Morphology of synapses in the autonomic nervous system
Authors: Smolen Arnold J. 
Published: February 2005

AbstractThe ultrastructure of synapses in the autonomic nervous system is reviewed. The synaptic organization of the parasympathetic ganglia is relatively simple. Preganglionic axons form synapses either on the soma or on short perikaryal processes of the ganglionic neurons. The presynaptic terminals have a cholinergic morphology and contain mainly small clear vesicles with a few large dense cored vesicles. A few neuropeptides have been localized to the large dense cored vesicles of these terminals. The postganglionic parasympathetic axons ramify within their target tissues where they form close associations, but not true synaptic contacts. Sites of release of transmitter are recognized morphologically as varicosities along the length of the axon that contain clusters of small clear vesicles with a few large dense cored vesicles. The organization of the sympathetic nervous system is somewhat more complex. In addition to acetylcholine, enkephalin also exists in these terminals, probably in the large dense cored vesicles. There are at least three types of ganglion cell neurons in the paravertebral portion of the sympathetic nervous system: those that contain norepinephrine alone, those that contain norepinephrine along with neuropeptide Y, and those that contain acetylcholine and vasoactive intestinal polypeptide. The first type provides innervation to the parenchyma of the target tissues, while the second mainly innervates blood vessels. The third type innervates the sweat glands. In the prevertebral ganglia, a fourth type of neuron exists that contains norepinephrine and somatostatin. This neuron probably innervates the gut. Preganglionic terminals of the cholinergic type form synaptic connections mainly with the dendrites of the sympathetic ganglion neurons. In addition to the types of synapses described for the paravertebral ganglia, neurons in the prevertebral ganglia receive synaptic connections from dorsal root ganglia and from the enteric nervous system. The sympathetic ganglia also contain interneurons that receive preganglionic synapses and form efferent synapses with some of the principal ganglion cells. The interneurons have been shown to contain a variety of transmitters, including norepinephrine, epinephrine, dopamine, serotonin, and a number of neuropeptides. The postganglionic sympathetic axons have a similar morphology to the parasympathetic axons. They form networks in their targets, and the axons display varicosities with concentrations of both small and large vesicles. After appropriate fixation, these vesicles are seen to possess dense cores. The morphology of the enteric nervous system has the greatest complexity. There are sensory neurons, interneurons and motor neurons of various types. In addition, it receives extrinsic connections from both the sympathetic and parasympathetic nervous system. The synaptic ultrastructure is correspondingly complex, with a large number of different, morphologically disticnt types of synaptic terminals. The function and transmitter content of these terminals is beginning to be elucidated.