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SILURAN PERIOD

444-416 million years ago

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The ancestors of bony fish modified a number of features of the nervous system and these changes were passed down to modern fish and tetrapods.

The early bony fish evolved the homologs of the ponto-bulbar body and mesencephalo-cerebellaris tract. All the axons of the olivo-cerebellar tract crossed the midline of the body and the cerebellar ventricle was reduced Ariens, p. 728-36). The midbrain was also modified in the early bony fish. A number of commissures formed in the midbrain, the tori semicirculares were further developed, connections were formed between the hypothalamus and midbrain regions which regulate autonomic function (Ariens, p. 1195), the sulcus limitans dividing dorsal sensory and ventral motor became less obvious (Ariens, p. 338), the red nucleus formed part of a rubrospinal tract (Butler, 1996, p. 216-7), and a nucleus isthmi formed which projects to and from the optic tectum (the mammalian nucleus parabigeminalis is considered its homolog) (Wiggers, 1991). The early bony fish developed a suprachiasmatic nucleus, preoptic nuclei, and dorsal and ventral hypothalamic nuclei around the third ventricle (Butler, 1996, p. 333). Early bony fish possessed at least one basal optic nucleus in the tegmentum (Butler, 1996, p. 290). The early bony fish possessed basal ganglia which had 2 major groups of projection neurons which released substance P and enkalphins (Medina, 1995).

The early bony fish modified their ANS pathways to include autonomic gray rami, myelinated white rami, sympathetic chain ganglia, rami communicates connect some chain ganglia, and splanchnic collateral ganglia near head (Ariens, p.172; Romer p.554). The ophthalmicus profundus became part of the trigeminal nerve (Romer p. 560) and chromaffin cells separated from ganglia (Kardong, p. 628). As early bony fish increased the complexity of innervation to the pectoral fin, a cervico-brachial plexus developed (Ariens). In the spinal cord, dorsal and ventral roots were always fused, although dorsal and ventral may not arise at exactly same level (although their points of origin were now closer than in lower vertebrates) (Ariens). The number of tracts in the spinal cord was increased with the formation of the ventrospinocerebellar tract, reticulospinal tract, vestibulospinal tract, spinothalamic tracts (homologous to the spino-mesencephalic and spinobulbar tracts of lower vertebrates)(Ariens, p. 178-9, 289). Within the spinal cord, motor neuron soma located more ventrally (Ariens, p. 280).


In early bony fish, the lens attached to the ciliary body (Romer, p. 510).