542-488 million years ago
THE BRAIN AND NERVOUS SYSTEM
Cephalochordates divided their brain into regions which served as the basic organizational pattern for the large brains of vertebrates (a telencephalon with olfactory processing, a diencephalon, a midbrain, and a hindbrain organized into segments). Cephalochordate cranial nerves are homologous to several of those found in vertebrates and the organization of the sensory and motor nerves of the spinal cord into dorsal and ventral roots is equivalent to the vertebrate condition.
The cephalochordate brain includes both a cerebral vesicle and a medulla-like posterior region. In the primitive chordates (such as lancelets), the telencephalon became the site for the processing of olfactory stimuli (Ariens). Gene expression patterns (such as those of AmphiFoxB and AmphiSim) indicate that the cerebral vesicle of Amphioxus is organized into regions homologous to the vertebrate diencephalon and midbrain. Posterior to this, there is a region of the neural tube which expresses Hox genes, as does the vertebrate hindbrain, although it does not form obvious rhomobmeres. Nevertheless, there are segmented blocks of tissue revealed by the expression pattern of AmphiFoxB which may represent an early stage in rhombomere evolution in which the signals for segmentation originated from surrounding tissues rather than retinoic acid gradients in the nervous tissue as in vertebrates (Mazet, 2002). The lancelet hindbrain also reveals segmentation in the expression pattern of islet1 (a gene important in developing motor neurons). Islet1 expression also reveals that Amphioxus may have homologs of the pineal gland and adenohypophysis (Jackman, 2000).
In Amphioxus, there is no distinction between gray and white matter in the spinal cord. Neuronal soma surround the central canal, including some which are very large (Weichert, 1970, p.613). The soma of motor neurons are located close to this central cavity, unlike the situation in jawless fish in which they are located in horns of gray matter. There are no dorsal root ganglia since the soma of sensory neurons are located within the spinal cord. (Weichert, 1970, p.637). The dorsal roots carry sensory information to the spinal cord and the ventral roots carry motor commands from the spinal cord, as in vertebrates (Ruppert, from Harrison, 1997, p. 474). The dorsal roots of spinal nerves do not fuse with ventral roots (nor do they leave the spinal cord from the same point) (Weichert, 1970, p.637). Affarent and efferent neurons innervating the viscera travel through dorsal roots (Weichert, 1970, p.634).
Mechanoreceptors, pressure receptors, thermoreceptors, and chemoreceptors
are known in lancelets (Ruppert, from Harrison, 1997, p. 483; Hoar, Vol