The most remarkable feature of the autonomic nervous system is that a major part of it is located outside the brain and spinal cord, in proximity to the visceral structures that it innervates. This position alone seems to symbolize its relative independence from the cerebrospinal system. In distinction to the somatic neuromuscular system, where a single motor neuron bridges the gap between the central nervous system (CNS) and the effector organ, in the autonomic nervous system there are always two efferent neurons serving this function, one (preganglionic) arising from its nucleus in the brainstem or spinal cord and the other (postganglionic) arising from specialized nerve cells in peripheral ganglia. Figure 25-1 illustrates this fundamental anatomic feature. Preganglionic neurons are part of CNS, forming a central autonomic network, which consists of reciprocally connected structures located at the cortex, hypothalamus, brainstem, and spine. Postganglionic neurons are divided into sympathetic and parasympathetic.
The autonomic nervous system, from an anatomic point of view, is divided into two parts: the craniosacral, or parasympathetic, and the thoracolumbar, or sympathetic (Figs. 25-2 and 25-3). Systems differ architecturally in that the ganglion in the sympathetic nervous system is located in a contiguous and interconnected, longitudinal chain (sympathetic chain) paravertebrally, whereas the parasympathetic ganglia are distributed in proximity to the structures they innervate. Moreover, the main neurotransmitter of the postganglionic connection to the end organ is norepinephrine in the case of the sympathetic nerves and acetylcholine for parasympathetic innervation. There are exceptions with regard to the sympathetic innervation of sweat glands (sudomotor), which are cholinergic. The neurotransmitter between the preand postneurons throughout the autonomic nervous system, sympathetic and parasympathetic, is acetylcholine as reiterated further on. These synapses between preand postganglionic cholinergic nerves are not blocked by atropine (nicotinic) whereas the postganglionic impulses are blocked by atropine (muscarinic).
Functionally, the two parts are complementary in maintaining a balance in the tonic activities of many visceral structures and organs. This rigid separation into sympathetic and parasympathetic parts, although useful for purposes of exposition, is physiologically not absolute. From a neurologist’s perspective, the two components are often affected together. Nonetheless, the notion of a balanced sympathetic and parasympathetic autonomic system has stood the test of time and remains a valid concept.
The Parasympathetic Nervous System (See Fig. 25-2)
There are two divisions of the parasympathetic nervous system: cranial and sacral. The cranial division originates in the visceral nuclei of the midbrain, pons, and medulla. These nuclei include the Edinger-Westphal pupillary nucleus, superior and inferior salivatory nuclei, dorsal motor nucleus of the vagus, and adjacent reticular nuclei.
Axons (preganglionic fibers) of the visceral cranial nuclei course through the oculomotor, facial, glossopharyngeal, and vagus cranial nerves. The preganglionic fibers from the Edinger-Westphal nucleus traverse the oculomotor nerve and synapse in the ciliary ganglion in the orbit; axons of the ciliary ganglion cells innervate the ciliary muscle and pupillary sphincter (see Fig. 13-9).