The nervous system in adult tunicates: current research directions

Abstract

This review covers 25 years of progress on structural, functional, and developmental neurobiology of adult tunicates. The focus is on ascidians rather than pelagic species. The ascidian brain and peripheral nervous system are considered from the point of view of ultrastructure, neurotransmitters, regulatory peptides, and electrical activity. Sensory reception and effector control are stressed. Discussion of the dorsal strand plexus centres on its relationship with photoreceptors, the presence in it of gonadotropin-releasing hormone and its role in reproductive control. In addition to hydrodynamic sense organs based on primary sensory neurons (cupular organs), ascidians are now known to have coronal sense organs based on axonless hair cells resembling those of the vertebrate acustico-lateralis system. The peripheral nervous system is remarkable in that the motor neuron terminals are apparently interconnected synaptically, providing the equivalent of a nerve net. Development of the neural complex in ascidians is reviewed, highlighting recent embryological and molecular evidence for stomodeal, neurohypophyseal, and atrial placodes. The nervous system forms similarly during embryogenesis in the oozooid and blastogenesis in colonial forms. The regeneration of the brain in Ciona intestinalis (L., 1767) is discussed in relation to normal neurogenesis. Finally, the viviparous development of salps is considered, where recent work traces the early development of the brain, outgrowth of nerve roots, and the targetting of motor nerves to the appropriate muscles.