Axons of sacral preganglionic neurons in the cat: I. Origin, initial segment, and myelination

Parasympathetic preganglionic neurons in the cat sacral spinal cord innervate intraspinal neurons and pelvic target organs. Retrograde tracing studies have revealed little of the morphology of their axons including their origin, initial segments, or their myelin, due to methodological limitations. Intracellular labeling of single neurons with neurobiotin or HRP has overcome these problems. Axons were studied in 24 preganglionic neurons. In 21 neurons the axon originated as a branch of a dendrite, without a detectable axon hillock, at distances from the soma ranging from 10 to 110 microm (average 34.1 microm ). In 3 neurons the axon was derived from the soma. Initial segments, present in all cells, ranged from 15 to 40 microm (average 26.8 microm). Nearly all axons followed the initial segment with unmyelinated segments that varied between 59 to 630 microm, followed by myelin and nodes of Ranvier. Internodal distances were variable and relatively short (average 93 microm). Axonal diameters measured over the intraspinal course in 18 axons averaged 1.3 microm (range 0.6-2.4 microm) and were relatively constant compared with other neurons. Spine-like protrusions were observed on the initial segments of 12 cells. Axon collaterals originated from unmyelinated sections and nodes of Ranvier. Antidromic action potentials showing initial segment, soma-dendritic inflections, did not differentiate between soma-derived and dendrite-derived axons. The data suggest that axons originating from a dendrite are the normal structure of preganglionic neurons in the lateral sacral parasympathetic nucleus. It is proposed that the particular structure of these axons may be part of a timing mechanism that coordinates preganglionic neurons with other spinal neurons involved in target organ reflexes.