The Muscular System PDF

The muscular System & muscle contraction Z 302 Medical physics  LEARNING OBJECTIVES, 1 Describe the different types of muscle Explain contractibility and extensibility Describe the layers of connective tissues packaging skeletal muscle Explain how muscles work wit...

The muscular System & muscle contraction Z 302 Medical physics  LEARNING OBJECTIVES, 1 Describe the different types of muscle Explain contractibility and extensibility Describe the layers of connective tissues packaging skeletal muscle Explain how muscles work with tendons to move the body Identify areas of the skeletal muscle fibers Describe excitation-contraction coupling M Amr El-Missiry 2 Types of muscles M Amr El-Missiry 3  Types of muscles The body contains three types of muscle tissue: – Skeletal muscle, – Cardiac muscle, – Smooth muscle M Amr El-Missiry 4  Prosperities of muscles Excitable elasticity. extensibility Contractility M Amr El-Missiry 5 Prosperities of muscles All muscles are : excitable – (their plasma membranes can change their electrical states (from polarized to depolarized) and send an electrical wave called an action potential along the entire length of the membrane). elasticity. – It can recoil back to its original length due to elastic fibers. extensibility – it can stretch or extend. Contractility – allows muscle tissue to pull on its attachment points and shorten with force. M Amr El-Missiry 6 Skeletal muscle The Three Connective Tissue Layers. Bundles of muscle fibers, called fascicles, are covered by the perimysium. Muscle fibers are covered by the endomysium. Each muscle is wrapped in a sheath of dense, irregular connective tissue called the epimysium M Amr El-Missiry 7  Epimysium: is a sheath of dense, irregular connective tissue wrap the muscle. Allows a muscle to contract and move powerfully while maintaining its structural integrity. The epimysium also Separates muscle from other tissues and organs in the area, allowing the muscle to move independently. M Amr El-Missiry 8  Skeletal muscle fibers Sarcolemma Sarcoplasm sarcoplasmic reticulum (SR) Cytoplasm Sarcomere A skeletal muscle fiber is surrounded by a plasma membrane called the sarcolemma, which contains sarcoplasm, the cytoplasm of muscle cells. A muscle fiber is composed of many fibrils, which give the cell its striated appearance. M Amr El-Missiry 9 Skeletal Muscle Fibers Sarcolemma is the plasma membrane of muscle fibers. Sarcoplasm is the cytoplasm of muscle cells sarcoplasmic reticulum (SR) specialized smooth endoplasmic reticulum, which stores, releases, and retrieves calcium ions (Ca++). Cytoplasm= sarcoplasm + SR Sarcomere is the functional contractile unite of muscle. M Amr El-Missiry 10 The Sarcomere M Amr El-Missiry 11 The Sarcomere The striated appearance of skeletal muscle fibers is due to the arrangement of the myofilaments of actin and myosin in sequential order from one end of the muscle fiber to the other. Each packet of these microfilaments and their regulatory proteins troponin and tropomyosin (along with other proteins) is called a sarcomere. M Amr El-Missiry 12 The Sarcomere Cont The Sarcomere. The region from one Z-line to the next Z-line, is the functional unit of a skeletal muscle fiber. When a skeletal muscle fiber contracts, myosin heads attach to actin to form cross-bridges followed by the thin filaments sliding over the thick filaments as the heads pull the actin, and this results in sarcomere shortening, creating the tension of the muscle contraction. M Amr El-Missiry 13 The Neuromuscular Junction Every skeletal muscle fiber is supplied by a motor neuron at the NMJ M Amr El-Missiry 14 The Neuromuscular Junction Excitation-Contraction Coupling All living cells have membrane potentials, or electrical gradients across their membranes. The inside of the membrane is usually around -60 to -90 mV, relative to the outside. This is referred to as a cell’s membrane potential. Neurons and muscle cells can use their membrane potentials to generate electrical signals. M Amr El-Missiry 15  Motor End-Plate and Innervation At the NMJ, the axon terminal releases ACh. The motor end-plate is the location of the ACh-receptors in the muscle fiber sarcolemma. When ACh molecules are released, they diffuse across a minute space called the synaptic cleft and bind to the receptors. M Amr El-Missiry 16 The T-tubule Narrow T-tubules permit the conduction of electrical impulses. The SR functions to regulate intracellular levels of calcium. Two terminal cisternae (where enlarged SR connects to the T-tubule) and one T- tubule comprise a triad—a “threesome” of membranes, with those of SR on two sides and the T-tubule sandwiched between them. M Amr El-Missiry 17 M Amr El-Missiry 18  Transmission of action potential 1- Signaling begins when a neuronal action potential travels along the axon of a motor neuron, and then along the individual branches to terminate at the NMJ. 2- At the NMJ, the axon terminal releases a chemical messenger, or neurotransmitter, called acetylcholine (ACh). M Amr El-Missiry 19  Transmission of action potential 3- The ACh molecules diffuse across a minute space called the synaptic cleft and bind to ACh receptors located within the motor end-plate of the sarcolemma on the other side of the synapse. 4- Once ACh binds, a channel in the ACh receptor opens and positively charged ions can pass through into the muscle fiber, causing it to depolarize, meaning that the membrane potential of the muscle fiber becomes less negative (closer to zero.) M Amr El-Missiry 20  LEARNING OBJECTIVES, 2 Describe the components involved in a muscle contraction Explain how muscles contract and relax Describe the sliding filament model of muscle contraction M Amr El-Missiry 21 Muscle Fiber Contraction and Relaxation M Amr El-Missiry 22 Contraction of a Muscle Fiber A cross-bridge forms between actin and the myosin heads triggering contraction. As long as Ca++ ions remain in the sarcoplasm to bind to troponin, and as long as ATP is available, the muscle fiber will continue to shorten. M Amr El-Missiry 23 Relaxation of a Muscle Fiber Ca++ ions are pumped back into the SR, which causes the tropomyosin to reshield the binding sites on the actin strands. A muscle may also stop contracting when it runs out of ATP and becomes fatigued. The release of calcium ions initiates muscle contractions. M Amr El-Missiry 24 The Sliding Filament Model of Muscle Contraction When a sarcomere contracts, the Z lines move closer together, and the I band becomes smaller. The A band stays the same width. At full contraction, the thin and thick filaments overlap. M Amr El-Missiry 25 M Amr El-Missiry 26  (a) The active site on actin is exposed as calcium binds to troponin. (b) The myosin head is attracted to actin, and myosin binds actin at its actin-binding site, forming the cross- bridge. M Amr El-Missiry 27  (c) During the power stroke, the phosphate generated in the previous contraction cycle is released. This results in the myosin head pivoting toward the center of the sarcomere, after which the attached ADP and phosphate group are released. M Amr El-Missiry 28  (d) A new molecule of ATP attaches to the myosin head, causing the cross- bridge to detach. M Amr El-Missiry 29  (e) The myosin head hydrolyzes ATP to ADP and phosphate, which returns the myosin to the cocked position. M Amr El-Missiry 30 M Amr El-Missiry 31 Skeletal Muscle Contraction (a) The active site on actin is exposed as calcium binds to troponin. (b) The myosin head is attracted to actin, and myosin binds actin at its actin-binding site, forming the cross-bridge. (c) During the power stroke, the phosphate generated in the previous contraction cycle is released. This results in the myosin head pivoting toward the center of the sarcomere, after which the attached ADP and phosphate group are released. (d) A new molecule of ATP attaches to the myosin head, causing the cross-bridge to detach. (e) The myosin head hydrolyzes ATP to ADP and phosphate, which returns the myosin to the cocked position. M Amr El-Missiry 32 Sources of ATP (Energy) Some ATP is stored in a resting muscle. As contraction starts, it is used up in seconds. More ATP is generated from creatine phosphate for about 15 seconds. M Amr El-Missiry 33 M Amr El-Missiry 34  As the ATP produced by creatine phosphate is depleted, muscles turn to glycolysis as an ATP source. Glycolysis is an anaerobic (non-oxygen- dependent) process that breaks down glucose to produce ATP. Accumulation of lactic acid contributes to muscle fatigue. M Amr El-Missiry 35 Muscle Strength The number of skeletal muscle fibers in a given muscle is genetically determined and does not change. Muscle strength is directly related to the amount of myofibrils and sarcomeres within each fiber. Factors, such as hormones and stress (and artificial anabolic steroids), acting on the muscle can increase the production of sarcomeres and myofibrils within the muscle fibers M Amr El-Missiry 36  Muscle tension The force generated by the contraction of the muscle (or shortening of the sarcomeres) is called muscle tension. Types of muscle contraction: – Isotonic contractions concentric contraction eccentric contraction – Isometric contraction M Amr El-Missiry 37 Types of Muscle contraction During isotonic contractions, – muscle length changes to move a load. During isometric contractions, – muscle length does not change because the load exceeds the tension the muscle can generate. M Amr El-Missiry 38 Motor Units Each muscle fiber is innervated by only one motor neuron. The actual group of muscle fibers in a muscle innervated by a single motor neuron is called a motor unit. M Amr El-Missiry 39  Types of motor units A small motor unit – is an arrangement where a single motor neuron supplies a small number of muscle fibers in a muscle. – the extraocular eye muscles. A large motor unit – is an arrangement where a single motor neuron supplies a large number of muscle fibers in a muscle. – the thigh muscles or back muscles M Amr El-Missiry 40 all-or-none law Motor Unit Follows “all-or-none ” principle – impulse from motor neuron will cause contraction in all muscle fibers it innervates or none. M Amr El-Missiry 41 The Length-Tension Range of a Sarcomere The Ideal Length of a Sarcomere. Sarcomeres produce maximal tension when thick and thin filaments overlap between about 80 percent to 120 percent. M Amr El-Missiry 42  The Frequency of Motor Neuron Stimulation A single action potential from a motor neuron will produce a single contraction in the muscle fibers of its motor unit. Twitch is isolated contraction (single contraction). M Amr El-Missiry 43 Single muscle twitch M Amr El-Missiry 44 The Frequency of Motor Neuron Stimulation Each twitch undergoes three phases. The latent period, – during which the action potential is being propagated along the sarcolemma and Ca++ ions are released from the SR. This is the phase during which excitation and contraction are being coupled but contraction has yet to occur. The contraction phase. – The Ca++ ions in the sarcoplasm have bound to troponin, tropomyosin has shifted away from actin-binding sites, cross- bridges formed, and sarcomeres are actively shortening to the point of peak tension. The relaxation phase, – when tension decreases as contraction stops. Ca++ ions are pumped out of the sarcoplasm into the SR, and cross-bridge cycling stops, returning the muscle fibers to their resting state. M Amr El-Missiry 45  Wave Summation and Tetanus (a) The excitation-contraction coupling effects of successive motor neuron signaling is added together which is referred to as wave summation. The bottom of each wave, the end of the relaxation phase, represents the point of stimulus. (b) When the stimulus frequency is so high that the relaxation phase disappears completely, the contractions become continuous; this is called tetanus. M Amr El-Missiry 46 Cardiac Muscle Tissue Intercalated discs are part of the cardiac muscle sarcolemma and they contain gap junctions and desmosomes. M Amr El-Missiry 47 Smooth Muscle M Amr El-Missiry 48 Smooth Muscle Because smooth muscle cells do not contain troponin, cross-bridge formation is not regulated by the troponin-tropomyosin complex but instead by the regulatory protein calmodulin. In a smooth muscle fiber, external Ca++ ions passing through opened calcium channels in the sarcolemma, and additional Ca++ released from SR, bind to calmodulin M Amr El-Missiry 49