The nervous system has two divisions, one is Central Nervous System (CNS), and the other is Peripheral Nervous System (PNS). The peripheral nervous system is further divided into the Somatic and Autonomic Nervous System. The autonomic nervous system regulates many body functions and acts almost unconsciously. It controls breathing, heart rate, peristalsis, vasomotor activity, and it is also involved in sexual activity.
The autonomic nervous system has three divisions. The first is the Sympathetic, the second is the Parasympathetic, and the third one is the Enteric branch. The sympathetic and the parasympathetic nervous system have opposite actions but this antagonistic role of theirs is what regulates some very important functions of the body.
Sympathetic Nervous System
The sympathetic nervous system is also called the “fight or flight” system of the body. The sympathetic nerves arise from the thoracolumbar region (thoracic and lumbar vertebras) of the spinal cord. These fibers are termed pre-ganglionic fibers. The pre-ganglionic fibers travel to a ganglion, paravertebral or prevertebral, and the post-ganglionic fibers arise from that ganglion. The post-ganglionic fibers then extend and innervate almost every organ system of the body. The sympathetic nervous system is antagonistic to the parasympathetic nervous system because the latter stimulates the body to “feed and breed”.
The main function of the sympathetic nervous system is the “fight and flight” response of the body in emergency situations.
- Increasing Heart rate
The sympathetic nervous system increases the heart rate in emergency situations. It does so by acting on the beta receptors of the heart. The release of catecholamines by the sympathetic nervous system increases the heart rate. It also increases the force of contraction of the heart by increasing the contractility of cardiac cells. In this way, the blood supply to skeletal muscles is greatly enhanced which is required to cope with emergencies.
- Dilation of bronchioles
The sympathetic nervous system dilates the bronchioles by acting on the beta-2 adrenergic receptors present in the smooth muscles of bronchioles. The smooth muscles relax, and the bronchi dilate which allows for greater alveolar oxygen exchange. The supply of oxygen to tissues and the brain is increased which helps in threatening situations.
The sympathetic nervous system causes vasoconstriction of the blood vessels present in the skin, the kidneys, and the digestive tract. The blood flow to these organs is reduced because it is not required in emergency situations. Vasoconstriction occurs when alpha-1 adrenergic receptors in vessels are activated by the sympathetic nervous system through the release of norepinephrine. On the other hand, the blood flow is increased in the heart, the lungs, and the brain because these organs require more blood flow as they are involved in intense physical activity.
- Renin secretion
The sympathetic nervous system increases the secretion of renin by acting through beta-1 adrenoreceptors. Renin is a proteolytic enzyme released by the kidneys into the bloodstream. It stimulates the formation of angiotensin, which in turn stimulates the release of aldosterone. This whole system is called the renin-angiotensin-aldosterone system and it influences vascular resistance and cardiac output.
- Sweat Secretion
Sweat glands are present in almost every part of the body. When the sympathetic nervous system is activated, it acts on the muscarinic receptors present in the sweat glands. The sympathetic nerves release acetylcholine which then triggers sweating. Sweating plays an essential role in the cooling of the body and it dissipates the excess heat during hyperthermia. In this way, the sympathetic nervous system plays an important part in the thermoregulation of the body.
- Dilation of pupil
The dilation of the pupil is called mydriasis. The sympathetic nervous system acts on the dilator pupillae muscles present in the eye and dilates the pupil. It also relaxes the ciliary muscles and decreases the bulging of the lens. The dilation of the pupil allows more light to enter the eye. This enhances the vision which is required in the “fight or flight” situations.
- Inhibition of peristalsis
The sympathetic nervous system reduces gastrointestinal secretion and motor activity and inhibits peristalsis. It acts through the alpha-1 and alpha-2 adrenergic receptors of smooth muscles and relaxation of smooth muscles cause inhibition of peristalsis.
The sympathetic nervous system also plays important roles in other organs. For example, it promotes emission before ejaculation in Ductus Deferens, constricts the urinary sphincter, and decreases urinary output.
Parasympathetic Nervous System
The parasympathetic nervous system is the “feed and breed” and “rest and digest” system that acts in the body. It is responsible for the activities of the body that happen when the body is at rest. For example, sexual arousal, urination, digestion, salivation, and other activities.
The parasympathetic nerve fibers arise from cranial nerves, especially the oculomotor nerve, facial nerve, glossopharyngeal nerve, and vagus nerve. The pelvic splanchnic nerves, present in the sacral region, also act as parasympathetic nerves. The parasympathetic nerve signals are carried to their target by a system of two neurons. The pre-ganglionic neuron arises from the central nervous system and it extends its axon to synapse with the dendrites of the post-ganglionic neuron. The presynaptic nerve fibers are long but the postsynaptic nerve fibers are usually short.
- Decreasing Heart Rate
The parasympathetic nervous system decreases the heart rate through the release of acetylcholine which acts on the M2 muscarinic receptors present on the heart. It reduces the atrial contractile forces and also reduces the conduction velocity of the sinoatrial node and the atrioventricular node. Ventricle muscles have sparse innervation of the parasympathetic nervous system, that is why they have a minimal effect.
The parasympathetic nervous system increases the secretion of saliva. The parasympathetic fibers innervate the parotid gland through the glossopharyngeal nerve. The submandibular and sublingual gland get their parasympathetic supply through the facial nerve. When parasympathetic activation occurs, these glands produce serous and watery saliva in contrast to the sympathetic nervous system which produces thickened saliva and inhibits salivation.
The parasympathetic nervous system increases lacrimation. The nerve fibers originate from the lacrimal nucleus in the pons and travel through the facial nerve and greater petrosal nerve to pterygopalatine ganglion. From the pterygopalatine ganglion, the parasympathetic fibers reach the lacrimal gland by traveling through the maxillary, zygomatic and lacrimal nerve, and cause lacrimation.
The parasympathetic activation releases the acetylcholine which acts on the muscarinic receptors present in the bronchioles. The reduction in the diameter of bronchioles occurs. This happens when the body is at rest and the need for oxygen has diminished.
- Constriction of Pupil
The parasympathetic nervous system acts on the constrictor muscles of the eye and causes contraction of the pupil in the presence of excessive light to save the retina. The ciliary muscles also contract, which facilitates accommodation and allow for closer vision.
- Acceleration of Peristalsis
The parasympathetic system accelerates the peristalsis and increases the blood flow to the gastrointestinal tract by dilating the blood vessels of git. The digestive gland secretion is also increased. The increase in salivary gland secretion and peristalsis mediate the digestion of food. It also increases the absorption of the nutrients indirectly.
- Sexual Activity
The pelvic splanchnic nerves which are part of the parasympathetic nervous system are involved in the erection of genital tissue. It relaxes the smooth muscles present in the fibrous trabeculae of the coiled arteries of the penis. The relaxation of smooth muscles allows the blood to fill the penis. It makes the penis rigid and prepared for sexual activity. The parasympathetic system also causes peristalsis of the urethral muscle which helps in emitting the semen.
- Urination and Defecation
The parasympathetic system causes peristaltic movements of the ureters and intestine. It helps in moving urine from the kidneys into the bladder. It also helps in moving food down the intestinal tract and it assists in excreting urine from the bladder or defecation.
Enteric Nervous System
The enteric nervous system (ENS) which is derived from the neural crest cells, is the third division of the autonomic nervous system. It has a mesh-like system of neurons that regulate and govern the function of the gastrointestinal tract. The ENS is also called the second brain of the body because it can operate independently of the brain and the spinal cord, although it may be influenced by the sympathetic or parasympathetic system.
- Control of Motility
The external muscle coat of the gastrointestinal tract mixes the food and propels the contents of the digestive tract. The enteric nervous system innervates this muscle coat and controls the activity through both excitatory and inhibitory neurons. Studies have shown that the enteric nervous system controls the motility of the small and large intestine, whereas the gastric motility control center is present in the brainstem.
- Fluid Exchange
The Enteric nervous system regulates the water and electrolyte exchange between gut lumen and tissues. It also regulates the local blood flow to the mucosa to balance the nutritive needs of the mucosa.
- Regulation of Secretion
Gastric and pancreatic secretions are regulated by both the parasympathetic nervous system and enteric nervous system. It largely depends on the vago-vagal reflex and the role of the enteric system is minor.
- Defense reactions
The ENS is also involved in many defense reactions of the gut including diarrhea, exaggerated colonic propulsive activity, and vomiting.
Disorders of Autonomic Nervous System
People with an autonomic disorder may have trouble regulating more than one system. The common symptoms are fainting, fluctuating blood pressure, and lightheadedness.
- Autonomic failure
It is a rare degenerative disorder of the autonomic nervous system. There is a general loss of autonomic functions. For example, there is reduced sweating and lacrimation, elevated blood pressure, and sexual dysfunction.
- Orthostatic hypotension
Orthostatic hypotension is the sudden drop in blood pressure on standing upright. it is a disorder in which the autonomic nervous system fails to constrict the blood vessels when a person stands up. The main complication of orthostatic hypotension is falling due to fainting.
- Afferent Baroreflex failure
The damage to the blood pressure sensing nerves in the neck leads to failure of the baroreflex. It causes fluctuations in the blood pressure, making it too high or too low. Symptoms of this autonomic disorder include fainting, headaches, and dizziness.
The autonomic nervous system is a part of the nervous system which largely acts unconsciously and controls some important functions including breathing, heart rate, peristalsis, sexual activity, etc. The autonomic nervous system is divided into the sympathetic, parasympathetic, and enteric nervous system. The sympathetic nervous system acts in emergencies and prepares the body for “fight and flight”. The sympathetic system acts on the alpha and beta receptors present in various organs of the body. It increases the heart rate and dilates the bronchioles when more oxygen is needed. It causes vasoconstriction in the blood vessels of the skin, the kidney, and the digestive tract but it causes vasodilation in coronary vessels and skeletal muscles.
The parasympathetic nervous system is described as the “feed and breed” and “rest and digest” system of the body. It is activated when the body is at rest and it increases salivation and lacrimation. It decreases the heart rate by acting through muscarinic receptors present in the heart. The pupil and the bronchioles are constricted when the parasympathetic system is activated. Other functions of this system include acceleration of the peristalsis, regulation of the sexual activity through pelvic splanchnic nerves, urination, and defecation.
The enteric nervous system is present in the lining of the gastrointestinal tract and it is capable of operating independently of the brain and the spinal cord. That is why it is called the second brain of the body. The functions of the enteric nervous system include control of gut motility, fluid exchange between gut lumen and tissues, regulation of blood flow to the mucosa, and defense mechanisms of the gut. It also plays a minor role in the regulation of gastric and pancreatic secretions. The autonomic nervous system sometimes fails due to degenerative and other causes and may lead to orthostatic hypotension, baroreflex failure, and pure autonomic failure.
- Schmidt, A; Thews, G (1989). “Autonomic Nervous System”. In Janig, W (ed.). Human Physiology (2 ed.). New York, NY: Springer-Verlag. pp. 333–370.
- Allostatic load notebook: Parasympathetic FunctionArchived 2012-08-19 at the Wayback Machine – 1999, MacArthur research network, UCSF
- Langley, J.N. (1921). The Autonomic Nervous System Part 1. Cambridge: W. Heffer.