On a molecular level, why is muscular relaxation an active process and not merely a passive act of immobility?

looney_tunes

"Relaxation" refers to the state when contraction is no taking place, but lots of other stuff is! "Normally, cessation of contractile activity and a state of relaxation follow electrical quiescence at the myoneural junction. The sarcoplasmic membrane returns to its resting electrical potential (about 60 mV more positive outside), as does the entire T tubule system and the SR membrane. Subsequently, sarcoplasmic calcium is pumped back into the SR cisternae by an extremely active ATP- driven calcium pump, which comprises one of the main proteins of the SR membrane. For each ATP hydrolyzed, 2 calcium ions are moved out of the sarcoplasm, with sarcoplasmic calcium ultimately falling below 0.1 micromolar, or 50- to 100-fold lower than the KD for calcium binding to Tn-C. The cisternal surface of the SR membrane also contains large quantities of a glycoprotein known as calsequestrin. Calsequestrin avidly binds calcium, decreasing its concentration in the cisternae, and thus favoring calcium accumulation. A final repository of sarcoplasmic calcium is the mitochondrial matrix. Mitochondria have a remarkably active calcium pump, driven by the electron transport--generated chemiosmotic potential. Under aerobic conditions this pump uses the energy of electron transport to sequester calcium in the mitochondrial matrix, in preference to the synthesis of ATP." http://www.med.unibs.it/~marchesi/muscle.html Oct 03 08, 11:18 PM

edmund80

Excellent, Anne! On a molecular level, calcium has to be pumped out of the troponin-tropomyosin complex to allow the sarcomere to "relax". This process involves the active expenditure of energy, which in part accounts for the rigidity of death (rigor mortis) in which there is no ATP (energy) available to remove the calcium ions, leaving the sarcomeres locked, rigid and "unrelaxed". http://books.google.com/books?id=SRptlOx7yj4C&pg=PA374&lpg=PA374&dq=muscle+relaxation+%2B+actin+myosin&source=web&ots=G5-gn4CUpI&sig=Arr3VllDbu--HxAX_hCw-CxMyxI&hl=en&sa=X&oi=book_result&resnum=10&ct=result#PPA380,M1 http://www.deathonline.net/decomposition/body_changes/rigor_mortis.htm Oct 03 08, 11:35 PM