The anatomy and physiology of the posterior stomach nerve (p.s.n.) in some decapod crustacea.

One of the major paired nerves in the decapod stomatogastric nervous system innervates the posterior part of the gastric mill. The morphology has been completely described previously only in Pugettia producta. This nerve is called the posterior stomach nerve (p.s.n.) in this paper. It is redescribed for Homarus vulgaris, and less detailed information is given for Palinurus vulgaris and Cancer pagurus. In these three species the p.s.n. contains numerous cell bodies. The majority of the cells are in one group and the long distal processes of these cells form a large proportion of the fibres in the nerve which runs to the gastric mill. Many of these fibres terminate by ramifying in the connective tissue which invests the ossicles of the gastric mill. The Brachyuran C. pagurus was used for most of the physiological experiments reported here. Only a small percentage of the animals exhibited spontaneous movements of the gastric mill when the carapace was removed and this activity ceased rapidly. However, a cycle of movements was usually observed in the active gastric mills and the p.s.n. contains many elements which respond to these normal movements. The p.s.n. does not appear to contain motor fibres. Intracellular recordings show that cells in the p.s.n. are proprioceptors. The distal processes of the major group of cells are morphologically dendrites, but they probably support action potentials. Repetitive electrical stimulation of a p.s.n. when it is isolated from the gastric mill evokes changes in the output from the stomatogastric ganglion. The p.s.n. sensory system could therefore function in the normal reflex control of the activity of the gastric mill. This investigation may assist in the analysis of the functions of the stomato­ gastric ganglion.