2. Sarcoplasmic reticulum is the SER of striated muscle cells, specialized to sequester calcium ions. In skeletal muscle, it consists of an anastomosing complex of membrane-limited tubules and cisternae that ensheathe each myofibril. At each A-I band junction, a tubular invagination of the sarcolemma termed a transverse tubule, or T tubule, penetrates the muscle fiber and comes to lie close to the surface of the myofibrils. On each side of the T tubule lies an expansion of the sarcoplasmic reticulum termed a terminal cisterna, A complex of 2 terminal cisternae and an intervening T tubule constitutes a triad, Triads are important in initiating muscle contraction.

3. Types of skeletal muscle fibers:

a. Red fibers contain more myoglobin and mitochondria and are capable of sustained contraction. Their contraction in response to nervous stimulation is slow and steady, resulting in their designation as slow fibers. They predominate in postural muscles and in the limbs.

b.White fibers contain less myoglobin and fewer mitochondria. They react quickly, with brief, forceful contractions, but cannot sustain contraction for long periods. They are thus termed fast fibers, They predominate in the extraocular muscles.