Musculoskeletal System IV - LECTURE NOTES

Musculoskeletal System IV - LECTURE NOTES - Musculoskeletal...

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Unformatted text preview: Musculoskeletal System IV Ligaments, Tendons and Muscle A. Ligaments and Tendons Ligaments and tendons are flexible structures that Mr the musculoskeletal system and are extremely strong in resisting tensile loads. Ligaments and tendons stabilize the skeleton and without them we would be mechanically hopeless. Tendon and ligament are 50th composed of dense fibrous connective tissue but they differ in morphology and fianctionmuict their .. relative movements. Tendons provide the connection between muscle and bone. I. Ligaments i. General Ligaments are tough bands conn ctive tissue that bind bones together and as such, originate and insert on bone. Ligaments function to maintainsorrect bone and j_o_int_ geometry. Ligaments and joint capsules are often referred to as passive jointsta’biljzers. WMth the articular contours, ligaments largely determine the range of motion at a joint. Ligament damage can occur when a joint is forced beyond this functional range. Many ligaments represent thickenings within the 'oint ca sule and their 'margins may be indistinct, and therefore (inflammable vammiigamgms. DWS may result in abnormal joint motion which can ultimately lead to cartilage degradation. "Jfi ii. Ligament Composition 55-65% H20 Collagen I (some III) 70-80% of dry weight Elastin 10-15% Proteoglycans 1-3 % Collagen is synthesized by fibroblasts which also possess the machinery to break down and remove old collagen. II. Tendons i. Gaa'al . Tendons connect muscle to bone and transmit forces developed by muscle contractions across joints to produce motion (or stabilize them). They originate in muscle, cross at least one joint (sometimes several) and insert into bone. Each muscle generally has two tendons, one proximal and one distal. The connection between muscle and tendon is the myotendinous junction and between the tendon and bone is the osteotendinous junction. e osteotendinous 'unction associated with the least movable , bone is commonly referred to as the g 111 of the muscle, and the junction with the more \ l movable bone is referred to as the WNOM. This can be somewhat subjective since Wdepending on the moVement being performed! Tendons vary considerably in size and shape and are brilliant white in color. Some are wide and flat, others are cylindrical, ribbon or fan shaped. Muscles designed to create powerful, resistive forces like the quadriceps muscles have short and broad tendons. In contrast, those designed for delicate moVements, like the finger flexors, have long and thin tendons. R- Tendons also play a role in proprioception. Mechanoreceptors in tendons are called golgi tendon organs. They are sensitive to the tension within a tendon, respond to force generated by the muscle, and provide feedback control that alters muscle activity. The golgi tendon organs are located in tendons near their insertion into the muscle. When the tendon is stretched, the sensory axons in the tendon organs excite both excitatory and inhibitory intemeurons in the spinal cord which inhibit the muscle in which tension has increased and excite the antagonistic muscle. , ii. Tendon Composition The composition of tendons is similar to that of ligaments with the exception of elastin which comprises less that 3% of the dry weight. In most locations the proteoglycan concentration is less than 2%. However, where tendons curve around bony surfaces and significant compressive forces occur, the cells respond by producing more proteoglycans. The increased proteoglycan concentration gives the tendon a more cartilagenous quality that helps to resist compression, decrease friction and enhance motion. Just like in the ligament, tendon collagen fibers demonstrate a wavy crimp pattern in the absence of load. 65-70% H20 Collagen I (some 111 and V) 75-85% of dry weight Elastin <3% Proteoglycans 1-2 % Fibroblasts in tendon are called tenoblasts and tenocytes and comprise 90-95% of the cells of the tendon. The other 5-10% includes, chondrocytes at pressure and insertion sites, synovial cells in tendon sheaths and vascular endothelial cells and smooth muscle cells of the arterioles in the endotenon and epitenon (these are discussed below). Newborn tendon has a very high cell to matrix ratio but in adults the cell-to matrix ratio decreases and the cells are long and elongated. B. Muscle Muscular tissues are found in many of the organ systems of the body and function to produce directed and organized movement- The contractile element of muscle is the muscle cell (myocyte or muscle fiber) which is highly specialized for this purpose and generally aligned in the direction of movement. All muscle cells consume body fuel (sugars and fats) to create a force by contracting and thus do mechanical work. Three structurally and functionally distinct types of muscle are found in vertebrates: 1. Smooth muscle: involuntary, non-striated 2. Cardiac muscle: involuntary, striated 3. Skeletal muscle: voluntary, striated 1. Smooth Muscle Smooth muscle is found as sheets or bundles in the walls of the gut, bile duct, ureters, urinary bladder, respiratory tract, uterus and blood vessels. The function of smooth muscle is to alter the volume of viscera, control intraluminal flow, pressure and the mixing of fluids and gases. Characteristics: . Spindle shaped cells, 5-20um in diameter, 20pm to 1mm or more in length One centrally placed nucleus Latticework of myosin, actin and intermediate filaments No striations Surrounded by a fine network of reticular fibers (type III collagen coated with proteoglycans and glycoproteins.) secreted by the muscle cells, blood vessels and nerves Innervated by the autonomic nervous system 0 Visceral or involuntary muscle 0 Some are connected by gap junctions which permit synchronous contraction o Arranged in single or multiple layers 2. Characteristics of Cardiac Muscle Cells: Cardiac muscle comprises the muscle of the walls of the heart. Innervated by the autonomic nervous system, involuntary control Cells are branched cylinders, 85-100um long, lSum in diameter Single central nucleus Surrounded by a fine network of reticular fibers and collagen Organization of contractile fibers is like skeletal muscle, fibers are striated Cells are joined end to end by intercalated disks Connected by gap junctions 3.0 Skeletal Muscle All multicellular animals rely on striated muscle to power their locomotion. Skeletal muscles exist in a variety of shapes and sizes including large and small, long and short, but their underlying microscopic structure is identical. a. Skeletal Muscle Fibers Skeletal muscle cells are huge multinucleate cells, commonly referred to as muscle fibers. The long thin muscle fibers of skeletal muscle are huge single cells that form from the fusion of many separate cells. The muscle cell is highly specialized to produce force and movement. Muscle "F iber Large multinucleate cell lO-lOOum in diameter, several centimeters long Up to 100 nuclei lie just beneath plasma membrane Plasma membrane is referred to as the sarcolemma Sarcolemma is surrounded by a basal lamina Myofibrils Basic contractile elements of the muscle cell Cylindrical, 1-2 pm in diameter, as long as the fiber Makes up the bulk of the cytoplasm Consist of a long repeated chain of tiny contractile units called sarcomeres. Fiber diameter determines its strength, and if fiber diameter is altered in mature muscle this suggests that the level of muscle use has changed. Muscle fiber length is highly variable and depends largely on the muscle architecture. Fiber length influences fiber contraction velocity, and the distance over which a fiber can shorten. b. T Tubules and Triads The sarcolemma has invaginations that extend the surface of the membrane deep into the muscle cell forming radially projecting tubes called transverse tubules or T tubules. Along its length the T-tubule associates with two cisternae which are specialized (expansions) regions of the sarcoplasmic reticulum (SR) which is the muscle equivalent of the endoplasmic reticulum and stores Ca2+. The SR forms a network of flattened tubules which surround each myofibril. The combination of the T tubule membrane and its two neighboring cistemae is called a triad. “Ihmm— mw'w-u‘ Sareolemma Tamlnalwb. clstemae ‘* Transverse tubule Sarco Iasrnic . ‘ reticu um 11 > ' 1 ; “73,4 K Mitochondria <l it: x : .hp‘ ’ f A B A. Organization of T-tubules and sarcoplasmic reticulum (SR) in a skeletal muscle fiber. T tubules penetrate to the center of the muscle cell and surround the individual myofibrils. The T tubule associates with two cisternae, specialized expanded regions of the SR. B. The combination of the T tubule membrane and its two neighboring SR cisternae is called a triad which plays a critical role in the coupling of excitation to contraction. Lodish et al Molecular Cell Biology (1995) Scientific American Books. c. Contractile Machinery i. Sarcomere The sarcomere is both the structural and the functional unit of skeletal muscle. A sarcomere is about 2.2um long and is formed from a miniature, precisely ordered array of parallel and partly overlapping thin and thick filaments. A chain of sarcomeres constitutes a myofibril, and the ordered arrangement of the filaments within each sarcomere gives the myofibril and thus the muscle fiber, its striated appearance. Each sarcomere contains two types of filaments; thick filaments, composed of myosin II, and thin filaments, containing actin. iii. Banding Patterns The partial interdigitation of the thick and thin filaments results in alternating light and dark bands along the axis of the myofibril. The light bands represent regions of the thin filament that do not lie alongside the thick filaments and are known as I hands (because they are isotropic to polarized light). The Z- disk is visible as a dark perpendicular line at the center of the I band. The dark bands represent the myosin filaments and are known as A hands (because they are anisotropic to polarized light). During contraction the A bands are unchanged in length whereas the I bands shorten. The A band is bisected by the M line. M line, or midline, is the location of specific proteins that link adjacent thick filaments to each other. Light micrograph (A) and electron micrograph (B) of longitudinally oriented skeletal muscle and schematic representation (C) of a sarcomere. From Dellmann and Eurell Textbook of Veterinary Histology (1998) Lippincott Williams and Wilkins. O Q l I I 5 I »§ ulsoslil "IIIQI‘ \O‘D5'if d. Excitation Contraction Coupling Excitation-contraction coupling is the process by which excitation triggers the increase in intracellular calcium and it is this increase in calcium that triggers contraction. The myocyte has control mechanisms to regulate calcium entry into the sarcoplasm as well as calcium removal once the stimulus for muscle contraction subsides. d.i. Irmervation: The Stimulus Motor Neurons Each fiber of a muscle can contribute to force production only if it is recruited by the brain. As such, the strength of contraction can be increased simply by engaging the simultaneous contraction of more cells. One motor nerve can branch into tens or even a thousand branches, each one terminating on a different muscle fiber. Cell body is in the ventral horn of the spinal cord Each cell body has one axon Peripheral nerve composed of many axons Axons branch near muscle Each branch innervates one muscle fiber usually midway along its length A motor unit with its cell body in the ventral horn of the spinal cord, sends out an axon that bifurcates to supply several muscle fibers (a motor unit). Boron et a1. Medical Physiology (2003) Saunders. Motor Units One motor neuron plus all of the fibers that it innervates is called a motor unit: ' o A single muscle can consist of hundreds of motor units Motor Unit = motor neuron + muscle fibers One nerve innervates many muscle fibers One muscle fiber is innervated by only one nerve Motor unit muscle fibers are intermingled with fibers of other motor units so motor units have overlapping territories Motor units generally contain only one fiber type 0 Muscle will contain a combination of slow and fast motor units. A. Single motor unit consisting of one motor neuron and the muscle fibers it innervates. B. Two motor units and their intermingled fibers. Vander et a1. (1995) Human Physiology McGrawHill. Neuromuscular Junction (Motor End Plate) An axon makes a single point of synaptic contact with a skeletal muscle fiber in a region called the neuromuscular junction. As it approached the sarcolemma, the axon divides into a tree-like patch of unmyelinated nerve processes with bulb shaped endings (boutons) that are referred to as the terminal arborization. d.ii. Ca2+ Release Mechanism Action potentials originate at the neuromuscular junction, propagate along the skeletal muscle membrane and down the T-tubules and causes release of calcium from the sarcoplasmic reticulum. d.iii. Control of Contraction Actin, T ropomyosin and T roponin Complex The backbone of the thin filament is a double stranded alpha helical polymer of actin but it also consists of tropomyosin and troponin. The coordinated interaction among troponin, tropomyosin and actin allows the actin-myosin interactions to be regulated by changes in calcium. ' Tropomyosin Two identical a helices that coil around each other and sit near the two grooves formed by the two alpha helical actin molecules. Runs full length of actin The role of tropomyosin is to interfere with the binding of myosin to actin. T roponin Heterotrimer consisting of: T: binds to tropomyosin C: binds Ca2+, 2 low affinity Ca2+ binding sites I: binds toactin, inhibits contraction Tropomyosin binds lengthwise along actin filaments and is associated with the troponin heterotrimer. In the absence of Ca2+, the tropomyosin-troponin complex blocks the binding of myosin to actin. Binding of Ca2+ to Troponin C shifts the complex, relieving this inhibition and allowing contraction to proceed. This represents the mechanism by which cross bridge cycling is regulated. If unregulated, the cycling would continue until the myocyte was depleted of ATP. "" Association of tropomyosin and troponins with actin filaments (A) Tropomyosin binds lengthwise along actin filaments and is associated with a complex of three troponins: troponin I (TnI), troponin C (TnC), and troponin T (T nT). In the absence of Ca2+, the . tropomyosin-troponin complex blocks the ' :...~. Myminlimhng binding of myosin to actin. Binding of Ca)+ to “'“W' TnC shifts the complex, relieving this inhibition and allowing contraction to proceed. (B) Cross-sectional view. Cooper et al. The cell: A molecular approach (2000) Sinauer. lmpnmy-xin —-- Inl~...‘ ‘lrnpuniu __ cmwlex "I: \ TuT' an Myoun binding mp. ' MYW" head Irommyosln d.iv. Force Generation: The sliding-filament model of contraction in striated muscle. A sarcomere is about 2.4 pm long at rest. It can be extended reversibly to more than 3 pm and can shorten to less than 2 mm. The shortening of the sarcomere is achieved by the actin and myosin filaments sliding over one another. Myosin has globular heads which bind to actin and then rotate to pull actin filaments towards the‘center of the sarcomere. Energy is supplied by ATP. When muscle is stimulated to contract it exerts a force tending to pull the attachment points at either end towards each other. This force is referred to as the tension developed by the muscle. The total force generated by a muscle is the sum 'of the forces generated by many independently cycling actin-myosin cross bridges. The number of simultaneously cycling cross bridges depends on the initial length of the muscle fiber and on the frequency of muscle cell stimulation. Z disc Actin Myosin Filament slidng -——-—> <— l‘tomrxfim Sliding-filament model of muscle contraction The actin filaments slide past the myosin filaments toward the middle of the sarcomere. The result is shortening of the sarcomere without any change in filament length. Cooper et al. The cell: A molecular approach (2000) Sinauer. 3.1 Parts of a Gross Muscle Each muscle has an origin (one or more) and an insertion (one or more). a. Origin: Usually has no tendon (if it does have a tendon it is usually short), origin is usually most proximal attachment of the muscle, origin is usually the least moveable part of the muscle, may be multiple or a broad flat tendon (aponeurosis). ‘ b. Belly: usually single but may be double or multiple. Examples include the digastricus (two bellies), the triceps brachii and the quadriceps femoris. c. Insertion: usually has a tendon which is commonly longer than the tendon of origin, usually the most distal attachment, usually the most moveable part, insertion may be multiple and may be aponeurotic. 3.2 Skeletal Tissue Organization Individual muscle fibers are bound'together in bundles or fascicles which make up the muscle. Within a fascicle, the sarcolemma of individual fibers is surrounded by a basal lamina and satellite cells and then further invested by a delicate layer of reticular fibers, the endomysium. The endomysium also contains an extensive capillary network and nerve fibers. Each fascicle is surrounded by dense irregular tissue called the perimysium which also contains blood vessels and nerves. The whole muscle is surrounded by a dense, irregular layer of connective tissue, which encapsulates the whole muscle, and is called the epimysium. All of the connective tissue layers are continuous with one another and with the extracellular matrix of tendons, thereby providing the means by which contractile forces are transmitted to other tissues. 3.3 Muscle Architecture Skeletal architecture can be defined as the arrangement of muscle fibers relative to the axis of force generation. An understanding of muscle architecture allows us to understand the functions of different muscles. There are three general types of fiber architecture: Parallel or Longitudinal: Muscles with fibers that extend parallel to the muscle force generating axis. Unipennate: Muscles with fibers that are oriented at a single angle relative to the force generating axis. The angle between the fiber and the force generating axis varies from O-30°. ' Multipennate: Muscles are composed of fibers that are oriented at several angles relative to the axis of force generation. Pennation can increase the packing of many fibers into a smaller space compared to fibers packed without pennation which is an advantage for areas of the body where space is at a premium. As the pennation angle increases, the force generated by each individual fiber decreases. This loss of force is counterbalanced by the fact that more muscle fibers can be packed into a smaller space. Fiber arrangement subserves function: parallel muscle (strap muscles) have a relatively long contractile distance but are relatively weak, whereas pinnate muscles have a short contractile distance and are relatively strong. As a general rule, the cross sectional area of a muscle is directly proportional to its maximal potential tension. Length of muscle fibers is also related to the velocity (speed) of shortening. Muscles with long fibers (strap muscles) will contract in the same amount of time as muscles with short fibers, but since the direction of shortening in the strap muscle is greater, the velocity of shortening is greater (Fig. 10.13). Muscle architectural types. ML=muscle length, FL = fiber length. Lieber (2002) Skeletal Muscle Structure, Function and Plasticity. Lippincott Williams and Wilkins. The arrangements of the attachments of muscle to bones at a joint also affect the force and speed of movements. The joint can be seen as a lever system where the relative length of the lever arms affects the force, speed and range of movements around that joint (Fig. 10.14). ...
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