Recent physiological evidence indicates that vagal postganglionic control of left ventricular contractility is mediated by neurons found in a ventricular epicardial fat pad ganglion. In the dog this region has been referred to as the cranial medial ventricular (CMV) ganglion [J.L. Ardell, Structure and function of mammalian intrinsic cardiac neurons, in: J.A. Armour, J.L. Ardell (Eds.), Neurocardiology, Oxford Univ. Press, New York, 1994, pp. 95–114; B.X. Yuan, J.L. Ardell, D.A. Hopkins, A.M. Losier, J.A. Armour, Gross and microscopic anatomy of the canine intrinsic cardiac nervous system, Anat. Rec., 239 (1994) 75–87]. Since activation of the vagal neuronal input to the CMV ganglion reduces left ventricular contractility without influencing cardiac rate or AV conduction, this ganglion contains a functionally selective pool of negative inotropic parasympathetic postganglionic neurons. In the present report we have defined the light microscopic distribution of preganglionic negative inotropic neurons in the CNS which are retrogradely labeled from the CMV ganglion. Some tissues were also processed for the simultaneous immunocytochemical visualization of tyrosine hydroxylase (TH: a marker for catecholaminergic neurons) and examined with both light microscopic and electron microscopic methods. Histochemically visualized neurons were observed in a long slender column in the ventrolateral nucleus ambiguus (NA-VL). The greatest number of retrogradely labeled neurons were observed just rostral to the level of the area postrema. TH perikarya and dendrites were commonly observed interspersed with vagal motoneurons in the NA-VL. TH nerve terminals formed axo-dendritic synapses upon negative inotropic vagal motoneurons, however the origin of these terminals remains to be determined. We conclude that synaptic interactions exist which would permit the parasympathetic preganglionic vagal control of left ventricular contractility to be modulated monosynaptically by catecholaminergic afferents to the NA-VL.
Ephaptic coupling refers to interactions between neurons mediated by current flow through the extracellular space. Ephaptic interactions between axons are considered negligible, because of the relatively large extracellular space and the layers of myelin that separate most axons. By contrast, olfactory nerve axons are unmyelinated and arranged in tightly packed bundles, features that may enhance ephaptic coupling. We tested the hypothesis that ephaptic interactions occur in the mammalian olfactory nerve with the use of a computational approach. Numerical solutions of models of axon fascicles show that significant ephaptic interactions occur for a range of physiologically relevant parameters. An action potential in a single axon can evoke action potentials in all other axons in the fascicle. Ephaptic interactions can also lead to synchronized firing of independently stimulated axons. Our findings suggest that ephaptic interactions may be significant determinants of the olfactory code and that such interactions may occur in other, similarly organized axonal or dendritic bundles.
The small, unmyelinated axons of olfactory sensory neurons project to the olfactory bulb in densely packed fascicles, an arrangement conducive to axo-axonal interactions. We recently demonstrated ephaptic interactions between these axons in the olfactory nerve layer, the layer of the olfactory bulb in which the axon fascicles interweave and rearrange extensively. In the present study, we hypothesized that the axons, which express connexins, may have another mode of communication: gap junctions. Previous transmission electron microscopy (TEM) studies have failed to demonstrate such junctions. However, the definitive method for detecting gap junctions, freeze fracture, has not been used to examine the interaxonal connections of the olfactory nerve layer. Here, we apply a combined approach of TEM and freeze fracture to determine if gap junctions are present between the olfactory axons. Gap junctions involving olfactory axons were not found. However, by freeze fracture, P faces of both the axons and ensheathing cells (glia that surround the axon fascicles) contained distinctive linear arrays of particles, aligned along the small columns of extracellular space. In axons, few intramembranous particles were present outside of these arrays. Multi-helix proteins, including ion channels and connexin hemichannels, have been shown to be visible as particles by freeze fracture. This suggests that the proteins important for signal transmission are confined to the linear arrays. Such an arrangement would facilitate ephaptic transmission, calcium waves, current oscillations, and paracrine communication and may be important for olfactory neural code processing.
Previous research from this laboratory has shown that substance P-immunoreactive (SP) terminals synapse upon negative chronotropic vagal preganglionic neurons (VPNs), but not upon negative dromotropic VPNs, of the ventrolateral nucleus ambiguus (NA-VL). Moreover, SP agonists injected into NA-VL cause bradycardia without decreasing AV conduction. In the current study, we have: (1) defined the electron microscopic characteristics of the SP neurons of NA-VL in dog; and (2) tested the hypothesis that SP nerve terminals synapse upon negative inotropic VPNs of NA-VL, retrogradely labeled from the cranial medial ventricular (CMV) ganglion. Numerous SP terminals and a few SP neurons were observed in the vicinity of retrogradely labeled neurons. SP terminals were observed forming synapses with unlabeled dendrites and with SP dendrites, but never with the retrogradely labeled neurons. Together, these results and earlier findings suggest that SP agonists may be able to induce bradycardia without decreasing AV conduction or ventricular contractility.
