GENERAL PRINCIPLES

 

The autonomic nervous system (ANS) is also known as the vegetative nervous system composed of nerve cells located in the central and the peripheral nervous systems. It controls involuntary actions as the cardiovascular activity, thermoregulation, intestinal motility, urination, reproductive activity, regulation of circadian rhythms and other endocrine functions. 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 hypothalamus (forms part of the limbic system) and the reticular formation. These areas are connected to the motor neurons of the brain stem and spinal cord by descending and ascending pathways in the reticular formation.

 

In the peripheral nervous system, the ANS is divided into sympathetic[4] and parasympathetic[5] systems. Both refer only to the efferent pathways. 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. 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 and blood vessels (with the exception of the brain vessels, heart, and reproductive organs).

 

The sympathetic and parasympathetic systems, involve two neurons: one preganglionic and, a second, postganglionic (derived from the neural crest). The body of the preganglionic neuron is located in the central nervous system and the postganglionic neuron has its body located in autonomic ganglia. Both neurons, preganglionic and postganglionic, communicate using the neurotransmitter acetylcholine. The postganglionic neuron acts on the target organ by 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 system that receives sympathetic and parasympathetic neurons due to the fact that the enteric nervous system is able to maintain its reflex functions after cutting its connections from the central nervous system[7].

 

The visceral afferent neurons derive from the neural crest and have their bodies located in the spinal ganglia of the dorsal root of the spinal nerves. For the cranial nerves, the body of the visceral afferent neurons is located in the distal glossopharyngeal and vagal ganglia, and in the geniculate ganglion of the facial nerve. In the case of the oculomotor nerve, it is located at the level of the trigeminal ganglion. However, the terms sympathetic and parasympathetic refer only to the efferent pathways.

 

 

Visceral pain

 

In the peripheral nervous system, visceral afferent fibers can be myelinic (from mechanical receptors) or amyelinic (from chemoreceptors). Unlike somatic pain, there are no areas in the cerebral cortex directly involved in visceral pain. Two possible explanations for the transmission of visceral pain in the central nervous system have been described: first, the nociceptive visceral afferent fibers (inn the PNS) synapse in somatic afferent neurons (in nthe CNS) associated with the nociception of dermatomes; second, the nociceptive stimulus triggers a visceral reflex that causes a spasm of the peripheral blood vessels that alters the metabolism of general somatic receptors. The result is a referred pain related to a dermatome.

 

Nociception of the thoracic and abdominal organs reaches the spinal cord through visceral afferent fibers that synapse on neurons of the somatic pathways. Otherwise, the existence of referred pain in the dermatomes of the trunk would not be possible since if the nociceptive signals traveled through the fibers of the vagus nerve, they would reach the solitary tract in the brain stem unrelated to the dermatomes. However, it is possible that nociceptive impulses from the thoracic and abdominal viscera may also be transmitted by fibers attached to the vagus nerve, but without representation of a specific trunk area. So the vagus nerve can induce other types of signals in the brain. Kollarik and Brozmanova point out that vagal afferent fibers may be involved in the prevention or alert of tissue damage[8].

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For pelvic viscera, nociceptive impulses are transmitted by fibers attached to the sacral parasympathetic nerves, with the exception of the vagina that projects all the afferent pathways alongside the pudendal nerve (somatic). 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.

 

 

[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