Muscles In Vertebrates

Type of animal science: Anatomy Fields of study: Histology, zoology

Vertebrate muscle tissue provides for many vital bodily functions. For these purposes, there are three major types of vertebrate muscle tissue: skeletal, cardiac, and smooth.

Principal Terms involuntary: functioning automatically;

not under conscious control motor neuron: a nerve cell that transmits impulses from the central nervous system to an effector such as a muscle cell motor unit: a motor neuron together with the muscle cells it stimulates tissue: a group of similar cells that executes a specialized function twitch: a rapid muscular contraction followed by relaxation that occurs in response to a single stimulus voluntary: capable of being consciously controlled

The ability of vertebrates to move their bodies and many of the contents of their bodies is a feature of major importance for their survival. The movements result from the contractions and relaxations of tissues specialized for the active generation of force: the muscles. Although initially it may seem sufficient to have only one type of muscle tissue, reflection on the functional requirements makes it clear that more than one type of muscular tissue is probably necessary. For example, the movement of the limbs should be under the conscious, voluntary activation and control of the animal. Otherwise, unwanted and uncoordinated random limb movements would result, or desired and possibly vital movements would not be forthcoming. In addition, the control of the limb movements should be precise, with as wide a range as possible for the forces which can be generated.

On the other hand, consider the movement of the blood through the circulatory system. This job must be continuously performed every minute of the animal's life. It would obviously be better to have this function run automatically, without the need for conscious, voluntary activation and control. The need for precise control of the forces generated here is also not as great as for the limb movements, nor is the need for a wide range of forces as great. The flow of blood to the various organs can be much better regulated by varying the diameter of the blood vessels at or near their entrance to the organs, thereby varying the flow into the organs, while the pumping forces generated to propel the blood remain relatively constant.

For the functions of controlling blood flow into organs and the mixing and passage of food through the digestive tract, it is again most reasonable to have automatic operation of the muscles involved, without the need for conscious, voluntary activation and control. To control the huge number of such muscles consciously is an impossible task in any case. Also, these muscles need to be able to change their lengths greatly, and sometimes, to maintain a maximal contraction for very extended periods; however, very rapid actions are not as important.

The preceding considerations make it appear necessary to have three fundamentally different types of muscle tissue. In fact, vertebrates do have three types of muscle tissue, whose different characteristics match these three sets of operational requirements: skeletal muscle tissue, cardiac muscle tissue, and smooth muscle tissue.

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