11.2: The Sympathetic Division of the ANS

Learning Objectives

By the end of this section, you will be able to:

• Name the components that generate the sympathetic responses of the autonomic nervous system
• Explain the bodily responses associated with the sympathetic divisions
• Describe the signaling molecules and receptor proteins involved in communication within the sympathetic division

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. To coordinate all these responses, the connections in the sympathetic system diverge from a limited region of the central nervous system (CNS) to a wide array of ganglia that project to the many effector organs simultaneously. The complex set of structures that compose the output of the sympathetic system make it possible for these disparate effectors to come together in a coordinated, systemic change.

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. It is referred to as the thoracolumbar system to reflect this anatomical basis. A central neuron 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 called the sympathetic chain ganglia that runs alongside the vertebral column. 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.

Figure \(\PageIndex{1}\) Sympathetic Division of the ANS. (Image and caption credit: "Autonomic Nervous System" by OpenStax is licensed under CC BY 4.0) / Cropped from original

As mentioned previously, the preganglionic neuron projects out to synapse onto a postganglionic neuron. A preganglionic neuron of the sympathetic division will release acetylcholine onto the postganglionic neuron. The postganglionic neuron will then respond by releasing epinephrine or norepinephrine to the target organ. The target organ will bind epinephrine or norepinephrine through adrenergic receptors on its cell membrane. The binding of epinephrine or norepinephrine to the adrenergic receptors of the target organ will then trigger the sympathetic response in that organ. An exception exists when the target organ is a sweat gland. A sympathetic postganglionic neuron will release acetylcholine if the target organ is a sweat gland. The sweat gland will bind to acetylcholine via muscarinic receptors and sweat will be released.

One type of preganglionic sympathetic fiber does not terminate in a ganglion. These are the axons from central 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 adrenal medulla will release norepinephrine/epinephrine into the bloodstream to enhance the sympathetic nervous system's effects.

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 and cause blood flow to the skeletal muscle to increase and blood flow to the digestive system to decrease. 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 variety of responses by the sympathetic nervous system can be seen in Figure \(\PageIndex{2}\) below.

Figure \(\PageIndex{2}\) Sympathetic Innervation: Neurons from the lateral horns of the spinal cord extend their preganglionic nerve fibers (solid lines) to the sympathetic chain ganglia on the lateral sides of the vertebral column. Axons may terminate here or pass through and terminate in prevertebral (collateral) ganglia that are anterior to the vertebral column. Axons from the ganglionic neurons, called postganglionic nerve fibers (dashed lines), then project to target effectors throughout the body. (Image and caption credit: "ANS - Sympathetic Division Innervation" by Jennifer Lange is licensed under CC BY-NC-SA 4.0. Image incorporates originals from Functional Neuroanatomy, KnowledgeWorks, Patrick J. Lynch, and Ron Slagter.)