<spoiler|Autonomic nervous system> <poem>
The autonomic nervous system is the part of the nervous system of the higher life forms that is not consciously controlled. It is commonly divided into two usually antagonistic subsystems: the sympathetic and parasympathetic nervous system.
The autonomic nervous system (ANS) (or visceral nervous system) is the part of the peripheral nervous system that acts as a control system, maintaining homeostasis in the body. These maintenance activities are primarily performed without conscious control or sensation. The ANS has far reaching effects, including: heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils, micturition (the discharge of urine), and sexual arousal. Whereas most of its actions are involuntary, some ANS functions work in tandem with the conscious mind, such as breathing. Its main components are its sensory system, motor system (comprised of the parasympathetic nervous system and sympathetic nervous system), and the enteric nervous system.
The human nervous system is comprised by two major divisions: the central nervous system and the peripheral nervous system. The central nervous system includes the brain and the spinal cord which are comprised of cells called neurons. The peripheral nervous system is comprised of neurons and a network of cell extensions (axons and dendrites), which could be compared to a wiring system. The largest part of the peripheral nervous system is located outside of the CNS.
The autonomic nervous system regulates bodily functions and the activity of specific organs. For example, the ANS plays a role in narrowing (constricting) and widening (dilating) blood vessels; increasing heart rate and the force of contraction in the heart's beating action. Another example is how the ANS controls constriction and dilation of airways (bronchioles) in the lungs. The ANS plays a role in many important physiological processes. A partial list includes: digestion, respiration, perspiration, constriction and dilation of the pupils, relaxation and contraction of the bowels and sphincters, erection and ejaculation, parturition (child birth), and tear formation.
Although the bodily functions that the ANS regulates are typically portrayed as being outside of voluntary control, they are not completely outside our awareness, and some schools of thought believe that one's state of mind impacts the functioning of the ANS. It remains open to debate whether the term “involuntary” nervous system is a precise description of the ANS. Many autonomic functions are beyond conscious control, but others are impacted voluntarily, such as the control of sphincters in urination (micturition).
The autonomic nervous system is divided into subsystems, the sympathetic (SNS) and the parasympathetic (PNS). The SNS and PNS often create opposite effects in the same organs or physiological systems, and can act as an aid in creating balance (homeostasis) within the body.
The SNS is frequently refered to as the “fight or flight” system, as it has a stimulating effect on organs and physiological systems. For example, the SNS narrows the amount of available space inside blood vessels while increasing heart rate and the force of the heart's contractions. Narrowing blood vessels creates a smaller space for blood to flow in, and helps to raise the pressure of the blood in the body. Increasing the strength of the heart's pumping action makes the blood flow more rapidly to locations in the body distant from the heart and lungs. In addition, the nerves that innervate the lungs, can widen the bronchioles, rapidly providing more oxygen (oxygenation) to the blood flowing in to pick up O2 and nutrients. Meanwhile, the SNS can give the body a boost of quick energy by stimulating glycogenolysis (it also helps the liver with lipolysis in adipose tissue). For reasons such as these, the sympathetic nervous system has typically been viewed as a system that mobilizes the body system for some type of action. Another example: the sympathetic nerves that innervate the pupils of the eyes can quickly widen (dilate) both pupils. This allows more light to enter the eyes.
The parasympathetic nervous system has sometimes been called the “rest and digest” response. The PNS slows and relaxes many functions of organs and body systems. For example, the PNS can cause blood vessels to widen, while slowing the heart beat and decreasing the force of the heart's contractions. These effects help to lower blood pressure by creating more space in the vessels, and slowing the force and rate of the pump (the heart). The PNS can divert blood back to the skin and the gastrointestinal tract when an urgent need for blood has passed. The PNS can narrow the bronchioles in the lungs when the need for oxygen has diminished. Similarly, it is the PNS that can contract the pupils. However, the PNS actually stimulates digestion, especially after such functions have been down-regulated by the SNS. (The PNS stimulates salivary gland secretion, and accelerates peristalsis). So, while in general the PNS has a calming effect on the body, it can stimulate activity as well.
Some anatomists refer to a third, or enteric, nervous system. The ENS regulates itself but can be impacted upon by both sympathetic and parasympathetic nerve fibers which are connected to ENS plexuses. The enteric nervous system is capable of operating on its own, even after having been severed from input from the SNS and PNS. This is why the enteric nervous system is sometimes referred to as a “second brain.” (Hospital Practice, The Enteric Nervous System: A Second Brain, Michael D. Gershon, MD, Columbia University)
The enteric nervous system regulates secretions of the intestinal glands, regeneration of the intestinal epithelium, and intestinal motility (movement of the gut). The ENS is sometimes considered the third part of the autonomic nervous system.
In order to reach the target organs and glands, the axons (largest “tentacle”) of neurons, in the SNS and PNS, often must travel long distances in the body. To accomplish this many axons link up with the axon of a second cell. The ends of the axons do not make direct contact, but rather link across a space, the synapse. As the location where many axons (and dendrites)meet and criss-cross is called a ganglion, in tracing the path of two-neuron linkages in the sympathetic nervous system, the terms “preganglionic” and “postganglionic” are used. Preganglionic nerves are the axons that are located before the intersection with the ganglion. Postganglionics are those axons of the second neuron that travel from the ganglia to the target organ or gland. In contrast to the voluntary motor nerves, which consist of only one cell, or neuron, the sympathetic and parasympathetic fibres have both a “preganglionic” and a “postganglionic” nerve cell.
A nerve impulse is transferred from cell to cell, at a synapse, by the chemical transmitter acetylcholine, or “ACh”. ACh is released from the first neuron and binds to a nicotinic acetylcholine receptor on the second. The latter transfers the impulse to an effector cell by releasing a second neurotransmitter. In parasympathetic fibres, the second transmitter is again ACh, while noradrenaline serves as the second transmitter in the sympathetic system. Preganglionic sympathetic fibres also end in the adrenal medulla, which functions as a giant ganglion which, instead of releasing a transmitter into a synapse, releases its second neurotransmitter, noradrenaline or adrenaline, directly into the blood stream.
The cell bodies of preganglionic autonomic nerve cells are situated in the central nervous system. Those of the sympathetic nervous system arise in the thoracal and lumbal segments of the spinal cord. The preganglionic parasympathetic cell bodies are situated in the brain stem (cranial parasympathetic) and in the sacral spinal cord (sacral parasympathetic).
The sympathetic axons build a chain of 22 ganglia, the so-called trunk of the sympathetic nerve, on each side of the spinal column. From these the splanchnic nerves run to the prevertebral ganglia, which lie in front of the aorta, at the level where its unpaired visceral arteries branch off. The left and right trunks of the sympathetic nerve fuse to form an unpaired ganglion in the pelvic area. Organs innervated by sympathetic fibres include the heart, lungs, esophagus, stomach, small and large intestine, liver, gallbladder and genital organs.
These organs are also innervated by the parasympathetic nervous system. The digestive system distal to the lower part of the colon is regulated by the sacral parasympathetic fibres via the pelvic ganglia. The more proximal digestive tract is controlled by the vagus nerve, the largest element of the cranial parasympathetic system. Like those of the vagus, other cranial parasympathetic fibers arise in the brain stem before exiting the skull with various cranial nerves, en route to the cranial parasympathetic ganglia and the innervation of the eye muscles and salivary glands.
Sympathetic and parasympathetic divisions typically function in opposition to each other. But this opposition is better termed complementary in nature rather than antagonistic. For an analogy, one may think of the sympathetic division as the accelerator and the parasympathetic division as the brake. The sympathetic division typically functions in actions requiring quick responses. The parasympathetic division functions with actions that do not require immediate reaction. Consider sympathetic as “fight or flight” and parasympathetic as “rest and digest”.
However, many instances of sympathetic and parasympathetic activity cannot be ascribed to “fight” or “rest” situations. For example, standing up from a reclining or sitting position would entail an unsustainable drop in blood pressure if not for a compensatory increase in the arterial sympathetic tonus. Another example is the constant, second to second modulation of heart rate by sympathetic and parasympathetic influences, as a function of the respiratory cycles. More generally, these two systems should be seen as permanently modulating vital functions, in usually antagonistic fashion, to achieve homeostasis. Some typical actions of the sympathetic and parasympathetic systems are listed below:
Sympathetic nervous system
Promotes a “fight or flight” response, corresponds with arousal and energy generation, inhibits digestion:
Parasympathetic nervous system
Promotes a *rest and digest“ response; promotes calming of the nerves and return to regular function, enhances digestion.