The Muscle cells, also known as muscle fibers, are the fundamental units of human muscles. Humans have three types of muscle: skeletal, smooth and cardiac. The muscle cells share eight properties in general that distinguish them from other cells. Those eight properties of muscles are as follows:
3) All or none law
4) Refractory period
8) Rigor mortis
Now we shall go for some discussion:
Ability of a tissue to respond to stimulation is called Excitability. Like nerve fibers, when a muscle fiber is stimulated by an adequate stimulus, it becomes active. That means, it generates an action potential which is transmitted throughout the sarcolemma. Excitability as the properties of muscles is expressed by two factors—rheobase and chronaxie. Rheobase is the threshold of intensity of electric current capable of exciting the tissue no matter how long it is given. Whereas Chronaxie is the threshold of duration required to excite a tissue when the strength of stimulus is double the rheobase. Excitability of a tissue is inversely proportional to its chronaxie. Skeletal muscles and multi-unit smooth muscles are normally excited through nervous stimulation only. But cardiac muscle and single unit smooth muscles are auto-excitable .
Contractility is a unique property of muscles by virtue of which when a muscle is excited, it contracts followed by relaxation. Skeletal muscles contract more quickly and forcefully than other muscles. So, their efficiency or capacity of taking work load is quite greater. On the other hand cardiac and smooth muscles show slow, continuous contractions.
All or none law
According to this law of properties of muscles, when a muscle is stimulated, either it responds maximally or it does not respond at all. If the stimulus is adequate in strength the response will be maximum and a further increase in the intensity of stimulus will not raise the degree of contraction any more if other conditions remain same. On the other hand, if the stimulus is inadequate, it will totally fail to bring out a response and the muscle will not contract at all.
Refractory period is the time gap during which a second stimulus fails to excite a tissue. In this property of muscles, in order to produce two successive responses, the second stimulus must fall after the refractory period from the first stimulus. In mammalian skeletal muscles, the refractory period is about 0.002 sec. Due to the short refractory period; the skeletal muscles show abridgment of contractions on repetitive stimulation. The refractory periods of cardiac and smooth muscles are much longer than skeletal muscles. In case of cardiac muscle, the refractory period is longest and extends throughout the contraction and relaxation periods.
When a skeletal muscle is stimulated by two stimuli in rapid succession, the contractile responses of the two stimuli are added together to produce a greater response; this properties of muscles are called summation. When two successive stimuli are given in a way that the second stimulus falls during the relaxation or contraction phase of the first response, the second response is super imposed on the first response producing greater force of contraction; this is called summation of contractions. If the second stimulus falls within the latent period of the first response, the effects of two stimuli are completely fused to produce a single contractile response which is greater than that would occur due to a single stimulus; this is called summation of stimuli.
During tetanus the muscle remains contracted throughout the period of stimulation and it does not get a chance at all to relax. Tetanus is the properties of muscle that sustained muscular contraction without intervening periods of relaxation. When the frequency of repetitive stimuli is lower so that each successive stimulus falls within the period of previous relaxation, the muscle shows incomplete relaxations between gradually increasing contractions; this is called incomplete tetanus. Thus incomplete tetanus is due to summation of repetitive contractions whereas tetanus is due to summation of repetitive stimuli. Because of their long refractory periods cardiac and smooth muscles are not easily totalized.
Fatigue is one of the properties of muscles which is a state of temporary loss of excitability of a tissue due to overwork. When a muscle is stimulated repeatedly; its contractility diminishes gradually and in due course it fails to respond. This happening is called fatigue. Muscular fatigue is developed due to two main reasons—(i) lack of supply of oxygen and food, and (ii) accumulation of waste products. Generally a fatigued muscle recovers the ability to contract after a period of rest.
Stiffening of the body after death due to a state of rigidity of muscles is called rigor mortis and are the terminal properties of muscles. It occurs 2-3 hours after death when all muscles of the body go into a state of contracture and become rigid without any stimulation. Rigor mortis disappears 24-48 hours after death when muscle proteins are decomposed due to autolysis caused by release of lysosomal enzymes.