GENERAL PRINCIPLES

 

The autonomic nervous system (ANS) is also known as the vegetative nervous system. It is composed of nerve cells located in the central and the peripheral nervous systems involved in the control of functions of the smooth musculature, heart, vessels and glands.

 

Langley[1] proposed that the ANS was form by motor pathways (efferent), however he was aware of the presence of afferent fibers due to the fact that the ANS functions on an anatomic basis of reflex arcs[2].

 

In the central nervous system the centers involved in the autonomic[3] control are: the cerebral cortex (involved in the recognition of dangerous situations), the limbic system (the main center is the hypothalamus) and the reticular formation. These areas are connected to the motor neurons of the brain stem and spinal cord by ascending and descending pathways in the reticular formation (reticulospinal tract). Its functions include the cardiovascular responses, thermoregulation, intestinal motility, urination, reproductive activity, regulation of circadian rhythms and other endocrine functions.

 

In the peripheral nervous system, the efferent ANS is divided into sympathetic[4] and parasympathetic[5] systems. The sympathetic system is the more developed, while the parasympathetic system reaches only specific organs[6]. The sympathetic system activates functions required for energy consumption, blood pressure and heart rate. The parasympathetic system is related to the preservation of the body reserves (activates the digestive system). Functionally, both systems are antagonistic in those organs that have a dual innervation. However, in some organs one system may dominate the other (as in the case of the parasympathetic system controlling of the urinary bladder) while others receive only one type of autonomic innervation (sympathetic) such as sweat glands.

 

The general visceral efferent pathway (GVE) in sympathetic and parasympathetic systems involves two neurons: one preganglionic and another postganglionic. The body of the preganglionic neuron is located in the central nervous system and the postganglionic neuron has its body located in autonomic ganglia, derived from the neural crest. Both neurons, preganglionic and postganglionic, communicate using the neurotransmitter acetylcholine. The postganglionic neuron acts on the target organ, releasing neurotransmitters (norepinephrine in the case of sympathetic neurons, and acetylcholine in parasympathetic neurons).

 

The enteric nervous system could be considered as a part of the ANS or as a third component that receives parasympathetic and sympathetic neurons.  However, the enteric nervous system is able to maintain its reflex functions after cutting its connections from the central nervous system[7].

 

The general visceral afferent (GVA) neurons are derived from the neural crest. The neurons included in the spinal nerves, have their bodies is located at the level of the spinal ganglia in the dorsal root of the spinal nerves. For the cranial nerves, the soma is located in the distal ganglia of the glossopharyngeal and vagus, and the geniculate ganglion of the facial nerve.

 

Olfaction and taste have been considered a special visceral afferent (SVA) component. Olfaction is transmitted trough the olfactory nerves and taste through the facial, glossopharyngeal and vagus nerves.

 

 

Visceral pain

 

There seems to be no areas in the cerebral cortex directly involved in visceral pain as there are in the case of somatic pain. In the peripheral nervous system, the visceral afferent fibers are myelinated (from mechanical receptors) and unmyelinated (from chemoreceptors). Once they reach the spinal cord or the brain stem two possible theories may explain the visceral pain transmission through the central nervous system: a) the nociceptive visceral afferents fibers make synapse on general somatic afferent neurons associated with somatic nociception of dermatomes, b) they form part of a visceral reflex causing spasm of peripheral blood vessels which alters the metabolism of general somatic receptors. The result is a referred pain related to a somatic dermatome.

 

Nociception from thoracic and abdominal organs reaches the spinal cord through sympathetic afferent fibers. Otherwise, it wouldn’t be possible to have referred pain in the trunk dermatomes. If the nociceptive signals would travel through vagal fibers, they would reach the solitary tract in the brain stem with no relation to trunk dermatomes.

 

For the pelvic viscera, the nociceptive signals travel through sacral parasympatic nerves with the exception of the vagina witch project all its afferent pathways via somatic pudendal nerve. The referred pain areas of pelvic viscera are located in the low abdomen related to lumbar dermatomes, and sacroperineal areas related to by sacral dermatomes.

 

It may be possible that nociceptive signals from thoracic and abdominal viscera also travel using vagal fibers but without representation of a specific area of the trunk as the vagus nerve is not related with dermatomes. These signals from the vagus may induce another type of signals in the cerebrum. Kollarik and Brozmanova point out that vagal afferent fibers may be involved in preventing or signaling tissue damage[8].

 

 

[1] John Newport Langley was a British physiologist (1852-1925) who advanced research in neurotransmitters and chemical receptors.

[2] pp. 7 of “The cardiorespiratory system. Integration of normal and pathological structure and function”. A.S.King. Blacwell Science 1999.

[3] From the Greek autos, "of ones's" and nomos, "law":

[4] From the Greek sympathein, "to respond for one self".

[5] From the Greek para, "besides".

[6] Page 5 of "The cardiorespiratory system" by King, A.S. Blackwell Science. 1999.

[7] Page 67 of "The cardiorespiratory system" by King, A.S. Blackwell Science. 1999.

[8] M. Kollarik, F.Ru. and M. Brozmanova. Vagal afferent nerves with the properties of nociceptors. Avton Neurosci. 2010. Feb. 16:153(1-2):12