15 - Motor Unit Recruitment the process of increasing the...

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Unformatted text preview: Motor Unit Recruitment the process of increasing the number of motor units that participate in muscle contraction. For weak contractions, only a few motor units are activated. For stronger contractions, more and more motor units are recruited. The number of muscle fibers per motor unit and the number of motor units per muscle vary widely, depending on the specific function of the muscle. For muscles involved in fine, delicate movements, there are very few (10-100) muscle fibers per motor unit. Thus, recruitment of each additional motor unit results in only a small increase in the muscles strength of contraction. These small motor units allow for a very fine degree of control over muscle tension. For muscles involved in powerful, coarsely controlled movement, such as those of the legs, a single motor unit may contain 1,500-2,000 muscle fibers. Recruitment of motor units in these muscles results in large incremental increases in whole-muscle tension. Lecture 17: Muscle Physiology Reading: chapter 8, section: skeletal muscle, pgs 272-279; 285-294 (pgs 277-283; 288-297, if using 5th edition) Muscle Metabolism - ATP is required as an energy source to drive two processes underlying muscular contraction and relaxation: the power stroke of the contractile apparatus and the active transport of Ca++ into the sarcoplasmic reticulum. The ATP made available for these processes is derived from 3 metabolic sources (Figure 8-22): Creatine Phosphate - this compound provides a reserve of high-energy phosphate that is utilized to synthesize ATP. It is the first source of ATP that is utilized by skeletal muscle cells when they contract. It is generated by the reversible enzymatic reaction utilizing creatine kinase: creatine kinase creatine phosphate + ADP ? creatine + ATP During periods of muscular rest, excess ATP, generated by glycolysis and oxidative phosporylation, is converted to creatine phosphate that is stored by the muscle cells as an energy reserve. Oxidative Phorphorylation - this metabolic reaction sequence takes place when sufficient oxygen is present, and can be fueled by glucose or fatty acids derived from glycogen or fat reserves, respectively. Muscle activity that is supported by this form of metabolism is referred to as aerobic because it requires oxygen. Some skeletal muscle cells have an abundance of an oxygen binding protein called myoglobin that facilitates the transport of oxygen from the blood. ...
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This note was uploaded on 10/29/2009 for the course NPB NPB 101 taught by Professor Weidner/wingfield during the Fall '08 term at UC Davis.

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