14.2: Divisions of the Autonomic Nervous System

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

• Compare and contrast the somatic and autonomic nervous systems
• Describe the functional differences between the sympathetic and parasympathetic divisions
• Outline the anatomical differences between the two divisions of the ANS
• Describe the preganglionic neurons, ganglia, nerves and pathways of the two divisions of the ANS

Comparison between the Somatic and Autonomic Nervous System

The nervous system can be divided into two functional parts: the somatic nervous system (SNS) and the autonomic nervous system (ANS). The major difference between these two systems is based on whether you are conscious of its process. The somatic nervous system consciously detects sensory stimuli from the special senses, skin and proprioceptors. The autonomic nervous system reflexively responds to visceral sensory stimuli, such as levels of carbon dioxide concentration in the blood or stretch caused by blood pressure, that you are not consciously aware of. Moreover, the motor efferent branches of these two systems innervate different target effectors. While the somatic motor neurons innervate and cause contraction of skeletal muscles, autonomic motor neurons innervate and control cardiac and smooth muscle, as well as glandular tissue. Thus, the motor response of the somatic nervous system is voluntary while the one of the autonomic nervous system is involuntary.

Another major difference between these two systems lies within the number of lower motor neurons that are involved in the response. In the somatic nervous system, a single lower somatic motor neuron of the brainstem or spinal cord extends from the CNS towards a skeletal muscle through a cranial or spinal nerve, respectively (Figure \(\PageIndex{1}\)). These somatic motor neurons have large myelinated axons that release acetylcholine (ACh) at neuromuscular junctions.

Figure \(\PageIndex{1}\): Comparison of Somatic and Visceral Motor Pathways.  The somatic motor pathway has a single efferent neuron between the CNS and the target effector organ whereas the visceral motor pathway has two efferent neurons. (Image credit: "Comparison of Somatic and Visceral Motor Pathways" by Jennifer Lange, with muscle by Justin Greene, is licensed under CC BY-NC-SA 4.0, modification of original by OpenStax College, CC BY 3.0, via Wikimedia Commons.)

In comparison, the autonomic nervous system is composed of a chain of two lower motor neurons (Figure \(\PageIndex{1}\)). The cell body of the first of the two ANS motor neurons is located in the brainstem or spinal cord and is called a preganglionic neuron. The axon of the preganglionic neuron extends outside of the CNS through cranial or spinal nerves forming a preganglionic fiber. This fiber projects to an autonomic ganglion of the peripheral nervous system. Preganglionic neurons have small myelinated axons that release acetylcholine (ACh) to excite a second motor neuron. The second motor neuron is called a ganglionic neuron. The cell body of the ganglionic neuron resides within the autonomic ganglion and its axon extends to an effector (cardiac muscle, smooth muscle, or gland) forming a postganglionic fiber. Ganglionic neurons have small unmyelinated axons that release either acetylcholine (ACh) or norepinephrine (NE) to either excite or inhibit an effector, depending on the type of receptors present on the effector.  The differences between the somatic motor and visceral motor systems are summarized in Table \(\PageIndex{1}\).

Table \(\PageIndex{1}\):  Comparison of Somatic and Visceral Motor Systems

Feature	Somatic Motor	Visceral Motor
Type of control	Voluntary	Involuntary
Number of neurons in pathway	One: extends from CNS to effector	Two: preganglionic from CNS to ganglion, ganglionic from ganglion to effector
Ganglia	None	Autonomic ganglia: sympathetic chain (paravertebral), prevertebral, intramural or terminal
Effectors	Skeletal muscle fibers	Smooth muscle fibers, cardiac muscle fibers, glands
Effector response	Excitation	Excitation or inhibition
Neurotransmitter(s) released	Acetylcholine (Ach)	Preganglionic: Acetylcholine Ach) Postganglionic: Acetylcholine (Ach) or norepinephrine
Myelination	Myelinated	Preganglionic: myelinated Postganglionic: unmyelinated
Simplified Pathway	(Image credit: "Somatic Motor Pathway" by Gabrielle Spurlock is licensed under CC BY-NC-SA 4.0, modification of original by BlueLink licensed under CC BY-NC 4.0 with notification of the original authors.)	(Image credit: "Autonomic Motor Pathway" by Gabrielle Spurlock is licensed under CC BY-NC-SA 4.0, modification of original by BlueLink licensed under CC BY-NC 4.0 with notification of the original authors.)

Divisions of the Autonomic Nervous System

The autonomic nervous system regulates many of the internal organs through a balance of two aspects, or divisions. The two divisions of the autonomic nervous system are the sympathetic division and the parasympathetic division. From a functional point of view, the sympathetic system is associated with the fight-or-flight response, while the parasympathetic activity is referred to by the epithet rest-and-digest. Homeostasis is the balance between the two divisions since one system complements the other. For example, the parasympathetic division will be more active when you need to conserve energy and replenish nutrient stores. The sympathetic division will activate when during exercise, stress or emergency situations. Many autonomic target effectors have dual innervation by both divisions of the autonomic nervous system, which determines their activity. For example, the heart receives connections from both the sympathetic and parasympathetic divisions: one causes heart rate to increase, whereas the other causes heart rate to decrease. From an anatomical point of view, both divisions use preganglionic and ganglionic neurons to innervate cardiac muscle, smooth muscle and glands. However, the location of preganglionic neurons within the CNS is different between the two divisions. Moreover, the location of the ganglia as well as the length of preganglionic and postganglionic axons differ in the two divisions. In the following sections, you will examine the functional and anatomical features of the sympathetic and parasympathetic divisions.

Sympathetic Division

To respond to a threat—to fight or to run away—the sympathetic system causes diverse 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 various organ systems of the body through connections emerging from the first thoracic (T1) and second lumbar (L2) spinal segments (Figure \(\PageIndex{2}\)). It is referred to as the thoracolumbar system to reflect this anatomical basis. Sympathetic preganglionic neurons are located in the lateral horns of any of these spinal regions and project to ganglia adjacent to the vertebral column through the anterior roots of the spinal cord.

Sympathetic Neurons, Ganglia and Nerves

The majority of ganglia of the sympathetic system belong to a network of sympathetic chain (or trunk) ganglia that runs lateral to the vertebral column (Figure \(\PageIndex{2}\)). For this reason, these ganglia can also be called paravertebral ganglia. The ganglia appear as a series of clusters of neurons linked by ascending and descending axonal bridges called sympathetic trunks. There are typically 23 ganglia in the sympathetic 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 thoracic and lumbar sympathetic preganglionic fibers travel sequentially through ventral roots, spinal nerves and bundles of myelinated axons called white rami communicans (singular = ramus communicans) to reach the correspondent paravertebral ganglia (Figure \(\PageIndex{2}\)). Here preganglionic sympathetic fibers either synapse with ganglionic neurons or often pass on through the sympathetic chain ganglion into one of its emerging nerves to synapse with ganglionic neurons elsewhere. Postganglionic fibers then travel through additional nerves to their destination in one of the organs. Many of the fibers from the postganglionic neurons in the sympathetic chain ganglia pass back into the spinal nerves through gray rami communicans composed of unmyelinated axons and carry sympathetic information through the spinal nerves.

Figure \(\PageIndex{2}\): Sympathetic Chain. The sympathetic chain is a series of ganglia connected to each other by the sympathetic trunks.  They are connected to the spinal nerve by the rami communicans. (Image credit: "Spinal Cord and Spinal Nerve Branching - ANS" by Jennifer Lange is licensed under CC BY-NC 4.0.)

The cervical and sacral paravertebral ganglia are not connected to the spinal cord directly through the spinal nerves, but through sympathetic trunks. Among the cervical ganglia, the superior cervical ganglion contains ganglionic neurons that innervate structures of the head and neck such the dilator pupillae and superior tarsal muscles of the eye, the lacrimal gland, mucous membranes of the nose, palate and mouth, and salivary glands. The middle and inferior cervical ganglia contain ganglionic neurons that innervate neck and thoracic organs such as the larynx, trachea, pharynx, smooth muscle of arteries and heart.

Preganglionic sympathetic axons extending from T5-L2 do not synapse in a sympathetic chain ganglion and instead continue through the chain anteriorly towards the abdominal and pelvic organs (Figure \(\PageIndex{3}\)). These axons typically terminate in three autonomic ganglia called prevertebral (or collateral) ganglia: the celiac ganglion, the superior mesenteric ganglion, and the inferior mesenteric ganglion. They are referred to as prevertebral because they are anterior to the vertebral column and descending aorta. The prevertebral ganglia are associated with controlling organs in the abdominal cavity, and are also considered part of the enteric nervous system. 

The neurons of the sympathetic autonomic ganglia are multipolar in shape, with dendrites radiating out around the cell body where synapses from the spinal cord neurons are made. Because the sympathetic ganglia are adjacent to the vertebral column, preganglionic sympathetic fibers are relatively short, and they are myelinated. Compared with the preganglionic fibers, postganglionic sympathetic fibers are long because of the relatively greater distance from the ganglion to the target effector. These postganglionic fibers are unmyelinated. 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 \(\PageIndex{3}\), the neural pathways of the sympathetic system are intentionally simplified.

Figure \(\PageIndex{3}\): 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 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.)

Sympathetic Pathways

A sympathetic preganglionic axon leaving the lateral horn of the thoracolumbar spinal cord enters the sympathetic chain ganglia, where it branches toward 10-20 targets. To continue with the analogy of the circuit diagram, there are four different types of “junctions” that connect the sympathetic preganglionic axons with their effectors. The type of pathway is determined by the location and type of target effector organ being innervated. In all cases, the preganglionic axon extends into the spinal nerve at the same level as its spinal cord segment. Then it can then either

1. synapse in the paravertebral ganglion and carry information through the spinal nerve at the same level (spinal nerve pathway),  
2. ascend to a more superior or descend to a more inferior paravertebral ganglion, synapse there and carry information through sympathetic nerves (sympathetic nerve pathway),
3. descend to a prevertebral (collateral) ganglion, synapse there and carry information through a splanchnic nerve (splanchnic nerve pathway), or
4. project directly to the adrenal medulla (adrenal medulla pathway).

All of these branches mean that one preganglionic neuron can influence different regions of the sympathetic system very broadly, by acting on widely distributed organs. In the following paragraphs, you will examine these four different pathways. Except for the adrenal medulla pathway, these connections are represented in Figure \(\PageIndex{4}\).

The spinal nerve pathway is the most direct connection. The sympathetic preganglionic nerve projects to the sympathetic chain ganglion at the same level as the target effector. The myelinated preganglionic fiber extending from the lateral horns of the spinal cord projects to the sympathetic chain ganglion through the ventral root and spinal nerve. Through the white ramus communicans, the fiber reaches and synapses with the ganglionic neuron in the sympathetic chain ganglion. The postganglionic fiber then projects to the target effector via the gray ramus communicans, which is formed by unmyelinated axons. An example of this type is spinal nerve T1 that synapses with the T1 sympathetic chain ganglion to innervate the skin (Figure \(\PageIndex{4}\).a). Indeed, this pathway generally innervates integumentary structures such as sweat glands, arrector pili muscles, and blood vessels of the skin in the neck, torso and limbs.

The postganglionic sympathetic nerve pathway occurs when the target effectors are located superior or inferior to the spinal segment at which the sympathetic preganglionic fiber emerges. With respect to the “wiring” involved, the synapse with the ganglionic neuron occurs at sympathetic chain ganglia superior or inferior to the location of the preganglionic neuron. In order to do this, the preganglionic fiber travels through sympathetic trunks to reach the superior or inferior sympathetic chain ganglion. The postganglionic fiber does not leave the ganglion through the gray ramus communicans. Instead, it extends away from the ganglion through a sympathetic nerve. An example of this is spinal nerve T1 that innervates the eye. The spinal nerve tracks up through the sympathetic trunks until it reaches the superior cervical ganglion, where it synapses with the ganglionic neuron and projects to the eye through a sympathetic nerve (Figure \(\PageIndex{4}\).b). This pathway innervates viscera of head (sweat glands, arrector pili muscles, blood vessels of the skin; dilator pupillae, tarsal muscle and gland of the eye; salivary glands) and neck, and thoracic organs such as esophagus, heart, lungs, thoracic blood vessels.

Not all axons from the central neurons terminate in the sympathetic chain ganglia. The splanchnic nerve pathway include branches from the ventral nerve root that continue through the sympathetic chain ganglion and on to one of the prevertebral (collateral) ganglia as the greater splanchnic nerve or lesser splanchnic nerve. For example, the greater splanchnic nerve at the level of T5 synapses with a prevertebral (collateral) ganglion outside the sympathetic chain before making the connection to the postganglionic nerves that innervate the stomach (Figure \(\PageIndex{4}\).c). This pathway innervates abdominopelvic organs such as stomach, intestines, kidneys, ureters, urinary bladder and reproductive organs.

Figure \(\PageIndex{4}\): Sympathetic Pathways. The axon from a central sympathetic neuron in the spinal cord can project to the periphery in a number of different ways. (a) represents the spinal nerve pathway where the preganglionic fiber projects out and synapses onto a ganglionic neuron at the same level which then carries the information out through the spinal nerve. (b) represents the postganglionic sympathetic nerve pathway where a preganglionic branch projects and synapses onto a more superior or inferior ganglion in the chain which then carries the information out through a sympathetic nerve. (c) represents the splanchnic nerve pathway where a preganglionic branch extends through the white ramus communicans, but does not terminate on a ganglionic neuron in the chain. Instead, it projects through one of the splanchnic nerves to a prevertebral (collateral) ganglion. (Image credit: "Sympathetic Pathways" by OpenStax is licensed under CC BY 4.0, modification by Jennifer Lange.)

There is one additional way that preganglionic sympathetic fibers can control their effector organs and it is through the adrenal medulla pathway. In this unique pathway the preganglionic fiber does not terminate in a ganglion but instead projects to the adrenal medulla, the interior portion of the adrenal gland (Figure \(\PageIndex{5}\)). These axons are still referred to as preganglionic fibers, but the target is not a ganglion per se. The cells in the adrenal medulla that are contacted by the preganglionic fibers are neurosecretory cells that develop from the neural crest along with the sympathetic chain ganglia. These cells in the adrenal medulla release epinephrine and norepinephrine into the bloodstream, rather than using axons to communicate with target structures. This hormonal component means that the sympathetic chemical signal can spread throughout the body very quickly and affect many organ systems at once. At the same time, these hormones remain in the bloodstream longer than neurotransmitters, prolonging the sympathetic effects.

Figure \(\PageIndex{5}\): Sympathetic Adrenal Medulla Pathway. Preganglionic axons from the lateral horns project through the sympathetic ganglia and terminate directly onto cells in the medulla of the adrenal gland. (Image credit: "Sympathetic Adrenal Medulla Pathway" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of original by OpenStax.)

Parasympathetic Division

The parasympathetic division of the autonomic nervous system is named because its central neurons are located away from (para- = “apart from”) the thoracolumbar region of the spinal cord which is dedicated to the sympathetic division. Indeed, the parasympathetic system can also be referred to as the craniosacral system because the preganglionic neurons are located in nuclei of the brainstem and the lateral horn of the sacral spinal cord (S2 to S4).  In Figure \(\PageIndex{6}\), the neural pathways of the parasympathetic system are intentionally simplified.

Parasympathetic Neurons, Ganglia and Nerves

The connections, or “circuits,” of the parasympathetic division are similar to the general layout of the sympathetic division with a few specific differences. The parasympathetic preganglionic fibers from the cranial region travel in cranial nerves, whereas parasympathetic preganglionic fibers from the sacral region travel in spinal nerves. The targets of these fibers are terminal ganglia, which are located near the target effector, and intramural ganglia, which are found within the walls of the target organ. Terminal ganglia receive input from cranial nerves or sacral spinal nerves. The terminal ganglia that receive input from cranial nerves are found in the head and neck, as well as the thoracic and upper abdominal cavities, whereas the terminal ganglia that receive sacral input are in the lower abdominal and pelvic cavities. The postganglionic fiber projects from the terminal ganglia a short distance to the target effector, or to the specific target tissue within the organ. 

The cranial nerves associated with the parasympathetic system are the oculomotor nerve (CN III), facial nerve (VII), glossopharyngeal nerve (CN IX) and vagus nerve (CN X). These nerves generate from particular nuclei of the brainstem. The remaining parasympathetic preganglionic axons originate from neurons of the lateral horns of the S2-S4 segments of the spinal cord. These axons project to terminal or intramural ganglia of pelvic organs. The specific ganglia and targets of each parasympathetic pathway are summarized in Table \(\PageIndex{2}\).

Table \(\PageIndex{2}\):  Ganglia and Targets of the Parasympathetic Nerves

Nerve	Parasympathetic Ganglion	Target(s)
CN III - Oculomotor nerve	Ciliary ganglion in posterior orbit	Sphincter pupillae of iris, ciliary muscle
CN VII - Facial nerve	Pterygopalatine ganglion - posterior to maxillary sinus	Lacrimal gland, glands in nasal cavities
	Submandibular ganglion - deep to posterior mandible, superior to submandibular salivary gland	Submandibular and sublingual salivary glands
CN IX - Glossopharyngeal nerve	Otic ganglion - inferior to foramen ovale	Parotid salivary gland
CN X - Vagus nerve	Terminal and intramural ganglia - thoracic and abdominal organs, gonads	Heart, respiratory tract, digestive tract, liver, pancreas, kidneys, etc.
Spinal nerves S2-S4	Terminal and intramural ganglia - pelvic organs	Rectum, anus, urinary bladder, reproductive tracts

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. Parasympathetic preganglionic axons tend to have fewer than 4 branches. The lack of divergent branches in parasympathetic preganglionic axons prevents a systemic response and facilitates discrete and localized effects on one group of organs at a time.

Figure \(\PageIndex{6}\): Parasympathetic Innervation. Neurons from brain-stem nuclei, or from the lateral horn of the sacral spinal cord, extend their preganglionic nerve fibers (shown in red) to terminal ganglia near the effector organs or to intramural ganglia within the walls of the effector organs. Postganglionic axons (shown in black) from these ganglionic neurons then project the short distance to target effectors (Image credit: "ANS - Parasympathetic 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.)

Everyday Connections

Fight or Flight? What About Fright and Freeze?

The original usage of the epithet “fight or flight” comes from a scientist named Walter Cannon who worked at Harvard in 1915. The concept of homeostasis and the functioning of the sympathetic system had been introduced in France in the previous century. Cannon expanded the idea, and introduced the idea that an animal responds to a threat by preparing to stand and fight or run away. The nature of this response was thoroughly explained in a book on the physiology of pain, hunger, fear, and rage.

When students learn about the sympathetic system and the fight-or-flight response, they often stop and wonder about other responses. If you were faced with a lioness running toward you, would you run or would you stand your ground? Some people would say that they would freeze and not know what to do. So isn’t there really more to what the autonomic system does than fight, flight, rest, or digest. What about fear and paralysis in the face of a threat?

The common epithet of “fight or flight” is being enlarged to be “fight, flight, or fright” or even “fight, flight, fright, or freeze.” Cannon’s original contribution was a catchy phrase to express some of what the nervous system does in response to a threat, but it is incomplete. The sympathetic system is responsible for the physiological responses to emotional states. The name “sympathetic” can be said to mean that (sym- = “together”; -pathos = “pain,” “suffering,” or “emotion”).

Concept Review

The nervous system can be divided into two functional parts: the somatic nervous system (SNS) and the autonomic nervous system (ANS). The differences between these two systems lie on multiple features: the conscious level of their processes, their targets, the number of lower motor neurons involved and the neurotransmitters used. The autonomic nervous system reflexively responds to visceral sensory stimuli, such as levels of carbon dioxide concentration in the blood or stretch caused by blood pressure, that you are not consciously aware of and involuntarily controls cardiac and smooth muscle, as well as glandular tissue. The autonomic nervous system has a chain of two lower autonomic motor neurons. The first neuron is called a preganglionic neuron and resides in the brainstem or lateral horns of the spinal cord. This neuron releases ACh to a second neuron called a ganglionic neuron that is located in ganglia. The axons of ganglionic neurons are called postganglionic fibers. These nerves extend to target effectors and release either ACh or norepinephrine (NE).

The primary responsibilities of the autonomic nervous system are to regulate homeostatic mechanisms in the body. The way we respond to the world around us, to manage the internal environment on the basis of the external environment, is divided between two parts of the autonomic nervous system. The sympathetic division responds to threats and produces a readiness to confront the threat or to run away: the fight-or-flight response. The parasympathetic division plays the opposite role. When the external environment does not present any immediate danger, a restful mode descends on the body, and the digestive system is more active.

The sympathetic output of the nervous system originates out of the lateral horn of the thoracolumbar spinal cord. An axon from one of these central neurons projects by way of the ventral spinal nerve root, spinal nerve and white rami communicans to a sympathetic chain (paravertebral) ganglion. The preganglionic fibers can synapse on ganglionic neurons here or extend to one of the prevertebral (collateral) ganglia via the splanchnic nerves (splanchnic nerve pathway). Postganglionic fibers of sympathetic chain ganglia can either return to the spinal nerve through the gray rami communicantes (spinal nerve pathway) or extend away from the ganglion through a sympathetic nerve (postganglionic sympathetic pathway). The sympathetic system also has a specialized preganglionic connection to the adrenal medulla that causes epinephrine and norepinephrine to be released into the bloodstream rather than exciting a neuron that contacts an organ directly (adrenal medulla pathway).

The parasympathetic output is based in the brainstem and sacral spinal cord. Neurons from particular nuclei in the brainstem carry parasympathetic information through four cranial nerves: oculomotor nerve (CN III), facial nerve (VII), glossopharyngeal nerve (CN IX) and vagus nerve (CN X). In the sacral spinal cord, preganglionic neurons of the lateral horn project out through pelvic splanchnic nerves. Cranial and sacral preganglionic fibers extend to terminal and intramural ganglia located close to or within the wall of target effectors. The postganglionic fibers of the ganglionic neurons then contact the target tissues within the organ to induce rest-and-digest responses. Due to the fact that parasympathetic ganglia are either close to or within the target organ, parasympathetic preganglionic axons are longer and postganglionic axons are shorter, compared to the sympathetic division.

Review Questions

Query \(\PageIndex{1}\)

 

Critical Thinking Questions

Query \(\PageIndex{2}\)

 

Glossary

Query \(\PageIndex{3}\)

Contributors and Attributions

OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at https://openstax.org/books/anatomy-and-physiology