Autonomic nervous system
The autonomic nervous system, sometimes called the visceral nervous system and formerly the vegetative nervous system, is a division of the nervous system that operates internal organs, smooth muscle and glands. The autonomic nervous system is a control system that acts largely unconsciously and regulates bodily functions, such as the heart rate, its force of contraction, digestion, respiratory rate, pupillary response, urination, and sexual arousal. The fight-or-flight response, also known as the acute stress response, is set into action by the autonomic nervous system.
The autonomic nervous system is regulated by integrated reflexes through the brainstem to the spinal cord and organs. These functions include control of respiration, cardiac regulation, vasomotor activity, and certain reflex actions such as coughing, sneezing, swallowing and vomiting. Those are then subdivided into other areas and are also linked to autonomic subsystems and the peripheral nervous system. The hypothalamus, just above the brain stem, acts as an integrator for autonomic functions, receiving autonomic regulatory input from the limbic system.
Although conflicting reports about its subdivisions exist in the literature, the autonomic nervous system has historically been considered a purely motor system, and has been divided into three branches: the sympathetic nervous system, the parasympathetic nervous system, and the enteric nervous system. The enteric nervous system however is a less recognized part of the autonomic nervous system. The sympathetic nervous system is responsible for setting off the fight-or-flight response. The parasympathetic nervous system is responsible for the body's rest and digestion response. In many cases, both of these systems have "opposite" actions where one system activates a physiological response and the other inhibits it. An older simplification of the sympathetic and parasympathetic nervous systems as "excitatory" and "inhibitory" was overturned due to the many exceptions found. A more modern characterization is that the sympathetic nervous system is a "quick response mobilizing system" and the parasympathetic is a "more slowly activated dampening system", but even this has exceptions, such as in sexual arousal and orgasm, wherein both play a role.
There are inhibitory and excitatory synapses between neurons. A third subsystem of neurons has been named as non-noradrenergic, non-cholinergic transmitters because they use other transmitters such as nitric oxide as a neurotransmitter. These functions are integral in autonomic function, in particular in the gut and the lungs.
Although the ANS is also known as the visceral nervous system and although most of its fibers carry non-somatic information to the CNS, many authors still consider it only connected with the motor side. Most autonomous functions are involuntary but they can often work in conjunction with the somatic nervous system which provides voluntary control. Overall, the ANS ensures the maintenance of vital functions and allows the body to effectively adapt to cycles of stress and recovery.
Structure
The autonomic nervous system has been classically divided into the sympathetic, parasympathetic and enteric nervous systems. More modern classifications recognize other networks that integral to different organs, such as the intrinsic cardiac nervous system.The sympathetic division emerges from the spinal cord in the thoracic and lumbar areas, terminating around L2-3. The parasympathetic division has craniosacral "outflow", meaning that the neurons begin at the cranial nerves and sacral spinal cord. These divisions are distinctive because they require a sequential two-neuron efferent pathway; the preganglionic neuron must first synapse onto a postganglionic neuron before innervating the target organ. The preganglionic, or first, neuron will have its nerve cell body in the central nervous system and will synapse at the postganglionic, or second, neuron's cell body. The postganglionic neuron will then form junctions within the target organ.
Sympathetic division
The sympathetic nervous system consists of cells with bodies in the lateral grey column from T1 to L2/3. These cell bodies are "GVE" neurons and are the preganglionic neurons. There are several locations upon which preganglionic neurons can synapse for their postganglionic neurons:- cervical ganglia
- thoracic ganglia and rostral lumbar ganglia
- caudal lumbar ganglia and sacral ganglia
- cervical cardiac nerves and thoracic visceral nerves, which synapse in the sympathetic chain
- thoracic splanchnic nerves, which synapse in the prevertebral ganglia
- lumbar splanchnic nerves, which synapse in the prevertebral ganglia
- sacral splanchnic nerves, which synapse in the inferior hypogastric plexus
Parasympathetic division
The parasympathetic nervous system consists of cells with bodies in one of two locations: the brainstem or the sacral spinal cord. These are the preganglionic neurons, which synapse with postganglionic neurons in these locations:- parasympathetic ganglia of the head: ciliary, geniculate,
- pterygopalatine, and submandibular,
- ottic in inner ear space
- tympanic nerve of VII with C9, C10, C5 in promontory plexus in middle ear space
- trigeminal ganglion specially sensory is common with other ones
- in or near the wall of an organ innervated by the vagus or sacral nerves plexus
- the postganglionic parasympathetic splanchnic nerves
- the vagus nerve, which passes through the thorax and abdominal regions innervating, among other organs, the heart, lungs, liver and stomach
Enteric nervous system
Development of the enteric nervous system
Development of the enteric nervous system involves migration of cells from the vagal section of the neural crest, eventually populating the entire gastrointestinal tract. Throughout development, tyrosine kinase activity has roles in formation and regulation of enteric ganglia to influence spontaneous, rhythmic, slow waves in the gastrointestinal tract.Structure of the enteric nervous system
The enteric nervous system is a division of the autonomic nervous system embedded in the gastrointestinal tract walls. Having about 200 million neurons, the ENS communicates with the central nervous system while regulating gut function independently. The core of this structure consists of two main interconnected neural networks or plexuses: the myenteric plexus and the submucosal plexus. The myenteric plexus extends the full length of the gut, primarily controlling motility and secretomotor functions, using nitric oxide to regulate smooth muscle in the ENS. The submucosal plexus has a role in secretory regulation by innervating intestinal endocrine cells and blood vessels.Intrinsic cardiac nervous system
Sensory neurons
The visceral sensory system - technically not a part of the autonomic nervous system - is composed of primary neurons located in cranial sensory ganglia: the geniculate, petrosal and nodose ganglia, appended respectively to cranial nerves VII, IX and X. These sensory neurons monitor the levels of carbon dioxide, oxygen and sugar in the blood, arterial pressure and the chemical composition of the stomach and gut content. They also convey the sense of taste and smell, which, unlike most functions of the ANS, is a conscious perception. Blood oxygen and carbon dioxide are in fact directly sensed by the carotid body, a small collection of chemosensors at the bifurcation of the carotid artery, innervated by the petrosal ganglion.Primary sensory neurons project onto "second order" visceral sensory neurons located in the medulla oblongata, forming the nucleus of the solitary tract, that integrates all visceral information. The nTS also receives input from a nearby chemosensory center, the area postrema, that detects toxins in the blood and the cerebrospinal fluid and is essential for chemically induced vomiting or conditional taste aversion. All this visceral sensory information constantly and unconsciously modulates the activity of the motor neurons of the ANS.
Innervation
Autonomic nerves travel to organs throughout the body. Most organs receive parasympathetic supply by the vagus nerve and sympathetic supply by splanchnic nerves. The sensory part of the latter reaches the spinal column at certain spinal segments. Pain in any internal organ is perceived as referred pain, more specifically as pain from the dermatome corresponding to the spinal segment.Motor neurons
Motor neurons of the autonomic nervous system are found in "autonomic ganglia". Those of the parasympathetic branch are located close to the target organ whilst the ganglia of the sympathetic branch are located close to the spinal cord.The sympathetic ganglia here, are found in two chains: the pre-vertebral and pre-aortic chains. The activity of autonomic ganglionic neurons is modulated by "preganglionic neurons" located in the central nervous system. Preganglionic sympathetic neurons are located in the spinal cord, at the thorax and upper lumbar levels. Preganglionic parasympathetic neurons are found in the medulla oblongata where they form visceral motor nuclei; the dorsal motor nucleus of the vagus nerve; the nucleus ambiguus, the salivatory nuclei, and in the sacral region of the spinal cord.
Function
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. The sympathetic system is often considered the "fight or flight" system, while the parasympathetic system is often considered the "rest and digest" or "feed and breed" system.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. In general, these two systems should be seen as permanently modulating vital functions, in a usually antagonistic fashion, to achieve homeostasis.
Higher organisms maintain their integrity via homeostasis which relies on negative feedback regulation which, in turn, typically depends on the autonomic nervous system. Some typical actions of the sympathetic and parasympathetic nervous systems are listed below.
| Target organ/system | Parasympathetic | Sympathetic |
| Digestive system | Increase peristalsis and amount of secretion by digestive glands | Decrease activity of digestive system |
| Liver | No effect | Causes glucose to be released to blood |
| Lungs | Constricts bronchioles | Dilates bronchioles |
| Urinary bladder and Urethra | Relaxes sphincter | Constricts sphincter |
| Kidneys | No effects | Decrease urine output |
| Heart | Decreases rate | Increase rate |
| Blood vessels | No effect on most blood vessels | Constricts blood vessels in viscera; increase BP |
| Salivary and lacrimal glands | Stimulates; increases production of saliva and tears | Inhibits; result in dry mouth and dry eyes |
| Eye | Stimulates constrictor muscles; constrict pupils | Stimulate dilator muscle; dilates pupils |
| Eye | Stimulates to increase bulging of lens for close vision | Inhibits; decrease bulging of lens; prepares for distant vision |
| Adrenal medulla | No effect | Stimulate medulla cells to secrete epinephrine and norepinephrine |
| Sweat gland of skin | No effect | Stimulate sudomotor function to produce perspiration |