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ORDOVICIAN PERIOD

488-444 million years ago

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THE NERVOUS SYSTEM

The early gnathostomes evolved a number of modifications in their nervous systems. These included a larger cerebellum, a more complex midbrain, additional nuclei in the diencephalon, an increase in the areas of the cerebrum dedicated to functions other than olfaction, a lens and iris in the eye, a saccule and utricle in the ear, and an organization of the gray matter of the spinal cord.

BRAIN
The trochlear nerve was reorganized so that its fibers no longer crossed the cerebellum and its nucleus was no longer located within the cerebellum. Its nucleus moved cranially to become closer to the nucleus of the oculomotor (Ariens, p. 710). Gnathostomes developed a mesencephalic nucleus for the trigeminal nerve, sensory components of facial and vagus (branchiomeric nerves) decreased, the facial nucleus moved more caudally, the trigeminal increased its innervation of the head, the roots of the abducens and trigeminal nerves separated, the vagal nucleus shifted caudally, separating from the glossopharyngeal nucleus, and the facial nucleus shifted cranially (Ariens, p. 523-33)

The cerebellum increased greatly in size and was composed of an unpaired corpus and 2 lateral auricles. The corpus cerebellum contained a pars anterior and pars posterior. The neurons of the cerebellar cortex formed molecular, Purkinje, and granular layers (Ariens, p. 720). The cerebellum increased its connections to the body through dorsal and ventral spinocerebellar tracts, although they were largely uncrossed (Ariens, p. 722).

In the early gnathostomes, the midbrain underwent a number of changes. The tectum was better developed, the basal region of the midbrain increased in size, cerebellar tracts to diencephalon passed through the midbrain, connections of the vestibulocochlear nerve to the hypothalamus traveled through the midbrain, the midbrain was able to affect body position through connections with cerebellum and medulla, the ependymal part of the midbrain was lost (Ariens, p. 1192-3), and the formation of cuneiform, intercollicular, red nuclei, a substantia nigra, and a locus coeruleus formed in the midbrain (Butler, 1996, p. 207-17)


The changes to the diencephalon in the early gnathostomes included the development of the velum transversum, increased development of lateral geniculate nucleus, connections between ventral thalamus and hypothalamus to the telencephalon, a ganglion sacci vasculosi in hypothalamus (Ariens, p. 1193-4). In the ventral thalamus, gnathostomes developed a nucleus intermedius, ventromedialis, and ventrolateralis (Butler, 1996, p. 262). The pineal projected to the habenular nuclei (Butler, 1996, p. 301-2) and the hypothalamus could be divided into a medial region around third ventricle and 2 inferior lobes (Butler, 1996, p. 332).

In early gnathostomes, the cerebrum underwent a variety of changes. There was greater development in the non-olfactory regions of the cerebrum (Romer p. 588) and olfactory fibers no longer projected to all pallial areas (Butler, 1994b). Other changes included the increase in interconnections between regions of telencephalon, the formation of a tractus olfacto-habenularis anterior and posterior, and increased connections between the dorsal thalamus and striatum (Ariens, p.1266). The area periventricularis ventrolateralis and the nucleus superficialis basalis in primitive gnathostomes may be equivalent to the dorsal and ventral striatal regions in mammals (Butler, 1996, p. 272). Gnathostomes formed medial and lateral septal nuclei (Butler, 1996, p. 440), the dorsal and medial pallium received visual input (Butler, 1996, p. 370), the striatum and pallidum formed distinct regions of basal ganglia (Medina, 1995), pyramidal cells were produced (in hippocampus at first) (Hassler, p. 117), and the bed nucleus of anterior hippocampal commissure developed (Hassler, p. 121).

The eyes of gnathostomes were much more developed than those of primitive vertebrates. Gnathostome modifications included smooth muscle in the iris, an attached lens (Romer, p. 508-9), and primitive eyelids (Romer, p. 516). Gnathostomes developed a saccule and utricle in the inner ear (Romer, p. 526).

SPINAL CORD
The spinal cord became more organized with distinct gray and white matter and the gray matter was organized into horns of spinal cord which were not present in primitive vertebrates (Hardisty p.313). The gnathostome spinal cord became round spinal instead of the more primitive condition of being flat (Ariens). Primitive gnathostomes evolved tracts homologous to spinotectal tract, dorsal spinocerebellar tract, spino-bulbar tract, and reticulospinal tract (Ariens, p. 279). Myelinization occurred in the spinal cord.


AUTONOMIC NERVOUS SYSTEM
The autonomic control over physiological function increased with ANS input to blood vessels and autonomic ganglia along the spinal cord (Romer p.554).