The Parasympathetic Nervous System Arises from the Brainstem and Spinal Cord
The parasympathetic nervous system generally has long preganglionic and short postganglionic axons. Preganglionic axons of the parasympathetic system leave the CNS by way of cranial nerves III, VII, IX, and X and through several sacral spinal nerves.
For this reason, it is called the craniosacral system (see Figure 13-2). The long preganglionic axons pass to parasympathetic ganglia in or near the target organ, where they synapse with short postganglionic neurons. (In the gastrointestinal system, postganglionic neurons participate in an extensive enteric neuron network called the intrinsic gastrointestinal nervous system, described in Chapter 27.)Most viscera receive both sympathetic and parasympathetic innervation. Although the parasympathetic system originates in brainstem and sacral regions, it can provide parasympathetic innervation to organs in the thoracic and lumbar parts of the body by way of the vagus nerve (cranial nerve X), which wanders to regions of the body cavity. The sympathetic thoracolumbar system can influence organs in cranial and sacral regions by way of preganglionic sympathetic axons that travel to sympathetic postganglionic neurons in cervical and sacral regions of the sympathetic ganglion chain.