Page 76 - Headlines Histology2024_Neat

impulses from the outer sarcolemma to the sER cisternae leading to release
of Ca++ ions that are necessary for myofibers contraction.
• Other cytoplasmic organelles include small inactive Golgi, dense glycogen
and well-developed sER.
Red and white fibers
Red myofibers
• They have a red color in fresh state due to their high myoglobin contents
that is an oxygen-storage molecule similar to hemoglobin.
• They are smaller in diameter and contain abundant cytochromes and
mitochondria. They have a rich blood supply.
• They are aerobic and predominate in muscle that can sustain their
contractile activity for a long time.
White myofibers
• They are larger in cross-section. They contain few mitochondria and
cytochromes and relatively little myoglobin hence, they are white in fresh
state.
• They have a relatively poor blood supply.
• They are rich in glycogen.
• They are anaerobic white fibers and constitute muscles that perform intense
but sporadic contraction. The white fibers contract more rapidly but they
also fatigue more rapidly.
• In most mammalian species, the skeletal muscles are composed of a mixture
of red and white fibers. In domestic poultry, some muscles are red such as
leg muscles, and others are white such as pectoral muscle. In wild fowl, the
pectoral muscles are composed of red fibers that are capable of sustained
flight.

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