vinculin. A review of the proteins forming the cytoskeleton is given in Maruyama (1985).
Other proteins of the muscle
The myofibrillar proteins form about 60% of the total muscle protein.
The remaining 40% consist mainly of the ordinary cellular proteins, especially the enzymes, normally found in all types of cell and, because they occur in the sarcoplasm, referred to as the sarcoplasmic proteins, and the stroma proteins (Table 3.7). The latter are rather insoluble and consist largely of the connective tissue components (collagen and elastin).
The myofibrillar and sarcoplasmic proteins can be extracted using appropriate-strength salt (e.g. potassium chloride) solutions. Fairly dilute salt solutions (0.04 M) extract the sarcoplasmic proteins. More concentrated solutions (0.4 M) are required to extract myosin. If the solution is then diluted to about 0.1 M the myosin molecules aggre- gate into ‘rods’ in which the molecules appear to arrange themselves in a way very similar to that in the thick filament, with the heads sticking out at the ends and a middle part consisting only of tails.
Actin can be extracted with 0.6 M potassium iodide. In concentrated salt solutions the molecules polymerize to form chains as in the thin filaments.
another. They are linked by the cross-bridges formed by the heads of the myosin molecules. These cross-bridges ‘ratchet’ along the actin chains of the thin filaments. With different degrees of contraction, different degrees of overlap of thick and thin filaments occur and different amounts of force are developed. The different degrees of over- lap are reflected in the sarcomere length and the distance between the visible striations. Maximum force is developed when all the available cross bridges on the thick filament are aligned with the thin filament (at a sarcomere length of about 2.1 µm). The striations therefore move closer together on contraction. A.F. Huxley and R.M. Simmons (1971) suggested an explanation of how the smooth force between the thick and thin filaments could be generated. They proposed the existence of elastic components in the cross bridge structure that would produce the continuous development of contraction actually seen.
Adenosine triphosphate (ATP) and the energy for contraction Both myosin and actin bind ATP. Each of the lobes of the head of the myosin molecule can bind an ATP molecule and each globular actin molecule also binds a molecule of ATP. However, on polymerization of the actin into chains; this ATP is hydrolysed to ADP. The myosin molecule acts as an ATP-ase but its activity is enhanced greatly by the presence of actin. The hydrolysis of ATP by the actomyosin ATP-ase is the mechanism by which contraction is fuelled. The details of this process by which chemical energy is converted into movement are, however, still unclear.
The sarcoplasmic reticulum and the control of contraction
The sarcoplasmic reticulum forms a complex membrane system surrounding each myofibril and is functionally continuous with the T-tubule system formed by the sarcolemma. It has a fundamental role in controlling the contractile process. Muscles normally contract in response to a nervous stimulation. This causes acetylcholine to be released by the neuromuscular junction. Acetylcholine is a neuro- transmitter, interacting with, or binding to, a receptor molecule on the surface of the sarcolemma. The acetylcholine produces a local depolarization of the muscle fibre membrane; in effect the polarity of the membrane reverses. Normally the inside of the cell is maintained at a negative potential ("90 mV) by a differential distribution of potassium (K+) and sodium (Na+) ions across the membrane. The concentration of K+is high inside the cell and that of Na+high outside.
During depolarization, K+ions move out and Na+ions move in.
The local region of depolarization spreads across the membrane surface. Because the sarcolemma is functionally continuous with the sarcoplasmic reticulum, the depolarization is transmitted into the cell and to the individual fibrils. Normally, the sarcoplasmic reticulum maintains the concentration of calcium ions (Ca2+) in the sarcoplasm very low (less than 0.1 µM) by active pumping. Depolarization causes a momentary very large release of Ca2+ from the sarcoplasmic reticulum back into the sarcoplasm. The 100-fold increase in Ca2+ (to about 10 µM) removes the normal inhibition exerted by tropomyosin and troponin on the attraction between myosin and actin and so causes a contraction. Details of the control of contraction are not fully understood. However, the calcium ions saturate troponin – C, changing the relative positions of the three subunits. This displaces the tropomyosin, exposing the binding site on the actin so allowing it to react with ATP on the head of the myosin molecule. The ATP-ase system is activated, ATP is hydrolysed and the actin and myosin molecules pull together. When the calcium ions are re-sequestered by the sarcoplasmic reticulum, inhibition returns and the contractile force ceases.
Anything that interferes with this chain of events can prevent muscle contraction. Some Indian tribes of South America hunt animals using arrows tipped with a poison called curare, which is made from a plant extract. Curare binds to the acetylcholine receptor on the muscle cell membrane, so blocking the effect of the acetylcholine, and thus paralysing the animal. The animal dies from asphyxiation because the chest muscles are paralysed. Nowadays, derivatives of curare are used as muscle relaxants in people undergoing surgery.
A review of the interaction between myosin and actin, the regula- tory roles of tropomyosin and troponin, and the role of calcium in controlling muscle contraction, is given in Weber and Murray (1973).
The physiology of the neuromuscular junction and the role of acetylcholine was described by Hubbard (1973).
The methods used to slaughter animals can affect carcass and meat quality, the animals’ welfare and the safety of the personnel operating the system. Many methods are influenced by religious beliefs. For example, some Jews and Muslims will not consume the meat from animals unless these have been slaughtered according to a set of religious codes. An important consideration is that in most forms of religious slaughter the animals are not stunned before they are killed by sticking and exsanguination. Stunning refers to rendering the animals unconscious and sticking refers to severing some of the major blood vessels in the neck or thorax so the animal bleeds to death (exsanguination). Stunning prevents any possibility that the animals should feel pain or distress during exsanguination.