Structure of peripheral synapses: autonomic ganglia

Abstract

Final motor neurons in sympathetic and parasympathetic ganglia receive synaptic inputs from preganglionic neurons. Quantitative ultrastructural analyses have shown that the spatial distribution of these synapses is mostly sparse and random. Typically, only about 1%-2% of the neuronal surface is covered with synapses, with the rest of the neuronal surface being closely enclosed by Schwann cell processes. The number of synaptic inputs is correlated with the dendritic complexity of the target neuron, and the total number of synaptic contacts is related to the surface area of the post-synaptic neuron. Overall, most neurons receive fewer than 150 synaptic contacts, with individual preganglionic inputs providing between 10 and 50 synaptic contacts. This variation is probably one determinant of synaptic strength in autonomic ganglia. Many neurons in prevertebral sympathetic ganglia receive additional convergent synaptic inputs from intestinofugal neurons located in the enteric plexuses. The neurons support these additional inputs via larger dendritic arborisations together with a higher overall synaptic density. There is considerable neurochemical heterogeneity in presynaptic boutons. Some synapses apparently lack most of the proteins normally required for fast transmitter release and probably do not take part in conventional ganglionic transmission. Furthermore, most preganglionic boutons in the ganglionic neuropil do not form direct synaptic contacts with any neurons. Nevertheless, these boutons may well contribute to slow transmission processes that need not require conventional synaptic structures.

Quantitative study of synaptic boutons on frog intracardiac neurons

“…Although it was considered that frog intracardiac neurons have no dendrites [4], previous study has shown abundance of dendrite-like processes on the hillock and the proximal axon of frog intracardiac neurons [1]. Th ese processes do not terminate with synaptic boutons [1].…”

Effect of streptozotocin-induced diabetes on the autonomic ganglia of albino rats

“…[42] Most neurons are noradrenergic and supply the stomach, spleen, pancreas, small intestine and mesen-teric blood vessels. [43] In the current study, we identified small, medium and large sized neurons.…”

Stereological and Allometric Studies on Neurons and Axo-Dendritic Synapses in Superior Cervical Ganglia

“…The first type provides innervation to the parenchyma of the target organs, while the second mainly innervates blood vessels. The third type innervates the sweat glands (Gibbins and Morris, 2006;Smolen, 1988).…”

“…Larger species possess SCG neurons which exhibit greater convergence, more complex arborizations and a greater proportion of axo-dendritic compared to axo-somatic synapses (Forehand, 1985;Purves and Lichtman, 1985;Purves et al, 1986). Therefore, in autonomic ganglia-including the SCG-a decision should be made to confine counts to axo-dendritic synapses rather than axo-somatic ones, since the former occur more frequently (Forehand, 1985;Gibbins and Morris, 2006;Loesch et al, 2010;Matthews, 1983).…”

“…However, even this appearance may not imply a functional synapse and the same is true for light microscopy (Gibbins and Morris, 2006). Ideally, a range of chemical markers needs to be localized and correlated with electrophysiological data for the same populations of neurons (Gibbins and Morris, 2006).…”