Echinodermata

MASHANOV, V. ; ZUEVA, O.; RUBILAR, T.; EPHERRA, L; GARCÍA-ARRARÁS, J. E.

Structure and evolution of invertebrate nervous systems

Oxford University Press

Año: 2015; p. 665 - 688

The central nervous system (CNS) of echinoderms is composed of the morphologically distinct ectoneural and hyponeural subsystems (plus the aboral subsystem in crinoids), which are nevertheless highly interconnected to form a single anatomical entity. In terms of histological architecture, each of these CNS components is a neuroepithelium composed of various types of unmyelinated neurons embedded in a supporting framework formed by radial glia. A large diversity of neuronal phenotypes producing a rich array of neurotransmitters have been described in both adult and larval nervous systems. Contrary to the general belief, echinoderm nerve cells can communicate with each other via ´classical´ chemical synapses. The radial glia are multifunctional cells, which share some morphological, immunocytochemical, and functional characteristics with radial glia of chordates. Sensory input to the echinoderm nervous system is mostly mediated through numerous receptors of various modalities scattered throughout the body. Specialized sense organs are rare and represented by light-sensitive structures and statocysts. The effectors that handle the CNS output include, besides the visceral and body wall musculature, echinoderm-specific mutable collagenous tissue, which is capable of changing its mechanical properties in response to neural stimuli. Indirectly developing echinoderms form a larval nervous system, which is mainly associated with locomotory ciliary bands and the gut. The larval nervous structures completely degenerate at metamorphosis, and the adult pentameric CNS develops de novo from the oral ectoderm. A hypothesis is discussed that each of the radial nerve cords of the pentameric echinoderm CNS might be homologous to the neural tube of chordates.