In chapter 1, I described the relationship between structure and function using two examples, muscles and kidneys, to illustrate. Structurally, muscle can be broadly classified as either striated or nonstriated. The two kinds of striated muscle are skeletal and cardiac. Nonstriated muscle is further characterized as visceral smooth muscle or vascular smooth muscle. Smooth muscle does not contain well-defined striations and is most commonly found in hollow tubular structures such as blood vessels, gut walls, and fallopian tubes. As well as the above distinctions, scientists classify different muscle types according to both physiological function and metabolic pathways for energy production and consumption.
Some skeletal muscles do not need to contract rapidly, but they do need to resist fatigue and to maintain tension for long periods of time. An example of this kind of muscle is the soleus of the lower leg. On the other hand, some muscles need to contract rapidly but at infrequent intervals. Examples of these kinds of muscle include the extraocular muscles of the eye and the extensor digitorum longus of the wrists and ankles, fingers and toes. Striated muscles are further characterized according to the rate at which they develop force once stimulated, the rate at which they shorten to lift a load, the rate at which they fatigue upon repetitive stimulation, and the rate at which they recover from fatigue. Muscles are therefore referred to as slow twitch (type I fibers whose rates of contraction upon stimulation are relatively slow) and fast twitch (type II fibers, with relatively fast rates of contraction). Fast-twitch fibers are further classified as type IIa (fatigue resistant or indefatigable) and type IIb (fatigable).
In their cytosol, slow-twitch muscles have a high content of the oxygenbinding protein myoglobin, they have many capillary networks surrounding each individual fiber, and they have a stable supply of oxygen. For these and related reasons (for example, many mitochondria per fiber), they are called