Autonomic nervous system

Anatomy

The central part of the ANS consists of centers within the brainstem and the spinal cord, while the peripheral part is made up of autonomic fibers and ganglia of the PNS. SNS centers are found within the thoracic and lumbar segments of the spinal cord, which is why it is also called the thoracolumbar division. On the other hand, PSNS centers are found within the brainstem and sacral segments of the spinal cord, which is why it is also called the craniosacral division.

Autonomic fibers belong to peripheral nervous system and they are either afferent or efferent. Visceral afferent (sensory) fibers convey impulses from the internal organs to the centers of the SNS and PSNS. According to the information they bring, the autonomic centers convey efferent impulses through the visceral efferent (motor) fibers to the visceral organs and constantly regulate their function. These impulses are conveyed through ganglia and pre- and postganglionic nerve fibers.

Preganglionic (first-order) neurons are found within the gray matter of the CNS. Their axons (preganglionic fibers) synapse with the bodies of the postganglionic (second-order) neurons, which are found within autonomic ganglia. A ganglion is a neural tissue outside of the CNS which comprises of the neuronal bodies of the second-order neurons whose axons (postganglionic fibers) provide autonomic innervation to the organs.
 
SNS ganglia are found close to the SNS centers, in contrast with PSNS ganglia which are farther from the PSNS centers. Therefore, preganglionic SNS fibers are short, while postganglionic SNS fibers are long as they have the longer route to pass in order to reach their target tissues. For the PSNS, it is the other way around–preganglionic fibers are long, while postganglionic fibers are short as ganglia are found very close to their target organs.

What is special for both divisions of the ANS is that the conduction of impulses from centers to periphery happens through a series of two multipolar neurons, instead of a single neuron which you typically see in the central nervous system. A first-order neuron, or preganglionic neuron, is in the ANS centers, and its axons synapse with a second-order neuron found within the autonomic ganglia. 

In terms of physiology, a couple of things are important:

• All preganglionic fibers of the ANS release acetylcholine as a neurotransmitter.
• Postganglionic PSNS fibers release acetylcholine, while postganglionic SNS fibers release norepinephrine (noradrenalin) (except for those that supply the sweat glands which release acetylcholine).

Sympathetic nervous system

The cell bodies of the SNS lays within the intermediolateral columns of the spinal cord gray matter (T1-L2/L3). In a transverse section of the spinal cord, the intermediolateral columns can be seen as the lateral horns of the spinal cord. The centers of the SNS give rise to preganglionic fibers, which synapse with SNS ganglia. SNS has two groups of autonomic ganglia: paravertebral and prevertebral.

Paravertebral ganglia are found on the left and right side of the body, parallel to the vertebral column (hence the naming paravertebral), and are linked together in a chain to form the left and right sympathetic trunk or sympathetic chain. Each trunk begins from the base of the skull with the superior cervical ganglion. The trunks unite at the level of coccyx and form the ganglion impar. 

Prevertebral ganglia (collateral ganglia, preaortic ganglia) lie anterior to the vertebral column, forming several plexuses around the major branches of the abdominal aorta, such as celiac ganglia around celiac trunk.

Preganglionic fibers leave the spinal cord through the anterior roots and anterior branches of spinal nerves as the white rami communicantes which then synapse with either paravertebral or prevertebral ganglia. Postganglionic fibers from the sympathetic trunk form the gray rami communicantes which enter the branches of all 31 spinal nerves.

Sympathetic innervation of the head and neck comes from the postganglionic fibers of the superior cervical ganglion of the sympathetic trunk and form multiple periarterial plexuses around the branches of the carotid arteries. Sympathetic innervation of the thoracic viscera comes from the cardiopulmonary splanchnic nerves, which contribute to cardiac, esophageal, and pulmonary plexuses. They are postganglionic fibers of the sympathetic trunk. 

Postganglionic SNS input for abdomen and pelvis comes from the abdominal and pelvic splanchnic nerves, which include the greater, lesser, and least thoracic splanchnic (T7-T11), and lumbar splanchnic nerves (T12-L3). Abdominal and pelvic sympathetic nerves are postganglionic fibers of the prevertebral ganglia. They form periarterial plexuses that surround the branches of the abdominal aorta.

Parasympathetic nervous system

The cell bodies of the PSNS are in the brainstem and S2-S4 segments of the spinal cord. PSNS has its ganglia placed near target organs of the abdomen and added to the branches of cranial nerves.

The brainstem centers provide cranial parasympathetic outflow. Preganglionic PSNS branches are added to the oculomotor (CN III), facial (CN VII), glossopharyngeal (CN IX), and vagus (CN X) nerves. They synapse with PSNS ganglia, which provide postganglionic fibers for the head and neck structures. The PSNS ganglia are the:

• Ciliary ganglion – added to the oculomotor nerve (CN III)
• Pterygopalatine ganglion – added to the facial nerve (CN VII)
• Otic ganglion – added to the glossopharyngeal nerve (CN IX)
• Submandibular ganglion – also added to the facial nerve (CN VII)

Sacral parasympathetic outflow originates from S2-S4 segments of the spinal cord. The preganglionic fibers exit the spinal cord via the anterior rami of spinal nerves, which form the pelvic splanchnic nerves. They synapse with PSNS ganglia found on or in the walls of their target organs. Thus, the postganglionic are very short. Sacral outflow supplies the descending colon, sigmoid colon, rectum, bladder, penis or clitoris.

Functions of divisions of the ANS

SNS is the part of the ANS which is mostly active during stress, while the PSNS dominates during rest. Thus, the common phrase that describes the body state during SNS domination is “fight or flight”, while for the PSNS is “rest and digest”.

We presented the functions of the SNS and PSNS in the table above, and since there are many of them, now we’ll extract the ones that are a must-know:

The SNS stimulates the “fight or flight” response by:

• Contracting smooth muscle
• Contracting cardiac muscle by stimulating the heart conduction system
• Decreasing gland secretions, except for sweat glands

Contraction of smooth muscle of the vessels will lead to constriction of the vessels, and thus increased blood pressure. Stimulation of the heart conduction system leads to an increased heart rate, thus increased cardiac output, which contributes to increasing blood pressure. Contraction of the smooth muscles of the bronchi will lead to bronchodilation, and together with decreasing the secretion of the bronchial glands, will provide maximal respiratory capacity and more oxygen for muscles when fighting or running away.

Also, contraction of the dilator of the pupil muscle will result in mydriasis (dilation of the pupil). This increases the ability to detect visual information and increases alertness. Effects on metabolism reflect on stimulation of energy consumption. All of these effects increase the alertness of the body and mobilize energy to prepare the body for a fight or flight from a dangerous situation (“fight or flight”).

On the other hand, PSNS domination will promote “rest and digest” actions. The PSNS relaxes smooth muscles, leading to vasodilation. It slows heart rate through its effect on the conduction system of the heart, which together with vasodilation will decrease blood pressure. Contraction of the sphincter of the pupil muscle will lead to miosis (constriction of the pupil), and contraction of the ciliary muscle will lead to accommodation of the eye (changing the optical power of the eye in order to maintain a clear image or focus on an object as its distance varies).

Increased gland secretion mostly reflects to increased function of the gastrointestinal tract. Release of digestive juices and enzymes will increase digestion, and increased blood flow through the intestines will increase absorption of nutrients. In addition, the PSNS promotes anabolism, which means that it will stimulate the production and storage of energy. As we see, the PSNS redistributes the bloodstream to the intestines to pick up as many nutrients as possible and store it in energy deposits. Redirection of blood flow and decreased blood pressure reduce the alertness of the CNS, which altogether presents as a state of relaxation (“rest and digest”).