Examination of a cross section through most skeletal muscles will reveal that it is spilt up into a number of compartments (known as fascicles) by connective tissue sheaths. Each of these fascicles is in turn made up of a large number of muscle cells which are the basic cellular units of a skeletal muscle. A single muscle cell is known as a muscle fibre. Each muscle fibre is formed during development by the fusion of a number of undifferentiated, mononucleated cells (known as myoblasts) into a single cylindrical, multinucleated cell. Adult skeletal muscle fibres are 10 to 100 micrometres in diameter, and can extend up to 20 cm in length.
As in other cells, the cell or plasma membrane of the muscle fibre (known as the sarcolemma) encloses the cytoplasm (known as sarcoplasm) which contains particularly high concentrations of glycogen (a polymer of glucose) and myoglobin (an oxygen-binding protein). The sarcoplasm contains a large number of mitochondria as well as some other highly specialised cytoplasmic organelles:
A. Myofibrils:
Each muscle fibre contains a large number of cylindrical bundles, approximately 1-2 micrometres in diameter. These myofibrils occupy approximately 80% of the volume of the sarcoplasm and run the entire length of the muscle fibre. Myofibrils are functionally important as they contain the myofilaments which comprise the contractile elements of the muscle fibre.
B. Sarcoplasmic Reticulum:
Each myofibril is surrounded by a series of 'sleeve-like' segments. At the end each segment are two enlarged regions known as lateral sacs, which are connected to each other by a series of smaller tubular elements. Sarcoplasmic reticulum serves as a storage site for calcium, which is essential for muscle contraction. In a typical skeletal muscle fibre, the sarcoplasmic reticulum contains around 10-3M Ca2+ which is significantly higher than the 10-7M Ca2+present in the sarcoplasm at rest. As we will see, the close relationship between the myofibrils and the sarcoplasmic reticulum as well as the high concentration of Ca2+ in the latter plays an important role in the control of muscle contraction.
C. Transverse (or T-) Tubules:
T-tubules are located at regular intervals along the length of muscle fibres. They are located directly between, and are closely linked with the lateral sacs of adjacent segments of the sarcoplasmic reticulum. The arrangement of these tubules is such that their lumen is continuous with the extracellular space. T-tubules play a very important role in the initiation of muscle fibre contraction by conducting electrical signals from the sarcolemma into the interior of the muscle fibre.
A schematic diagram of a short length of a muscle fibre with a portion of the sarcolemma removed which illustrates the close relationship between the T-tubules, sarcoplasmic reticulum and the myofibrils is shown above.