The Myelinated Axon

The ability of the nervous system to generate and propagate repetitive nerve impulses at a high speed depends on the myelinated axon—a highly complex biological structure, which has been characterized as ". . . the most unique cyto-logical feature in the animal kingdom . . ." (Fig. 1) (Schnapp and Mugnaini, 1978). A large myelinated axon represents an

Figure I Light micrograph from a longitudinal section from adult cat spinal cord white matter showing a node-paranode-juxtaparanode region of a large fiber. The axon (Ax) is constricted at the node and the paranodes. Note the density of the nodal-paranodal axoplasm. The juxtaparanodal myelin sheaths turn in toward the axon and delimit a narrow and deep node gap (arrows) (x2,000). (With permission from Hildebrand, 1971.)

Figure I Light micrograph from a longitudinal section from adult cat spinal cord white matter showing a node-paranode-juxtaparanode region of a large fiber. The axon (Ax) is constricted at the node and the paranodes. Note the density of the nodal-paranodal axoplasm. The juxtaparanodal myelin sheaths turn in toward the axon and delimit a narrow and deep node gap (arrows) (x2,000). (With permission from Hildebrand, 1971.)

extraordinary union between cellular components with remarkable dimensions. A human S1 a-motoneuron with a diameter of 50 |m has a soma volume of about 65,400 |m3. The volume of the dendritic tree may amount to three to four times the soma volume (Ulfhake and Cullheim, 1988). If the axon has a diameter of 12.5 | m and a length of 1.3 meter (distance from the S1 segment to a plantar muscle), and disregarding the terminal branching of the axon, its volume is about 160 million |im3, 2,400 times the soma volume. If this axon has an average nodal spacing of 1,300 |im, there are altogether 1,000 mainly PNS-type myelin sheaths along its course. Assuming that the average myelin sheath has 119 lamellae, each outrolled sheath measures 1.3 x 5.5 = 7.1 mm2. If we add all the myelin in the 1,000 internodes we get a strip of myelin that measures 1.3 x 5,500 mm. During the myelination period, oligodendrocytes synthesize myelin at a fast rate. It has been estimated that one oligodendrocyte produces 5,000 | m2 of myelin membrane per day and 105 myelin protein molecules per minute (Pfeiffer et al., 1993). According to other calculations, a rat brain oligodendrocyte produces twice its own weight of myelin membrane per day at the peak of myelination (Norton and Poduslo, 1973; Norton and Cammer, 1984).

0 0

Post a comment