MUSCLES AND MUSCLE TISSUE

Transcription

1 MUSCLES AND MUSCLE TISSUE The muscular system provides for movement of the body and its parts (as muscles shorten), maintains posture, generates heat and stabilizes joints. The various types of muscles differ in the organization of their cells, location within the body, function and basis of activation. GENERAL CLASSIFICATION OF ALL (HUMAN) MUSCLE TISSUES There are three types of muscle tissue in the human body: Skeletal (striated): they are the muscles of the body s frame. They are voluntary. Their cells are multinucleate. Largest of the three types of muscle cells (some are one foot/30 cm long). Skeletal muscle tissue can contract rapidly and with great force, but wears out easily and must rest after short periods of activity. Most skeletal muscles span joints and are attached to bones or other structures in at least two places: (a) the insertion (the movable bone), which will move toward (b) the origin (the less movable or immovable bone). Smooth (visceral): they are located in walls of vessels and internal organs. These muscles are involuntary. Smaller cells than skeletal muscle fibers. The myofilaments are not arranged in orderly sarcomeres. Therefore, smooth muscle lacks striations. A greater number of thin fibers; significantly more that the thick filaments. Slow, sustained contractions due to lack of transverse tubules to distribute Ca ++. Calmodulin regulates Ca ++ distribution in place of calsequestrin. Propels materials along a definite tract. Spindle-shaped cells. Single nucleus in each cell. Types of smooth muscle include: (1) single-unit (visceral) and (2) multi-unit. Cardiac muscle: the muscle tissue of the heart. Involuntary. Single nucleus. Striated; branched fibers (cells). Cushioned by small bundles of CT. Arranged in figure eight shaped bundles. Special junctions called intercalated discs join cells. Intercalated discs are sarcolemma thickenings with desmosomes and gap junctions. They provide a rapid transfer of the nerve impulse so that the upper and lower portions of the heart can act as two separate units. Same arrangement of actin + myosin as skeletal muscle. Two networks: upper and lower chambers. Cardiac muscle contracts without ACh stimulation because it has its own built-in electrical system to stimulate contraction. CHARACTERISTICS OF SKELETAL MUSCLE TISSUE Prefixes such as myo-, sarco-, or mys- all refer to muscle tissue and/or muscles in general. The plasma membrane, ER and cytoplasm of a skeletal muscle cell are very specialized and are called the sarcolemma, sarcoplasmic (SR) reticulum and sarcoplasm. All skeletal muscle cells are elongated and are therefore called fibers. The cells in a skeletal muscle are parallel to one another. Structures called t-tubules are extensions of the sarcolemma and run parallel to the sarcolemma. The muscle cell s sarcoplasm contains special units called myofibrils, which are made of myofilaments. Myofibrils run longitudinally through the cytoplasm. The ability of these cells (fibers) to shorten depends on two types of myofilaments (microfilaments of muscle cells) composed of actin and myosin. The two types of myofilaments are called thin and thick. The two major proteins of myofilaments have distinctive features: (1) actin and (2) myosin. The proteins in the myofilaments do not extend the length of the fiber; rather they are stacked in compartments called sarcomeres. The structure of the sarcomere includes: (1) Z-line, (2) I-band (isotropic), (3) A-band (anisotropic), (4) H-zone and (5) M-line. 23

2 CONNECTIVE TISSUES SUPPORTING SKELETAL MUSCLES Skeletal muscles can exert tremendous power because when they stretch thousands of their fibers are bundled together by connective tissue. Therefore, they do not rip apart when they contract. There are two sets of CT wrappings for skeletal muscles: fascia and its subsections, which are known as the mysium layers. Fascia is the sheet or broad band of fibrous CT beneath the skin. It is divided into three layers: (1) superficial fascia, (2) deep fascia (the -mysiums ) and (3) subcutaneous (visceral) fascia. The mysiums are also classified into three categories: (1) endomysium, (2) perimysium and (3) epimysium. SLIDING FILAMENT THEORY OF MUSCLE CONTRACTION A motor unit is a motor neuron plus all the muscle fibers (cells) it stimulates, usually up to 150 cells. Neurons are cells of the NS that provide stimulation to help body cells/tissues/organs (such as muscles) get the energy they need to do their job. Axons are long extensions of a neuron (as long as 3 feet). These axons branch when entering skeletal muscle. Where the end of one axon meets the sarcolemma is the area termed the motor end plate. The area of contact between a neuron and a muscle fiber is called the neuromuscular junction. Synaptic vesicles are structures that release a neurotransmitter called acetylcholine (ACh). The sliding-filament theory summarizes the process of muscle contraction thusly: Relaxed muscle fibers have a low level of Ca ++ in their sarcoplasm. ADP + P binds to myosin cross-bridges (heads) and the cross-bridges are therefore prevented from attaching to actin. The tropomyosin-troponin complex remains attached to actin. The axonal endings of the neuron approach, but do not touch, the muscle membrane. This close contact area is called the neuromuscular junction. The space between the sarcolemma and the axon is called the synaptic cleft. Axonal endings contain synaptic vesicles that carry neurotransmitters. In the case of the nerveskeletal muscle junction, the neurotransmitter is usually ACh (acetylcholine). A nerve impulse entering the dendrites of a neuron brings with it a small amount of Ca ++, which will cause the synaptic vesicles of the neuron s axon to release ACh. When the impulse passes through the cell body and reaches the axon, Ca ++ channels open. This allows Ca ++ to flow into the terminal region of the axon. Ca ++ causes the synaptic vesicles in the axon to fuse with the axonal membrane, thereby allowing the release of ACh into the synaptic cleft by active transport (exocytosis). ACh diffuses across the myoneural junction and combines with receptor sites on the motor end plate. The motor end plate is a trough-like structure with many infoldings to increase its surface area. This great amount of folding increases the surface area of the sarcolemma. The motor end plate helps form the neuromuscular junction. ACh alters the motor end plate and initiates an impulse that spreads over the surface of the sarcolemma and goes into the t-tubules. The t-tubules convey the impulse to the sarcoplasmic reticulum and the reticulum releases Ca ++ from storage into the sarcoplasm around the myofilaments. Ca ++ binds to troponin, thereby weakening the link between troponin and actin. This creates a free receptor site on actin and provides the opportunity for myosin heads to attach to actin. Myosin then acts as an enzyme and catalyzes the release of P from the combination of ADP + P that is attached to myosin when it is at rest. The phosphate removed from the ADP + P combination is used to power the attachment of the myosin heads to the actin molecules. 24

3 Z-lines are drawn towards one another as the myosin heads interact with actin and the sarcomere thereby shortens. ACh-ase (AChE; acetylcholinesterase) begins destroying ACh. The absence of ACh inhibits nerve impulse conduction from axon terminals to the motor end plate. When the impulse ends, Ca ++ is actively transported back into the sarcoplasmic reticulum by a transport protein called calsequestrin. Removal of Ca ++ stops the enzymatic action of the myosin. CLASSIFICATION OF TYPES OF MUSCLE FIBERS There are several types of muscle fibers, including slow and fast. Slow have a pink coloration (due to the presence of myoglobin) and fast have a white coloration. Human skeletal muscles include a mixture of slow and fast fiber types; therefore they look pink. Most skeletal muscle cells are primarily the fast fiber types. There are different amounts of each type of fiber in the various muscles of the body. Types of fibers include: (1) slow oxidative (type I), (2) fast oxidative (type II, intermediate, type II FR and fast resistant) and (3) fast glycolytic (type II-B, Type-II FF and fast-fatigue). ENERGY ACQUISITION FOR MUSCLE OPERATIONS A great deal of energy is used to power the muscles of the body. Energy is acquired from nutrients via the process of cellular respiration and creatine phosphate production. The normal energy-releasing process occurs in the cytoplasm and mitochondria of a cell and is termed cell respiration. Cell respiration is a series of chemical reactions that change glucose into pyruvate (in the cytoplasm) and then process the pyruvate in the mitochondria to make energy to recharge ATP (that s why mitochondria are called the powerhouses of a cell). There are three sub-sets of reactions in the process of cell respiration: glycolysis, the TCA (a/k/a Krebs or citric acid cycle) and the electron transport system (cytochrome system, ET chain, ETS, or ETC). Glycolysis occurs without oxygen in the cytoplasm and forms pyruvate from glucose. If enough oxygen is present, the pyruvate can be processed and will enter the mitochondria. If a sufficient amount of oxygen is not available, then pyruvate will be anaerobically processed (fermentation pathway) in the cytoplasm to form lactic acid. Resting muscle cells need little energy and produce more ATP than they need. When contracting, ATP synthesis must be accelerated to meet expanded energy demands. Often ATP is used up faster than it can be produced. Thus, muscle cells must maintain reserves of ATP. ATP can be (and is) generated by three pathways during muscle activity: (1) aerobic respiration, (2) anaerobic respiration and (3) creatine phosphate (CP, PC, and phosphocreatine). PROCESS OF MUSCLE FATIGUE Fatigue is caused by excessive lactic acid production. Lactate must be processed back into pyruvate and shunted back into aerobic respiration. A change in ph caused by the lactic acid build-up affects AChE and stimulates a shunt process. 25

4 ADJUSTING MUSCLE TENSION Key terms: All or none principle, threshold stimulus (liminal), subliminal, twitch, treppe, tetanus, isotonic and isometric. 26

5 DISORDERS/DISEASES Charley horse Fibromyositis Hernia Muscle contusion. Trauma induced tearing of muscle with bleeding into the tissues (hematoma formation). A/k/a fibromyalgia. Inflammation of muscles and their CT s. Nonspecific symptoms. Protrusion of an organ through its body cavity wall. Muscular dystrophy Myalgia General term. Refers to a group of inherited muscle diseases that all cause muscle enlargement due to replacement of functional muscle tissue with non-contractile CT, such as adipose tissue. Duchenne form of MD is caused by lack of protein known as dystrophin. General muscle pain resulting from any type of disorder. Myasthenia gravis Myopathy Autoimmune disease involving muscle membrane acetylcholine (ACh) receptors. Inhibits communication between skeletal muscles and their nerves. General term for any disease of a muscle. Spasm Involuntary twitch due to chemical imbalance. Results in inflammation. Strain Tetanus A/k/a pulled muscle. Due to excessive stretching and/or tearing of the muscle. Inflammation due to overuse. Sustained muscle contraction. Can result from normal muscle function but also is term used to refer to Clostridium tetani infection (anaerobic) of skeletal muscle tissue. 27