1 Only appointments for producers with active licenses on PDB will be renewed. 2  Appointment Renewal Invoices and the list of appointment renewals due will be available on the NIPR website beginning 8:00 AM Central Time, November 1, 2015 through 4:00 PM Central Time December 31, 2015. 3  NIPR website link is: www.nipr.com.
Myofibrils are composed of long proteins including actin, myosin, and titin, and other proteins that hold them together. These proteins are organized into thick and thin filaments called myofilaments, which repeat along the length of the myofibril in sections called sarcomeres.
3. Sustained contractions are produced by the asynchronous stimulation of different motor units. Mechanisms of Contraction. I. Skeletal muscle cells, or fibers, contain structures called myofibrils. A. Each myofibril is striated with dark (A) and light (I) bands; there are Z lines in the middle of each I band.