Zoology Notes: 011 Chapter 7

Chapter 7. The Muscular System Almost half (40% - 50%) the body's weight is muscle. Muscles are the part of our body that allows us to move. They are made up of special tissues that can contract, or shorten, when they receive a signal from the brain. The muscles are attached to bones by stretchy tissue called tendons. When the muscles contract, they pull on the tendons which pull on the bones and cause our limbs to move. Muscle actions can be voluntary or involuntary. Involuntary muscles, such as the heart, diaphragm and intestines, are automatically controlled by the brain. You don't have to think about making them work. For example the heart beats between 60 and 80 beats every minute without you having to think about it. Voluntary muscles, such as your arms and legs can be controlled by your thoughts. All muscle actions are controlled by your brain, which sends and receives signals through your nervous system. Myology is the study of the structures and functions of muscles. Muscles are highly specialized organ, which are characterized by their property of contracting in definite manner when stimulated. When they contract, they effect movement of the body as a whole; of blood (circulation); of food through digestive tract; of urine through the urinary tract; and of the chest, diaphragm, and abdomen during respiration. The two key words, then, are contraction and movement. Muscles move by transforming chemical energy into movement. Movement is essential to life, and takes many forms, from cytoplasmic streaming and the growth of neurons at the cellular level, to the long distance flight of the albatross or the explosive performance of a sprinter. Although only a few families of proteins are responsible for movement in the biological world, muscle has developed to optimize this function, and is packed with movement-related proteins. There are many types of muscles, but they fall into three categories: skeletal muscle (or striated muscle), responsible for locomotion, flight etc; cardiac muscle, which has a vital role and is able to function for a century or more, without ever taking a break, and smooth muscle (or involuntary muscle) which lines the walls of the arteries to control blood pressure, or controls the digestion of food by causing movement of the intestine. Gross muscles vary in shape and form according to their function. Some are long and tapering, some are broad and sheet-like, others are short and thick, and still others form sphincters with circular arrangement of the fibers. The less movable end of a muscle is called the origin, the more movable end the insertion. The middle mass is the belly, and the shining connective tissue that encloses the muscle is the fascia. A muscle’s only action is to contract which causes it to become shorter and thicker in the belly. Types of Muscle Skeletal Muscle. Skeletal muscles, as the name suggests, are usually associated with the skeletal system. They are called striated muscle because of the presence of alternating dark and light bands along the length of its fibers. They are called voluntary muscles because they are subject to voluntary control (CNS). This type of muscle is composed of long fibers surrounded by a membranous sheath, the sarcolemma. The fibers are elongated, sausage-shaped cells containing many nuclei and clearly display longitudinal and cross striations. Skeletal muscles are supplied with nerves from the central nervous system, and because they are partly under conscious Fig. 7.1. Skeletal muscle. 36

control, they are also called voluntary muscle. Most skeletal muscles are attached to portions of the skeleton by connective-tissue attachments called tendons. Contractions of skeletal muscles serve to move the various bones and cartilages of the skeleton. Skeletal muscles form most of the underlying flesh of vertebrates. An entire muscle consists of a number of bundles of muscle fibers known as fasciculi. Each muscle fiber is multinucleated and is surrounded by an electrically polarized membrane, the sarcolemma. Surrounding each fiber and filling in the space between fibers within a fasciculus is a delicate connective tissue called the endomysium. Each fasciculus is bounded by a stronger connective tissue, the epimysium, enveloping the whole muscle.

Fig. 7.2. Myofibril showing the different parts of a sarcomere.

When viewed under the microscope, the skeletal muscle fiber is seen to have regular striations. These striations are due to transverse alternating dark and light bands on the myofibrils, which are parallel threadlike structures in the sarcoplasm (muscle cytoplasm) of a muscle fiber. Myofibrils, the smallest element of the muscle fiber visible under the light microscope, are the contractile units of the fiber. Myofibrils have subunits, the thick and thin filaments. The thick filaments composed largely of the protein myosin and the thin filaments are composed of three proteins: actin (the principal one), topomysin and troponin. The dark band, or A-band, of the myofibrils corresponds to the thick element; the light band, or I-band, corresponds to the regions where there are only thin filaments. The additional markings are of importance: the Z-line, a narrow band in the central region of the Iband, represents a structure to which the thin filaments are attached on both side, and the H-zone, a lighter region located in the central region of each A-band. The area between two adjacent Z lines, called a sarcomere, represents the repeating unit of myofibril. Smooth Muscle. Visceral, or smooth, muscles are composed of spindle-shaped cells, each having a central nucleus. The cells have no cross striations, although they do

Fig. 7.3. Smooth muscles.

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exhibit faint longitudinal striations. Stimuli for the contractions of smooth muscles are mediated by the autonomic nervous system. Smooth muscles are found in the skin, internal organs, reproductive system, major blood vessels, and excretory system. They are found in the digestive and respiratory tracts and other hollow structures, such as the urinary bladder and blood vessels. Other locations include the iris and ciliary muscle of the eye and the pilo-erector muscle of the skin. Since the contraction in smooth muscle is ordinarily not induced at will, the muscles are described as involuntary. Each smooth muscle cell, also called a fiber, contains a single large nucleus and is smaller than a fiber of skeletal muscle. Cross-striations are absent in smooth muscle. Cardiac Muscle. Cardiac (heart) muscles are involuntary muscles, possessing the striated appearance of voluntary muscle. The striations result from the same arrangement of thick and thin filaments as in skeletal muscles. Cardiac muscle fibers have a single, centrally placed nucleus and are

branched at their ends. Muscle fibers are functionally linked at the branched ends by junctional specialization with inter-digitations of cell membrane called intercalated discs. The resulting three-dimensional network is generally referred to as a functional syncytium. This muscle tissue composes most of the vertebrate heart. The cells, which show both longitudinal and imperfect cross striations, differ from skeletal muscle primarily in having centrally placed nuclei and in the branching and interconnecting of fibers. Cardiac muscles are not under voluntary control. They are supplied with nerves from the autonomic nervous system, but autonomic impulses merely speed or slow its action and are not responsible for the continuous rhythmic contraction characteristic of living cardiac muscle. The mechanism of cardiac contraction is not yet fully understood. Fig. 7.3. Cardiac muscle fiber.

An important characteristic of cardiac muscle cells is the slow return of the membrane to a resting state following excitation by an electrical impulse. During the recovery period it is insensitive, or refractory, to another impulse. The long refractory period of a cardiac muscle prevents tetanization, which of course, would interfere with its rhythmic pumping action. Mechanism of Muscle Contraction Muscles of the animal body are characterized by their ability to contract, usually in response to a stimulus from the nervous system. The basic unit of all muscle is the myofibril, a minute, threadlike structure composed of complex proteins. Each muscle cell, or fiber, contains several myofibrils, which are composed of regularly arranged myofilaments of two types, thick and thin. Each thick myofilament contains several hundred molecules of the protein myosin. Thin filaments contain two strands of the

Fig. 7.4. Actin and myosin.

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protein actin. The myofibrils are made up of alternating rows of thick and thin myofilaments with their ends interleaved. During muscular contractions, these interdigitated rows of filaments slide along each other by means of cross bridges that act as ratchets. The energy for this motion is generated by densely packed mitochondria that surround the myofibrils. The generally accepted conception of how muscle contracts, the “sliding filament” model, is based on elegant electron microscopy. According to this model, the contraction is brought by the sliding of the thin (actin) filaments at each end of sarcromere toward each other between the stationary thick (myosin) filaments. This draws the Z-lines closer together, shortening the sarcomere. As a sarcomere shortens, the I-band of each myofibril narrows as the thin filaments move toward the center of the sarcomere, while the A band is unaltered. The H-zone of the Aband, the lighter, central region not penetrated by thin filaments Fig. 7.5. The Sliding Filament Model of in relaxed muscles disappears as thin filaments come to muscle contraction. completely overlap the thick filaments in the contracted state. Types of Muscle Contraction 1. 2.

Isotonic contraction - contraction with shortening of muscle against constant load (tension is greater than the load and there is movement). Isometric contraction - contraction without shortening of muscle against constant load (tension is greater than the load and there is no movement).

Muscle Attachment 1. 2.

Origin – point of attachment which is more proximal and more fixed Insertion - point of attachment which is more dorsal and more movable.

Types of Muscle as to Their Action There are more than 640 muscles, and they hardly ever work alone. Muscles can get shorter and pull, but they cannot push. So most muscles are arranged in opposing teams. One team pulls the body part one way and the other team pulls it back again. As each team pulls, the other team relaxes and gets stretched. Muscles band together to form: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

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Extensors - muscles which straighten a limb or a joint. Flexors - muscles which bend a limb or a joint. Abductors - muscles that pull a limb away from the median plane. Adductors - muscles that pull a limb towards the median plane. Levators - muscles that raise a part of the body. Depressors – muscles that lower part of the body. Dilators - muscles that increase the size of an opening. Constrictors - muscles that decrease the size of an opening. Sphincters - muscles that open and close an opening (whether voluntary or involuntary). Protractors - muscles that push a part of the body away from its base. Retractors - muscles that pull a part of the body toward its base. Rotators - muscles that turn a part about its axis producing a rotary movement. a. Types of Rotators i. Supinators - rotators the turn a part upward or dorsad. ii. Pronators - rotators that turn a part downward or ventrad.