Axoplasmic organelles at nodes of Ranvier. II. Occurrence and distribution in large myelinated spinal cord axons of the adult cat

Abstract

The occurrence and distribution of axoplasmic organelles in large myelinated axons of the ventral, the lateral and the dorsal funiculi of L₇ spinal cord segments of the cat have been studied using electron microscopy (EM). Most organelles were found to be concentrated to the paranode-node-paranode (pnp)-regions and they showed their highest relative concentration in the constricted part of these regions, i.e. at the nodes of Ranvier. In the paranode-node-paranode-regions of the lateral and dorsal funiculi, large dense bodies predominated distal to the nodal mid-level and vesiculo-tubular membranous organelles proximal to it. This pattern of organelle distribution, a proximo-distal (with reference to the neuron soma) segregation of the organelles, was only faintly indicated in the paranode-node-paranode-regions of the alpha motor axons of the ventral funiculus. These paranode-node-paranode-regions were, apart from a weak proximo-distal segregation of a few organelles, characterized by deposits of electron dense granules and clusters of large round mitochondria. We conclude that there are two types of organelle accumulation and distribution in the paranode-node-paranode-regions of large spinal cord nerve fibres of the cat. One type is found in the lateral and dorsal funiculi, i.e. in axons with terminal (synaptic) fields inside the blood-brain-barrier. The other type is found in the alpha motor axons of the ventral funiculus, i.e. in axons with their terminal field in the PNS and thus outside the blood-brain barrier. It should be noted that retrogradely transported material in the alpha motor axons has passed through a long sequence of paranode-node-paranode-regions equipped with Schwann cells before it reaches the CNS, while material transported retrogradely in the axons of the dorsal and lateral funiculi has not. The following discussion includes a comparison of the organelle accumulation and distribution in these two types of CNS paranode-node-paranode-regions with the organelle accumulation and distribution observed in the paranode-node-paranode-regions of PNS axons.