Answer
On the basis of speed of twitch, skeletal muscles may be divided into two types; fast twitch glycolytic fibers (FTF) and slow twitch oxidative fibers (STF). Slow twitch fibers function mainly to maintain posture, bear weight and support the body. Examples of this kind of muscle are the soleus, the erector spinae and the quadratus lumborum. These fibers reach peak tension in about 100 msec. Fast twitch fibers take only about 8 msec to reach peak tension. Examples of fast twitch muscles are muscles of the hand and of the eye-- iris, and extra ocular muscles ( superior and inferior obliques, the medial and lateral recti).
Characteriistics of Slow Twitch Fibers
They react slowly to stimulation
They are adapted for endurance and fatigue resistance
They produce ATP by aerobic respiration
They need much oxygen
They are supplied blood by dense capillary networks
The long thin myofibrils of STF facilitate oxygen diffusion; in addition, they have many mitochondria, and abundant myoglobin. Other characteristics are that their sarcoplasmic recticulum releases and reabsorbs Ca++ ions slowly, and their myosin ATPase is slow acting. This last characteristic makes for slow cross-bridge formation.
Fast twitch glycolytic fibers are adapted for quick responses. They are found in eye muscles as well as in larger muscles like the biceps brachii and other elbow flexors.
Some characteristics of FTFs
They ave extensive sarcopasmic recticuum(SR)
They re;lease and reabsorb Ca++ions quickly
Their myosin ATPASe is fast acting and it hydroplyses ATP quickly with resultant fast cross-bridge recycling
They have less myoglobin than FTF , so their fibers are white.
Work Step by Step
Energy production in FTF is by glycolysis and anerobic fermentation-- an inefficient method of producing ATP. These fibers have large stores of glycogen which they use for glycolysis. ATP production is enhanced by the donation of Pi ions from creatine phosphate; these combine with ADP molecules for a rapid regeneration of ATP . The fibers in these muscles are thicker and stronger than those of STFs; diffusion is slower, but they have no need to deliver oxygen rapidly to the deep regions of the cytoplasm.
