THE NERVOUS SYSTEM
The brain of the first fish was larger and more differentiated
than that of their chordate ancestors. The new features which had evolved
include cerebral hemispheres, olfactory pathways equivalent to those of
vertebrates, a semicircular canal, additional cranial nerves, features
of the autonomic nervous system, and the ability to taste sour, bitter,
and salty tastes.
The medulla developed a choroid plexus (Hardisty, p. 312-3). The thalamus
developed a habenular complex (Butler, 1996, p. 303). There is disagreement
over whether modern hagfish possess a cerebellum; the development of this
brain region reached a more definitive stage by the evolution of the vertebrates
(Butler, p. 184). The midbrain included a nucleus of the superior raphe
and an interpeduncular nucleus (Butler, 1996, p. 207, 213). There was
significant development in the cerebrum in the first craniates. Cerebral
hemispheres developed (Ariens, p. 1251) and the major regions of the cerebrum
were present: the pallium (both the medial/hippocampal region and the
dorsal & lateral/cortex region) and the subpallium (both the striatum
and septum) (Kardong, p. 646). There were distinct nuclei in the tegmentum
(Butler, 1996, p. 216-7), paired olfactory placodes which formed olfactory
bulbs (Ariens, p. 1244), and a preoptic nucleus (Ariens, p.1247).
SPINAL CORD AND NERVES
Although early craniates possessed a spinal cord, its primary function
was for the control of simple reflexes operating locally. There were very
few fibers which traveled either to or from the brain--only a few fibers
from midbrain and medulla pass through the spinal cord (Ariens, p. 278).
The dorsal and ventral roots of spinal cord joined to form mixed nerves
(although this condition would be absent in some jawless fishes such as
lampreys; Ariens;Romer p. 547) and collateral ganglia developed (Kardong,
The early craniates possessed rudiments of the sensory systems found in
higher vertebrates. Olfactory pathways connected to the hippocampus. (Hardisty,
p. 316). Visual pathways connected the retina to the optic tectum (Butler,
1996, p. 244). One semicircular canal had developed (Romer, p. 526). The
first craniates possessed all basic vertebrate tastes except for sweet
(Ariens, p. 157).
As the head became more important, the cranial nerves better developed.
The nuclei of cranial nerves V through X were located in the medulla.
The trigeminal nerve developed large maxillo-mandibular and ophthalmic
branches (Ariens). The vestibulocochlear nerve developed 2 main branches,
the anterior and posterior rami (Ariens, p. 500; Hardisty, p. 312-3).
AUTONOMIC NERVOUS SYSTEM
The early craniates possessed features of the autonomic nervous system
which would be retained throughout the vertebrates. They possessed chromaffin
cells which could secrete epinephrine to mediate responses to stress,
although these cells would be located in the heart prior to the evolution
of gnathostomes (Hardisty, p. 359). Autonomic innervation regulated the
activities of the gut just as in higher vertebrates: ACh stimulated the
gut while NE inhibited it (Hardisty, p. 360). Autonomic fibers were included
in the vagus nerve (Romer p. 547).