quinta-feira, fevereiro 04, 2010

The evolution of the nervous system in a phylogenetic context for non-vertebrate deuterostomes

The rudiments of a nervous system with the main functional divisions only came to existence in basal craniates (the most inclusive clade containing Myxiginiformes). A fundamental key unavoidably linked to the evolution of the nervous system is locomotion. An animal moving though the space needs to have a representation of what is surrounding it. In deuterostomes, such is the case in tunicate larvae. Before settling in a fixed spot the tunicate larvae are able to move around the 3-dimensional space with the help of a postanal tail. In order to have a clearer notion to where it is going the larvae have a cluster of photosentive cells in the foremost part of the anterior part of the body, which is then linked to a central command that regulates muscle action, this corresponds to the neural tube and notochord. Incredibly, once the tunicate larva settles it eats its own “brain” as energy supply since a “visual” representation of the surrounding environment is not necessary anymore to perform the basic tasks for survival; tunicates are filter-feeders. Nevertheless a ganglion and a subneural gland are maintained in some species and their function is poorly understood.
Amphioxus, and other cephalochordates, is more mobile (i.e. higher levels of activity) and expectadly retain their nervous system through all stages of ontogeny. Cephalochordates are essentially flat swimmers that bear lateral muscles that are controlled by a notochord and neural tube. By sending signals to the lateral mucles the amphioxus can control its turgidity while in locomotion. Again, a pigment spot in the foremost part of the body allows these creatures to acquire electromagnetic information about the surrounding world. Nevertheless, for the most part cephalochordates are passive filter feeders that live penetrated in the sand with the anterior portion of the body protruding to acquire small food items.
Craniates, again, associated to higher levels of activity, further expand their perception of the environment in a number of ways. Hagfish for example have an accurate sense of smell that enables them to search for carcasses to feed and tactile protuberant organ at the tip of the body. These animals are entirely mobile, and therefore, linked to the sensory organs they have a tripartite brain. Other evolutionary novelties were acquired, namely the neurogenic placodes and cranial nerves possibly increasing the efficiency of the nervous system. Importantly, neurogenic placodes are pockets of neurons that are linked to the ampullary organs and to the lateral line that is electrically sensible. In basal craniates gustatory, tactile, olfactory sense systems were already present. Also neural crest cells have a role on 1) the formation of the gill arch skeleton, 2) cranial motor neurons, 3) sensory neurons and 4) dentine and enamel formation.
Echinodermata developed a radial “nervous” system unparalleled in other deuterostomes, several “nerves” radiate from the mouth to the tip of the arms and are composed of interconnected neurons.
Enteropnests (Hemichordata) are essentially tactile animals and they have developed a subepidermal layer of neurons which are more densely concentrated either in the dorsal and ventral part of the body. These layers in conjunction with the stomochord served the base to postulate the origin of the neurogenic placodes and neural crests in craniates.

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