Carcass meat consists of lean, fat and bone, together with connective tissue. The fat can be subcutaneous (lying under the skin of the animal), intermuscular (lying between individual muscles) or intra- muscular (occurring within the body of the muscle). Subcutaneous fat is relatively easy to trim to produce leaner-looking meat; intermuscular fat is more difficult to remove simply. Intramuscular fat is also referred to as marbling fat because when abundant it gives a marbled appear- ance to the lean.
Muscles and their structure
The most important component of meat is the muscle. Indeed, when people think of meat they often only consider the muscle and the terms meat and muscle are sometimes used interchangeably. Muscles removed from a carcass frequently have a complex shape resulting from the complexity of their operation in the live animal and their attachment to the skeleton. The way the carcass is handled post mortem, allowing some parts of the musculature to contract while others are stretched, adds to this complexity. Nevertheless, all muscles have the same basic structure, consisting of muscle cells (fibres) bound together into bundles which are themselves often arranged in larger groupings. This arrangement is defined by connective tissue sheaths that eventually transmit the force developed by contractions of the individual fibres to the skeleton. The muscle fibres usually run in a direction parallel to the long axis of the muscle but can run obliquely.
The muscles are attached to the skeleton through the tendons. Note
that while tendons join muscle and bone, ligaments join different bones together.
Because muscles can only contract, they are only able to pull on the skeleton, not push. Hence they are effectively arranged in opposing pairs or groups. A simple example is that of the biceps and triceps muscles of the human upper arm. The biceps contracts to bend or raise the forearm and is referred to as a flexor muscle. The triceps contracts to straighten the forearm and is known as an extensor muscle. As their names suggest, the biceps has two heads each with a tendon attaching it to the shoulder blade, the triceps three heads and three tendons attaching it to the shoulder blade and long bone (humerus) of the upper arm. The import- ance to meat of muscles operating in opposing groups of flexors and extensors is that, in the carcass post mortem, muscles that are in a highly contracted state are likely to be balanced by a group of relaxed muscles.
The state of contraction can considerably influence meat texture.
The muscle is supplied with blood through an artery and vein and is innervated by nerves. The nerves consist of numerous nerve fibres originating in the central nervous system and ending in specialized end plates called neuromuscular junctions.
Some important muscles
A good account of the anatomy of the muscles and fat depots of the carcasses of meat animals is given in Gerrard (1980). An extremely detailed description of the muscles of the ox is that of Butterfield and May (1966). Muscles are given Latin names which describe their characteristics or position; hence musculus longissimus dorsi. Often the musculus is abbreviated to ‘m.’ or omitted altogether. The m.
longissimus dorsi (LD), sometimes referred to as the m. longissimus thoracis et lumborum, runs the whole length of the back and is the main muscle seen when ‘chops’ or ‘rib-steaks’ are cut from the posterior rib region and the loin. It lies dorsal to the transverse process of the vertebrae (Fig. 3.4). Perhaps rather confusingly, it is sometimes referred to as the ‘eye’ muscle. On beef carcasses it is possible to access it if each side of the carcass is ‘quartered’ by cutting between, for example, the last and last-but-one ribs so dividing the side into fore and hindquarters. It forms the ‘striploin’ joint.
The m. psoas major (PM) or psoas muscle forms the ‘fillet’. It runs posteriorly under the transverse processes of the vertebrae in the loin region from the level of the head of the last rib. It is therefore easily accessible on the inside of the dressed carcass. The m. supraspinatus (SS) and m. infraspinatus (IS) are the muscles of the shoulder blade or scapula. The SS runs dorsal to the spine on the scapula; the IS runs ventral to the spine. Because the spine can be felt from the outside of
the forequarter, the muscles are relatively straightforward to locate. The m. biceps femoris (BF), m. semimembranosus (SM), m. semitendinosus (ST) and m. adductor (AD) are all large muscles of the hind limb. The SM and AD are on the medial (inside) part of the limb, the BF on the lateral (outside) part. In beef the BF forms part of the rump joint. A portion of the SM is often exposed when the tissue around the anus is cut to release the rectum prior to removal of the gut during carcass dressing. It can also be exposed on the cut medial surface when the carcass is split into two sides (Fig. 3.5). The SM forms the topside joint in beef. The AD is also cut during splitting.
The most important muscle in poultry is the breast muscle (m.
pectoralis). Modern strains of broiler chicken and turkey have been selected to have very large breast muscles even though their major role – to flap the bird’s wings in flight – is not required. The m. pectoralis is attached to the sternum, or keel, and the humerus. Pulling down on the humerus pulls the wing down. The wing is raised by a smaller muscle, the m. supracoracoideus, that lies behind the m. pectoralis.
This is also attached to the keel and humerus. However, the tendon to the humerus is inserted on the opposite side of the bone and runs over a smooth, rounded surface in the bone of the shoulder girdle. It operates like a rope through a pulley in effect to change its direction of operation. The articulation of the wing with the body is strongly supported by the ‘wishbone’ formed of the fused clavicles.
Fig. 3.4. Tracing of a photograph of a pork chop showing the longissimus dorsi muscle.
The connective tissue sheaths
The structure of the muscles is largely defined by sheaths of connec- tive tissue. There are three levels of organization (Fig. 3.6). Individual muscle fibres are surrounded by a fine network of connective tissue, the endomysium. Bundles of fibres are surrounded by the perimysium, and the whole muscle is contained within the epimysium. In some cases the epimysium may appear to extend into the body of the muscle.
This is so with the beef LD where there is an epimysial extension into the dorsal surface of the muscle.
The main component of the connective tissue is collagen, together with the protein elastin. The LD of an ox contains about 0.5% collagen and 0.1% elastin. These form fibres that are embedded in an amorphous ground substance. Collagen fibres are straight, inextensible and non-branching. Elastin fibres are branched and elastic. Elastin Fig. 3.5. Tracing of a photograph of the medial surface of the hind part of the left side of a pig carcass to show the exposed surfaces of the m. semimembranosus and m. adductor. The m. biceps femoris lies ‘behind’ these two muscles and would be accessible from the lateral aspect of the carcass by making an incision through the skin of the leg.
occurs especially in the walls of blood vessels and in ligaments. It also contributes to the elasticity of skin. A particularly good source is the neck ligaments of cattle, the ligamentum nuchae. Collagen can form very strong structures. As well as the connective tissue sheaths of the muscles it is a major component of skin, and is the reason why leather, made from tanned skin, is so strong.
The microscopic structure of the muscle fibre
Detailed accounts of the microscopic structure of skeletal muscle are given in many standard physiology textbooks, for example, Reece (1991). Each fibre is functionally equivalent to one cell even though it has been formed by the fusion of several myoblasts (see Chapter 2). The fibre may be several, or even tens of centimetres, long but is usually only about 60–100 µm in diameter. In young animals the diameter may be much less. Fibres contain all the organelles normally found in living cells: nuclei (more than one because each fibre is effectively formed from more than one cell), mitochondria and an extensive sarco- plasmic reticulum (equivalent to the endoplasmic reticulum of other types of cell) all within the sarcoplasm (cytoplasm; Fig. 3.7). The mitochondria contain the enzymes involved in aerobic metabolism.
The sarcoplasmic reticulum acts as a store for calcium ions: these are Fig. 3.6. Schematic cross-section of part of a muscle to show the connective tissue sheaths.
released to initiate muscle contraction and reabsorbed (or sequestered) to stop it. The sarcoplasm also contains lysosomes, which act as a reservoir of various proteolytic enzymes, and granules of glycogen.
Bounding the cell is a membrane, the sarcolemma (plasmalemma). The sarcolemma folds in to give a system of tubules that form a network through the fibre (the T-tubules) particularly in the region of the Z lines (or Z discs – see later). The system comes into intimate contact with distended regions of the sarcoplasmic reticulum to form ‘triads’. The T-tubules and sarcoplasmic reticulum form a functionally continuous system. The nuclei lie just below the sarcolemma.
A unique feature of muscle fibres is that, embedded in the sarco- plasm, are regularly arranged fibrils. In a single fibre there might be between one and two thousand fibrils each about 1 µm in diameter and running longitudinally. Together the fibrils may occupy about 80% of Fig. 3.7. Schematic representations of part of a fibre and one of its constituent fibrils.
the volume of the fibre. Each fibril is itself made up of smaller elements called filaments. These are of two sorts, thick filaments (about 15 nm in diameter) consisting mainly of the protein myosin and thin fila- ments (about 7 nm in diameter) consisting mainly of the protein actin.
Under certain conditions actin and myosin can react together to produce contraction of the system and therefore the whole muscle.
When they are in this state they are sometimes referred to in combina- tion as actomyosin. Fibres, fibrils and filaments are sometimes also given the prefix myo- to indicate their relation to muscle, hence:
myofibre, myofibril and myofilament.
Red and white fibres
Based on their complement of enzymes, and the activities of these enzymes, muscle fibres can be categorized into different metabolic types. The essential difference is between fibres with a predominantly oxidative (aerobic) metabolism and those with a mainly glycolytic (anaerobic) metabolism (see Chapter 5). Aerobic metabolism requires the availability of oxygen; anaerobic metabolism can occur in the absence of oxygen. Oxidative fibres have more mitochondria, con- taining red cytochromes, and a higher concentration of the red pigment myoglobin in their sarcoplasm. Oxidative fibres therefore appear red in colour. Cytochromes are an essential part of the mechanism (oxidative phosphorylation) by which nutrients are oxidized by the cell to liberate energy. Myoglobin transports oxygen within the cell, just as the haemoglobin of the red blood cells trans- port oxygen from the lungs to the rest of the body. Glycolytic fibres contain relatively small amounts of cytochromes or myoglobin and therefore appear white.
Some fibres have both oxidative and glycolytic metabolic capa- bility. They may appear pink in colour and are sometimes referred to as intermediate fibres. Red, oxidative fibres often contain fat droplets whereas white, glycolytic fibres have a high glycogen content. Red fibres are characterized histochemically by having relatively weak ATP-ase and phosphorylase activity but strong activity of enzymes involved in aerobic metabolism like cytochrome oxidase and succinic dehydrogenase. ATP-ases hydrolyse ATP and phosphorylases are important in the initial stages of glycogen breakdown. White fibres, in contrast, have strong ATP-ase and phosphorylase activity but weak aerobic enzyme activity. Intermediate fibres tend to have strong ATP- ase and glycolytic enzyme activity, but variable activity of the enzymes of aerobic metabolism.
Different muscles contain different proportions of the different fibre types and this determines the macroscopic colour of the muscle.
In poultry and pigs, the differences in colour of different muscles in the carcass are more noticeable than the differences in ruminants. In broiler chickens and turkeys especially, the breast meat (m. pectoralis) is very pale compared with the meat on the legs. In pigs, the LD in the back is pale and the AD in the ham, for example, is dark. Ruusunen and Puolanne (1988) measured the proportions of red, intermediate and white fibres in the LD and AD of Finnish and German pigs (Table 3.5). Both muscles contained all three fibre types. The proportion of red fibres was lower, and the proportion of white fibres higher, in the paler LD, but two-thirds of the fibres in the dark AD were white and both muscles contained about 12% intermediate fibres. The figures for the pigs from the two countries were remarkably consistent. However, the actual designation of the different fibre types can be influenced by the particular histological staining method used to differentiate them, and different proportions of fibres, particularly white and intermediate types, have been reported by other workers.
From a functional point of view, red fibres tend to be ‘slow twitch’ and white fibres ‘fast twitch’. Slow-twitch and fast-twitch refer to the contraction speed. Slow-contracting fibres are charac- teristic of postural muscles whereas fast-contracting fibres are found especially in muscles whose main role is in rapid but intermittent movement. The relation between contraction speed and the meta- bolic characteristics of the fibre, as reflected in its categorization as red, white or intermediate, is actually rather more complex than implied here. Two types of fast-twitch fibres are sometimes recog- nized. Type IIA are fast-twitch red and Type IIB are fast-twitch white fibres. They are differentiated from Type I fibres which are slow- twitch red (Fig. 3.8).
There are morphological differences between fibre types.
Generally, white fibres have a greater diameter. Red fibres have a larger number of capillaries associated with them, as might be imagined from their reliance on oxidative metabolism, and therefore a requirement for more oxygen during activity.
Table 3.5. Fibre type composition of two pig muscles (data from Ruusunen and Puolanne, 1988; LD = m. longissimus dorsi, AD = m. adductor).
Percentage red Percentage intermediate Percentage white LD
Finnish pigs 6.3 13.9 79.8
German pigs 5.8 10.3 83.9
AD
Finnish pigs 22.2 11.7 66.1
German pigs 20.5 11.7 67.8
The banding pattern seen in muscles
A characteristic feature of photographs of longitudinal sections of skeletal muscles when seen under the microscope is the regular trans- verse bandings or striations. These have given the name ‘striated’ as an alternative to ‘skeletal’ muscle. Unless specially stained with dyes, muscles do not usually appear striated when in correct focus under the ordinary light microscope. However, the striations can be seen by using either a polarizing or a phase contrast microscope. The reason for this is that the striations are caused by alternating bands of protein which have a higher or lower refractive index. The bands with high refractive index are birefringent. Birefringent materials alter the plane of vibration of light passing through them and this can be detected in the polarizing microscope.
Birefringent materials are described as optically anisotropic. Non- birefringent materials are isotropic. The dark bands in muscle seen under the polarizing microscope therefore became known as A-bands (after anisotropic) and the intervening (clear) bands as I-bands (after isotropic). It is now known how these bands relate to the structural relationships of the thick and thin filaments. The A-band is formed by the thick filaments, together with the overlapping thin filaments, and the I-band by mainly just the thin filaments (Fig. 3.9). Because the
Fibre Types in Muscle
Type I Type II
• slow twitch
• aerobic
• low glycogen content
• fast twitch
Ila fast aerobic
Ilb fast anaerobic
• high glycogen content
• high glycogen repletion rate
• not affected by stress
• low glycogen content
• slow glycogen repletion
• more susceptible to stress-induced depletion of glycogen
anaerobic
• low myoglobin content
• white
• strong ATP-ase activity aerobic
• high myoglobin content
• red
• weak ATP-ase activity
• strong cytochrome oxidase activity
Fig. 3.8. A summary of the main characteristics of the different fibre types in muscle.
myofibrils are exactly aligned, the banding pattern continues across the fibres.
Early microscopists also identified various other lines and zones, including the H zone, and the M and Z lines. The Z line is of particular functional significance. It is really a disc through which the thin fila- ments pass (and is often therefore also referred to as the Z disc).
Adjacent Z lines delineate the functional unit of the myofibril – referred to as a sarcomere. A myofibril consists of thousands of sarco- meres. The sarcomere length, defined by the distance between Z lines, varies with the state of contraction of the muscle but averages between about 1.5 and 2 µm. Differences in the degree of contraction also lead to variation in the details of the banding pattern observed. These differences are now known to reflect differences in the degree of over- lap of the thick and thin filaments.
Fig. 3.9. The banding pattern of the muscle fibril and the correspondence with its fine structure shown schematically.
The arrangement of the thick and thin filaments
The thick and thin filaments interdigitate in a very regular way so that each thick filament is surrounded by six thin ones and each thin filament by three thick ones (Fig. 3.10). The actual pattern seen in transverse section will vary with the position of the cut. Close to the Z line only thin filaments will be visible; in the centre of the sarcomere only thick filaments. Also, in reality the thick and thin filaments are very close together. This is necessary to allow the interaction of myosin and actin molecules during contraction. The ‘cross-bridges’ visible in some electron micrographs of muscles are formed by part of the myosin molecules – the heads – linking on to the actin molecules. H.E. Huxley (1969), in a classic paper, gave a full explanation of the structural organization of the myofibril in terms of the relationship between thick and thin filaments, the role of the cross bridges in contraction, and the relationship of the ultrastructural details of organization to the structure seen in microscopic preparations.