Myofibril and Endomembrane System

Structure

Myofibrils are arranged in bundles enclosed by a plasma membrane called the sarcolemma. Each myofibril is surrounded by a sarcoplasmic reticulum and by transverse tubules, which are collectively known as the endomembrane system (Wirtz et al., 2012).

The sarcoplasmic reticulum is a specialized type of smooth endoplasmic reticulum found in skeletal muscle. It forms a network of tubules that extend through the muscle fiber. The flattened tubules surround, but do not make direct contact with the myofibrils in a muscle fiber.

Transverse tubules are extensions of the sarcolemma, made up of interconnecting rings, each of which surround individual myofibrils.

Key Features/Anatomical Relations

Myofibrils are arranged in bundles enclosed by a plasma membrane called the sarcolemma. This structure is collectively called a muscle fiber. A connective tissue layer, the endoneurium, surrounds the sarcolemma.

Each myofibril is made up of numerous sarcomeres that are positioned end to end. The light regions of the sarcomere do not refract light and are considered isotropic. These regions are called I-bands. I-bands are composed of thin filaments of the protein actin and its associated proteins, such as tropomyosin and troponin. I-bands are dissected by a Z-line. The area between successive Z-lines are called sarcomeres. The sections of thick filament that are not superimposed by the thin filaments when the myofibril is relaxed is called the H-band (Laing and Nowak, 2005).

The sarcoplasmic reticulum (SR) is an organelle that consists of a network of loose flattened tubules with various compartments, many of which contain Ca2+. The two ends of the SR are wider, to from terminal sacs, called cisternae. Two cisternae of adjacent sarcoplasmic reticula and one transverse tubule form a functional unit called a triad, which operates to elicit the release of Ca2+ from SR stores.

Function

The protein bands of the myofibril facilitate contraction and relaxation of the muscle by gliding the thin actin and thick myosin filaments over each other.

The primary function of transverse tubules is to facilitate the conduction of signals from outside the fiber to the myofibrils. Transverse tubules are the site for excitation-contraction coupling. When an electric signal excites the sarcolemma, the signal travels down along the T-tubules to the sarcoplasmic reticulum, which releases Ca2+ for contraction.

The sarcoplasmic reticulum functions as an intracellular calcium store. Depolarization of the sarcoplasmic reticulum membrane results in the release of calcium ions (Ca2+). This release of calcium ions triggers the cascade required for contraction.

List of Clinical Correlates

—Myofibrillar myopathy

—Actin myopathy

—Hyaline body myopathy

—Muscular dystrophy

References

Dorland, W. (2011) Dorland's Illustrated Medical Dictionary. 32nd edn. Philadelphia, USA: Elsevier Sauders.

Laing, N. G. and Nowak, K. J. (2005) 'When contractile proteins go bad: the sarcomere and skeletal muscle disease', Bioessays, 27(8), pp. 809-22.

Wirtz, K. W. A., Packer, L., Gustafsson, J. Å., Evangelopoulos, A. E. and Changeux, J. P. (2012) New Developments in Lipid-Protein Interactions and Receptor Function. Nato Science Series A:: Springer US.