Anatomy of The Autonomic Nervous System

The autonomic nervous system (ANS) happens to be a component of the peripheral nervous system that regulates important activities like heart rate, respiration, and digestion. It also participates in the immediate response to stress, where it collaborates with your endocrine system to prepare the body for fight or flight. It’s further separated into sympathetic and parasympathetic components. 

The autonomic nervous system sends and receives signals from internal organs like the lungs and liver. It acts spontaneously and is typically regarded as being beyond the scope of a deliberate command. The ANS regulates processes such as drooling, sweating, altering pupil size, controlling heart rate, weeping, and secreting hormonal fluids.

As a result, the ANS varies from your somatic nervous system (one more branch of your peripheral nervous system), which is responsible for directing voluntary bodily motions because of the activities of the sympathetic vs parasympathetic nervous systems. Although the majority of your ANS’s operations are automated, they can collaborate with your somatic nervous system. Your ANS gathers inputs from either environmental stimulation or your body. 

Your limbic system provides autonomic regulatory feedback to the hypothalamus, something that is located directly above your brain stem (a collection of structures in-depth inside your brain which happen to be associated with operations like fear, memory, and emotion). This signal is used by your hypothalamus to govern most of your ANS’s activities.

Structure and Function

Sympathetic Nervous System

Sympathetic nervous system neurons contain cell bodies inside your spinal cord’s intermediolateral rows, also known as the lateral horns. Presynaptic fibers exit your spinal cord via anterior roots and go to the anterior rami of the T1-L2 spinal nerves, where they connect to your sympathetic trunks through white rami communicantes. 

Here from, the fibers of your sympathetic nervous system could indeed descend or ascend the sympathetic trunk to something like an inferior or superior paravertebral ganglion, pass to adjoining anterior spinal nerve rami via gray rami communicantes, or cross your trunk avoiding synapsing and proceed through an abdominopelvic splanchnic nerve to achieve prevertebral ganglia. Due to the obvious sympathetic ganglia’s central placement, presynaptic fibers are often smaller than their postsynaptic equivalents.

Pre and postganglionic neurons synapse inside paravertebral ganglia, which occur as nodules all through the sympathetic trunk, close to the spinal column. Whereas the numbers might vary depending on the person, there are 3 cervicals, twelve thoracic, 4 lumbar, and 5 sacral ganglia. The cervical ganglia comprise the superior, intermediate, and inferior cervical ganglia.

The latter, the inferior cervical ganglion, and the very first thoracic ganglion in the sympathetic nervous system may combine to produce the stellate ganglion. Splanchnic nerves happen to be all nerves that are proximal to the paravertebral ganglia. These transport afferent and efferent fibers from the CNS to the viscera. The postsynaptic fibers going for the thoracic cavity are carried by the cardiopulmonary splanchnic nerves.

Abdominopelvic splanchnic nerves are formed when nerves that primarily affect the abdominal and pelvic viscera pass through to the paravertebral without synapsing. The larger, lesser, least, and lumbar splanchnic nerves are among these nerves. Finally, presynaptic neurons synapse in prevertebral ganglia closer to their target organ. The nerve plexuses that surround the aortic branches include the prevertebral ganglia. 

The aorticorenal, celiac, and inferior and superior mesenteric ganglia are among them. The celiac ganglion is stimulated by the larger splanchnic nerve, the aorticorenal by the smaller and least splanchnic nerves, and the superior and inferior mesenteric by the lesser and lumbar splanchnic nerves. The celiac ganglion innervates the following foregut-derived organs: distal esophagus, proximal duodenum, stomach, pancreas, biliary system, liver, adrenal glands, and spleen.

The superior mesenteric ganglion innervates the following midgut derivatives: the jejunum, distal duodenum, ileum, appendix, cecum, ascending colon, as well as the proximal transverse colon. Finally, the inferior mesenteric ganglion innervates the following organs derived from the hindgut: the distal transverse, descending, and sigmoid colon; the rectum and upper anal canal; and the bladder, external genitalia, and gonads.

There are some prominent exceptions to the two-neuron general rule for SNS and PNS circuits. Sympathetic vs parasympathetic nervous system postganglionic neurons that synapse onto the ENS from a three-or-more neuron chain. Presynaptic sympathetic fibers bound for the adrenal medulla go through the celiac ganglia and synapse directly onto chromaffin cells. 

These one-of-a-kind cells act as postganglionic fibers, secreting epinephrine straight into the venous system. Postganglionic sympathetic neurons secrete NE, which binds to adrenergic receptors in the target tissue. The affinity of NE for the receptor is influenced by the receptor subtype, beta-3, beta-2, beta-1, alpha-2, or alpha-1, as well as the organs in which they are expressed.

As previously established, the SNS allows your body to deal with stress by activating the “fight-or-flight” reaction. This response is principally responsible for blood vessel regulation. Vessels happen to be tonically innervated, and an elevation in sympathetic impulses usually results in vasoconstriction, the inverse of vasodilation. The exclusions include coronary arteries and veins that feed the external genitalia and skeletal muscles, which have the opposite effect on your autonomic nervous system.  

The combination of beta and alpha-receptor activation in the sympathetic nervous system is responsible for this conflicting result. Beta-receptor activation enhances coronary artery dilatation in a normal state, however, this impact is diminished by alpha-receptor-mediated vasoconstriction. Inside a pathologic condition, like coronary artery disease, alpha-receptor activity increases while beta-activity decreases.

As a result of sympathetic activation, the coronary arteries might contract. Sympathetic activity raises heart rate and contractile strength, but it also raises metabolic requirements, which is deleterious to cardiovascular output in those who are already sick. Even within non-stressful conditions, the SNS is continually functioning. 

The SNS is functional during the regular breathing cycle, in case of increased stated tonic activation of blood vessels. The sympathetic activity works in tandem with the PNS to widen the airways upon inspiration, providing for an optimal influx of air.

The sympathetic nervous system controls immunity by innervating immune organs like the spleen, lymph nodes, and thymus. This impact has the potential to either increase or decrease inflammation. The autoimmune system’s cells largely express beta-2 receptors, whereas the innate immune systems respond to stimuli and alpha-2 and alpha-1 adrenergic receptors. Macrophages are activated by alpha-2 adrenergic receptor stimulation and inhibited by the beta-2 adrenergic receptor stimulation.

The bulk of postganglionic sympathetic neurons are noradrenergic and emit one or more peptides such as neuropeptide Y or somatostatin. NE/neuropeptide Y neurons innervate cardiac blood arteries, regulating blood flow, and NE/somatostatin neurons of the celiac and superior mesenteric ganglia feed the submucosal ganglia of the colon, controlling gastrointestinal motility.

These peptides are thought to modify the reaction of the postsynaptic neuron to the main neurotransmitter. Peptides are also linked to cholinergic sympathetic postganglionic neurons. These neurons happen to be most typically found in innervating sweat glands and precapillary resistance vessels in skeletal muscle, and they produce the vasoactive intestinal polypeptide in addition to ACh. Paravertebral sympathetic neurons have also been developed to create calcitonin gene-related peptide, a potent vasodilator.

Parasympathetic Nervous System

Parasympathetic nervous system fibers leave the CNS through the cranial nerves (CN) III, VII, IX, and X, as well as the S2-4 nerve roots. The parasympathetic ganglia are divided into four pairs and are all found in the head. The iris and ciliary muscles of the eye are innervated by CN III through the ciliary ganglion in the parasympathetic nervous system. 

The pterygopalatine ganglion innervates the nasal, lacrimal, pharyngeal, and palatine glands.  It also innervates the submandibular and sublingual glands via the submandibular ganglion. The otic ganglion innervates the parotid glands through CN IX.  Every other presynaptic parasympathetic fiber synapses in a ganglion near or on the target tissue’s wall, resulting in presynaptic fibers that are lengthier than postsynaptic fibers.

The parasympathetic nervous system gets its name from the placement of these ganglia: “para-” means next to, so “parasympathetic.” The vagus nerve, CN X, accounts for about 75% of the parasympathetic nervous system and gives parasympathetic input to the majority of the thoracic and abdominal viscera, with sacral parasympathetic fibers innervating the descending and sigmoid colon and rectum. In the medulla oblongata, the vagus nerve has four cell bodies.

The vagus nerve induces heart relaxation in a variety of ways. It reduces contractility in the atria but not in the ventricles. It mostly slows conduction through the atrioventricular node. Carotid sinus massage works by this method to inhibit reentry in Wolff-Parkinson-White syndrome. The PNS’s other important role is digesting. Parasympathetic fibers to the head stimulate salivation, but those to the ENS increase peristaltic and secretory activity. 

The vagus nerve also has an impact on the respiratory cycle. Parasympathetic nervous system nerves activate during expiration in a nonpathological condition, constricting and stiffening airways to prevent collapse. Through these functions, the PNS has been linked to the beginning of postoperative acute respiratory distress syndrome.

Enteric Nervous System (ENS) 

The ENS is made up of two ganglionated plexuses: the myenteric (Auerbach) plexus and the submucosal plexus (Meissner). The myenteric plexus is located between the gastrointestinal tract’s longitudinal and circular smooth muscles, whereas the submucosal plexus is located within the submucosa. The ENS is self-contained and operates by local reflex activity, but it frequently receives input from and delivers feedback to the SNS and PNS.  

Postganglionic sympathetic neurons and preganglionic parasympathetic neurons can both send signals to the ENS. The submucosal plexus regulates the passage of water and electrolytes through the intestinal wall, whereas the myenteric plexus coordinates the contraction of the gut’s cyclical and transverse muscle cells to create peristalsis.

The ENS generates motility via a reflex circuit comprising the circular and longitudinal muscles. The reflex circuits are mediated by nicotinic synapses between interneurons. When the circuit is activated by the bolus being present, inhibitory neurons in the longitudinal muscle fire and excitatory neurons in the circular muscle, resulting in a narrow stretch of colon proximal to the bolus, called the propulsive segment.

Concurrently, inhibitory neurons in the circular muscle fire and excitatory neurons in the longitudinal muscle, resulting in the “receiving segment” of the gut where the bolus will proceed. This procedure is repeated for each part of the bowel.

The ENS has numerous characteristics similar to that of the CNS. Enteric neurons, like those in the CNS, can be bipolar, pseudounipolar, or multipolar, with neuromodulation occurring through excitatory and inhibitory interaction. Similarly, ENS neurons employ about 30 neurotransmitters that are comparable to those found in the CNS, the most frequent of which are cholinergic and nitrergic transmitters.

Clinical Disorders

Your autonomic nervous system regulates involuntary processes such as heartbeat and blood vessel dilation or constriction. When something goes wrong in this system, it can lead to major issues such as problems with blood pressure, heart issues, breathing and swallowing difficulties, male erectile dysfunction, etc. Autonomic nervous system diseases can arise on their own or as a consequence of unrelated diseases, such as Parkinson’s, alcoholism, or diabetes. 

Problems might impact either a section of the system or the entire system, as in complicated regional pain syndromes. Some are transient but may deteriorate over time. These illnesses can be fatal if they interfere with your breathing or heart function. When an underlying condition is addressed, certain autonomic nervous system diseases improve. Often, however, there is no cure. In that case, the goal of treatment is to improve symptoms.

Conclusion

If a person is suffering from the aforementioned symptoms and wants to know if it is due to a malfunctioning autonomic nervous system, there are a variety of tests that may be performed, depending on the symptom. An ECG, for example, may be used by a doctor to evaluate electrical activity inside the heart if a patient is having abnormal cardiac rhythms. However, to prevent or control such conditions, it is important to know the anatomy of the autonomic nervous system.