Adenosine triphosphate (ATP) is the immediate energy source for muscle contraction. There are three main pathways forming ATP for contraction:
• Phosphorylation of ADP by creatine phosphate (CP)—very rapid and occurs at the onset of contraction. It is limited by the content of CP in the cell, only lasts a few seconds and lasts until OP and GP can take over
• Oxidative phosphorylation (OP) in mitochondria—requires oxygen and supplies most of ATP requirements during moderate exercise levels
• Glycolytic phosphorylation (GP) in the cytoplasm—does not require oxygen and normally only produces small numbers of ATP molecules per mole of glucose, but can produce greater quantities if the enzymes and substrate are available. Glycolytic phosphorylation produces lactic acid and incurs an 'oxygen debt' which may be repaid by a prolonged elevation of oxygen usage after muscle activity has ceased.
The energy sources and substrates used during muscle contraction are summarized in Figure MP.12. Metabolic Substrates During Contraction
The principal fuel for muscle metabolism is glycogen. The following points summarize substrate utilization:
• Glycogen mobilization is stimulated by Ca2+ and adrenaline
• Muscle glycogen only lasts about 10 min during moderate exercise
• For the next 30 min blood glucose and fatty acids provide substrate equally
• After this fatty acids become the predominant fuel Muscle Fibre Types
Muscle fibres are classified according to their mechanical performance and their metabolic performance. Mechanical performance is reflected by maximal shortening velocity, which is dependent on the myosin-ATPase activity in the fibre. Metabolic performance is determined by whether ATP formation is mainly oxidative or glycolytic. A high oxidative capacity in a fibre will give it the ability to resist fatigue, since this will avoid the accumulation of an oxygen debt during prolonged contraction. On this basis three main types of fibre may be identified:
• Type I—slow oxidative fibres which are red in colour
• Type II—fast glycolytic fibres which are white in colour
• Type III—fast oxidative fibres
Type I fibres are metabolically and mechanically relatively slow compared with type II fibres, but they have greater oxidative capacity and are capable of sustained work rates over prolonged periods without incurring a significant oxygen debt. Type II fibres, on the other hand, are fast metabolically and mechanically, capable of intense work rates but only for short periods as they readily accumulate a significant oxygen debt. Functionally, type I fibres predominate in muscles associated with continuous slow sustained contractions, such as the paraspinal muscle columns responsible for maintaining posture. Type II fibres are found mainly in muscles performing short, rapid movements such as the oculomotor muscles and the small muscles of the hand. A comparison of the properties of these muscle fibres is given in Figure MP.13.
In practice, few muscles are made up exclusively of red or white fibres. Classically, sprinters have relatively more white fibres and marathon runners relatively more red. With periods of inactivity there is a relative increase in the number of white fibres.
Electrical impulses are transmitted from the motor neurone to the muscle by the release of acetylcholine (Ach). This transducer process is similar to that of synaptic transmission. Ach is the only neurotransmitter involved in skeletal neuromuscular transmission. It diffuses across the junctional gap and interacts with specific receptors on the post junctional membrane of the motor end plates.
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