Evolution of the Autonomic Nervous System

The autonomic nervous system of vertebrates is the central nervous system outflow that cooridinates and integrates the activity of the somatic and visceral divisions of the body in a manner that is consistent with the behavioral strategy being employed.

In moments of danger and exertion the autonomic nervous system exerts control over the baseline activity of the viscera.

While in moments of security and comfort the enteric nervous system provides hedonic modulation of emotional state and somatosensory evaluation within the central nervous system.

In this work extensive use will be made of the Polyvagal Theory of Stephen W. Porges (, , , , ).

The Polyvagal Theory delineates the evolution of the vertebrate behavior as it relates to the autonomic nervous system and it´s regulation of the heart by the cranial nerves of the mid and hindbrain.

Contrary to the standard division of the autonomic nervous system into two divisions - sympathetic and parasympathetic - that innervate the viscera; we will find it more appropriate to adopt a single CNS efferent outflow division at the origin of vertebrates that will transition to a two-fold efferent outflow division at gnathostomes and a three-fold efferent outflow division within mammals.

The Points of Fusion and Cooridination of Action

The Central Nervous System (CNS) of extant vertebrates is a vestige of the dorsal nerve cord of the "Somatic Animal" and is dedicated to environmental interactions.

The CNS is primarily metamerically orgranized with each unit being traversed by a sensory and a motor plate integrated by a interneuronal reticulum.

The Enteric Nervous System and The Visceral Animal

The Enteric Nervous System (ENS) of extant vertebrates is a vestige of the nerve-net that governed the behavior of the "Visceral Animal" and is represented by the "myenteric-submucosal plexus" of extant vertebrates, located within the wall of a gastrointestinal tract.

Michael Gershon ( ) has referred to the enteric nervous system as the "Second Brain" of vertebrates, due to it´s autonomous functioning in the abscence of central nervous system input and a the possession of a wide range of endogenous neurotransmitters.

The Enteric Nervous System:

• possesses a large array of neuroactive transmitters and peptides,
• is dedicated to trophic visceral functions,
• may operate independently of the CNS,
• and is capable of sophisticated behaviors in the absence of exogenous neural influences.

The First Autonomic Division - The archaic Unmyelinated Parasympathetic

The somatovisceral fusion points correspond to anatomical positions in the CNS that give rise to the original vertebrate autonomic division, the archaic unmyelinated Parasympathetic Nervous System:

• The Hindbrain-Pharyngeal Complex
• The Sacral Plexus

Early in vertebrate evolution there was little coupling between the somatic and visceral bodies except at the hindbrain-pharyngeal and sacral fusion points; and no sympathetic nervous system components were present.

Agnathans are modern representatives of this condition.

The archaic vertebrates lacked the neuroanatomy required for the "somatic" central nervous system to exert coordinated control over the "visceral" tract in times of perceived danger; limiting the primary behavioral response under conditions of environmental stress to immobilization stratagies (i.e. freezing, feigning death, or catatonic state).

Interestingly, Graber ( ) suggested that the cloacal nerve at the sacral end of the vertebrate animal be classified as the "Thirteenth Cranial Nerve" due to the similarity of it´s nerve components to that of the facial and trigeminal cranial nerves.

With the comming of mammals, a new division of the parasympathetic nervous system emerges; the myelinated parasympathetic nervous system.

This system will act to modulate the extreme expression of the older unmyelinated parasympathetic division and sympathtetic division, giving mammals the capacity for realtime environmental foraging and subtle engagement behaviors.