2/28/18. Muscular System. Introduction. Anatomy. Chapter 20

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1 Muscular System Chapter 20 1 Introduction Shortening or lengthening of a muscle results from changes in relative positions of one small part of a muscle cell to another To understand contraction, we will examine muscles from the cellular level to the organ level 2 Anatomy Skeletal muscles Associated structures - Tendons - Aponeurosis Related fascial structures 3 1

2 Physiology Movement Posture maintenance - Contributes to joint stability Muscles are the most metabolically active structures in the body Heat production (thermogenesis); important in maintaining body temperature 4 Skeletal Muscle Anatomy Muscle fibers: - Sarcolemma: Equivalent of a cell membrane in a typical cell - Sarcoplasm: Equivalent of cytoplasm in typical cells - Sarcoplasmic reticulum: System of interconnected tubes; store and release calcium ions - T-tubules: Impulses travel deep into the cell during muscle contraction - Sarcomere: Basic unit of contraction; defining borders are Z lines or Z disks 5 Muscle Fiber Modified from Patton KT, Thibodeau GA: Anatomy & physiology, ed 7, St. Louis, 2010, Mosby. 6 2

3 Sarcolemma Modified from Patton KT, Thibodeau GA: Anatomy & physiology, ed 7, St. Louis, 2010, Mosby. 7 Skeletal Muscle Anatomy Muscle fiber = muscle cell - Distinct, threadlike shape - Often run the length of the muscle Myofibril - Strands within muscle fibers containing sarcomeres Sarcomere: Basic unit of contraction - Each one contains myofilaments 8 Close-up of a Sarcomere Modified from Patton KT, Thibodeau GA: Anatomy & physiology, ed 7, St. Louis, 2010, Mosby. 9 3

4 Thin Myofilaments Actin: Resembles a twisted strand of beads; contains binding sites for myosin Tropomyosin: Covers actin's binding sites Troponin: Acts like nails to keep tropomyosin in place Modified from Patton KT, Thibodeau GA: Anatomy & physiology, ed 7, St. Louis, 2010, Mosby. 10 Thick Myofilaments Myosin - Shaped like golf clubs with the golf shafts bundled together - Heads sticking out of the bundle are angled toward thin filaments 11 Regions within the Sarcomere I bands include thin filaments and Z lines A bands run the length of thick filaments - Contain the H zone (center of sarcomere) 12 4

5 Skeletal Muscle Anatomy Fasciculi: Bundles of muscle fibers - Groups of fasciculi form muscle organs or skeletal muscles Muscle organization review from macroscopic to microscopic structures - Muscle fascicle muscle fiber myofibril sarcomere myofilaments 13 Connective Tissues: Fascia Superficial fascia: Fascial layer just beneath skin Deep fascia: Extends from superficial fascia and surrounds muscles Myofascial: Skeletal muscles and related fascial coverings 14 Tendons Tendon: Cords anchoring muscle to bone Aponeurosis: Broad, flat tendon Tendon sheaths: Tubes that wrap tendons crossing multiple joints 15 5

6 Retinacula Retinacula: Retaining bands - Stabilize tendons and keep them in place - Found primarily in elbows, knees, ankles, and wrists 16 Connective Tissue Coverings Muscle and fibers are protected by connective tissue - Epimysium: Surrounds an entire muscle - Perimysium: Surrounds fasciculi - Endomysium: Surrounds each individual muscle fiber 17 Neuromuscular Junction Where motor neuron excites sarcolemma and initiates contraction Three parts: - Motor end plate: Contains receptor sites for neurotransmitters (acetylcholine [ACH]) - Motor neuron: Responsible for sending impulses that stimulate muscles to contract - Synaptic gap: ACH moves from synaptic vesicles across the synaptic gap and connects receptors on the motor end plate to propagate the stimulus 18 6

7 Neuromuscular Junction and Motor Unit 19 Neuromuscular Junction and Motor Unit 20 Muscle Contraction Processes: - Excitation of the sarcolemma - The actual contraction (muscle shortening) - Relaxation of muscle back to precontraction state 21 7

8 Excitation of the Sarcolemma Contraction begins with nerve impulse Nerve impulse reaches end of motor neuron Acetylcholine stored in synaptic vesicles is released into synaptic cleft ACH binds to receptor sites on motor end plate Generated impulse travels through T-tubules into sarcoplasmic reticulum Sarcoplasmic reticulum releases stored calcium ions, which contain sarcomeres 22 Contraction Tropomyosin slides off to expose binding site Myosin heads attach to actin and bridge the gap between thin filaments - After cross-bridging, myosin heads toggle (like a light switch) Thin filaments slide over thick filaments to center of sarcomere (H zone) If adenosine triphosphate (ATP) is present, myosin heads detach, bind to next exposed site, and toggle again 23 Muscle Contraction 24 8

9 Relaxation After calcium is released into sarcoplasm, it s pumped back Tropomyosin slides back over binding sites on actin Myosin heads return to precontraction state Muscle is now at rest 25 Motor Units and Recruitment All-or-none response: All muscle fibers in a motor unit will contract to fullest ability or not at all Motor unit: Single motor neuron and all fibers to which it attaches Recruitment: Motor unit activation based on units needed to perform an action - If more strength is required, more motor units are recruited 26 Adenosine Triphosphate Universal unit of energy in the cell Between contractions, myosin heads are ready for action Myosin binds to actin when calcium ions are present Produced by mitochondria 27 9

10 Oxygen and Glucose Oxygen is stored on myoglobin on the sarcoplasm until needed Myoglobin: Red respiratory pigment similar to hemoglobin in red blood cells - Fibers containing large amounts are deep red (often called red muscle) - Fibers with fewer amounts are lighter in color (often called white muscle) Glucose is needed to produce ATP Excess glucose is stored as glycogen in liver and muscles 28 Anaerobic Respiration Occurs as soon as muscles start contracting Does not contribute a large amount of energy Lasts 30 to 60 seconds Creates the energy equivalent of 2 ATP molecules Lactic acid is the end product of anaerobic glycolysis 29 Aerobic Respiration Creates the equivalent of 36 ATP molecules Mitochondria brings in oxygen and expels carbon dioxide Continues as long as oxygen is available through cellular respiration 30 10

11 Types of Skeletal Muscle Fibers Two types: - Type 1 - Type 2 31 Type 1: Slow Twitch (Red Muscle) Contain large amounts of mitochondria and myoglobin Contract more slowly Resist fatigue of sustained activity Most back muscles and muscles of the lower extremity are Type 1 Marathon runners have high ratio in lower extremities 32 Type 2: Fast Twitch (White Muscle) Contract more quickly for shorter periods of time Fatigue quickly due to rapid ATP depletion Low amounts of myoglobin and fewer numbers of mitochondria and capillaries give them a lighter color Muscles of upper extremity contain many type 2 fibers 33 11

12 Muscle Fiber Arrangement Modified from Patton KT, Thibodeau GA: Anatomy & physiology, ed 7, St. Louis, 2010, Mosby. 34 Parallel/Fusiform Tapered off at both ends or spindle-shaped Fibers run parallel along long axis of the bone Most skeletal muscles have parallel fibers Parallel muscles also called fusiform muscles Example: Rectus abdominis (anterior abdominal wall) - Contains tendinous intersections to help maintain its length and for added strength 35 Convergent Fibers join at one end and spread out at other end (like a fan) Can pull in several directions instead of just one Example: Pectoralis major (located in the chest) 36 12

13 Spiral and Circular Spiral - Fibers twist between points of attachment - Example: Latissimus dorsi (lower back) and levator scapulae (lateral neck) Circular - Rounded fiber arrangement - Example: Orbicularis oris (around mouth) and orbicularis oculi (around eyes) 37 Pennate Arranged around a central tendon with fibers extending diagonally Gives muscle featherlike appearance Types are unipennate, bipennate, and multipennate - Example: Flexor pollicis longus (forearm [unipennate]), rectus femoris (anterior thigh [bipennate]), and deltoid and scapularis (shoulder [multipennate]) 38 Parts of a Skeletal Muscle Most skeletal muscles cross at least one joint while attached to two other structures When a muscle contracts, it pulls on these bones and causes movement at the joint Parts of a muscle: - Belly - Origin - Insertion 39 13

14 Parts of a Skeletal Muscle Belly - Wide central portion that contains contractile units or sarcomeres Origin - Tendinous attachment on a more stable (less movable) bone during contraction Insertion - Tendinous attachment more moveable during contraction 40 Parts of a Skeletal Muscle Modified from Patton KT, Thibodeau GA: Anatomy & physiology, ed 7, St. Louis, 2010, Mosby. 41 Parts of a Skeletal Muscle Modified from Patton KT, Thibodeau GA: Anatomy & physiology, ed 7, St. Louis, 2010, Mosby

15 Classifying Muscle by Number of Joints They Cross When a muscle crosses a joint, it acts on that joint to create movement Classifications are: - Uniarticular: Cross only one joint - Biarticular: Cross two joints - Multiarticular: Cross three or more joints 43 Muscle Actions Some muscles contract, while others offer support by either relaxing and lengthening or stabilizing joints Muscles assume differing roles depending on the required task 44 Prime Movers and Synergists Prime mover - Muscle causing specific movements; also called agonists - Example: Brachialis is prime mover in elbow flexion Synergists - Facilitate prime movers by performing the same movements at the same time - Example: Pronator teres is a synergist to biceps brachialis because they both cause elbow flexion 45 15

16 Fixators and Antagonists Fixators - Specialized synergists that stabilize joints so that prime movers can exert their actions - Example: Deltoid stabilizes shoulder joint Antagonists - Opposing muscle that relaxes, lengthens, and contracts to produce movement 46 Muscle Actions Adapted from Muscolino JE: Kinesiology, St. Louis, 2006, Mosby. 47 Isotonic Contractions Muscles generate force and cause length to change Also called dynamic contractions because they involve movement Types: - Concentric contractions: Muscles shorten while generating force - Eccentric contractions: Muscles lengthen and elongate while generating force 48 16

17 Isotonic Contractions Modified from Patton KT, Thibodeau GA: Anatomy & physiology, ed 7, St. Louis, 2010, Mosby. 49 Isometric Contractions Muscle generate force as muscle length stays the same No movement occurs Often occur in fixators and stabilizers 50 Isometric Contractions Modified from Patton KT, Thibodeau GA: Anatomy & physiology, ed 7, St. Louis, 2010, Mosby

18 Stretching and Stretch Receptors Stretching: Extending muscle to its fullest length Stretch receptors: - Muscle spindles - Golgi tendon organs 52 Muscle Spindles Activated when muscle is stretched rapidly Respond by stretching muscle reflexively Located within muscle belly 53 Golgi Tendon Organs Detect tension; activated when tension exceeds a certain threshold and triggers a lengthening reaction Respond by inhibiting contraction, which causes muscles to relax Located in musculotendinous junctions 54 18

19 Posture and Muscle Tone Posture: Body positioned over a base of support Muscle tone: Continued partial contraction of skeletal muscles - Good baseline tone enables us to stand, sit, walk, and run - Muscles with less tone are flaccid; muscles with more tone are spastic 55 Posture and Muscle Tone Tonus: Continued partial contraction - Flaccid: Less-than-normal tone - Spastic: More-than-normal tone 56 19