Bioenergetic Provision of Energy for Muscular Activity Greg D
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G Model YPRRV 687 1–8 Paediatric Respiratory Reviews xxx (2009) xxx–xxx Contents lists available at ScienceDirect Paediatric Respiratory Reviews 12 Mini-symposium 3 4 Bioenergetic provision of energy for muscular activity Greg D. Wells 1,*, Hiran Selvadurai 2, Ingrid Tein 3 5 6 1 Department of Physiology and Experimental Medicine, The Hospital for Sick Children, Toronto, Canada & Department of Anesthesiology, The Toronto General Hospital, Canada 7 2 Paediatrics & Child Health, Children’s Hospital, Westmead, Canada 8 3 Division of Neurology, Department of Pediatrics, The Hospital for Sick Children, University of Toronto, Canada ARTICLE INFO SUMMARY Article history: A complex series of metabolic pathways are present in human muscle that break down substrates from Available online xxx nutritional sources to produce energy for different types of muscular activity. However, depending on the activity in which an individual is engaged, the body will make use of different energy systems that Keywords: have been adapted for the particular activity. More specifically, utilization of bioenergetic substrates Q1 exercise depends on the type, intensity, and duration of the exercise. The aerobic oxidative system is used for physiology longer duration activities of low to moderate intensity, the anaerobic glycolytic system is used for short aerobic to moderate duration activities of higher intensity, and the high energy phosphagen system is used for anaerobic short duration activities of high intensity. The efficiency and effectiveness of these pathways can be respiratory enhanced through physical activity and training. It is these bioenergetic pathways that are the focus of metabolism this review. ß 2009 Published by Elsevier Ltd. 8 9 31 INTRODUCTION can be enhanced through physical activity and training. It is these 32 10 bioenergetic pathways that are the focus of this review. 33 11 Humans are capable of performing amazing feats. Sprinters run 12 down the track with astonishing speed and power; power lifters AN OVERVIEW OF MUSCLE PHYSIOLOGY 13 make hundreds of kilograms look like a sack of potatoes; 34 14 swimmers traverse an entire lake or channel against the elements; Muscle tissue - the contraction specialist - provides a prime 35 15 hurdlers gracefully clear all obstacles in their way; and some example of how the structure of a tissue is well-adapted to perform 36 16 basketball players even seem to defy the laws of gravity. Before a specific function. With approximately 324 muscles, and with 37 17 muscles can produce movement by pulling on their attachments to muscle constituting 30-35% and 42-47% of body mass in women 38 18 bones, they must first obtain a source of energy to sustain such a and men, respectively the importance of muscular activity is 39 19 movement. A complex series of metabolic pathways are present in obvious. The various types of muscle tissue support numerous life 40 20 human muscle that break down substrates from nutritional functions such as ventilation, physical activity and exercise, 41 21 sources to produce energy for different types of muscular activity. digestion, and of course, pumping life-sustaining blood throughout 42 22 However, depending on the activity in which an individual is the body via specialized cardiac muscle. Skeletal muscle (also 43 23 engaged, the body will make use of different energy systems that termed striated muscle) connects the various parts of the skeleton 44 24 have been adapted for the particular activity (see Fig. 1).1 More through one or more connective tissue tendons and is the type of 45 25 specifically, utilization of bioenergetic substrates depends on the muscle used to produce movement during exercise. During muscle 46 26 type, intensity, and duration of the exercise.2 The aerobic oxidative contraction, skeletal muscle shortens and, as a result of the 47 27 system is used for longer duration activities of low to moderate tendinous attachments, functions to move the various parts of the 48 28 intensity, the anaerobic glycolytic system is used for short to skeleton with respect to one another via joints. This allows changes 49 29 moderate duration activities of higher intensity, and the high in position of one skeletal segment in relation to another, thus 50 30 energy phosphagen system is used for short duration activities of creating movement. Skeletal muscle is comprised of numerous Q251 31 high intensity. The efficiency and effectiveness of these pathways multinucleated cylinder-shaped cells called muscle fibres (myofi- 52 brils), and each fibre is made up of a number of myofilaments. 53 UNCORRECTEDPhysically, they PROOF range in size from under a hundred microns in 54 diameter and a few millimeters in length to a few hundred microns 55 * Corresponding author. Division of Respiratory Medicine, Rm. 4534, The Hospital across and a few centimetres in length. Each cell (fibre) is 56 for Sick Children, 555 University Avenue, Toronto, Ontario, Canada M5G 1X8. 57 Tel: +1 416 710 4618; Fax: +1 416 813 5109. surrounded by a connective tissue sheath called the sarcolemma, E-mail addresses: [email protected] (G.D. Wells), and a variable number of fibres are enclosed together by a thicker 58 [email protected] (H. Selvadurai), [email protected] (I. Tein). connective tissue sheath (the perimysium) to form a bundle of 59 1526-0542/$ – see front matter ß 2009 Published by Elsevier Ltd. doi:10.1016/j.prrv.2009.04.005 Please cite this article in press as: Wells GD, et al. Bioenergetic provision of energy for muscular activity. Paediatr. Respir. Rev. (2009), doi:10.1016/j.prrv.2009.04.005 G Model YPRRV 687 1–8 2 G.D. Wells et al. / Paediatric Respiratory Reviews xxx (2009) xxx–xxx Figure 1. Contributions of the three energy systems to activities of different durations and power outputs. 59 60 fibres called fascicles. Each fibre contains not only the contractile Figure 2. An electron micrograph of the inside of a muscle fibre. The actin and 61 machinery needed to develop force (sarcomeres), but also the cell myosin filaments, mitochondria (white arrows) and lipid droplets (black arrows) 62 organelles necessary for cellular respiration (mitochondria). Each are clearly visible (Images courtesy of M. Tarnopolski). 63 fibre is activated through electrical impulses transmitted by 91 64 nerves, the motor nerves and motoneurons in particular. A group of fibres. Each of the fibre types can be described on the basis of its 92 65 fibres activated via the same nerve is termed a motor unit. Also predominant metabolic pathway by which it derives energy, with 93 66 located outside each fibre is a supply of capillaries from which the the type I slow twitch fibres being described as oxidative (Type I or 94 67 cell obtains nutrients and eliminates waste. SO), and the fast twitch fibres being subdivided into two sub- 95 68 Within each myofibril, a number of contractile units called groups, fast twitch oxidative-glycolytic (Type IIa or FOG) and fast 96 69 sarcomeres, are organized in series, i.e., attached end to end. Each twitch glycolytic (Type IIb or FG). Each of the three fibre types 97 70 sarcomere is comprised of three types of protein myofilaments: the described above have distinctive morphological, contractile, and 98 71 thick filament system is composed of myosin protein which is metabolic characteristics (see Table 1). 99 72 connected from the M-line to the Z-disc by the protein titin. It also 73 contains myosin-binding protein C which binds at one end to the THE CHEMISTRY OF ENERGY PRODUCTION IN HUMAN MUSCLE 74 thick filament and the other end to the protein actin. The thin 100 75 filaments are assembled by actin monomers bound to nebulin, in a All energy in the human body is derived from the breakdown of 101 76 process that also involves tropomyosin. Nebulin and titin give complex nutrients such as carbohydrates, fats, and proteins.3 The 102 77 stability and structure to the sarcomere (see Fig. 2). When a signal end result of the breakdown of these substances is the production 103 78 comes from the motor nerve activating the fibre, the neurotrans- of the adenosine triphosphate (ATP) molecule, the energy currency 104 79 mitter acetylcholine is released and travels across the neuromus- of the body. ATP provides all the energy for fuelling biochemical 105 80 cular junction. The action potential then travels along T processes of the body such as muscular work or the digestion of 106 81 (transverse) tubules until it reaches the sarcoplasmic reticulum. food. The capacity to perform muscular work (work = force exerted 107 82 The action potential from the motor neuron changes the x distance moved) is dependent on supplying sufficient energy at 108 83 permeability of the sarcoplasmic reticulum, allowing the flow of the required rate for the duration of the activity. 109 84 calcium ions into the sarcomere. Outflow of calcium from the Energy is liberated for work when the chemical bond between 110 85 sarcoplasmic reticulum allows the heads of the myosin filaments ATP and its phosphate sub-group is broken through hydrolysis 111 86 to temporarily attach themselves to the actin filaments, a process when catalyzed by the enzyme ATPase: 87 termed ‘‘cross bridge formation’’. The movement of the cross ATPbreakdown : ATP þ!ADP þ Pi þ Energy (1) 88 bridges causes a movement of the myosin filaments in relation to 112113114 89 the actin filaments, leading to shortening of the sarcomere, and ATP is broken down in a process called ‘‘ATP turnover’’. Water 115 90 muscle contraction. Human skeletal muscle is composed of a (H2O) hydrolyzes the