Sarcoplasmic Extensive(Rapid calcium Less extensive
Reticulum release)
Blood supply Less extensive More extensive
Mitochondria Fewer Increased number
Myoglobin Small amount Large amount
Contraction Adapted for very rapid intense Adapted for slow continuous
Contraction for short muscle activity (depends on
period(depends on anaerobic aerobic metabolism)
metabolism)
Fatigue Rapid Delayed
Example Occular muscle Soleus muscle
,PHYSIOLOGY DR.ELSHERIF
¾ Human skeletal muscles contain mainly 2 types of muscle fibers:
1) Slow (Red = type I1) Fibers characterized by the following:
a) Small muscle fibers innervated by small slowly conducting motor
neurons.
b) Contain large numbers of oxidative enzymes (high mitochondrial
volume).
c) Surrounded by more extensive capillaries to supply extra amounts of
oxygen.
d) Have low ATPase activity.
e) Contain a higher concentration of myoglobin, which stores oxygen
until need.
The above data provide type I fibers with:
Slow contractile mechanism.
Large capacity for aerobic metabolism and a high resistance to
fatigue.
2. Fast (pale = type II b) Fibers characterized by the following:
a) Larger fibers innervated by large rapidly - conducting motor neurons.
b) Contain extensive sarcoplasmic reticulum for rapid release of calcium
ions.
c) Have large amounts of glycolytic enzymes for rapid release of energy
by the glycolytic process.
d) Have high ATPase activity.
e) Contain less blood supply, less myoglobin content, and fewer
mitochondria.
The above data provide type lI b fibers with:
Rapid contractile mechanisms.
Less resistance to fatigue
, PHYSIOLOGY DR.ELSHERIF
NB:
1. Most muscles of the body contain both types of muscle fibers.
2. Muscles adapted for long posture -marinating contractions e.g., back muscles &
soleus muscle are composed mainly of slow fibers. Muscles specialized for fine skilled
movements e.g., external ocular muscles & some hand muscles are composed mainly of
fast fibers.
3. Muscle groups with a high percentage of fast fibers exert more force and a greater
velocity than those with predominant slow fibers
4. Aging is associated with a loss of muscle mass. Aging results in a loss of fast fibers
and a relative increase in slow fibers.
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