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A micro-mechanics model for non-isotropic, open-celled foams is developed using an elongated tetrakaidecahedron (Kelvin model) as the repeating unit cell. Assuming the cell edges possess axial and bending rigidity, the mechanics of deformation of the elongated tetrakaidecahedron lead to a set of equations for the Young's modulus, Poisson's ratio and tensile strength of the foam in the principal material directions. These equations are written as a function of the cell edge lengths and cross-section properties, the inclination angle and the strength and stiffness of the solid material. This micro-mechanics model employs an elongated Kelvin model geometry which is more general than that employed by previous authors, as the size and shape of the repeating unit cell are defined by specifying three independent dimensions. As a result, the model accounts for an additional variation in the unit cell shape which is not accounted for in the previous models. The effect of this additional shape parameter on the non-isotropic stiffness and strength behavior is demonstrated and the advantages of this more general micro-mechanics model are illustrated. Published by Elsevier Ltd.

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SUMMARYThree pairs of ligaments support not only the heart of spiny lobsters but also ligamental nerve plexuses, the complex terminal aborizations of segmental nerves. Segmental nerves 1-4 project from the thoracic ganglia into the pericardial cavity and ultimately ramify along the strands of the anterior, medial, and posterior ligaments. In each branch, a core of large axons sends fibers to terminate in a surrounding cortex of fine and varicose secretory processes. Electron micrographs reveal at least five distinct populations of granule-filled neuronal profiles, many with vesicles clustered at membrane thickenings adjacent to the epineural sheath.The ligamental nerve plexuses synthesize and accumulate octopamine, dopamine, 5-HT, and acetylcholine. Octopamine and 5-HT are predominant, comprising 33% and 65%, respectively, of the synthetic activity devoted to the four amines. Thus, the anatomy, ultrastructure, and neurochemistry of the ligamental nerve plexuses establishes their homology with the pericardial organs of other Crustacea. Octopamine and 5-HT are released by a Ca++-dependent mechanism upon electrical stimulation of preterminal nerve trunks, and, in uiuo, would be swept immediately through ostia into the heart. These observations, when considered with known effects of octopamine and 5-HT on crustacean cardiac activity, neuromuscular transmission, muscle tension, and cyclic AMP metabolism provide a strong case for hormonal actions a t target sites throughout the animal. Segmental nerve processes in the dorsal nerve trunk ramify into a plexus around the dorsal nerve apparatus, a small muscular bulb that lies recessed in the cardiac surface. The dorsal nerve, carrying excitatory and inhibitory input

The neuropeptide proctolin has distinguishable excitatory effects upon premotor cells and motorneurons of Homarus cardiac ganglion. Proctolin's excitation of the small, premotor, posterior cells is rapid in onset (5-10 s) and readily reversible (less than 3 min). Prolonged bursts in small cells often produce a "doublet" ganglionic burst mode via interactions with large motorneuron burst-generating driver potentials. In contrast to small cell response, proctolin's direct excitatory effects upon motorneuron are slow in onset (60-90 s to peak) and long-lasting (10-20 min). The latter include: a concentration-dependent (10(-9)-10(-7)M) depolarization of the somatic membrane potential; increases in burst frequency and enhancement of the rate of depolarization of the interburst pacemaker potential. Experiments on isolated large cells indicate: the slow depolarization is produced by a decrease in the resting GK and proctolin can produce or enhance motorneuron autorhythmicity . A two- tiered non-hierarchical network model is proposed. The differential pharmacodynamics exhibited by the two cell types accounts for the sequential modes of ganglionic burst activity produced by proctolin.