Unique role of skeletal muscle contraction in vertical lymph movement in anurans

Electromyographic (EMG) activity of skeletal muscles that either insert on the skin or are associated with the margins of subcutaneous lymph sacs was monitored for two species of anurans, Chaunus marinus and Lithobates catesbeiana (formerly Bufo marinus and Rana catesbeiana). Our hypothesis was that contraction of these muscles varies the volume, and hence pressure, within these lymph sacs, and that this pressure is responsible for moving lymph from ventral, gravitationally dependent reaches of the body to dorsally located lymph hearts. EMG activity of M. piriformis, M. gracilis minor, M. abdominal crenator, M. tensor fasciae latae, M. sphincter ani cloacalis, M. cutaneous pectoris and M. cutaneous dorsi was synchronous with pressure changes in their associated lymph sacs. These muscles contracted synchronously, and the pressures generated within the lymph sacs were sufficient to move lymph vertically against gravity to the lymph hearts. The pressure relationships were complex; both negative and positive pressures were recorded during a contractile event, a pattern consistent with the addition and loss of lymphatic fluid to the lymph sacs. Severing the tendons of some of the muscles led to lymph pooling in gravitationally dependent lymph sacs. These data are the first to: (1) describe a function for many of these skeletal muscles; (2) document the role of skeletal muscles in vertical lymph movement in anurans; and (3) reinterpret the role of the urostyle, a bony element of the anuran pelvic girdle.