LPT 321: Meat Science (1+1)

SARCOPLASMIC RETICULUM AND "T" TUBULES

• The endoplasmic reticulum of the muscle, the Sarcoplasmic reticulum (SR) is very well developed and is a membranous system of tubules and cisternae (flattened reservoir for Ca²⁺) that forms a closely meshed network around each myofibril.
• Periodically along the length of the muscle fibre, and around its entire circumference, invagination of the sarcolemma forms the network of tubules that are called transverse tubules. These are usually called as T system or as T tubules.
• Thus SR and transverse tubules (t-tubules) are two separate distinct membrane systems and the t tubules are associated with the sarcolemma while the SR is intracellular in nature.
• The reticulum membranes of the SR are the storage sites of Ca²⁺ in resting muscle fibres.
• The SR consists of several distinct elements.
  • Thin tubules oriented in the direction of the myofibrillar axis, constitute the longitudinal tubules of the reticulum.
  • In the H zone region of the sarcomere the longitudinal tubules converge forming a perforated sheet that is called a fenestrated (window-like opening) collar.
  • At the junction of the A and I bands the longitudinal tubules converge and join with a pair of larger, transversely oriented, tubular elements called terminal cisternae.
  • The longitudinal tubules extend in both directions from the fenestrated collar to the terminal cisternae.
  • The two tubular elements comprising the terminal cisternae lie parallel to each other with one tubule of the pair transversing the A band and the other transversing the adjacent I band, of the sarcomere.
  • A "T" tubule also runs transversely across the sarcomere, at the A-I band junction and lies between the two tubular elements of the terminal cisternae pair.
  • The central "T" tubule and the two tubular elements of the terminal cisternae collectively form a structure known as the triad.
  • Each sarcomere has two triads, one in each half sarcomere at the A-I band junction.
  • The triads encircle each myofibril at the A-I band junction of mammals, birds and in some fish.
  • In some species the triads are located at the Z lines.
• The SR volume varies from one muscle fibre to another, but it is estimated as constituting approximately 13 percent of the total fibre volume.
• The T tubules on the other hand, comprise only about 0.3 percent of the fibre volume.