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.
Causes
The disorder is caused by mutations in part of a gene (CLCN1) encoding the ClC-1 chloride channel, resulting in muscle fiber membranes having an unusually exaggerated response to stimulation (hyperexcitability).Three cases have been reported who were diagnosed with Thomsens myotonia and proved on genetic testing not to have mutations in the chloride gene but rather in the alpha-subunit of the voltage gated sodium channel (SCN4A). Like chloride channel mutations, patients with sodium channel mutations may have a very varied phenotype, making a diagnosis more difficult. Mechanisms
Myotonia congenita is caused in humans by loss-of-function mutations in the gene CLCN1. This is the gene encoding the protein CLCN1, that forms the ClC-1 chloride channel, critical for the normal function of skeletal muscle cells. This gene is also associated with the condition in horses, goats, and dogs. In short, in lack of sufficient functional chloride channels, the muscle fiber membrane becomes hyper-excitable and continues to be electrically active (firing action potentials) when stimulated, for longer periods of time, than a normal muscle fiber. This results in prolonged contraction/delayed relaxation of the muscle.The dysfunctional Cl− channels are located in the muscle fiber membrane and do not affect the motor nerve innervating the muscle. However, many studies have shown that denervation of muscle fibers alter the resting membrane conductance, but whether this affects myotonia in the muscle has been subject to heavy debate, and results from experiments are inconclusive.In skeletal muscle fibers, a large transverse tubule system with a high surface-area to volume ratio exists.