Forms of muscle irritability, contractility, conductivity, metabolic activity

All forms of muscle possess the following physiologic properties: irritability, contractility, conductivity, and metabolic activity. The specific function of muscle is to shorten for the purpose of movement, hence, its most specialized property is contractility. This property is most marked in skeletal muscle.

When a muscle responds, certain rather definite changes occur within it, namely, mechanical change, chemical change, thermal change, and the production of electricity and sound.

During contraction muscle changes its shape but its volume remains unchanged.

In response to a single stimulus, a muscle gives a single twitch, the record of which is known as a myogram. The myogram of a frog's muscle shows a latent period of 0.01 second, a contraction phase of 0.04 second, and a relaxation phase of 0.05 second. The various phases of the response of human muscles, although varying somewhat from those of the frog muscles, have about the same relative duration.

Following a series of repeated stimuli (two to five per second), the first few contractions progressively increase in extent to a maximum--treppe-after which the responses soon become less and less effective as fatigue develops. Fatigue prolongs each of the phases of the response--especially the relaxation phase. As the relaxation phase is lengthened the succeeding stimulus begins to reach the muscle before relaxation is completed. The new contraction begins at this point leaving a contraction remainder or contracture.When the stimuli are applied more rapidly the muscle may be held in a state of continuous contraction or complete tetanus. If a partial relaxation is permitted, the tetanus is now incomplete and is known as incomplete tetanus. The importance of tetanus lies in the fact that practically every voluntary and reflex response in man is of this nature. They possess three fundamentally important characteristics, namely, maintenance, fusion of separate contractions, and a summation of single responses.Although the mechanical response is continuous and maintained, it can be shown by the electrical change and sound produced that, within certain limits, the internal response is discontinuous. This discontinuous phase is, however, limited by an inherent period of forced rest imposed upon the muscle due to what is known as the refractory period.Within certain limits, a rise or fall of temperature increases or decreases respectively the rate of response. Again it is the relaxation phase which is especially affected. At a certain low temperature activity is reversibly suspended; at a certain high temperature it is irreversibly lost. These temperatures differ from animal to animal.The skeletal muscle fiber responds as a unit, that is, when stimulated it either gives a maximal contraction or fails to respond at all,--the "all-ornothing" phenomenon. Graded response is a function of the whole muscle, the strength of the response being determined by the number of fibers acting at any given time. Heart muscle in its entirety obeys the same law.