The Pharyngeal Hindbrain Complex and the Sacral Plexus

Early in his career , Alfred Sherwood Romer noted that in early vertebrates there was a clear division between the two divisions of the bodyplan and that initially neurophysiological communication between the "Somatic Animal" and the "Visceral Animal" is limited to the points of anatomical fusion between these divisions ( ).

Within the vertebrate nervous system the points of fusion correspond to the following anatomical structures:

• The Pharyngeal-Hindbrain Complex
• The Sacral Plexus

The pharyngeal-hindbrain complex originates at the point of fusion between the neural components of the pharyngeal arch system and the central nervous system.

In humans and other terrestrial vertebrates this point can be seen most clearly during embryogenesis at the pharyngula stage of development - a highly conserved developmental state which most closely recapitulates the vertebrate bodyplan archetype.

Pharyngeal Components of the Pharyngeal-Hindbrain Complex

The pharyngeal arch complex comprises the primary cardiorespiratory system of pre-tetrapod vertebrate

Each pharyngeal arch has the following components:

• A pharyngeal cleft covered with epithelia of ectodermal origin and a pharyngeal pouch covered with epithelia of endodermal origin.


• A striated muscle group.


• A dorsal and a ventral cartilage.


• A Special Visceral Afferent (SVA) and
  a Special Visceral Efferent (SVE) nerve.


• A pharyngeal arch artery connecting the dorsal aorta to the ventral aorta.

In terrestrial vertebrates the pharyngeal arch complex becomes dissociated into a number of semi-autonomous systems as the now obsolete respiratory complex is co-opted for new roles:

• The Cardiopharyngeal Respiritory System: consisting of the pharyngeal aortic arches, the heart, the arterial and venous circulatory systems.


• The Neuromuscular System: consisting of the three special columns of the midbrain-hindbrain complex and their corresponding sensorimotor connections to the pharyngeal arch muscles. (Special divisions of cranial nerves V, VII, IX, and X).


• The Musculoskeletal Pharyngeal System: consisting of the pharyngeal arch cartilages and their corresponding striated arch muscle.

The Neural Components of the Pharyngeal-Hindbrain Complex

The pharyngeal-hindbrain complex is intimately associated with the pons and midbrain where hedonic input is integrated with sensorimotor activity thereby forming the basis for value-driven cognition and behavior.

The pharyngeal-hindbrain complex consists of:

• The three special cell columns of the brainstem and the special cranial nerve componenets


• The four general columns of the brainstem and the general cranial nerve components


• The reticular formation of the brainstem


• The Global Neurotransmitter Systems and their distributed efferents.

The three special columns are the neural components belonging to the pharyngeal arch complex and the sensory apparatus of the visceral bodyplan.

The four general columns belong to the somatic body and extend from the top of the midbrain to the tail end of the organism; only the midbrain and hindbrain portions will be considered to belong to the pharyngeal-hindbrain complex.

The Reticular Formation is responsible for integrating the afferent and efferent projections of the seven neural cell columns of the hindbrain complex and controlling the output of the global neurotransmitter system.

The global neurotransmitter systems are responsible for cooridinating and binding the sensorimotor activity of the somatic nervous system in a manner that is consistent with the hedonic state of the visceral nervous system.

Evolution and Development of the Pharyngeal-Hindbrain Complex

A large amount of vertebrate evolutionary change is associated with modifications and transformations of the primitive pharyngeal arch components.

It has recently been reported that the cranial nerve system of vertebrates may be homologous to the motoneurons of the somatic muscle of the pharyngeal arch system in sessile adult ascidians.

Derivatives of the Mammalian Embryonic Pharnygeal Arch System

Cranial nerves V, VII, IX, and X are elaborations on the nerves innervating the striated muscles of the pharyngeal arch complex.

The pharyngeal arch muscles are the only striated visceral muscles possessed by vertebrates, all other visceral muscles are smooth or cardiac muscles.

In higher vertebrates, the striated pharyngeal arch muscles show the greatest degree of evolutionary modification within the visceral muscles.

The Archaic Unmyelinated Parasympathetic Nervous System

Interestingly, the archaic outflow from the pharyngeal-hindbrain complex and the sacral nerve plexus corresponds to the unmyelinated components of the parasympathetic nervous system; a featured conserved in all vertebrates.

The unmyelinated parasympathetic nervous system constitutes the sole source of direct central nervous system influence over the visceral body in the early vertebrates, while the pharyngeal-hindbrain complex provides direct hedonic input to the central nervous system from the visceral body.
Stephen W. Porges (, , , ) has proposed that the autonomic nervous system of mammals is polyvagal and retains modified, yet highly conserved neurophysiological and anatomical features of at least two prior evolutionary stages of autonomic and metabolic control.

The pharyngeal-hindbrain complex comprises the core of the autonomic nervous system and in mammals has a heirarchy of three phylogenetically determined layers of structure:

• An Archaic Unmyelinated Parasympathetic (all vertebrates - this is the archaic condition)
• A Myelinated Sympathetic (at gnathostomes)
• A Myelinated Parasympathetic (at mammals)

In gnathostomes and higher vertebrates this region is distinguished from the rest of the central nervous system by the prescence of special myelinated somatic and visceral cranial nerves that accompany remodelling of the pharyngeal arch complex.

Also of interest is a suggestion by Graber ( ) 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 the special components of the facial and trigeminal cranial nerves.