes, J. Anat. (Lond.), 83 (1949) 336-350. 9 WALL, P. D., LETTVIN, J. Y., McCULLOCH, W. S., AND PlX'rs, W. H., Factors limiting the maximum impulse transmitting ability of an afferent system of nerve fibers. In C. CHERRY (Ed.), Information Theory, Third London Symposium, Butterworths, London, 1956, pp. 329-344. 10 WAXMAN,S. G., Closely spaced nodes of Ranvier in the teleost brain, Nature (Lond.), 227 (1970) 283-284. 11 WAXMAN,S. G., An ultrastructural study of the pattern of myelination of preterminal fibers in teleost oculomotor nuclei, electromotor nuclei, and spinal cord, Brain Research, 27 (197 l) 189-201. 12 WAXMAN,S. G., AND BENNETI', M. V. L., An analysis of the pattern of myelination of some preterminal fibers in the teleost central nervous system, J. Cell Biol., 47 (1970) 222a. 13 WAXMAN,S. G., PAPPAS, G. D., AND BENNET, M. V. L., Morphological differentiation of active and inactive nodes of Ranvier in a neurogenic electric organ, Proc. 12th Ann. Meeting Amer. Soc. Cell Biol., (1970) in press. 14 WH1TI~AR,M., lnternode length in the skin plexuses of the fish and the frog, Quart. J. micr. Sci., 93 (1952) 307-314. (Accepted June 24th, 1971)
Brain Research, 32 (1971) 445-448
445
Closely spaced nodes of Ranvier in the mammalian brain The distinction between white and gray matter in the central nervous system is based on the presence of tracts of myelinated fibers in the former and relative absence of these in the latter. Nevertheless, neuropil and nuclear regions of the brain and spinal cord contain some myelinated fibers. Because of their complex geometry, there have been few attempts to study patterns of myelination of fibers in neuropil and nuclear regions of the CNS (cf. ref. 4). The one systematic study of fibers from preoptic area, hypothalamus, hypoglossal root, and pyramidal tract of the adult opossum, showed a roughly linear relationship between axon diameter and internode distance, and noted that central fibers and peripheral fibers of comparable diameter (fixed and stained in a similar manner) showed comparable internode distances3. Recent studies 10-1z of preterminal fibers in teleost brain and spinal cord have demonstrated, however, that internode distances in neuropil and nuclear regions can be significantly shorter than previously reported for teleost peripheral nerve s or mammalian CNS3, 6. Internode distances are also relatively short in the skin plexuses of the fish and frog, where there is a progressive decrease in length of myelin segments as the fiber approaches its termination 14, and in teleost neurogenic electric organs 1. The present study was undertaken in order to determine whether there are fibers in mammalian gray matter which deviate in their pattern of myelination from peripheral nerve fibers or fibers in main tracts of the CNS. On the basis of a preliminary survey of fibers in the mammalian brain, we present evidence here which suggests that internode distances in mammalian gray matter can be relatively short. Furthermore, our evidence suggests that shcrt internode distances are characteristic of certain fibers and do not represent variation in internode length along a given fiber. Fifteen #m sections were prepared from the brains of kangaroo rats (Diplodomys merriami), perfused with 10~ formalin and stained by a modified Bodian protargol technique 2, and from the brains cf cats fixed by intravital perfusion with solutions of 2.5 ~o glutaraldehyde in phosphate buffer, followed by immersion in 1 ~o OsO4 in buffer. These were examined by light microscopy. In the case of protargol preparations, Bodian's criteria 3, i.e. the presence of constriction at nodes, were used for identification of nodes and determination of internode distances. Since the protargol method stains only axon cylinders and not myelin, axon diameters (i.e. diameter of the axon without the myelin sheath) were determined with this method. In glutaraldehyde-osmium preparations, nodes could be identified as distinct interruptions of the myelin sheath, associated with constriction of the axon. In these preparations, both axon diameters and fiber diameters (diameter including the sheath) could be measured. The majority of fibers examined, exhibited internode distances comparable to those reported previously for fibers of similar diameter in opossum brain 8. However, careful examination of reticular formation, oculomotor nucleus, and striatum revealed some fibers with internodes considerably shorter than reported previously. The ratio of internode distance to fiber diameter (including the myelin sheath) was as low as 18 for some fibers. One fiber with a short internode distance is shown in Fig. 1.
Brain Research, 32 (1971) 445-448
446
SHOR 1" ('OMMUNICAI I~)NS
Fig. 1. Myelinated fiber from tile mesencephalic reticular f o r m a t i o n of the cat. Two nodes of Ranvier (indicated by arrows) are separated by a distance of approximately 350 Mm; the diameter of the fiber is greater than 12 Mm. The nodes are s h o w n at higher magnification in the insets. Glutaraldehyde o s m i u m , x 440; Insets, ;,~ 1000. Fig. 2. Myelinated fiber from the o c u l o m o t o r nucleus of the k a n g a r o o rat, Diplodomys merriami. N o d e s appear as constricted regions in the a x o n in this protargol preparation. T w o relatively short internodes are s h o w n (nodes indicated by arrows); a third consecutive short internode on the same fiber is n o t included in the figure. ~ 360.
SHORT COMMUNICATIONS
447
Fiber diameter (average of 10 equidistant measurements) is 12.6/~m; the distance between nodes of Ranvier is approximately 350/~m, Since earlier reports have shown that an occasional short internode can be intercalated between longer internodes in both peripheral nerve 7 and main fiber tracts of the mammalian spinal cord 6, observations of successive internodal segments were necessary to distinguish between fibers with an occasional short internode and fibers characterized by uniformly short internodes. In some cases it was possible to follow a fiber along several internodal segments, all of which were relatively short. Fig. 2 illustrates two internodes from a fiber in which 3 consecutive short internodes were observed. Axon diameter along the internodes (average of 9 equidistant measurements) is 3.4/~m, and the internode distances vary between 205 and 215 #m (a variation of about 5 ~). The consistency of the internode spacing in fibers of the type illustrated in Fig. 2 suggests that the relatively short internode distances we have observed can represent a characteristic pattern of myelination of certain central fibers, and do not necessarily represent variation of internode distance along single fibers. Our results suggest that there may be a population of fibers in nuclear regions and neuropil of the mammalian brain which are characterized by a relatively short distance between nodes of Ranvier. The possible effects of variation of internode distance on conduction velocity have been discussed in a previous reporO 1. Conduction velocity le and safety factorS, 9 in preterminal fibers are functions of geometrical parameters, including internode distance and nodal area. Studies on neurogenic electrocytes in sternarchid fishes have demonstrated a functionally significant structural differentiation of nodes of Ranvier along single nerve fibers1,13; thus, the pattern of myelination along a given fiber needs not be invariant. Observations of nodal spacing in teleost neuropil, which are such as to increase conduction velocity less than maximally, suggest that variations in patterns of myelination may modulate spatiotemporal patterning of impulses 11. The occurrence of closely spaced nodes in mammalian brain, and particularly in reticular formation, suggests that variations in the geometry of the central myelin sheath may provide a mechanism for 'velocity matching' or more complex transformations of neural information in the axons of mammalian integrative neurons. This work was supported in part by Grants NB-07512, 5T5-GM-1674 and T01-GM-0102 frcrn the U.S. National Institutes of Health. We wish to thank Dr. G. D. Pappas for helpful advice and for providing laboratory facilities. We also benefited from discussions with Drs. M.V.L. Bennett and D. P. Purpura. Department of Anatomy, Albert Einstein College of Medicine, Bronx, N.Y. 10461 (U.S.A.)
STEPHEN G. WAXMAN RICHARD J. MELKER
1 BENNETT, M. V. L., Comparative physiology: electric organs, Ann. Rev. Physiol., 32 (1970)
471-528. 2 BODIAN,D., A new method for staining nerve fibers and nerve endings in mounted paraffin sections, Anat. Rec., 65 (1936) 89-97. Brain Research, 32 (1971) 445--448
448
SHORT COMMUNICA I IONS
3 BOD1AN, D., A note on nodes of Ranvier in the central nervous system, J. comp. Neurol., 94 (1951) 475-484. 4 BUNGE, R. P., Glial cells and the central myelin sheath, Physiol. Rev., 48 (1968) 197-251. 5 CHUNG, S. H., RAYMOND, S. A., Arid LETTV1N, J. Y., Multiple meaning in single visual units, Brain Behav. Evol., 3 (1970) 72-101. 6 HESS, A., AND YOUr~G, J. Z., The nodes of Ranvier, Proc. roy. Soc. B, 140 (1952) 301-320. 7 LtmifiSKA, L., Short internodes intercalated in nerve fibers, Acta Biol. exp. (Warszawa), 18 (1958) 117-136. 8 THOMAS, P. K., AND YOUNCJ, J. Z., Internode lengths in the nerves of fishes, J. Anat. (Lond.), 83 (1949) 336-350. 9 WALL, P. D., LETTVIN, J. Y., McCULLOCH, W. S., AND PlX'rs, W. H., Factors limiting the maximum impulse transmitting ability of an afferent system of nerve fibers. In C. CHERRY (Ed.), Information Theory, Third London Symposium, Butterworths, London, 1956, pp. 329-344. 10 WAXMAN,S. G., Closely spaced nodes of Ranvier in the teleost brain, Nature (Lond.), 227 (1970) 283-284. 11 WAXMAN,S. G., An ultrastructural study of the pattern of myelination of preterminal fibers in teleost oculomotor nuclei, electromotor nuclei, and spinal cord, Brain Research, 27 (197 l) 189-201. 12 WAXMAN,S. G., AND BENNETI', M. V. L., An analysis of the pattern of myelination of some preterminal fibers in the teleost central nervous system, J. Cell Biol., 47 (1970) 222a. 13 WAXMAN,S. G., PAPPAS, G. D., AND BENNET, M. V. L., Morphological differentiation of active and inactive nodes of Ranvier in a neurogenic electric organ, Proc. 12th Ann. Meeting Amer. Soc. Cell Biol., (1970) in press. 14 WH1TI~AR,M., lnternode length in the skin plexuses of the fish and the frog, Quart. J. micr. Sci., 93 (1952) 307-314. (Accepted June 24th, 1971)
Brain Research, 32 (1971) 445-448