Muscular System

1. Muscular System

2. Muscle • Makes up about 40-50 % of body weight • Myo – muscle • Myology – study of muscles • Sarco – “flesh”

3. Types of muscle: • Skeletal muscle • Attached primarily to bones • Striated • Voluntary • Cardiac muscle • Wall of the heart • Striated • Involuntary • Autorhythmicity – the ability to contract by itself • Influenced by neurotransmitters and hormones

4. Smooth muscle • Located in viscera – blood vessels, skin, abdominal organs, etc. • Nonstriated • Involuntary • May have autorhythmicity • Influenced by neurotransmitters and hormones

5. Functions of muscle tissue • Motion • Movement of substances within the body • Stabilizing body position and regulating organ volume • Thermogenesis (heat production) • Heat is a “by-product” of muscle activity

6. Characteristics of Muscle Tissue • Excitability (irritability) – the ability of muscles and nerves to respond to a stimulus by producing electrical signals called impulses or action potentials • Conductivity – the ability of a cell to conduct action potentials (electrical current) along the plasma membrane • Contractility • Extensibility • Elasticity

7. Anatomy and innervation of skeletal muscle tissue • Connective tissue components: • Fascia (“bandage”) –sheet or band of fibrous C.T. under the skin or around organs • Superficial fascia (subcutaneous fascia): • Areolar C.T. and adipose tissue • Stores water and fat • Reduces heat loss (insulates) • Protects against trauma • Framework for nerves and blood vessels

8. Deep fascia: Dense irregular C.T. – holds muscles together and separates them into groups 3 layers: Epimysium – surrounds the whole muscle Perimysium – separates muscle into bundles of muscle fibers – fascicles Endomysium – covers individual fibers

11. These three layers come together to form cords of dense, regular connective tissue called tendons. Tendons attach muscle to the periosteum of bones. • When the connective tissues form a broad, flat layer, the tendon is called an aponeurosis.

14. Microscopic Anatomy • Muscle cells are called muscle fibers or myofibers • Plasma membrane – sarcolemma • Cytoplasm – sarcoplasm • Myoblasts fuse to form one myofiber – several nuclei • Myofibrils run lengthwise

15. Myofibrils are made of filaments • Thin filaments – primarily actin • Thick filament – myosin • Elastic filaments • Sarcomeres are the basic, functional units of striated muscle fibers.

19. Each thick filament is surrounded by interacts with 6 thin filaments

21. Thick filaments • Made of about 200 molecules of myosin • Each myosin molecule has two “heads” with peptide chains (“light chains”) • Each head has an actin binding site and an ATP binding site • The ATP site splits ATP and transfers energy to myosin head; which remains charged (“cocked”) until contraction.

24. Thin Filaments • Actin molecules form a helix • Each actin molecule has a myosin binding site • Other proteins: • Tropomyosin – long, filamentous protein, it wraps around the actin and covers the myosin binding sites • Troponin – a smaller molecule bound to tropomyosin, it has calcium binding sites.

25. Other proteins: • Titin –suspends thick filaments • Nebulin – stabilizes thin filaments during contraction • Α – actinin – anchor thin filaments to Z lines • Dystrophin – supports sarcolemma during contraction • Integrins – membrane-spanning proteins • Laminin – link between integrins and extracellular matrix

27. Sarcoplasmic reticulum • Specialized smooth E. R. • Tubes fuse to form cisternae • In a relaxed muscle, S.R. stores Ca++ (Ca++ active transport pumps) • When stimulated, Ca++ leaves through Ca++ release channels.

28. Transverse tubules (T-tubules) • Infoldings of sarcolemma that penetrate into muscle fiber at right angles to filaments. They are filled with extracellular fluid. • T-tubules and the cisternae on either side form a triad.

30. Blood and nerve supply • Muscle contraction uses a lot of ATP • To generate ATP, muscles need oxygen • Each muscle fiber is in close contact with one or more capillaries • Motor neurons – originate in brain and spinal cord; cause muscle contraction

31. Motor unit • A Motor Unit is made of one motor neuron and all the muscle fibers it innervates. • These cells all contract together. • A single motor unit can have 2 – 2,000 muscle fibers. • Precise movements are controlled by small motor units, and large movements by large motor units.

33. Neuromuscular Junction (NMJ) • Nerves communicate with muscles and other organs at structures called synapses. • Synaptic cleft – gap between neuron and sarcolemma • Axon releases a chemical called a neurotransmitter – Acetylcholine (Ach) • Axon branches into axon terminals. • At the end of each axon terminal is a swelling called the synaptic end bulb.

36. The region across the synaptic cleft from the synaptic end bulb is called the motor end plate. • The sarcolemma of the motor end plate is folded and contains many receptors for ACh . • When a nerve impulse reaches the synaptic end bulbs, it causes synaptic vesicles to fuse with the membrane and release ACh by exocytosis.

39. Acetylcholine diffuses across the synaptic cleft, and binds with receptors on the motor end plate. • This binding causes the receptor to change shape, and opens Na+ channels in the membrane. • When enough Na+ channels are opened, an electrical current is generated and is carried along the sarcolemma. This is called a muscle impulse or muscle action potential. This electrical activity can be recorded in an electromyogram.

40. Have you played mousetrap ?

41. Sliding Filament Mechanism • When a nerve impulse reaches an axon terminal, the synaptic vesicles release acetylcholine (ACh) • ACh crosses the synaptic cleft and binds with receptors on the motor end plate. • This binding opens channels that allow sodium to rush in, beginning a muscle action potential in the sarcolemma.

42. The action potential or impulse travels down the sarcolemma and into the T-tubules, causing the sarcoplasmic reticulum to release Ca++ into the sarcoplasm. • The Ca++ binds to the troponin, which changes shape, pulling the tropomyosin away from the myosin binding sites on the actin. • The activated myosin attaches to the actin, forming actin/myosin crossbridges. • The myosin head moves toward the center of the sarcomere, pulling the actin filaments past the myosin. This is called a power stroke.

43. When the myosin heads turn, they release ADP, and ATP binds to the heads. • When ATP binds, it causes the myosin to release the actin. • ATP is split, and the myosin heads again bind to the actin, but further down the filament. • The myosin again pulls the actin. • This action is repeated many times. • The Z lines (discs) get closer together as the actin and myosin filaments slide past each other, and the muscle fiber shortens.

48. Relaxation • ACh is broken down by an enzyme called acetylcholinesterase. • Action potentials are no longer generated, so the Ca++ release channels in the S.R. close. • Ca++ active transport pumps take Ca++ out of the sarcoplasm and into the S.R. where it binds to a protein called calsequestrin.

49. As the Ca++ levels in the sarcoplasm fall, troponin releases tropomyosin, which falls back and covers the myosin binding sites on the actin. • The thin filaments slip back into their relaxed positions.

50. Rigor mortis • After death, muscle cells begin autolysis, and Ca++ leaks out of the S.R. • This causes muscles to begin to contract. • Since the body is dead, no more ATP is produced. • Without the ATP to recharge the myosin heads, they remain linked to the actin, and neither relax nor contract any further. • After about 24 - 72 hours it disappears as the tissues begin to disintegrate.