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Physiology Lect(4) Asst.Lect/Shaimaa Hussein Muscle tissue If you weight 120 pounds about 50 pounds of your weight comes from your muscle. Under the microscope these muscles appear an bundles of fine threads with many crosswise stripes. Each fine thread is a muscle cell or, as it is usually called (a muscle fiber). This type of muscle tissue has three names striated muscle because of its cross stripe, skeletal muscles because it attaches to bone and voluntary muscle because its contraction can be controlled voluntarily. Besides skeletal muscle the body also contain two other kinds of muscle tissue:1- Cardiac muscle 2-and nonstriated,Smooth or involuntary muscle. Cardic muscle as its names suggests composes the bulk of the heart. Cardic muscle branch frequently. Nonsrtriated or smooth muscle lacks the cross stripes which seen in skeletal muscle. It called involuntary because we normally do not have control over its contraction. This muscle forms an important part of blood vessel walls and of many internal organ(viscera) such as the gut. Muscle cells specialize in the function of contraction or shortening. Every movement we make is produced by contraction of skeletal muscle cells. Contraction of cardiac muscle cells keep the blood circulating through do many things for instance, move food into and through the stomach and intestine and make a major contraction to the maintenance of normal blood pressure. Skeletal muscle A skeletal muscle is an organ composed mainly of striated muscle cells and connective tissue. Most of skeletal muscle attach to two bones that have a movable joint between them. Most muscle extend from one bone a cross a joint to another bone. Also one of the two bones moves less easily than the other. The muscle s attachment to this more stationary bone is called its origin. Its attachment to the more movable bone is called the muscle s insertion. The rest of the muscle( all of it 1 except its two ends) is called the body of the muscle. Tendons anchor muscles firmly to bone. Made of dense fibrous connective tissue. Small fluid filled sacs called bursae lie between some tendons and the bones. These small sac are made of connective tissue and lined with synovial membrane. The synovial membrane secretes a slippery. Lubricating fluid that fills the bursa. 2 3 Microscopie Structure Muscle tissue consist of specialized contractal cells or muscle fibers that are grouped together and arranged in a highly organized way. Each skeletal muscle fiber is itself e filled with two kinds of very fine and thread like structure called thick and thin myofilaments. The thick are formed from a protein called myosin and thin are composed of the protein actin. Find the label Sarcomere. The sarcomere as the basic functional or contractile unit of skeletal muscle. The submicroscopie structure of a sarcomere consist of numerous actin and myosin myofilaments arranged so that when viewed under microscop, dark and high stripes or cross-striae are seen. The repetitise units or sarcomere are separated from each other by dark band called Z lines. Look at the structure of sarcomere in figure that the thick and thin myofilaments overlap each other to form a dark area are called A band. A light area between the Z line and A band is called I band. Each A band is composed of both thick and thin myofilaments and each I band is composed only of thin. Another specialized structure found in skeletal musccle and cardiac muscle is the system of transverse tubule (T-tube). During the contraction process energy obtained from ATP molecules eneble the two type of myofilament to slide toward each other and shorten the sarcomer. The explantation of muscle contraction resting from the movement of thick and thin sliding-filament theory. Function : The three primary function of the muscular system are: 1-movement 2-posture or muscle tone 3-heat production 1-Movement: Muscle move bones by pulling on them. Because the length of skeletal muscle becomes shorter as the fibers contract the bone 4 to which muscle attached move coloser together. The shortening of the muscle pulls the insertion bone toward the origin bone. 2-Posture: We are able to maintain our body position because of a specialized type of skeletal muscle contraction called tonic contraction. Skeletal muscle tone maintains posture by contraction the pull of gravity. 3-Heat production: Healthy survival depends on our ability to maintain a constant body temprature. A fever or elevation in body temperature of only a degree or two above 37c will result in illness. Any decrease below normal, a condition called hypothermia. The contraction of muscle fiber produces most of the heat required to maintain our body temperature. Energy required to produce a muscle contraction obtained from ATP. Most of energy released during the break down of ATP during muscle contraction is used to shorter the muscle fibers however some of energy is lost as heat during the reaction. It is this heat that help us to maintain our body temperature at a constant level. 5 6 7 Exaltation of muscle cells: Skeletal muscle is stimulated to contract by nerve impulses traveling at a rate of about 100 meter per second. When a stimules reaches the membrane of muscle cells. The permeability of the membrane to sodium and potassium ions is altered. The result is a change in the balance of electrical charges, creating an electrical potential or action potential. The action potential moves along the muscle cell membrane spreading down in to T. tubes. When the action potential part regin of the sarcoplasmic reticulum to release calcium ions (Ca++) in the sarcoplasm. It take roughly amillisecond (1/1000 second) Ca ion act as mediaters between excitation, an electrical event and contraction, a mechanical event. The contraction process: The sliding filament theory of contraction proposed in the 1950 by H.E. Huxely and based on electron microscop studies. Acording to this theory muscle shortening occur when the thin filament of a sarcomer slide over thick filament so that greater overlap exist between them. Actin and myosin are protein molecule that bring about the generation of force in the contraction process. Actin is a globular protein, actin molecule are arranged like two string of bread wound in a spiral to form an actin filament. Myosin is much large molecule with a globular head and along tall picee. The head of myosin molecule project laterally from a thick filament toward the surrounding actin filament. These heads are called cross-bridges. الجسور العرضية The head of each myosin molecule contain an enzyme capable of releasing energy from ATP. Magnesium ion are required for this enzyme to function. The protein troponin and tropomyosin which are closely associated with actin also are important in regulation the attachment of actin to the cross- bridges. Calcium ions can a change in its molecular configuration. This in turn alteres the configuration of tropomyosin to which tropomyosin binds.This 8 shift exposes previously covered area called active site on the surface of actin molecule. Myosin cross-bridges are then able to bind to there active site. The step in the contraction process show in figure (a) ATP is bound to head of a myosin molecule.Thought energy is available for contraction no contraction occure because the troponin-tropomyosin complex on the thin filament blocks the myosin binding sites on the actin. Following excitation Ca ions released into sarcoplasm cause the troponintropomyosin complex to withdraw into agroove between the chains of actin molecule. (b) An enzyme in the myosin head splits ATP to ADP and phosphate energizing the cross-bridge and cross-bridges bind to actin. (c) Each time across-bridge completes a swiveling movement , a new molecule of ATP replaced the ADP. The formation of a new ATP- myosin bound breaks the existing cross-bridge attachment and allow a new one to form. If Ca++ remain available in sufficient quantity c,d,e occur over and over again rate of 50-100 times/second. These repetitive steps constitute the contraction. *It is important to note that in the contraction process,no shortening of either the actin or the myosin filaments occurs. The sarcomere shortens because of the sliding of the actin filaments produced by cross-bridge movements. H-zones and I-bands, but the width of the A-band remains constant. *The most signifigantevent in this process is the cyclic changes in the position of the myosin heads and the resultant movement of the filaments in relation to each other. This movement is a little like the movement of the oars مجدااof a boat. حركدة الججداا على الزورق Rowing التجدافwith oars puches a boat across the water. In some what the same way that myosin and actin filaments slide along one another. 9 10 11 The relaxation process: Like contraction, relaxation also requires ATP. When a muscle is stimulated, the SR releases Ca++ for only a few milliseconds (repetitive stimuli are required for Ca++ to continue to be released). As soon as stimulation ceases فتوق, Ca++ beigns to be returned to the SR by active transport (a process that requires ATP). However, contraction continues to occur for several handred milliseconds, or until enough Ca++ is removed from the troponin- tropomyosin complex to allow these molecules to block reactive sites on myosin when myosin cannot attach to actin the muscle relaxes. However, the actin filaments slide back to their relaxed position, and the sarcomeres lengthen only when an external force (load) is exerted on the muscle. The Motor unit: A motor unit consists of a single motor neuron(nerve cell) and the muscle fibers it innervates. Motor units controlling precise دقيدmovements such as those in the fingers have 3-6 muscle fibers / motor neroun. Motor units controlling gross movements such as those in the back have several handred fibers/ motor neroun. Motor units in some leg muscles such as the gastrocnemius muscle have as many as 1900 muscle fibers/motor neroun. The branch of an axon that serves a particular muscle fibers has many small terminals branches that lie in grooves in the muscle cell membrane. The portion of the muscle plasma membrane that lies beneath تحدthese nerve ending is called the motor end plate. The terminal ends of the axon and the motor end plate together constitutes the myoneural junction. As a nerve impulse reaches the terminal end of the axon, small sacs called synaptic vesicles fuse with the axon membrane and release a chemical transmitter (acetyl-cholin). A.ch diffuses across the synaptic cleft (the space between the axon membrane and the motor end plate). In this area the muscle cell membrane is folded into subneural clefts and has 12 A.ch. receptor sites on its surface. When a sufficient number of these sites are stimulated by A.ch., the motor end plate becomes depolarized. زوال أستقطاب To understand depolarization, it is important to realize that a resting muscle cell membrane is polarized that is it has a small positive charge at the outer surface and a small negative charge at the inner surface. This polarized state is maintained by the transport of Na+ and K+ the membrane. A.ch.acts to depolarize the muscle cell membrane by causing it to become suddenly more permeable to Na+. The positive Na+ entering the cell reduce the electrical charge across the membrane (the membrane is then said to be depolarized). When depolarization of the motor end plate reaches a certain level, it creates an action potential. This action potential is propagated along the T-tubules and causes the SR to release Ca++. Along with receptor sites for A.ch. the motor end plate also contain an enzyme (cholin esterase) that breaks down A.ch. About 5 milliseconds after the release of A.ch., cholinesterase has broken A.ch. down into it components parts (acetate and choline). The effect of cholinesterase is to prevent continous stimulation of a muscle fiber after the release of A.ch. Aportion of the choline diffuses back to the axon and is reused to synthesize more A.ch. for transmission of subsequent impulses. Muscle Tension: The amount of tension (force) the force produced by a whole muscle when it contraction. The amount of tension that can be developed by a skeletal muscle depends on several factors: 1- Frequency of stimulation of muscle fibers by motor neurons. 2- The number of motor fibers contraction (number of active motor units) at a particular time. 3- The components of the muscle fibers themselves: a- contractile elements: are those components that are actively involved in muscle contraction (thin and thick myofilaments). 13 The tension generation by contractile elements is called active tension. b- Elastic elements: are structures that are capable of being stretched. They include the C.T. surrounding each motor fibers and the tension that attaches muscle to bone . The tension generation by elastic elements is called passive tension. 4- The length of the muscle fibers at the time of contraction. Muscle Tone : A sustained partial contraction of portionsof a skeletal muscle in response to activation of stretch receptors مسدتقالت الدداresult in muscle tone and occurs even in relaxed muscle. Tone is essential for maintaining posture. For example: when the muscle in the back of the neck are in tonic contraction, they keep the head in the anatomical position and prevent it from slumping forwared onto the chest, but they do not apply enough force to pull the head back into hyperextension. The degree of tone in a skeletal muscle is monitored by receptors in the muscle called muscle spindles. They provide feedback information on tone to the brain and spinal cord so that adjustments can be made. The load : is the force exerted on a muscle by a weight. Eg. When you pick up a book, the book is the load and the force produced by the muscle in your arm is the tension. Thus, load and tension are opposing forces. Types of muscle contraction: 1- Isotonic contraction (same tention): When the tension in a muscle in a muscle is sufficient to lift a load, the contraction is said to be isotonic. One the other hand, during isotonic contraction, the muscle is shorten. 2-Isometric contraction (same length): When the tension in a muscle is used to support a load (a book you are carrying,for example) or to push against an immovable object, the contraction is said to be isometric. In isometric contraction the muscle maintains the same length but develops tension during the period of the contraction. 14 Simple muscle twitch: There are three phases of twitch can be distinguish: 1- The latent period: is the time between the stimulation and the beginning of the contraction During this period: a-the nerve action potential is conducted to the axon terminals. b-A.ch. is released and binds to the motor end plate. c- The action potential passes over the membrane. d- Ca++ inhibits the action of troponin and tropomyosin or Ca++ exposes the myosin active site. 2-The contraction period: is the time during which tension is rising or shortening is occurring. 3- The relaxation period: is the time during which tension is declines or muscle back to its original length. Energy for Contraction and Relaxation: 1- Phosphagen system a-ATP is the immediate source of energy for contraction. When a muscle action potential stimulates a motor fiber, ATP, in the presence of enzyme ATPase, breaks down into ADP+P, and energy is released. Like other cells of the body, motor fibers synthesize ATP as follows ADP+P+ energy ATP *The amount of ATP present in skeletal muscle fibers is sufficient to maintain muscle contraction during vigorous exercise for only 5-6 seconds. b- Creatine phosphate: keletal muscle fibers contain a high energy molecule called phosphocreatine that is used to generate ATP rapidly. The amount of phosphocreatine is about 2-3 times greater than ATP. Phosphocreatine creatine+P+energy ADP+P+energy ATP *The transfer of energy from phosphocreatine to ATP takes place in a fraction of a second. *Phosphagen system provide only enough energy for muscle to contract maximally for about 15 seconds. 15 2-Glycogen-Lactic acid system When muscle activity is continued so that even the supply of phosphocreatine is depleted, then the source of energy is glucose, which is derived from the break down of glycogen and also picked up from the blood. Glycogen is always present in skeletal muscle and the liver. In some cases there may no be sufficient oxygen for the complete catabolism of pyruvic acid. Then most of the pyruvic acid is converted to lactic acid and energy that can be used to produce ATP. *The glycogen-lactic acid system can provide sufficient energy for about 30-40 seconds of maximal muscle activity. 3-Aerobic system If sufficient oxygen is present, pyruvic acid can be completely catabolized to form CO2,H2O and energy(ATP) in the mitochondria. This aerobic system along with glycogen, is used for prolonged muscular activity and will continue as long as nutrients and adequate oxygen last. These nutrients include fatty acid (from fat) and amino acids (from proteins) as well as glucose derived from glucose derived from glycogen breakdown or delivered to the muscle by way of blood. In summary 1-The phosphogen system will provide enough ATP to sustain maximal muscular activity for about 15 seconds. 2-The glycogen-lactic acid system will provide sufficient ATP to support about 30-40 seconds of maximum muscular activity. 3-The aerobic system will provide sufficient ATP for a prolonged activity as sufficient oxygen and nutrients are available. 16 17