Functional subdivision of the venom gland musculature and the regulation of venom expulsion in rattlesnakes

Abstract

A combination of histology, whole muscle force physiology, glycogen depletion, and venom expulsion analyses using transonic probes to measure venom flow and fluid pressure transducers to measure venom pressure was performed on the m. compressor glandulae and m. pterygoideus glandulae. The m. pterygoideus glandulae has less than one-third the cross-sectional area of the m. compressor glandulae, and produces approximately one-fifth the total twitch and tetanic force; however, in situ surface stimulation of the muscle produces venom flow and pressure levels that are similar to those produced by the m. compressor glandulae. The similarity in venom output following stimulation reflects in part the functional role of the larger m. compressor glandulae in jaw adduction, but also the functional subdivisions within this muscle. The m. compressor glandulae is divided into a series of columnar fascicles that run from the surface of the muscle to the venom gland. The combined results of clearing and staining and glycogen depletion studies suggest that these fascicles may represent functional compartments. Identical stimulations applied to different regions of the m. compressor glandulae result in up to a six-fold difference in venom expulsion. This functional specialization may play a role in the regulation of venom flow during offensive and defensive strikes.