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University of British Columbia
Cellular, Anatomical and Physiological Sciences
CAPS 391
Bruce Matheson

MUSCULAR SYSTEM Functions: Movement of the body > skeletal muscles attached to bones responsible for body movement (walking, running, chewing) Maintenance of posture > skeletal muscles maintain tone, keeps us sitting/standing erect Respiration > skeletal muscles of thorax carry out respiratory movements Production of body heat > Skeletal muscles contract, give off heat as by-product, maintains body temp. Communication > Skeletal muscles involved in speaking, writing, typing, gesturing, smiling Constriction of organs and vessels > contraction of smooth muscles in walls of eternal organs/vessels cause structures to constrict. Propels/mixes food and water in digestive tract, removes materials from organs, regulates blood flow through vessels. Contraction of the heart > Contraction of cardiac muscles causes heart to beat, propelling blood to parts of the body. Properties of muscle: Contractility: ability of muscle to shorten forcefully. Causes structures that its attached to to move or increase pressure inside a hollow organ/vessel. Shortens forcefully, lengthens passively (other forces such as gravity, contraction of opposing muscle, pressure of fluid in hollow organ cause it to lengthen) Excitability: Capacity of muscle to respond to a stimulus. (from nerves we consciously control) Ex. Waving to a friend. Smooth/cardiac muscle contract without stimuli, respond to stimulation by nerves/hormones. Extensibility: muscle can be stretched beyond its normal resting length and still be able to contract (stretch to reach and grab a dropped pencil) Elasticity: Ability of muscle to recoil to its original resting length after it’s been stretched. Ex. Taking deep breath because exhalation is recoil of respiratory muscles back to resting position). Types of Muscle Tissue Skeletal > 40% of body weight, body locomotion, facial expression, posture, respiratory functions. Nervous system voluntarily or consciously controls these. Smooth Muscle > widely distributed, walls of hollow organs and tubes, interior of eye, walls of bvs. Propels urine through urinary tract, mixes food in stomach/small intestine, dilates/constricts pupils. Cardiac Muscle > only in heart, provides major force for moving blood through circulatory system, autorythmic (contract spontaneously at somewhat regular intervals/nervous and hormonal stimulation not always required for contraction). Not consciously controlled by nervous system, controlled involuntarily by endocrine/autonomic nervous systems. Skeletal Muscle Structure Skeletal muscle fibers and small CT, bvs, nerves. CT fibers surrounding muscle extend to become tendons. Composed of bundles (Fasciculi), each is surrounded by heavier CT layer (perimysium), entire muscle surrounded by CT (epimysium). Epimysium > dense collagenous CT Fascia > CT sheaths within the body Muscular fascia > superficial to epimysium, separates muscles or muscle groups (dense irregular collagenous CT). Motor Neurons > stimulate muscles to contract. Cell bodies are in brain and spinal cord, axons extend to skeletal muscle fibers through nerves. Axons branch in perimysium projecting towards centre of muscle fibers. Contact points between muscle fibers (synapses/neuromuscular junctions). Each motor neuron innervates more than one muscle fiber. Artery and 1 or 2 veins extend with a nerve through CT layers of skeletal muscles. Supply capillary beds surrounding muscle fibers, blood carried away by veins. Skeletal Muscle Fibers Single, long, cylindrical, cell containing several nuclei located around periphery near plasma membrane. Large muscles have large diameter fibers, small have small diameter. Contains mixture of small/large diameter fibers. Striated > alternating light and dark bands in longitudinal section, striped appearance * Origin of Muscle Fibers Fibers develop from Myoblasts > fusion of myoblast precursor cells result in multiple nuclei. Contractile proteins accumulate within cytoplasms of myoblasts > form muscle fibers, nerves grow into area and innervate developing muscle fibers. Hypertrophy > enlargement of muscles after birth results from increase in size of muscle fiber* NOT NUMBER * Histology of Muscle Fibers Sarcolemma > plasma membrane of muscle fiber surrounded by 2 delicate CT layers. External lamina > deeper, thinner layer of reticular (collagen) fibers, VERY thin Endomysium > reticular fibers, much thicker, second layer Transverse Tubules (T Tubules) > along the surface of the sarcolemma, tubelike invaginations, at regular intervals along muscle fiber, extending inward to it. Associated with Sarcoplasmic Reticulum Sarcoplasmic Reticulum > smooth endoplasmic reticulum associated w/ t tubules Sarcoplasm > aka CYTOPLASM in muscle cells (other organelles (mitochondria, glycogen granules) packed in cells) Sarcoplasm has Myoflibrils > bundles of protein filaments, threadlike structures, extend from one end of muscle fiber to another. Contains 2 protein filaments: Myofilaments > Actin + Myosin Actin Myofilaments > thin Myosin Myofilaments > thick Actin + Myosin form units called Sarcomeres* joined end to end to form myofibrils. Actin + Myosin Myofilaments Actin > composed of 2 strands of fibrous actin (F actin), series of Tropomyosin molecules and Troponin Molecules. 2 strands of F actin > coiled forming double helix, extends the length of actin myofilament. Each F actin strand > polymer of 200 globular units called Globular Actin (G Actin) G actin monomer > has active site where myosin molecules bind during muscle contraction Tropomyosin > elongated protein, winds along grove of
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