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ORDOVICIAN PERIOD

488-444 million years ago

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The early vertebrates wrapped their nervous system in a meninx and evolved a special type of cell called neural crest cells. Many of the modifications of their nervous system were important for their jawed descendants such as a cerebellum in the brain, modifications of cranial nerves, better developed spinal tracts, and a diencephalon with a thalamus, hypothalamus, and pineal body.

The first vertebrates surrounded their nervous system with a single meninx (Hardisty, p. 308). Many of the structures of the head and brain which make vertebrates unique are produced from a type of embryonic cell known as neural crest cells whose existence has not yet been demonstrated in more primitive craniates (Ahlberg, p. 23). Glial cells evolved to support the function of neurons (Hardisty, p. 335). GABA functioned as an inhibitory neurotransmitter while glutamate was excitory (Hardisty, p. 328). Primitive vertebrates used oculomotor and trochlear nerves to control eye muscle movement (since hagfish eyes are degenerate, the use of these nerves in more primitive craniates is difficult to establish; Hardisty 355-7). Primitive vertebrates evolved a nucleus for the accessory nerve (Ariens, p. 587).

The ancestors of the first vertebrates evolved a region of the brain known as the cerebellum. In primitive vertebrates, it is located at the anterior end of fourth ventricle as it is in the embryos of higher vertebrates (Hardisty, p. 327). The cerebellum of primitive vertebrates possessed an outer layer of large cells, which may represent the precursors of the Purkinje cells of gnathostomes, and an inner layer of small cells (Ariens, p. 707). A number of cerebellar connections in primitive vertebrates were homologous to those that are found in gnathostomes including the bulbocerebellar tract, crossed and uncrossed tectocerebellar tracts, cerebellotectal tract, cerebellotoral tract , bello-tegmental connections and the lobo-cerebellaris tract (Ariens, p. 708). As in gnathostomes, the vestibular nerve projected to cerebellum (Ariens, p. 708).
As in the brains of gnathostomes, the midbrain of primitive vertebrates contained the nuclei of the oculomotor and trochlear nerves and the trochlear nerve is unusual in departing from the dorsal region of the brain (Hardisty, p. 311). The midbrain also evolved a torus semicirularis (equivalent to the inferior colliculus in mammals). (Butler, 1996, p. 229). The region which unites the diencephalon to the midbrain developed regions known as the pretectum and posterior tuberculum. (Butler, 1996, p. 259). In primitive vertebrates, the epiphysis and the pineal projected to the pretectum. (Butler, 1996) and visual input was processed in the pretectum (Butler, 1996, p. 281, 302).

The diencephalon evolved additional regions and could be divided into a thalamus, hypothalamus, and epithalamus (with a pineal body and a habencular nucleus; Hardisty, p. 310). The diencephalon contained large reticular cells, a nucleus interpeduncularis, part of the tectum ependymal, optic tract projections to the tectum, a lateral geniculate nucleus, a dorsal and ventral thalamus, neural hypophysis, pars intermedius, and a lateral hypothalamus (Ariens, p. 1192). The geniculate primordium of the thalamus received visual input (Hardisty, p. 319). The hypothalamus contained the preoptic nucleus, mammillary body, and neurons capable of secretion (Hardisty, p. 320-1). The hypothalamus integrated visceral input (Romer p. 586).
Primitive vertebrates evolved medial, dorsal, and lateral subdivisions of the pallium (Butler, 1996, p. 263 ). Bipolar neurons connected the olfactory bulbs to the olfactory epithelia and the olfactory pathway was structured similarly to that of higher vertebrates (Hardisty, p. 316). The anterior commissure evolved to connect the cerebral hemispheres (Romer, p. 594). Other modifications included new striatal regions, a medial olfactory nucleus, and a lobus subhippocampus (Ariens, p. 1247).

The spinal cords of primitive vertebrates increased their connections with the brain through spinocerebellar and tectospinal tracts (Hardisty, p. 328). The dorsal root was more restricted to sensory information and there were dorsal root ganglia (Hardisty, p. 357).

The automomic nervous system became more developed in early vertebrates (Hardisty, p. 359). The heart was innervated by the vagus nerve (although ACh and vagal stimulation stimulate the heart, unlike the situation in higher vertebrates; Hardisty, p. 358). Enteric ANS plexuses controlled the gastrointestinal tract (Kardong, p. 628).

Primitive vertebrates evolved a second semicircular canal (Romer, p. 526; Kardong, p. 682) and rods in the eye (Kardong, p. 665). The ancestral condition for vertebrates seems to have been 3 retinal opsins (rhodoposin, a color opsin with maximum absorption <500 nm, and a color opsin with maximum absorption >500 nm) in addition to extraretinal opsins (Nathans, 1999).