Introduction to the Vertebrate Central Nervous System Structures

This section will detail a basic structural model for the vertebrate nervous system.

The primary intention is to gain an understanding of the relationship between the physiology of the vertebrate nervous system organization and the behavior repertoire available to the organism.

The core structural organinization of the vertebrate system is expected to be highly conserved but modified by newer evolutionary structures that make available new degrees of freedom within the physiological and behavioral repertoire of the organism.

The basic organizing principles of the sturctural model will be derived from clues of provided by the organization and development of the urochordates as well as a consideration of the primitive condition of cephlachordates and the jawless craniates.

The sturctural model will undergo saltatory changes at several key historical points, the most important of which include the emergence of gnathostomes, tetrapods, amniotes, and mammals.

The Central Nervous System and The Somatic Animal

The Central Nervous System (CNS) of extant vertebrates is a vestige of the dorsal nerve cord of the "Somatic Animal" and is primarily responsible for cooridinating interactions between the organism and the environment.

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

Additional communication is mediated by diffusable circulating factors released from chromaffin tissues, i.e. hormones, neuropeptides, etc...

With the coming of gnathostomes, the sympathetic nervous system will emerge and gradually exert more and more control over the visceral nervous system during moments of somatic exertion.

The Common Gnathostome Nervous System

The Ascidian Nervous System and the Primitive Vertebrate

The nervous systems of Ascidians and Vertebrates share several basic characteristics, such as:

• A neural plate that gives rise to a neural tube and centralized axial nervous system,
• A tripartite regional organization of the cerebral vesicle/brain, and
• Similiar regional expression of genes during development.

In ascidians, the larval central nervous system is remodelled during metamorphosis and is transformed into the mature adult's neural complex.

The mature ascidian nervous system consists of a cerebral ganglion with a neural gland.

There has long been speculation that the neural gland is homologous with the hypophysial system of vertebrates.

It has also been speculated that the acustico-lateralis line of vertebrates has been derived from the ectodermal placodes that fuse to form the atrial siphon of the adult Ascidian.

It is thought that the vertebrate brain evolved from the anterior neural tube of ascidians.

The primitive condition of the vertebrate senses is thought to resemble that of the urochordates and protochordates.

The Sensory Alar and Motor Basal Plates

Early embryological inductive signals in development of the neural tube lead to segregation of the neural tube into two laminar plates whose boundary is the sulcus limitans:

The Alar Plate

• The Alar Plate located dorsal to the sulcus limitans and extends from the telencephalon to the caudal end of the spinal cord.
• The Alar Plate gives rise to the sensory components of the CNS.
• The alar plate starts at the posterior end of the neural tube and extends further than the basal plate, with the result that the telencephalon is comprised entirely of alar plate derivatives.

The Basal Plate

• The Basal Plate located ventral to the sulcus limitans and extends from the Hypothalamus to the caudal end of the spinal cord.
• The Basal Plate gives rise to the lower motor neurons of the CNS and the four distinct types of interneurons to the interneuronal network.

The General Somatic and Visceral Columns

The plates become further segregated so as to form somatic and visceral columns within each plate.

This results in the formation of four longitudinal cell columns on each side that run, from the posterior end of the neural tube to the anterior end of the diencephalon.

Over the course of evolution leading from primitive agnatha to mammals, the point of exit for visceral motor fibers has shifted from the dorsal horn in lampreys to the ventral horn in mammals.

Dorsal Afferent Alar and Ventral Efferent Basal

• The General Afferent Cell Columns (GSA and GVA) are derivatives of the sensory alar plate and recieve afferents from the somatic and visceral sensory transducers.
  
  Neuron cell bodies located in the general afferent columns project to the interneuronal network of the spine as well as to the pharyngeal hindbrain complex and higher brain centers.


• The General Efferent Cell Columns (GSE and GVE) are derivatives of the motor basal plate and neurons within these columns send axonal efferents to the somatic and autonomic structures.
  
  Neurons located in the general efferent columns recieve neuronal projections from the interneuronal network of the spine as well as from the pharyngeal hindbrain complex and higher brain centers.

Somatic Vs. Visceral

• The General Somatic Cell Columns (GSA and GSE) are relatively contiguous and are functionally dedicated to somatic sensorimotor behavior related to the environment.


• The General Visceral Cell Columns are often disrupted or non-contiguous and are functionally dedicated to providing the CNS autonomic control over the ENS via the ANS.

The Pharyngeal-Hindbrain Complex

It is of further interest to note a general exception to the rule that motor nuclei and neurons are associated with the basal plate while sensory nuclei and neurons are associated with the alar plate.

• A specific subset of primary motor neurons in the basal plate, those associated with the branchiomotor nuclei (originating developmentally in the medial part of the basal plate), migrate into the alar plate during development and exit the hindbrain at the points of entry for their afferent counterparts.

The Three Special Columns of the Pharyngeal Arch Complex

In the brainstem-midbrain region there are seven cell columns in contrast to the four columns of the spinal nervous system.

The three additional bilaterally distributed longitudinal cell columns are inserted between the general somatic and general visceral columns of the CNS and correspond to pharyngeal arch neural components and archaic sensory neural components of an tunicate-like anscestor.

These three cell columns are restricted to the brainstem-midbrain region and are either adjacent to or surrounded by the brainstem reticular system.

Two of the cell columns (SVA and SVE) are associated with the pharyngeal arch complex and extend the entire length of the brainstem and midbrain.

The third cell column (SSA) is associated with the auditory/lateral-line/vestibular system and unlike the other special columns which extend the entire brainstem-midbrain region, this column extends from middle of the brainstem to the tectum of the midbrain.

The Cell Column Insertion Hypothesis

Nuclei Derived From the General and Special Cell Columns of the Pharyngeal-Hindbrain Complex

Evolutionary Pharyngeal-Hindbrain Complex Hypothesis