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

A combination of histology, whole muscle force physiology, glycogen depleti on, and venom expulsion analyses using transonic probes to measure venom fl ow and fluid pressure transducers to measure venom pressure was performed o n the m. compressor glandulae and m. pterygoideus glandulae. The m. pterygo ideus 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 prod uces venom flow and pressure levels that are similar to those produced by t he m. compressor glandulae. The similarity in venom output following stimul ation reflects in part the functional role of the larger m. compressor glan dulae in jaw adduction, but also the functional subdivisions within this mu scle. The m. compressor glandulae is divided into a series of columnar fasc icles that run from the surface of the muscle to the venom gland. The combi ned results of clearing and staining and glycogen depletion studies suggest that these fascicles may represent functional compartments. Identical stim ulations applied to different regions of the m. compressor glandulae result in up to a six-fold difference in venom expulsion. This functional special ization may play a role in the regulation of venom flow during offensive an d defensive strikes. (C) 2000 Wiley-Liss, Inc.