The bat cricothyroid muscle is believed to participate in the production of the short bursts of frequency modulated ultrasound which these animals use as an echolocation device. The evidence seems to indicate that this muscle must be extremely fast acting. It possesses a very well developed sarcoplasmic reticulum, consisting of intercommunicating longitudinal and transverse tubular elements. The transverse elements, situated at the level of the junction between the A and the I bands, are tripartite complexes of tubules called triads, and these are sometimes replaced by more complex structures, the pentads. The intermediate element of the triad appears as a slender continuous tubule, which can be shown to come into close contact with the sarcolemma and also to share with it certain common staining properties. The longitudinal components of the reticulum consist of very numerous tubules which link successive triads to each other and anastomose to form multiple layers of close-meshed reticula in the interfibrillar sarcoplasm. Both the longitudinal and the transverse elements of the sarcoplasmic reticulum form a continuous network across the muscle fiber. It is suggested that the extraordinary development of the sarcoplasmic reticulum in the bat cricothyroid is related to the unusual physiological properties of this muscle.

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