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Myology Obert Tada Department of Livestock & Wildlife Management Midlands State University Muscle A contractile form of tissue. It is one of the four major tissue types, the other three being epithelium, connective tissue and nervous tissue. Muscle contraction is used to move parts of the body, as well as to move substances within the body. Types of Muscles There are three general types of muscle: Cardiac muscle is a specalized kind of muscle found only within the heart. Skeletal muscle or "voluntary muscle" is anchored by tendons to bone and is used to effect skeletal movement such as locomotion. Smooth muscle or "involuntary muscle" is found within structures such as the intestines, throat and blood vessels. Types of Muscles (cont’d) Cardiac and skeletal muscles are "striated" in that they contain sarcomeres and are packed into highly regular arrangements of bundles; smooth muscle has neither. Striated muscle is often used in short, intense bursts, whereas smooth muscle sustains longer or even near-permanent contractions. Skeletal muscle is further divided into two subtypes: Skeletal Muscle Divisions Type I; slow oxidative, "slow twitch", or "red" muscle dense with capillaries and is rich in mitochondria and myoglobin, giving the muscle tissue its characteristic red color can carry more oxygen and sustain aerobic activity are associated with endurance; produce ATP more slowly. Type II; glycolytic, "fast twitch", or "white" muscle less dense in mitochondria and myoglobin can contract more quickly and with a greater amount of force than Type I muscle metabolize ATP more quickly can only sustain short, anaerobic bursts of activity before a buildup of lactic acid in tissue which begins to interfere with muscular contraction and causes pain. Characteristics of muscle types Fibre Type Type I fibres Type II A fibres Type II B fibres Contraction time Slow Fast Very Fast Size of motor neuron Small Large Very Large Resistance to fatigue High Intermediate Low Activity Used for Aerobic Long term anaerobic Short term anaerobic Force production Low High Very High Mitochondrial density High High Low Capillary density High Intermediate Low Oxidative capacity High High Low Glycolytic capacity Low High High Major storage fuel Triglycerides CP, Glycogen CP, Glycogen Anatomy Muscle is composed of muscle cells (also called "muscle fibers"). Within the cells are myofibrils; myofibrils contain sarcomeres, which are composed of actin and myosin. Individual muscle cells are lined with endomysium. Muscle cells are bound together by perimysium into bundles called fascicles; the bundles are then grouped together to form muscle, which is lined by epimysium. Muscle Fibre each muscle fibers contains: an array of myofibrils that are stacked lengthwise and run the entire length of the fiber. mitochondria an extensive endoplasmic reticulum many nuclei (becoz each muscle fiber develops from the fusion of many cells, the myoblasts). not a single cell, its parts have special names: sarcolemma for plasma membrane sarcoplasmic reticulum for endoplasmic reticulum sarcosome for mitochondrion sarcoplasm for cytoplasm Muscle Fibre striated appearance of the muscle fiber is created by a pattern of alternating dark A bands (bisected by H zone) and light I bands (bisected by Z line). each myofibril is made up of arrays of parallel filaments. thick filaments, 15nm diameter, composed of the protein myosin. thin filaments, 5nm diameter composed of protein actin along with smaller amounts of two other proteins (troponin and tropomyosin). Anatomy (cont’d) Muscle spindles are distributed throughout the muscles and provide feedback sensory information to the central nervous system. Skeletal muscle is arranged in discrete groups, examples of which include the biceps brachii. It is connected by tendons to processes of the skeleton. In contrast, smooth muscle occurs at various scales in almost every organ, from the skin in which it controls erection of body hair to the blood vessels and digestive tract in which it controls the caliber of a lumen and peristalsis. Micro-structure of Muscle Skeletal Muscle Attachment Physiology The three types of muscle have significant differences, but all use the movement of actin against myosin to produce contraction and relaxation. In skeletal muscle, contraction is stimulated by electrical impulses transmitted by the nerves, the sensory nerves and motoneurons in particular. All skeletal muscle and many smooth muscle contractions are facilitated by the neurotransmitter acetylcholine. Muscles and muscular activity account for most of the body's energy consumption. Muscles store energy for their own use in the form of glycogen, which represents about 1% of their mass. This can be rapidly converted to glucose when more energy is necessary. Nervous Control (sensory leg) Vertebrates move muscles in response to voluntary and autonomic signals from the brain. Deep muscles, superficial muscles, muscles of the face and internal muscles all correspond with dedicated regions in the brain. muscles react to reflexive nerve stimuli that do not always send signals all the way to the brain. Nerves that control skeletal muscles in mammals correspond with neuron groups along the primary motor area of the brain's cerebral cortex. Commands are routed though basal ganglia and modified by input from the cerebellum before being relayed through the pyramidal tract to the spinal cord and from there to the motor end plate at the muscles. Deeper muscles such as those involved in posture often are controlled from nuclei in the brain stem and basal ganglia. Nervous Control (motor leg) Muscle memory, Proprioception is the "unconscious" awareness of where the various regions of the body are located at any one time. Several areas in the brain coordinate movement and position with the feedback information gained from proprioception. The cerebellum and nucleus ruber in particular continuously sample position against movement and make minor corrections to assure a smooth projection. Muscle contraction Occurs when a muscle cell (a muscle fiber) shortens. Locomotion is possible only through the repeated contraction of many muscles at the correct times. For most muscles, contraction occurs as a result of conscious effort originating in the brain. The brain sends signals, in the form of action potentials, through the nervous system to the motor neuron that innervates the muscle fiber. However, some muscles (such as the heart) do not contract as a result of conscious effort (autonomic). Muscle contraction Also, it is not always necessary for the signals to originate from the brain. Reflexes are fast, unconscious muscular reactions that occur due to unexpected physical stimuli. The action potentials for reflexes originate in the spinal cord instead of the brain. There are three general types of muscle contractions, skeletal muscle contractions, heart muscle contractions, and smooth muscle contractions. Skeletal muscle contractions Steps: 1. 2. 3. 4. An action potential reaches the axon of the motor neuron. The action potential activates voltage gated calcium ion channels on the axon, and calcium rushes in. The calcium causes acetylcholine vesicles in the axon to fuse with the membrane, releasing the acetylcholine into the cleft between the axon and the motor end plate of the muscle fiber. The acetylcholine diffuses across the cleft and binds to nicotinic receptors on the motor end plate, opening channels in the membrane for sodium and potassium. 5. 6. 7. Sodium rushes in, and potassium rushes out. because sodium is more permeable, the muscle fiber membrane becomes more positively charged, triggering an action potential. The action potential on the muscle fiber causes the sarcoplasmic reticulum to release calcium. The calcium binds to the troponin present on the thin filaments of the myofibrils. The troponin then allosterically modulates the tropomyosin. Normally the tropomyosin physically obstructs binding sites for cross-bridge; once calcium binds to the troponin, the troponin forces the tropomyosin move out of the way, unblocking the binding sites. The cross-bridge (which is already in a ready-state) binds to the newly uncovered binding sites. It then delivers a power stroke. 9. ATP binds the cross-bridge, forcing it to change conformation in such a way as to break the actin-myosin bond. Another ATP is split to energize the cross bridge again. 10. Steps 7 and 8 repeat as long as calcium is present on thin filament. 11. All the time, the calcium is actively pumped back into the sarcoplasmic reticulum. When no longer present on the thin filament, the tropomyosin changes conformation back to its previous state, so as to block the binding sites again. The cross-bridge ceases binding to the thin filament, and the contractions cease. 8. Smooth muscle contraction 1. Contractions are initiated by an influx of calcium which binds to calmodulin. 2. The calcium-calmodulin complex binds to and activates myosin light-chain kinase. 3. Myosin light-chain kinase phosphorylates myosin light-chains, causing them to interact with actin filaments. • This causes contraction. Smooth muscle contraction The calcium ions leave the troponin molecule in order to maintain the calcium ion concentration in the sacoplasm. As the calcium ions are being actively pumped by the calcium pumps present in the membrane of the sarcoplasmic reticulum creating a deficiency in the fluid around the myofibrils. This causes the removal of calcium ions from the troponin. Thus the tropomyosin-troponin complex again covers the binding sites on the actin filaments and contraction ceases. Control of Cardiac Muscle Contraction Cardiac or heart muscle resembles skeletal muscle in some ways: it is striated and each cell contains sarcomeres with sliding filaments of actin and myosin. myofibrils of each cell (and cardiac muscle is made of single cells — each with a single nucleus) are branched. The branches interlock with those of adjacent fibers by adherens junctions. These strong junctions enable the heart to contract forcefully without ripping the fibers apart Control of Cardiac Muscle action potential that triggers heartbeat is generated within the heart itself. Motor nerves (of the autonomic nervous system) run to the heart but simply modulate — increase or decrease — the intrinsic rate and the strength of the heartbeat. even if the nerves are destroyed (as they are in a transplanted heart), the heart continues to beat. action potential that drives contraction of heart passes from fiber to fiber through gap junctions. Control of Cardiac Muscle Significance: All the fibers contract in a synchronous wave that sweeps from the atria down through the ventricles and pumps blood out of the heart. Anything that interferes with this synchronous wave (such as damage to part of the heart muscle from a heart attack) may cause the fibers of the heart to beat at random — fibrillation. the refractory period in heart muscle is longer than the period it takes for the muscle to contract (systole) and relax (diastole). Control of Cardiac Muscle Cardiac muscle has a much richer supply of mitochondria than skeletal muscle. reflects its greater dependence on cellular respiration for ATP. Cardiac muscle has little glycogen and gets little benefit from glycolysis when the supply of oxygen is limited. thus anything that interrupts the flow of oxygenated blood to the heart leads quickly to damage — even death — of the affected part (heart attack). Control of Smooth Muscle Smooth muscle is made of single, spindle-shaped cells. each smooth muscle cell contains thick (myosin) and thin (actin) filaments that slide against each other to produce contraction of the cell. thick and thin filaments are anchored near the plasma membrane (with the help of intermediate filaments). Smooth muscle (like cardiac muscle) does not depend on motor neurons to be stimulated. However, motor neurons (of the autonomic system) reach smooth muscle and can stimulate it — or relax it — depending on the neurotransmitter they release (e.g. noradrenaline or nitric oxide, NO). Control of Smooth Muscle Smooth muscle can also be made to contract by other substances released in the vicinity (paracrine stimulation) e.g.: release of histamine causes contraction of the smooth muscle lining air passages (triggering an attack of asthma) by hormones circulating in the blood E.g.: oxytocin reaching the uterus stimulates it to contract to begin parturition. The contraction of smooth muscle tends to be slower than that of striated muscle. It also is often sustained for long periods. This, too, is called tonus with a mechanism not like that in skeletal muscle.