Physiology of Finfish and Shellfish (2+1)

3.2.11. Muscle Physiology

As far as we know, the biochemical mechanism of muscle contraction is the same in all muscles. Two proteins, actin and myosin, are part of the machinery, and ATP is the immediate energy source for the contraction.

The broadest classification is based on the presence or absence in the muscle of regular cross microscope. Vertebrate skeletal muscles and heart muscle are stariated; muscles of the internal organs – in the walls of the bladder, internal organs – in the walls of the bladder, intestine, blood vessels, uterus, and so on – are unstriated (also called smooth). The heart muscle, although striated, is often considered as a separate type because it differs from skeletal muscle in characteristic ways, the functionally most important being that a beginning contraction of the heart muscle spreads to the entire organ.

Striated muscle: The organization of striated muscle is shown in diagram form in Fig. The muscle is made up of a large number of parallel fibers, which are between 0.1 and 0.01mm in diameter, but may be several centimeters long. These fibers in turn are made up of thinner fibrils. These fibers in turn are made up of thinner fibrils. These fibrils have characteristics cross striations the so-called Z-lines, which are repeated at completely regular intervals.

The region between two Z-lines is called a sarcomere. In vertebrate muscle the distance between the Z-lines, the sarcomere length, tends to be very uniform, about 2.5 µm in a relaxed muscle. Many invertebrate muscles have similar sarcomere length, but some have much longer sarcomeres, 10 or 15 µm or even longer.

From the narrow Z-line very thin filaments extend in both direction, and in the center these thin filaments are interspersed with somewhat thicker filaments. The result is a number of less conspicuous bands located between the Z-lines; the appearance of these bands changes with the state of contraction of the muscle.

In vertebrates the thin filaments are about 0.005 µm in diameter and the thick filaments are about twice that size, about 0.01 µm in diameter. The length of the thick filaments is about 1.5 µm. The thin filaments in different muscles differ somewhat more in length. They are often between 2 and 2.6 µm from tip to tip (or about half this length measured from the Z-line to the tip). The arrangement of the filaments is extremely regular and well ordered.

In striated muscle the thick filaments consist of myosin and the thin filaments of actin. The thick and thin filaments are linked together by a system of molecular cross linkages, and when the muscle contracts and shortens, these cross linkages are rearranged so that the thick filaments slide in between the thin filaments, reducing the distance between the Z-lines (Huxley 1969).

Cardiac muscle: Cardiac muscle is cross-striated, like skeletal muscle, but its functional properties differ in two important respects. One is that, when a contraction starts in one area of the heart muscle, it rapidly spreads throughout the muscle mass. Another important property of cardiac muscle is that a contraction is immediately followed by a relaxation period during which the muscle cannot be stimulated to contract again. As a result, a long-lasting contraction, like the sustained contraction of a skeletal muscle, does not occur. These two properties are essential for the normal rhythmic contraction of the heart, which we shall return to shortly.

Smooth muscle: Smooth muscle lacks the cross striations characteristic of skeletal muscle, but the contraction depends on the same proteins as in striated muscle, actin and myosin, and on a supply of energy from ATP.

Smooth muscle has not been as extensively studied as striated muscle. There are several reasons for this. One is that, smooth muscle is often interspersed with connective tissue fibers. Another is that smooth muscle fibers do not form neat parallel bundles that can readily be isolated and studied. Also, smooth muscle consists of much smaller cells; the fibers are often only a fraction of a millimeter long.

Substances for energy storage

The immediate source of energy for a muscle contraction is adenosine triphosphate (ATP), a compound that is the primary energy source for bearlry every energy-requiring process in the body. ATP is the only substances, the muscle proteins can use directly. When the terminal phosphate group of this compound is split off, the high energy of the bond is available for the energy of muscle contraction.

In spite of its great importance, ATP is present in muscle in very small amounts. The total quantity may be sufficient for no more than ten rapid contractions; it follows, therefore, that ATP must be restored again very rapidly, for otherwise the muscle would soon be exhausted. The source is another organic phosphate compound, creatine phosphate, which is present in larger amounts. Its phosphate group is transferred to adenosine diphosphate (ADP) and thus the supply of ATP is restored.

The creatine phosphate must, however, eventually be replenished, and the ultimate energy source is the oxidation of carbohydrates of fatty acids. Carbohydrate is stored in the muscle in the form of glycogen, which is often present in an amount of between 0.5 and 2% of the wet weight of the muscle and provides an amount of energy perhaps 100 times as great as the total quantity of creatine phosphate. In the absence of sufficient oxygen, the glycogen can still yield energy by being split into lactic acid, but the amount of energy is then only a small fraction, about 7%, of that available from complete oxidation.

Contraction of the muscle causes a force to be exerted on the points of attachment; because no change in length takes place, this us called an isometric contraction. If, on the other hand, we attach to one end of the muscle a weight it can lift, the muscle shortens during contraction; because the load remains the same throughout the contraction, this is called an isotonic contraction.

The molecular events during muscle contraction are better known than most other processes in the living organism. To describe these events, we must refer to the structural composition of the thick and thin filaments.

The thick filaments consist of the protein myosin. Each myosin molecular resembles a thin rod with a globular “head”. One thick filament contains several hundred myosin molecules, lined up as shown in Fig() with the heads facing in both directions away from the center of the filament. This leaves a bare zone in the middle and the heads protruding all along the remainder of the thick filament.

The thin filaments are more complex, with three important protein actin, a relatively osmall globular protein, arranged in the filament as a twisted double strand of beads. Each actin molecule is slightly asymmetrical, and in each filament all the actin is lined up facing in the same direction. This directionality is important for muscle contraction.

The next important protein in the thin filament is tropomyosin, long and thin molecules attached to each other end to end, forming a very thin threadlike structure that lies in the grooves between the double helix of the actin molecules. The length of one tropomyosin molecule is such that it extends over seven actin molecuels. The single thin filament is about 1 µm long and contains about 400 actin molecules and 60 tropomyosin molecules.

A third protein molecule, troponin, is attached to each tropomyosin molecule. Troponin, a calcium-binding protein, is a key to the contraction process. When troponin binds calcium ions, it undergones a conformational change that is essential for the interaction between the myosin heads of the thick filaments and the action of the thin filaments (Squire 1975).

The interaction between thick and thin filaments consists of a cyclic attachment and detachment of cross bridges between the two filaments. The myosin heads attach to the thin filaments at a certain angle; they undergo a conformational change that makes the bridge swivel to a different angle, pulling the thin filament past the thick (Huxley 1973). The cross bridges on opposite sides of the bare zone in the middle of the thick filament swivel in opposite directions, pulling at the opposing ends of the thin filaments, thereby shortening the distance between the Z-lines. This decrease in the distance between the Z-lines causes the shortening of the muscle.

How is concentration triggered?

Normally, muscle contraction is initiated when a nerve impulse arrives at the neuromuscular junction (the motor end-plate). The impulse spreads rapidly as an electric depolarization over the surface of the muscle fiber (the sarcolemma), momentarily abolishing the normal surface potential of about – 60 mV. This depolarization is almost instantaneously communicated contraction to take place.

The muscles of fish also have twitch and tonic fibers. Pelagic fish such as mackerel and tuna, which swim continuously at relatively low speeds, have two types of fibers, separated in different muscle masses, which have strikingly different appearances. The tonic muscle is deep red, because of a high concentration of myoglobin. It is located along the sideline and stretches in toward the vertebral column. The basal swimming during cruising is entirely executed by this red muscle; the large mass of white muscle (twitch type) represents a reserve of power prolonged periods of slow swimming, however, there is no change in their glycogen level; this indicates that they are not used at all during cruising (Bone 1966).

The separation into red and white swimming muscle has an interesting counterpart in an invertebrate. In squid, the muscles of the mantle show a clear separation between layers that have a high density of mitochondria and oxidative enzymes and a layer low in mitochondria but high in glycolytic enzymes. This metabolic differentiation is analogous to the red muscle of fish that support steady swimming and the white muscle that supports burst-type swimming (Mommsen et al., 1981.)