3. muscle structure, contraction and energy metabolism3. muscle structure, contraction and energy...

MEAT418/518 Meat Technology - 3 - 1 ©2009 The Australian Wool Education Trust licensee for educational activities University of New England

3. Muscle Structure, Contraction and Energy Metabolism

Graham Gardner

Learning objectives This lecture provides an introductory understanding of the structure of skeletal muscles in terms of the factors that affect the quality of meat obtained from them. The biochemistry of muscular contraction is also introduced. By the end of this lecture you should know: • The composition of muscle including water, lipid, carbohydrate, and protein. • The organisational structure of muscle, including the grouping of fibres into bundles, and bundles

into entire muscles. • Myofibre structure within muscle, including the basic layout of the sarcomere. • Mechanisms of contraction, the proteins and organelles involved, and how it is regulated.

Key terms and concepts Myosin, actin, troponin, tropomyosin, z-disk, sarcomere length, myofibril, sarcoplasmic reticulum, t-tubule, acto-myosin crossbridge.

3.1 Introduction Most of the “edible” protein of an animal is present in muscle, of which there are three distinct types - skeletal, smooth and cardiac muscle. Economically, skeletal muscle is by far the most important due to the quantity present, and will therefore be the major focus of the chapters to follow. The properties of muscle post-mortem, and therefore of meat, depend upon the structure, function, and regulation of function in living muscle. Knowledge of the structural and functional aspects of muscle enables proper understanding of differences between cuts of meat within a carcase, and the ways in which pre-slaughter animal management and post-slaughter processing techniques will affect meat quality.

Figure 3.1 An electron microscope image (25,000 x) of skeletal muscle.

Source: Hopkins and Thompson (2001).

Notes – Lecture 3 – Muscle Structure, Contraction andEnergy Metabolism

3 - 2 – MEAT418/518 Meat Technology ©2009 The Australian Wool Education Trust licensee for educational activities University of New England

Figure 3.1 clearly demonstrates the characteristic banding in skeletal muscle. The sarcomere structure from Z-disk to Z disk (A-bands, Z-disks, and I-bands) and mitochondria are also shown (Hopkins & Thompson 2001c).

3.2 Composition of muscle Lean muscle of various species generally consists of about 75% water, and 20% protein, with the remainder made up of lipid and soluble non-protein substances. The ratio of protein to water is relatively constant over the animal's lifetime, except for the immediate post-natal period. Water and fat contents of muscle are inversely related - thus as fat content increases, the water content decreases. Muscle also contains carbohydrate in the form of glycogen which is a branched polymer of glucose molecules representing about 1–2% of muscle weight. Glycogen is an important energy store, but also affects meat quality due to its influence on ultimate pH of the muscle post mortem. Table 3.1shows the major components of skeletal muscle. Minor constituents include minerals, vitamins, and low molecular weight nitrogen-containing compounds. Table 3.1 Chemical composition of Muscle. Source: Gardner, (2005).

Constituent Range % Form Changes Water 70-78 Free & bound to proteins Decreases with age and fat

content Protein 15-22 Myofibrillar, caroplasmic stormal Decrease with fat content Liquid 1-13 Depot fat

Membrane liquids etc Highly variable Inversely related to water content

Carbohydrate 1-2 Glycogen, monosaccharides Relatively constant 3.3 Muscle structure Skeletal muscle is made up of groupings of muscle fibres surrounded and supported by connective tissues. simply fibres. Muscle cells are formed at birth by the fusion of many singly-nucleated precursor cells. The end-to-end fusion of these cells yields the long narrow shape and multiple nuclei of the muscle fibres, which are typically 40 to 50 µm in diameter (range 10-100 µm) and several mm long (range 1-40 mm). Each muscle fibre is enveloped in connective tissue (endomysium) and muscle fibres are arranged longitudinally into bundles which are also enveloped in thin sheets of connective tissue (perimysium). Smaller bundles, termed primary bundles, are grouped into larger secondary bundles which may be grouped into even larger tertiary bundles. These are also surrounded by perimysium. Entire muscles are covered by a heavy sheath of connective tissue (epimysium) which thickens as it blends into the tendon connecting the muscle to either bone or other muscles. The three levels of connective tissue (epi-, peri- and endomysium ) are a continuum which mainly consist of fibrous proteins known as collagen.



Figure 3.2 Diagram of a muscle in cross section. Source: Gardner, (2005).

Figure 3.2 demonstrates the arrangement of the epimysium, perimysium and endomysium in relationship to the muscle fibres. Muscle fibres are long, multinucleated cells and are also called myofibres. Several blood capillaries are located in the endomysium around each muscle fibre. The strength of a muscle is independent of fibre length, but closely related to fibre number.

Figure 3.3 Diagram of the gross structure of a muscle of the leg. Source: Rowe, CSIRO.



Figure 3.4 Diagram of the muscle structure of a human arm.

Source: http://www.sirinet.net/~jgjohnso/amuscle.html.

Figure 3.4 depicts the differentiation of muscle within a human arm, starting at the level of the entire muscle, then the fibre bundles, then the individual fibres themselves, and lastly the individual myofibre units within each muscle cell showing the overlapping structures of actin and myosin. (Source http://www.sirinet.net/~jgjohnso/amuscle.html) Each muscle cell or fibre is surrounded by a cell membrane, the sarcolemma, which is in close proximity to the endomysium. The nuclei are on the surface of the cell, just underneath the sarcolemma. A structure called the basement membrane, links the collagenous fibres of the endomysium to the sarcolemma. The basement membrane contains about 40% by weight of collagen with the remainder consisting of complex polysaccharides. Because of these structures, contact between individual muscle cells is rare. Contact is maintained via the basement membrane and the endomysium. Muscle fibres do not attach directly to the bones that they move - the connective tissues of the muscle blend with massive aggregates of connective tissue forming tendons which attach to the skeleton. The mode of connection between contractile proteins and tendons is not fully understood. Muscle fibre orientation Muscle fibres taper at both ends and the tapered portion can extend up to 50% of the length of the fibre (ie 25% at each end) depending upon the length of the fibre. In long muscles, fibres do not extend the length of the muscle from one point of tendon insertion to the other. They are arranged in end-to-end series with their tapering ends significantly overlapping adjacent fibres. This is shown in figure 3.5.



Figure 3.5 Diagrammatic representation of entire semitendinosus muscle of the goat demonstrating fibre overlap. Source: Gans and de Vree, (1987).

3.4 Fibre bundles and meat texture The size of muscle fibre bundles determines the texture of a muscle. In muscles capable of finely adjusted movement, such as those of the eye, the texture is fine, whereas in those performing grosser movements it is coarser. To meat graders, the texture of meat is a property related to the size and degree of separation of the muscle bundles (fasciculi). Their prominence is related to the depth or thickness of the connective tissue around them. Texture becomes increasingly coarse as animals age due to muscle hypertrophy expanding the size of the muscle bundles, thus young animals have a finer texture. Muscle texture is a factor in some meat grading systems, although MSA has found that it has little predictive value for palatability, thus it is no longer part of the MSA grading criteria. Subcellular organisation of muscle fibres The muscle fibre or cell is the fundamental organisational unit of muscle. In isolation it can contract if stimulated and perform all the functions of muscle, although it normally operates in larger functional units. A muscle fibre is composed of smaller sub units called myofibrils which comprise the majority of the volume of the cell (80-87%). Each myofibril contains a number of (smaller) long thin filaments called myofilaments, which consist of actin and myosin. Other cell elements include mitochondria and a membrane system, the sarcoplasmic reticulum. When skeletal muscle is examined using a light microscope, alternating light and dark bands are seen (see figure 3.6) indicating that it contains a repeating structure.

Figure 3.6 A cross-section of a human muscle with different microscope settings. Source: http://neuromedia.neurobio.uda.edu/campbell/lutz/Muscle.htm#LABSKELETAL.



The nuclei can be seen as the darker purple bodies along the muscle fibre. The magnified insert shows the cross-striation pattern. (Source http://neuromedia.neurobio.ucla.edu/campbell/lutz/ Muscle.htm#LABSKELETAL)The units which “form” this repeating structure are termed sarcomeres, which are typically 2 to 3 mm in length. When the muscle fibre is examined at higher magnification the structured detail becomes clearer as seen below:

Figure 3.7 Transmission electron micrograph from psoas muscle of rabbit. Parts of seven myofibrils and their characteristic banding patterns can be seen. The A-band (A), I-band (I), Z-

line (Z), H-zone(H) and M-line (M) are labelled. Source: Pearson and Young (1989). Printed with permission from Elsevier.

A sarcomere is defined as the portion of the fibre lying between two consecutive dark discs, the Z-discs (see figure 3.8 below). Each sarcomere contains a number of areas which can be distinguished under higher magnification. These result from the basic structure which consists of two sets of filaments which overlap ¾ of the length of the thick and thin filaments.

Figure 3.8 Structure of a myofibril, showing the overlapping thin (actin) and thick (myosin) filaments. Source http://www.humboldt.edu/~wva1/ images%20muscle.

Visualised in cross section, sarcomeres will reveal various patterns depending on where the section is taken. The thick filaments consist of myosin and appear dark and the thin filaments are actin and appear light. In the region where these two types of filaments overlap, there are three thick filaments surrounding each thin filament and six thin filaments surrounding each thick. These structural details are shown in the figure 3.9 below.



Figure 3.9 Two dimensional diagramatic representation of a myofibril, showing the overlapping thin (actin) and thick (myosin) filaments. Source: http/://www.humbolt.edu/

~wva1/images%20muscle. Fine structure of a single myofibre showing one sarcomere — 2D view

Figure 3.10 Three dimensional diagramatic representation of a myofibril, showing the

overlapping thin (actin) and thick (myosin) filaments. The disks are drawn to intersect the myofibril (from left to right) at the I-band, H-zone, M-line, and outer edge of the A-band where

the myosin and actin filaments overlap Source: http://www.humboldt.edu/~wva1/images %20 muscle.

Figure 3.11 Transects showing the arrangement of thin (actin) and thick (myosin) filaments

Source: http://www.humboldt.edu/~wva1/images %20muscle.



The distances between Z-discs varies as muscles contract or lengthen thereby changing the sarcomere length. The basis of muscle contraction resides in the back and forth movement of thick and thin filaments (myosin and actin) relative to each other. These filaments are not actively interacting in relaxed muscle. As rigor mortis sets in following death, the sarcomeres shorten and assume a fixed length. Final sarcomere length in post mortem muscle is a major determinant of meat tenderness.

3.5 Muscle proteins Muscle cells contain proteins which serve many different functions. We are mostly interested in the contractile proteins and their control. Table 3.2 Types of myofibrillar protein. Source: Gardner, (2005).

Protein % Total Location Function Major Contractile Proteins

Myosin 50 Contraction Actin 20 Contraction

Regulatory Proteins Tropomyosin 3 Thick Filaments Regulate Troponin Complex 4.5 Thick Filaments Contraction

Myofibrillar proteins · Cytoskeletal proteins These support and stabilise the contractile apparatus of the cell, both

laterally and longitudinally i.e. they are structural. The major protein in this group is titin, which holds the thick filaments laterally. It comprises 8% of the myofibrillar protein.

· Sarcoplasmic proteins These are soluble proteins in the cytosol, mainly glycolytic enzymes, myoglobin and some haemoglobin. Myoglobin is the oxygen-carrying protein of muscle.

· Stromal proteins These are the proteins of the connective tissue, mainly collagen and elastin and are discussed later.

Muscle contraction The distance between Z-discs varies as muscles contract or lengthen thereby changing the sarcomere length. The basis of muscle contraction resides in the back and forth movement of thick and thin filaments (myosin and actin) relative to each other. There is no interaction of these filaments in relaxed muscle. As rigor mortis sets in following death, the sarcomeres shorten and assume a fixed length. Final sarcomere length in post mortem muscle is a major determinant of meat tenderness.



Figure 3.12 Cross-bridges, consisting of the myosin heads, cover the surface of the thick filaments except near the centre. Modified from http://www. essentialcellbiology.com.

Thick and thin filament structure of the myofibril.

The myosin and actin filaments overlap with the same polarity on either of the midline. The actin filaments are anchored by their plus end to the Z disc, while the mysoin filaments are bipolar. During contraction the actin and myosin filaments slide past one another without shortening as shown below. The sliding motion is driven by the myosin heads “walking” towards the end of the adjacent actin filament.

Figure 3.13 Animated diagram showing how the actin and myosin filaments slide over one another and shorten the length of the sarcomere.

Source: http://bio.winona.msus.edu/berg/animtns/slidfila.htm

The muscle filaments are anchored to the connective tissue matrix and therefore as the sarcomere shortens tension is created and the entire muscle shortens.



Figure 3.14 A legend of the components shown in the following animation. Source: http://www.sci.sdsu.edu/movies/actin_ myosin html

Figure 3.15 Animation of a myosin head ratcheting up on the actin filament. Source: http://www.sci.sdsu.edu/movies/actin_ myosin html

ATP is required to break the chemical bond and for the myosin head to move further up the actin filament There is still some confusion over the structural changes that take place during contraction, particularly with respect to the myosin filaments. Some theories suggest that the myosin head pivots at a point close to where it stems from the myosin filament. Other theories suggest a more uniform change throughout the myosin molecule, as depicted in the two figures below. However, research to elucidate these structural changes during contraction is continuing.



Figure 3.16 3–D view of myosin interacting with the actin filaments The actin filaments are the blue/green twisted rods surrounding the “brown” myosin filament. The myosin heads are

depicted in purple and red. Source: http://www.ibib.waw.pl/~leszek/.

Figure 3.17 The structural mechanism of contraction within the myosin filament. Source: http://www.ibib.waw.pl/~leszek/

This animation depicts conformational “flexing” within the entire myosin filament, effectively pulling the rest of the filament past the actin filament



Figure 3.18 Interior structure of a muscle cell. Source: Gardner, (2005).

Figure 3.18 depicts the contractile myofibrils (light brown) consisting of myosin and actin. The sarcoplasmic reticulum is the pink coloured membrane surrounding all the myofibrils, mitochondria are the organelles depicted in red, and the t-tubules are the light green channels transferring electrical impulses from the cell membrane through the cell interfacing with the sarcoplasmic reticulum. Muscle contraction is controlled by changing the concentration of calcium ions (Ca++) in the muscle cell. The normal intracellular concentration is below about 10-7 M and is maintained at this level by an intracellular membrane system called the sarcoplasmic reticulum (SR), which envelops each myofibril. Periodically, along the length of the muscle fibre, invaginations in the sarcolemma form a network of tubules which are called the transverse or T-tubules because they run transverse to the direction of the fibres. The SR also contains small longitudinal tubules which are oriented to run parallel to the fibre axis. These are also part of larger transversely oriented tubular elements, the terminal cisternae, which lie parallel to each other—one transversing the A-band and one the I-band of the sarcomere. These two tubular elements and the central T-tubule which lies between them together form a triad structure. Each sarcomere has two triads, one at each A-band and I-band junction.

3.6 Events in muscle contraction Nerve supply to the muscle fibres is via the perimysium. Contraction is triggered by a nerve impulse. The action potential moves from the motor end plate and is carried to the interior of the muscle cell via the T-tubules. Nervous innervation of muscle cells is shown in Figure 3.19 below.



Figure 3.19 Neuromuscular junction. Source: http://www.udel.edu/Biology/Wags/histopage/empage/em/em.htm

The depolarisation of the membrane by the nerve impulse triggers the release of Ca++ from the SR with its concentration rising from 10-7 to 10-5 M. A four sub unit protein called the ryanodine receptor opens a channel to release the Ca++. This diffuses to the myofibrils and initiates the contraction. The released Ca++ is then quickly pumped back into the SR by a Ca++ pump embedded in the membrane. The triads are the main site of Ca++ binding during relaxation, and Ca++ is released as a consequence of the nerve stimulus which initiates contraction. Major steps in a muscle contraction cycle 1. Membrane depolarisation (nerve impulse) spreads across surface of cell and down T-tubules. 2. Ryanodine receptors open (triad) and release Ca++ into cell cytosol. 3. Ca++ diffuses to myofibrils and binds to troponin, a regulatory protein that forms a comphase

with myosin (tropomyosin). 4. Ca++ induces a conformational change in troponin which physically moves tropomyosin, allowing

myosin heads to bind to actin. 5. A power stroke occurs, with the filament sliding, the muscle shortens, ADP +Pi are released (from

myosin). 6. SR pumps Ca++ back, troponin and tropomyosin move back to their original positions, myosin-

actin binding ceases, ADP is bound to myosin and the muscle relaxes. 7. ADP is hydrolysed to ADP+Pi by myosin and the system is ready for next contraction signal.

3.7 Energy metabolism in muscle Working muscle sources its energy in a similar fashion to most of the other cells of the body, relying on three primary metabolic systems: 1. The phosphagen reserve — a combination of ATP and Creatine Phosphate. 2. Anaerobic metabolism — the glycolytic catabolism of carbohydrate to lactic acid. 3. Aerobic metabolism — utilising carbohydrates, lactic acid, fatty acids and amino acids as a fuel.



To understand the limitations for muscle cell metabolism we must consider these three main metabolic systems in a quantitative fashion. The phosphagen reserve This represents the combined amounts of cellular ATP and Creatine Phosphate. Adenosine Triphosphate (ATP) ATP is the primary source of energy for muscle contraction as well as most other energy requiring processes within the cell. The basic structure of ATP is represented in figure 3.20 and the bonds attaching the last two phosphate radicals to the molecule are high-energy bonds each storing about 7300 Calories of energy per mole of ATP. This energy is released when these bonds are broken, providing the energy necessary for cellular functions to take place (ie muscle contraction, protein biosynthesis, etc).

Figure 3.20 Interconversion of ATP with ADP. Source: Gardner, (2005).

The ATP in muscle cells is in very short supply. Even a well-trained athlete only has sufficient ATP to sustain maximal muscle power for about 3 seconds, enough to cover 25 meters at a sprint. Therefore, except for a few seconds at a time, it is essential that ATP is regenerated from ADP continuously within the muscle cell. Figure 3.20 shows how ATP is broken down to adenosine diphosphate (ADP) releasing 7300 Calories of energy per mole of ATP. ADP can also be broken down to adenosine monophosphate (AMP), also releasing 7300 Calories of energy per mole of ADP. Thus ATP can be thought of as a charged battery, which has been partially spent when converted to ADP and fully spent when converted to AMP. Creatine Phosphate Creatine phosphate is another molecule that has a phosphate group attached via a high energy bond (see figure below), each bond containing about 10,300 Calories per mole as opposed to ATP with only 7300.



Figure 3.21 Creatine Phosphate – a high energy molecule. Source: Gardner, (2005).

When ATP levels begin to fall creatine phosphate is broken down to creatine and phosphate ion, releasing enough energy to directly convert one ADP to ATP. This conversion occurs at such a rapid rate that the energy from creatine phosphate can be made instantaneously available for muscle contraction, just as is the energy stored in ATP. During recovery following a period of high intensity demand for ATP creatine phosphate is regenerated, this inter-conversion catalysed by the enzyme creatine kinase (see figure 3.22).

Figure 3.22 Creatine Phosphate Catabolism. Source: Gardner, (2005).

The combined amounts of cell ATP and creatine phosphate, called the phosphagen reserve, is sufficient to provide maximal muscle power for 8 to 10 seconds, almost enough to cover 100 meters at a sprint. Thus the energy from the phosphagen reserve is used for maximal short bursts of muscle power.



Anaerobic metabolism Glycolysis is considered to be one of the most ancient energy pathways, present in all living cells. It involves the catabolism of one molecule of glucose into two molecules of pyruvate, this process occurring without the use of oxygen and therefore termed anaerobic metabolism. The pathway of glycolysis and the catabolism of the substrates involved are outlined below.

Figure 3.23 Glycolytic pathway. Source: Gardner, (2005).

Muscle cells source their glucose either from the blood, or from their own stores of glucose present as muscle glycogen. In an anaerobic state (ie there is no oxygen present) the pyruvate formed from the catabolism of glucose is converted to lactic acid and the entire pathway yields a total of 2 ATP per molecule of glucose catabolised. This lactic acid then diffuses out into the interstitial fluid and blood, where it is then carried back to the liver for re-conversion to glucose. A unique feature of the glycolytic pathway is that its velocity can be increased by 400–500 times, enabling it to respond to periods of intensive energy demand particularly when the body cannot supply oxygen to the muscle tissues at a rate fast enough to meet this demand, as is the case during a 400 meter sprint. Therefore when large amounts of ATP are required for short to moderate periods of muscle contraction, this anaerobic glycolysis mechanism can be used as a rapid source of energy. This energy is not available as rapidly as the phosphagen system, but about half as rapid.



Aerobic metabolism The tricarboxylic acid cycle (TCA; also called the Krebs cycle, and the citric acid cycle) occurs exclusively in the mitochondria and coupled with oxidative phosphorylation generates enormous quantities of ATP. For oxidative phosphorylation to occur oxygen must be present, and thus the TCA/oxidative phosphorylation pathway is termed aerobic. Under aerobic conditions, the pyruvate formed from glycolysis (discussed above in anaerobic metabolism) is converted into acetyl CoA which then enters the TCA cycle and is catabolised to CO2, H2O, and ATP. Thus in the presence of oxygen the yield of ATP from the catabolism of one molecule of glucose is as much as 38 ATP, a far more efficient return from glucose catabolism than in the anaerobic state where only 2 ATP are formed. The other primary source of acetyl CoA is derived from fat, therefore aerobic metabolism will continue indefinitely so long as substrate (fat, glucose, lactate, amino acids) and oxygen are available. A limitation of this pathway is that its rate of ATP production is relatively slow, largely due to limitations in the rate at which oxygen can be supplied to working muscle. Thus the three major metabolic systems can be compared based on their rate of energy supply (as outlined in table 3.3). Thus the phosphagen reserve is primarily utilised for burst exercise, anaerobic metabolism is relied upon for sustained high intensity exercise, and aerobic metabolism is utilised for lower intensity exercise as well as providing the ATP for the normal day to day functioning of muscle. Table 3.3 Rate of energy supply from the three main metabolic systems of muscle. Source: Gardner, (2005).

Metabolic System Rate of ATP supply (mol of ATP) The phosphagen reserve 4 Anaerobic metabolism (glycolysis) 2.5 Aerobic Metabolism (glycolysis & TCA) 1

Readings

There are no readings for this topic.

Summary Summary Slides are available on web learning management systems Skeletal muscle fibres (cells) vary in length from 1 to 40 mm and in diameter from 10 to 100 microns. Their cells have multiple nuclei with approximately 35 ovoid shaped nuclei present in each millimetre. The sarcolemma (the plasm membrane surrounding the cell) surrounds the sarcoplasm, which bathes the myofibrils, mitochrondria, nuclei and other structural elements of the muscle cell. Individual muscle fibres are surrounded by connective tissue called endomysium, and grouped together to form muscle bundles. Muscle bundles are surrounded by another layer of connective tissue called perimysium, and grouped together to form entire muscles, which in turn are surrounded by a thicker layer of connective tissue called the epimysium. Skeletal muscle is a highly specialised tissue designed to convert chemical energy into mechanical work. Contractional force generated chemically within skeletal muscle is transferred via the connective tissues to other muscles or onto the rigid framework of the bones to produce movement. Contraction in skeletal muscle is controlled by nerve impulses which cause the release of calcium. Calcium enables the two contractile proteins myosin and actin to interact, causing the sliding action of the thick (myosin) and thin (actin) filaments resulting in muscle contraction. Skeletal muscle derives its energy from three major metabolic pathways, the phosphagen system which is composed of ATP and creatine phosphate, anaerobic metabolism via glycolytic breakdown of glucose to lactic acid, and aerobic metabolism which incorporates the TCA cycle utilising acetyl CoA derived from pyruvate and fatty acids.



References Anon 2000. The chilled, vacuum-packed meat cold chain. Meat Technology Update Newsletter 00/5.

Food Science Australia, Brisbane Bobbitt, J., Haines, H., Hodgeman, R. and Roache, T. (2006) Alternative Meats – Novel flavours,

products and safe delivery. RIRDC Publication No 06/008 RIRDC Project No DAV-216A Gans, C. and de Vree, F. 1987. Functional bases of fibre length and angulation in muscles. Journal

of Morphology, vol 192, pp 63-85.

Garland Science Publishing Website http://www.essentialcellbiology.com.Danvers,M.A.USA

Ham, A.W. 1965. Histology. 5th Edition, Lippincott, Philadelphia, Pennsylvania.

Hopkins, D.L. and Thompson, J.M., 2001c. Inhibition of protease activity. Part 2. Degradation of myofibrillar, myofibril examination and determination of free calcium levels. Meat Science, vol 59, pp 199-209. Printed with permission from Elsevier.

Hopkins, D.L., Hegarty, R.S, Farrell, T.C. 2005a. Relationship between sire EBV’s and the meat and eating quality of meat from their progeny grown on two planes of nutrition. Effect of sire and plane of nutrition on lamb meat quality. Australian Journal of Experimental Agriculture. Vol 45 pp 522-533

Humboldt State University, California, USA College of Natural Resources and Sciences. Retrieved 25th August, 2006 from http://www.humboldt.edu/~wva1/images%20muscle

Hwang I.H. and Thompson J.M. 1998. Ultrastructural characteristics of beef loin muscle treated with high or low voltage electrical stimulation. Presented at 44th International Congress of Meat Science and Technology, Barcelona, Spain 44: 698-699.

Institute of Biocybernetics and Biomedical Engineering. Polish Academy of Sciences, Warsaw, Poland. Leszek Chmielewski. Retrieved 25th August, 2006 from http://www.ibibwaw.pl/~leszek/

Johnson, J.G. The World of Biology - How Do Muscles Work? Retrieved 25th August, 2006 from http:/www.sirinet.net/~jgjohnso/amuscle.html

MLA Fact Sheet: Sheepmeat eating quality: A guide for Australian producers Peason A.M. and Young R.B. 1989. Muscle and Meat Biochemistry. Academic Press, Sydney.

Printed with permission from Elsevier.

Romans, J.R., Costello, W.J., Carlson, C.W., Greaser, M.L., Jones, K.W. 1994. The meat we eat. Interstate Publishers, Danville, Illinois.

San Diego State University College of Science Biology 590-Human Physiology. Actin Myosin Crossbridge 3D Animation. Retrieved August 25th, 2006, from http://www.sci.sdsu.edu/movies/actin_myosin.html

Slomianka, L., 2006. Blue Histology. School of Anatomy and Human Biology. The University of Western Australia. Retrieved 25th August from http://www.lab.anhb.uwa.edu.au/mb140/.

University of California, Los Angeles, Department of Neurobiology, School of Medicine, Dr. J.H. Campbell. Retrieved 25th August 2006 from http://neuromedia.neurobio.ucla.edu/Campbell/lutz/Muscle.htm#LABSKELETAL

University of Delaware Histology, Department of Biological Sciences, Mammalian Histology, Dr. R.C. Wagner. Retrieved 25th August, 2006 from http://www.udel.edu/Biology/Wags/histopage/empage/em/em.htm

Winona State University, Winona, Minnesota, USA. Biology Department, Prof. Steven Berg's Animations. Retrieved 25th August, 2006 from http://www.bio.Winona.msus.edu/berg/animtns/slidfila.htm

3. muscle structure, contraction and energy metabolism3. muscle structure, contraction and energy...

Documents