Ultrastructural features of neurons and nerve fibres in the spiral ganglia of cats

Summary

Most neurons in the cat's spiral ganglia are bipolar myelinated cells; a few are unmyelinated. The usual cytoplasmic organelles are found in both types of neurons. However unmyelinated cells and some of the myelinated ones are especially rich in neurofilaments and neurotubules. The perikarya of myelinated cells are surrounded by 4–25 lamellae contributed by more than one Schwann cell. Conversely, one Schwann cell can ensheath more than one neuron. Perikaryal myelin consists of a combination of the loose and compact variety. Irregularities in loose myelin probably account for the variation in number of layers among segments of an ensheathment; some lamellae change direction to form myelin convolutions within a neuronal covering. Unmyelinated neurons are covered incompletely by 1–3 layers of Schwann cells. Processes of these cells remain unmyelinated whereas those of myelinated neurons become more heavily myelinated distal to the first node of Ranvier. Regions of the axolemma of unmyelinated and myelinated processes are free of Schwann cell covering and are separated from the interstitial space by basement membrane only. The physiology of the eighth nerve relevant to our findings are discussed.