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CAMBRIAN PERIOD

542-488 million years ago

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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 vertebrates, the boundary of the diencephalon and mesencephalon includes a structure known as the subcommissural organ (SCO) which secretes products important in the development of the nervous system, including the spinal cord. A protein secreted here, SCO-spondin, is homologous with F-spondin secreted from the floor plate and thrombospondin. The SCO-spondin gene is present in all chordates and there is some evidence that it exists in more basal deuterostomes as well. The infundibular organ in Amphioxus may be homologous to the subcommissural organ of vertebrates (Gobron, 1999).
In lancelets, the first two cranial nerves (the terminal nerve [designated as cranial nerve 0] and a homolog of the olfactory) are sensory only and innervate the tip of the head (Willey, p. 90-1; Ariens). Lancelets also possess homologs of the facial, glossopharyngeal, and vagus nerves. In lancelets and vertebrates the components of the trigeminal don't innervate visceral structures or mucus membranes but send at least some fibers to the cutaneous membrane. The trigeminal nerves move more caudally during development in lancelets and vertebrates. Lancelets possess homologs of the musculature innervated by the trigeminal nerve (Ariens, p. 380-1, 517).
Amphioxus possesses a diencephalon although it is not well differentiated from the cerebrum. (Ariens, p. 868). Lancelets possess a region similar to the hypothalamus (Murakami, 2005).

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 V).