Cardiac muscle fibers of the hummingbird and finch have no transverse tubules and are smaller in diameter than those of mammalian hearts. The fibers are connected by intercalated discs which are composed of desmosomes and f. adherentes; small nexuses are often interspersed. As in cardiac muscle of several other animals, the junctional SR of the couplings is highly structured in these two birds but, in addition, and after having lost sarcolemmal contact, the junctional SR continues beyond the coupling to extend deep into the interior of the cells and to form belts around the Z-I regions of the sarcomeres. This portion of the sarcoplasmic reticulum, which we have named "extended junctional SR," and which is so prominent and invariant a feature of cardiac cells of hummingbirds and finches, has not been observed in chicken cardiac cells. The morphological differences between these species of birds may be related to respective differences in heart rates characteristic for these birds.
CARDIAC MUSCLE : Its Ultrastructure in the Finch and Hummingbird with Special Reference to the Sarcoplasmic Reticulum
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Paul H. Jewett, J. R. Sommer, E. A. Johnson; CARDIAC MUSCLE : Its Ultrastructure in the Finch and Hummingbird with Special Reference to the Sarcoplasmic Reticulum . J Cell Biol 1 April 1971; 49 (1): 50–65. doi: https://doi.org/10.1083/jcb.49.1.50
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