Cnidarian Nerve Nets and Neuromuscular Efficiency.

Abstract

Cnidarians are considered "nerve net animals" even though their nervous systems include various forms of condensation and centralization. Yet, their broad, two-dimensional muscle sheets are innervated by diffuse nerve nets. Do the motor nerve nets represent a primitive organization of multicellular nervous systems, do they represent a consequence of radial symmetry, or do they offer an efficient way to innervate a broad, two-dimensional muscle sheet, in which excitation of the muscle sheet can come from multiple sites of initiation? Regarding the primitive nature of cnidarian nervous systems, distinct neuronal systems exhibit some adaptations that are well known in higher animals, such as the use of oversized neurons with increased speed of conduction, and condensation of neurites into nerve-like tracts. A comparison of neural control of two-dimensional muscle sheets in a mollusc and jellyfish suggests that a possible primitive feature of cnidarian neurons may be a lack of regional specialization into conducting and transmitting regions.