7.2 Divisions of the Autonomic Nervous System

Parasympathetic Division of the Autonomic Nervous System

The parasympathetic division of the autonomic nervous system is named because its preganglionic neurons are located on either side of the thoracolumbar region of the spinal cord (“para-” meaning “beside” or “near”). The parasympathetic system can also be referred to as the craniosacral system (or outflow) because the preganglionic neurons are located in nuclei of the brainstem and the lateral horn of the sacral spinal cord.

The preganglionic fibers from the cranial region travel in cranial nerves, whereas preganglionic fibers from the sacral region travel in spinal nerves. The targets of these fibers are terminal ganglia, which are located near—or even within—the target effector. These ganglia are often referred to as intramural ganglia when they are found within the walls of the target organ. The postganglionic fiber projects from the terminal ganglia a short distance to the target effector or to the specific target tissue within the organ. Comparing the relative lengths of axons in the parasympathetic system, the preganglionic fibers are long and the postganglionic fibers are short because the ganglia are close to—and sometimes within—the target effectors (Figure 7.2.1).

The cranial component of the parasympathetic system is based in particular nuclei of the brain stem. For example, in the midbrain, the Edinger–Westphal nucleus is part of the oculomotor complex, and axons from those neurons travel with the fibers in the oculomotor nerve (cranial nerve III) that innervate the extraocular muscles. The preganglionic parasympathetic fibers within cranial nerve III terminate in the ciliary ganglion, which is located in the posterior orbit. The postganglionic parasympathetic fibers then project to the smooth muscle of the iris to control pupillary size.

Sympathetic Division of the Autonomic Nervous System

To respond to a threat—to fight or to run away—the sympathetic system causes divergent effects as many different effector organs are activated together for a common purpose. More oxygen needs to be inhaled and delivered to skeletal muscle. The respiratory, cardiovascular, and musculoskeletal systems are all activated together. Additionally, sweating keeps the excess heat that comes from muscle contraction from causing the body to overheat. The digestive system shuts down so that blood is not absorbing nutrients when it should be delivering oxygen to skeletal muscles.

The sympathetic division of the autonomic nervous system influences the various organ systems of the body through connections emerging from the thoracic and upper lumbar spinal cord. A preganglionic in the lateral horn of any of these spinal regions projects to ganglia adjacent to the vertebral column through the ventral spinal roots. The majority of ganglia of the sympathetic system belong to a network of sympathetic chain ganglia (also known as paravertebral ganglia) that runs alongside the vertebral column. The ganglia appear as a series of clusters of neurons linked by axonal bridges. There are typically 23 ganglia in the chain on either side of the spinal column. Three correspond to the cervical region, 12 are in the thoracic region, four are in the lumbar region, and four correspond to the sacral region. The cervical and sacral levels are not connected to the spinal cord directly through the spinal roots but through ascending or descending connections through the bridges within the chain.

A diagram that shows the connections of the sympathetic system is somewhat like a circuit diagram that shows the electrical connections between different receptacles and devices. In Figure 7.2.2, the “circuits” of the sympathetic system are intentionally simplified.

The cell bodies of the preganglionic neurons reside in the lateral horn and leave the cord via the ventral root. The neurons can then synapse on the sympathetic chain ganglia (also known as paravertebral ganglia) or the collateral ganglia (also known as prevertebral ganglia). The paravertebral ganglia are on either side of the vertebral column. The prevertebral ganglia are anterior to the vertebral column. The synapses with the trunk ganglia can occur at the same level or at higher or lower level. This means there are three different types of “junctions” that operate within the sympathetic system, meaning the synapses between the pre- and postganglionic neurons can occur at three different locations (Figure 7.2.3). The neurons can synapse at the same level, at a higher or lower level, or in a collateral ganglion.

The fibers which synapse with the chain are known as the white rami communicantes (singular ramus communicans); they are mostly myelinated and therefore referred to as white. The postganglionic neurons exit via branches or fibers known as the gray rami communicantes, which are unmyelinated axons, and continue to their effectors. As mentioned, there are two types: ones that synapse at the same level and ones that synapse at higher or lower level.

The first type is most direct: the sympathetic nerve projects to the chain ganglion at the same level as the target effector (Figure 7.2.3a). An example of this type is spinal nerve T1 that synapses with the T1 chain ganglion to innervate the trachea. The axon from the preganglionic neuron (the preganglionic fiber shown as a solid line) synapses with the postganglionic neuron (with the postganglionic fiber shown as a dashed line). This neuron then projects to a target effector—in this case, the trachea.

In the second type, the target effectors are located superior or inferior to the spinal segment at which the preganglionic fiber emerges. With respect to the “wiring” involved, the synapse with the postganglionic neuron occurs at chain ganglia superior or inferior to the location of the preganglionic neuron. An example of this is spinal nerve T1 that innervates the eye. The spinal nerve tracks up through the chain until it reaches the superior cervical ganglion, where it synapses with the postganglionic neuron (Figure 7.2.3b).

Lastly, not all axons from the preganglionic neurons terminate in the chain ganglia. Additional branches from the ventral nerve root continue through the chain and on to one of the collateral ganglia as the splanchnic nerves, including the greater splanchnic nerve, lesser splanchnic nerve, and least splanchnic nerve. For example, the greater splanchnic nerve at the level of T5 synapses with a collateral ganglion outside the chain before making the connection to the postganglionic nerves that innervate the stomach (Figure 7.2.3c).

Collateral ganglia are situated anterior to the vertebral column and receive inputs from splanchnic nerves as well as preganglionic sympathetic neurons. They are associated with controlling organs in the abdominal cavity and are also considered part of the enteric nervous system. The three collateral ganglia are the celiac ganglion, the superior mesenteric ganglion, and the inferior mesenteric ganglion (Figure 7.2.3). The word celiac is derived from the Latin word “coelom,” which refers to a body cavity (in this case, the abdominal cavity), and the word mesenteric refers to the digestive system.

One type of preganglionic sympathetic fiber does not terminate in a ganglion. These are the axons from preganglionic sympathetic neurons that project to the adrenal medulla, the interior portion of the adrenal gland. These axons are still referred to as preganglionic fibers, but the target is not a ganglion. The adrenal medulla releases signaling molecules into the bloodstream, rather than using axons to communicate with target structures. The cells in the adrenal medulla that are contacted by the preganglionic fibers are called chromaffin cells. These cells are neurosecretory cells that develop from the neural crest along with the sympathetic ganglia, reinforcing the idea that the gland is, functionally, a sympathetic ganglion.

The projections of the sympathetic division of the autonomic nervous system diverge widely, resulting in a broad influence of the system throughout the body. As a response to a threat, the sympathetic system would increase heart rate and breathing rate, increase blood flow to the skeletal muscle, and decrease blood flow to the digestive system. Sweat gland secretion should also increase as part of an integrated response. All of those physiological changes are going to be required to occur together to run away from the hunting lioness—or the modern equivalent.

The sympathetic preganglionic fibers release ACh, which excites the ganglionic neuron through the nicotinic receptor. The axon from the ganglionic neuron—the postganglionic fiber—then projects to a target effector where it will release norepinephrine to bind to an adrenergic receptor, causing a change in the physiology of that organ in keeping with the broad, divergent sympathetic response. As you might recall, there are two types of α-adrenergic receptors, termed α₁, α₂, and there are three types of β-adrenergic receptors, termed β₁, β₂, and β₃. The receptor expressed depends on the organ. See Table 7.3 for a summary. The postganglionic connections to sweat glands in the skin and blood vessels supplying skeletal muscle are, however, exceptions; those fibers release ACh onto muscarinic receptors.