There are three kinds of muscles in our bodies: (1) Voluntary muscles, which include the skeletal and facial muscles; (2) involuntary muscles, which are found among the muscles in the blood vessels, intestines, stomach, bladder and uterus, and (3) heart muscle.
Voluntary muscles are also termed "striated" or striped muscles, because under a microscope they show alternating light and dark stripes or ridges running crosswise. Most of these muscles are long and narrow, as in the arms and legs; but others are sheetlike, as those of the abdomen and the back. Voluntary muscles are endowed with a rich nerve and blood supply so that they can accomplish their purpose. Some, such as respiratory muscles, might be said to be both voluntary and involuntary. We use them voluntarily when we take a deep breath, but most of the time, and especially when we are asleep, they work without any volition on our part.
By way of contrast, involuntary muscles are termed "smooth," for they are without these striped ridges. With the aid of the electron microscope a striking and purposeful difference has been discovered between the structure of the voluntary and the involuntary or smooth muscles. In smooth muscles the filaments that do the actual work of contracting are arrayed in parallel, overlapping one another, and at an angle of as much as 10 percent to the long axis of the muscle cell. This oblique arrangement of the filaments in smooth muscles may give them as much as ten times the strength that they would otherwise have and so enables them to sustain forceful contractions over long periods of time. On the other hand, the way the filaments lie in voluntary or striped muscles, in series or end to end, permits them to move with greater velocity or speed. And the amount of contraction can be far greater than in smooth muscle.
The muscle of the heart is in a class by itself. Because of its great work load it has a special kind of construction, making it the strongest muscle in man. In woman, only the muscle of the uterus, needed to expel a baby at birth, is said to be stronger. Heart muscle is constructed after the pattern of voluntary muscle, but it functions as an involuntary muscle.
Voluntary muscles are also termed "striated" or striped muscles, because under a microscope they show alternating light and dark stripes or ridges running crosswise. Most of these muscles are long and narrow, as in the arms and legs; but others are sheetlike, as those of the abdomen and the back. Voluntary muscles are endowed with a rich nerve and blood supply so that they can accomplish their purpose. Some, such as respiratory muscles, might be said to be both voluntary and involuntary. We use them voluntarily when we take a deep breath, but most of the time, and especially when we are asleep, they work without any volition on our part.
By way of contrast, involuntary muscles are termed "smooth," for they are without these striped ridges. With the aid of the electron microscope a striking and purposeful difference has been discovered between the structure of the voluntary and the involuntary or smooth muscles. In smooth muscles the filaments that do the actual work of contracting are arrayed in parallel, overlapping one another, and at an angle of as much as 10 percent to the long axis of the muscle cell. This oblique arrangement of the filaments in smooth muscles may give them as much as ten times the strength that they would otherwise have and so enables them to sustain forceful contractions over long periods of time. On the other hand, the way the filaments lie in voluntary or striped muscles, in series or end to end, permits them to move with greater velocity or speed. And the amount of contraction can be far greater than in smooth muscle.
The muscle of the heart is in a class by itself. Because of its great work load it has a special kind of construction, making it the strongest muscle in man. In woman, only the muscle of the uterus, needed to expel a baby at birth, is said to be stronger. Heart muscle is constructed after the pattern of voluntary muscle, but it functions as an involuntary muscle.
