Axons of sacral preganglionic neurons in the cat: II. Axon collaterals.

Abstract

Axon collaterals were identified in 21 of 24 preganglionic neurons in the lateral band of the sacral parasympathetic nucleus of the cat. Following the intracellular injection of HRP or neurobiotin the axons from 20 of these neurons were followed and 53 primary axon collaterals were found to originate from unmyelinated segments and from nodes of Ranvier. Detailed mapping done in the five best labeled cells showed bilateral axon collaterals distributions up to 25,000 microm in length with 950 varicosities and unilateral distributions up to 12,561 microm with 491 varicosities. The axon collaterals appeared to be unmyelinated, which was confirmed at EM, and were small in diameter (average 0.3 microm). Varicosities were located mostly in laminae I, V, VII, VIII and X and in the lateral funiculi. Most varicosities were not in contact with visible structures but some were seen in close apposition to Nissl stained somata and proximal dendrites. Varicosities had average minor diameters of 1.3 microm and major diameters of 2.3 microm. Most were boutons en passant while 10-20% were boutons termineaux. EM revealed axodendritic and axoaxonic synapses formed by varicosities and by the axons between varicosities. It is estimated that the most extensive of these axon collaterals systems may contact over 200 spinal neurons in multiple locations. These data lead to the conclusion that sacral preganglionic neurons have multiple functions within the spinal cord in addition to serving their target organ. As most preganglionic neurons in this location innervate the urinary bladder, it is possible that bladder preganglionic neurons have multiple functions.