Muscle contraction relaxation phase

Generalized seizures include absence, myoclonic, and tonic-clonic. Manifestations of a generalized tonic-clonic seizure include alternate contraction (tonic phase) and relaxation (clonic phase) of muscles, a loss of consciousness, and abnormal behavior. Myoclonic seizures involve sudden, forceful contractions involving the musculature of the trunk, neck, and extremities. Absence seizures, previously referred to as petit mal seizures, are seizures characterized by a brief loss of consciousness during which physical activity ceases. The seizures typically last a few seconds, occur many times a day, and may go unnoticed by others. 

In the relaxation phase of muscle contraction, the S-1 head of myosin hydrolyzes ATP to ADP and Pj, but these products remain bound. The resultant ADP-Pj-myosin complex has been energized and is in a so-called high-energy conformation. 

It has been postulated that 2-PAM exerts its cardiac action in rabbit atria through its alteration of calcium metabolism. The relaxation phase of skeletal muscle contraction seems to be directly affected by the sarcoplasmic reticulum because of its ability to sequester calcium actively.29,46 a similar role has been suggested for the sarcoplasmic reticulum in cardiac muscle. 6,83 The onset of muscle contraction takes place when calcium reaches a crit-cal concentration. This contraction is later reduced by the increased calcium-sequestering activity of the sarcoplasmic reticulum. Thus, 2-PAM can affect this process by decreasing the rate of calcium uptake by the sarcoplasmic reticulum, which results in increasing the time required to reduce the calcium concentration enough to allow relaxation to take place. This was demonstrated by the Increase in the relaxation phase. It was suggested that this... 

I call magnesium the relaxer because it stimulates the relaxation phase in muscle tissue, including the heart muscle, just as calcium stimulates the contraction phase. Many people are familiar with milk of magnesia, an antacid suspension of magnesium hydroxide sometimes used as a cathartic, and with Epsom salts, a bath salt of magnesium sulfate that produces wonderful relaxation. Dr. C. Norman Shealy considers magnesium a crucial neurochemical, which I will discuss in the next few pages. 

The present review will deal with the properties of these five systems and their behavior on the addition of adenosine triphosphate (ATP), adenosine diphosphate (ADP), inosine triphosphate (ITP), inorganic triphosphate, and pyrophosphate. The really important question is how far the interaction phenomena with ATP can be regarded as a model for contraction in living muscle—a model for contraction as a whole in the case of the earlier of these stages, and a model of the molecular mechanism in the case of the later stages. For the relaxation phase, no model is yet known (c/. however. Section II, Ah and appendix). 

Fig. 1. Schematic diagram showing the appearance of vertebrate striated muscle as it appears under phase-contrast microscopy, (a) Relaxed, (b) contracted.

The relaxation and contraction phases will obviously be easier to study when it becomes possible to isolate them experimentally. For the contractile phase, such isolation now appears possible by progressively breaking up the whole muscle into simpler components. We may think of this breaking up as going through five stages in the first stage we have... 

Somewhat similar considerations were also applied to acto-myosin systems, for which enhanced fluidization was predicted due to actin-driven sliding of filaments.For actin networks, it has long been known that the addition of myosin motors can cause network contraction and eventually a macroscopic phase separation, a process dubbed super-precipitation. Under certain conditions, however, enhanced fluidization due to an increased rate of stress relaxation was foimd in solutions of non-CTOss-linked actin that was activated by minifilaments of skeletal muscle myosin II. The gels were foimd to soften at low frequencies. This can be explained by an increase in the rate of stress relaxation in the entangled networks when motors actively slide the filaments arormd the obstacles. This sliding can also lead to the formation of bundled or oriented/polar-ized states of actomyosin gels. There bave been a number of hydrodynamic-like theories developed for such fluidlike active gels. "... 

---