Metabolic Muscular and Nervous Systems

The immediate source of energy for muscular contraction is the high-energy phosphate compound called adenosine triphosphate (ATP). Although ATP is not the only energy-carrying molecule in the cell, it is the most important one, and without sufficient amounts of ATP most cells die quickly. The three main parts of an ATP molecule are: an adenine portion, a ribose portion, and three phosphates linked together. The formation of ATP occurs by combining adenosine diphosphate (ADP) and inorganic phosphate (Pi). This formation requires a large amount of energy to and it is called a high-energy bond. In order for a muscle to contract, the enzyme ATPase breaks the ATP bond and releases energy which is used to do work. ATP is the energy produced from the breakdown of food into a useable form of energy required by cells.

Muscle cells store limited amounts of ATP. Therefore, because muscular exercise requires a constant supply of ATP to provide the energy needed for contraction, metabolic pathways must exist in the cell to be able to produce ATP rapidly. Muscle cells can produce ATP by three metabolic pathways: creatine phosphate (CP), formation of ATP, formation of ATP through the degragation of glucose or glycogen (glycolysis), and oxidative formation of ATP. The formation of ATP through the CP pathway or glycolysis is called anaerobic metabolism because they do not use oxygen. Oxidative formation of ATP by the use of oxygen is called aerobic metabolism.

As rapidly as ATP is broken down to ADP and Pi during exercise, ATP is reformed through the CP reaction. However, muscle cells only contain small amounts of CP, so the total amount of ATP formed through this action is limited. The combination of stored ATP and CP is called the ATP-CP system and provides energy for muscle contraction during short-term high-intensity exercise. CP is reformed only while you are recovering from exercise. For this process to occur, there has to be ATP present.

A second metabolic pathway capable of producing ATP rapidly without the involvement of oxygen is called glycolysis. Glycolysis involves the breakdown of glucose or glycogen to form two molecules of pyruvic acid or lactic acid. Glycolysis is an anaerobic pathway used to transfer energy from glucose to rejoin Pi to ADP. Glycolysis produces a net gain of two molecules of ATP and two molecules of pyruvic or lactic acid. Although the end result of glycolysis is energy producing, you must add ATP at two points at the beginning of the pathway. In conclusion, glycolysis is the breakdown of glucose or glycogen into pyruvic or lactic acid with the net production of two or three ATP. This depends on whether the pathway began with glucose or glycogen. Since oxygen is not directly involved in glycolysis, the pathway is considered anaerobic. However, in the presence of oxygen in the mitochondria, pyruvate can participate in the aerobic production of ATP. In addition to being an anaerobic pathway capable of producing ATP without oxygen, glycolysis is the first step in the aerobic degragation of carbohydrates.

Although several factors serve to control glycolysis, the most important rate-limiting enzyme in glycolysis is phosphofructokinase (PFK). PFK is located near the beginning of glycolysis. When exercise begins, ADP/Pi levels rise and enhance PFK activity, which serves to increase the rate of glycolysis. In contrast, at rest when cellular ATP levels are high, PFK activity is inhibited and glycolytic activity is slowed. Further, high cellular levels of free fatty acids also inhibit PFK activity. Similar to the control of the ATP-CP system, regulation of PFK activity operates through negative feedback. Another important regulating enzyme in glycolysis is phosphorylase, which is responsible for degrading glycogen to glucose. This reaction provides the glycolytic pathway with the necessary glucose at the origin of the pathway. At the beginning of exercise, calcium is released from the sarcoplasmic reticulum in muscle. This rise in sarcoplasmic calcium concentration indirectly activates phosphorylase which immediately begins to break down glycogen to glucose for entry into glycolysis.

In addition, phosphorylase activity is stimulated by high levels of the hormone epinephrine. Epinephrine, released at a faster rate during heavy exercise, results in the formation of cyclic AMP. It is cyclic AMP, not epinephrine, that directly activates phosphorylase. Therefore, the influence of epinephrine on phosphorylase is indirect.

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