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The muscular system Muscle tissue facts Muscles make up half of your body weight. Lines the insides of hollow organs. They take chemical energy (ATP) and directly convert it to mechanical energy. Important prefixes Mys or myo- muscle Sarco- flesh (referring to muscle) Types of Muscle tissues There are three types of muscle tissues: • Skeletal • Cardiac • Smooth Skeletal Muscle Tissue Skeletal muscle tissue is bundled to create skeletal muscle, which are organs that attach directly to the skeleton. They have obvious stripes called striations. Often are muscles that can be voluntarily controlled. The only muscle type that has voluntary control. *Key words to remember: • Skeletal: striated, voluntary Cardiac Muscle Tissue Found only in the heart. Striated, but involuntary. contracts normally at a steady rate set by the hearts pacemaker depending on the situation. Key words to remember: • Cardiac: striated, involuntary Smooth muscle tissue Found in the walls of hollow visceral organs (like stomach). It has the role of forcing fluids and other substances through internal body channels. No striations Slow sustained contractions. Key words to remember: • Visceral, nonstriated and involuntary Special characteristics of muscle tissue Irritability: able to respond to the outside environment. Usually responds to chemical and electrical impulses. Contractility: is the ability to shorten forcibly when stimulated. Extensibility: the ability to stretch or extend. Elasticity: the ability for muscle to recoil to its original shape after stretching. General Muscle Functions Producing movement. Maintaining posture and body Position. Stabilizing joints. Generating heat. Protection (skeletal muscle) Skeletal Muscles Gross Anatomy Skeletal muscle Each muscle in skeletal muscle is its own discrete organ. Each is made up of several kinds of tissues. Nerves and blood supply In general, each muscle is served by at least one nerve, one artery, and one or more veins. Skeletal muscles tend to be highly vascularized, because they expend a lot of energy when they contract. Connective tissue sheaths Each muscle fiber is wrapped in an individual sheath to support the entire muscle. This is important since this will help prevent muscles from bursting, except for when a contraction is too strong. Sheath types Epimysium-dense overcoat that surrounds the whole muscle. Perimysium and fascicles- fascicles are groups of muscle cells that look like a bundle of sticks. Each fascicle is connected by dense fibrous tissue called the perimysium. Endomysium- a sheath of connective tissue that surrounds each individual muscle fiber. Consists of dense connective tissue. Remember Muscles span over joints. The less moveable part of a muscle is called the origin, and the more moveable area is the insertion. Muscles can attach directly, where muscle is fused directly to bone, or indirectly, where muscle attaches to bone through a tendon. Skeletal Muscles Microscopic Anatomy Inside skeletal muscle tissue Sarcoplasm- cytoplasm of muscle cells that contains high amounts of glycosomes, which store glucose for situations of high activity. Myoglobin- a red pigment that stores oxygen. *very similar to hemoglobin. myofibrils Rod-like fiber that run parallel to the muscle fibers. There are hundreds of thousands in each muscle cell. Striations Striations Are repeating series of dark and light bands in the muscle tissue. Dark bands are called A bands Light bands are called I bands H zone, M line, and Z disc H zone- light region in the middle of an A band. M line- bisection of the H zone Z disc- dark interruption in the I band. Sarcomere Sarcomere- the smallest functional unit of the muscle tissue. Thicker parts of the of tissue contain myosin, which runs down the length of the I band. Thin filaments contain actin, which extends through the I band and partly into the A band. Sarcoplasmic reticulum A type of smooth ER that interconnects each myofibril. Sliding Filaments Model of Contraction During muscle contraction, thin and thick filaments begin to over lap greatly. There is very little overlap in the thick and thin filament when the tissue is relaxed. For a skeletal muscle fiber to contract 1. It must be stimulated by a nerve ending. 2. It must generate an electrical current called an action potential through the sarcolemma (thin membrane that covers the muscle fiber). 3. a quick moment of calcium ions spike and act a trigger for muscle contraction to occur. Nerve stimulus of muscle contraction Nerve cells in skeletal muscle are called somatic motor neurons. The place where somatic motor neurons join to a muscle through large branches the axon of the nerve cell is called a neuromuscular junction. The nerves do not connect with each other and are separated by a space called a synaptic cleft. The cleft is filled with a gel like substance that contains acetylcholine (a neurotransmitter, abbreviated Ach). Acetylcholine (Ach) will land on the thick parts of the sarcolemma that contain large Ach receptors. When Ach diffuses across the membrane, it causes a change in electrical potential in the membrane. The enzyme acetylcholesterase will break down acetylcholine so that it does not continue muscle fiber contraction when not necessary. Channels involved in muscle contraction Calcium channels trigger the release of Ach. Ach binds to Ach receptors to open sodiumpotassium channels. High sodium influx causes electrical potential. The influx of sodium depolarizes other sodium channels and further depolarizes the sarcolemma and creates an action potential. Transmission of action potential along T-tubules causing a conformation change in voltage sensitive proteins which cause calcium channels to release calcium into cytosol. Muscle fiber contraction: cross bridge activity The attachment of myosin to actin is called cross bridge formation. It requires calcium to occur. When intracellular calcium is low, muscle is relaxed. Myosin has a special blocking molecule called tropomyosin. As calcium levels rise, tropomyosin will be removed by troponin. Troponin will not move the tropomyosin unless it binds to calcium ions to itself. This acts as the go ahead to move tropomyosin by guaranteeing enough calcium is present. Contraction of Skeletal Muscle Principles of Muscle Mechanics The principles of skeletal muscle contraction and general muscle contraction are basically the same. The force exerted by a muscle on an object is called muscle tension. The weight of the object is called the load. Contracting muscle does not always shorten and move a load. If you develop muscle tension and the load does not move, this is referred to as isometric contraction. If you develop muscle tension and the load you are attempting to lift or move does in fact move, it is referred to as an isotonic contraction. Motor units The functional unit of a motor neuron and all of the muscles it supplies is called a motor unit. When the motor neuron fires, all of the muscles it associates with will contract simultaneously. The number of muscle fibers to motor units tends to vary depending on the location and how precise control needs to be. The motor unit to muscle fiber ratio would be small when a fine muscle is required to move like an eyelash. Motor units and the muscles they control are not necessarily right next to each other. This is important because a single motor neuron will not cause a strong contraction in a single area of a muscle, but a weak contraction over the entirety of the muscle. Muscle twitch The response of a motor unit to an action potential of its motor neuron is called muscle twitch. It consists of three stages: • The latent period • The period of contraction • The period of relaxation The latent period Muscle tension increases but no response is seen. Lasts only a few milliseconds. Contraction period Cross bridges activate. If the build of tension is greater than the load being lifted, then muscle will shorten. Last from 10-100 milliseconds. Period of relaxation Calcium reenters the sarcoplasmic reticulum. Muscle contraction declines. Muscle relaxes and returns to latent state. Muscle Metabolism Muscle metabolism Muscles use ATP when they contract to perform the cross bridge movement, and for the operation of the calcium pumps of the sarcoplasmic reticulum. They can get that ATP from three different means: • Direct phosphorylation of creatine phosphate. • glycolysis (anaerobic) • Aerobic respiration Direct phosphorylation Reaction of creatine phosphate and ADP. creatine kinease Creatine phosphate + ADP Stats: creatine + ATP • Oxygen use: none • Products: 1 ATP per CP, creatine • How long energy lasts: 15 seconds. Anaerobic respiration: Glycolysis Forms ATP without oxygen, forming lactic acid in the process. Energy source is glucose. Glucose breaks down through glycolysis to create pyruvic acid, which is then broken down into pyruvic acid and released in the blood. Stats: • Oxygen use: None • Products: 2 ATP per glucose, lactic acid • How long it lasts: ~60 seconds Aerobic respiration Uses glucose and oxygen to produce ATP. Glucose + oxygen carbon dioxide + water + ATP Stats: • Oxygen Use: Yes (required) • Products: 32 ATP per glucose, carbon dioxide and water • How long it lasts: hours What is used during sports activities? The length of time a muscle can undergo aerobic respiration without tiring is called aerobic endurance. At the point where a muscle can no longer continue to support itself under just aerobic respiration, it will reach its anaerobic threshold. Muscle Fatigue Muscle fatigue is the physiological inability for a muscle to contract. Several things cause this including mineral imbalances. Contracture- the complete loss of ATP which prevents rapid muscle contraction. Permanent contracture= rigor mortis Temporary contracture= writer’s cramp. Oxygen Deficit For a muscle to return to normal, oxygen levels must be restored, lactic acid must be converted back to glucose or glycogen, and ATP and creatine phosphate must be resynthesized. An oxygen deficit is the oxygen needed to begin the healing process. Force of Muscle Contraction The force of a muscle contraction is affected by four things: 1. The number of muscle fibers stimulated. 2. The relative size of the fibers. 3. The frequency of stimulation. 4. The degree of muscle stretch. 1. Number of stimulated muscle fibers More fibers= bigger contraction 2. The size of muscle fibers The bulkier the muscle, the more tension it can have. Regular resistance exercise will lead to hypertrophy (bigger muscles) 3. Frequency of Fire Force generated by cross bridges is called internal tension. Internal tension stretches the connective tissue sheaths (called contractile components). They become tight and transfer tension called external tension to the load and when the contraction ends the noncontractile components help return the muscle to it’s original shape. 4. Degree of muscle stretch The degree of muscle stretch in relationship to the amount of force produced is called the length tension relationship. Velocity and duration of muscle fibers Speed of contraction: • slow fiber- high aerobic rate, low anaerobic rate, slow movement, long endurance. • fast fiber- high anaerobic rate, low aerobic rate, fast movement, low endurance. Major pathways of forming ATP: • Oxidative fibers- rely on oxygen and ATP generation • Glycolytic fibers- relies on anaerobic glycolysis to form ATP. So…… So you can classify skeletal muscle fibers as: • Slow oxidative fibers • Fast oxidative fibers • Fast glycolytic fibers To learn these, refer to the velocity of the muscle fiber, and the pathways for forming ATP. Muscle Adaptation to exercise Aerobic/endurance exercise will (over time) increase the number of capillaries in the muscle, causing a slight increase in muscle size, however… If you are looking for bulging muscle than anaerobic processes like weight lifting will increase muscle size and give you that ripped look. That ripped look Overuse injuries Muscles do not heal over night. Prolonged and extended use repeatedly under high intensity can cause muscle to tear, causing intense pain and temporary loss of muscle function. Smooth Muscle Smooth muscle Lines the walls of hollow organs (like the intestines). The only major exception is the heart, which is made up of cardiac tissue. Microscopic anatomy of smooth muscle In smooth muscle there is no sheath that covers all of the smooth muscle, however, there is a small amount of connective tissue (endomysium) that is found between smooth muscle tissue. Smooth muscle fibers will alternate contraction and relaxation in the tissues. When muscle contracts, the organs will dilate and shorten, pushing substances (like food) downward. This action is known as peristalsis. Smooth muscle is autonomic (automatic) and is involuntarily controlled. Myosin arrangement in smooth muscle Myofilaments in the smooth muscle have four major differences when comparing them to skeletal muscle. They are: • 1. Thick filaments are fewer but have myosin heads along their entire length. • 2. No troponin complex within the filaments. • 3. Thick and thin filaments are arranged vertically, which cause muscle contraction to move in a cork screw like manner. • 4. Smooth muscle fibers contain a lattice-like arrangement of noncontractile intermedeiate filaments that resist tension. Smooth muscle contraction Smooth muscle is able to contract in a “smooth” and synchronized fashion due largely to the fact that there are gap junctions that link the cells in the smooth muscle tissue together allowing for an action potential to move across the tissue easily. Contraction of smooth muscle is similar to that of skeletal muscle in that: • 1. Actin and myosin interact by the sliding filament mechanism. • 2. The final trigger for contraction is a rise of intracellular calcium. • 3. The sliding process is energized by ATP. Regulation of Contraction Two major factors regulate contraction of smooth muscle. They are: • 1. Neural regulation- just like skeletal muscle. AP is generated by Neurotransmitter binding Increase in intracellular calcium Ach diffusion through muscle 2. Hormones and local chemical factors- smooth muscles that have no nerve supply will rely on a change in chemicals or hormones to signal contraction. Types of Smooth Muscle Single unit smooth muscle- lines the walls of hollow organs except the heart. Multiunit smooth muscle- lines the insides of the lungs and in large arteries, eye muscles, and arrector pilli muscles. Developmental Aspects of Muscle Muscle tissue progenitor Most all muscle tissue cells are produced by a cell called a myoblast. When myoblast become innervated, they release a growth hormone called agrin is released. The imbedded neurons in the myoblast will help determine the type of muscle tissue.
