For example, if a person has not spoken for awhile and then wants to speak, their tongue may be stiff at first causing the words to come out a little garbled, but after a few seconds of trying to speak, the tongue muscle will loosen up and then they can speak normally for the remainder of the time that they are conversing.Patients report that repeated contraction of muscle alleviates present myotonia with each contraction, such that myotonia is almost absent after a few contractions of the same muscle. The effect lasts about five minutes. There have been several proposed mechanisms for this phenomenon, but none have been demonstrated conclusively; one hypothesis is that the Na+/K+-ATPase is stimulated during the myotonic activity by increased intracellular Na+ in the cytosol of the muscle cell, increasing the activity of the Na+/K+-ATPase. However, in experiments with patients where the Na+/K+-ATPase had been blocked in the underarm by infusion of the Na+/K+-ATPase-blocker Ouabain, no effect on warm-up was observed. Another hypothesis states that the few remaining functional chloride channels in muscle may become more active with increased muscle activity.It has been proposed that inactivation of sodium channel protein type 4 subunit alpha, residing in skeletal muscle, could play an important role in the warm-up phenomenon. In particular, slow inactivation of the channel is believed to have a spatial and temporal extent that is correlated to warm-up and therefore may provide a plausible cause.
Such defects can be caused by air entrained in the melt, steam or smoke from the casting sand, or other gasses from the melt or mould. (Vacuum holes caused by metal shrinkage (see above) may also be loosely referred to as blowholes). Proper foundry practices, including melt preparation and mould design, can reduce the occurrence of these defects. Because they are often surrounded by a skin of sound metal, blowholes may be difficult to detect, requiring harmonic, ultrasonic, magnetic, or X-ray (e.g., industrial CT scanning) analysis. Pouring metal defects
Pouring metal defects include misruns, cold shuts, and inclusions. A misrun occurs when the liquid metal does not completely fill the mould cavity, leaving an unfilled portion. Cold shuts occur when two fronts of liquid metal do not fuse properly in the mould cavity, leaving a weak spot. Both are caused by either a lack of fluidity in the molten metal or cross-sections that are too narrow. The fluidity can be increased by changing the chemical composition of the metal or by increasing the pouring temperature. Another possible cause is back pressure from improperly vented mould cavities.Misruns and cold shuts are closely related and both involve the material freezing before it completely fills the mould cavity. These types of defects are serious because the area surrounding the defect is significantly weaker than intended. The castability and viscosity of the material can be important factors with these problems.