2. Fascia extend beyond ends of muscle; give rise to: tendons, which are fused to periosteum

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1 Chapter 9: Muscular System Notes I. Introduction: A. What are they? organs B. Why do we need them? to move C. How do they work? use chemical energy to contract D. Three kinds: smooth (visceral), skeletal, cardiac II. Structure of a Skeletal Muscle A. Each muscle is an organ, composed of 1. skeletal muscle tissue 2. connective tissue 3. nervous tissue 4. blood B. Connective Tissue Coverings: 1. Fascia a. layers of dense connective tissue b. surround & separate each muscle 2. Fascia extend beyond ends of muscle; give rise to: tendons, which are fused to periosteum of bone 3. Aponeuroses: broad sheets of connective tissue that connects muscles to each other 4. Epimysium: layer of connective tissue around each whole muscle 5. Perimysium: surrounds individual bundles (fascicles) within each muscle 6. Endomysium covers each individual muscle fiber (cell) C. Skeletal Muscle Fibers 1. description: single long cylinder with rounded ends 2. Sarcolemma cell membrane 3. Sarcoplasm: (cytoplasm of muscle fiber) contains myofibrils made up of: 1

2 a. myosin thick filaments b. actin thin filaments c. striations due to the organization of actin & myosin (sarcoplasm has many mitochondria and nuclei) 4. Sarcoplasmic reticulum: network of membranous channels around each myofibril (the sarcolemma s endoplasmic reticulum) a. T tubule invaginations of the sarcolemma open to outside of the muscle fiber (allows extracellular fluid in) b. Cisternae thickened areas where actin & myosin filaments meet c. Arrangement allows extracellular fluid in d. Sarcoplasmic retulum and transverse tubules activate muscle contraction e. When fiber is stimulated: length of sarcomere shortens, causing contraction. f. *Sarcomere extends from Z line to Z line* D. Neuromuscular Junction 1. What is it? site where motor neuron & muscle fiber meet a. Motor end plate (1) formed by: muscle fiber membrane (2) sarcolemma tightly folded (3) many nuclei and mitochondria b. Synaptic clefts: recesses or gaps of motor end plate; branched motor neuron fibers project into them c. Cytoplasm of motor neuron contains: numerous mitochondria & synaptic vesicles storing neurotransmitters 2. Sketch see p 292 2

3 E. Motor Units 1. made up of: motor neuron & many muscle fibers it controls 2. when stimulated: muscle fibers of motor unit contract all at once III. Skeletal Muscle Contraction A. Result of Muscle contraction: 1. shortening of sarcomeres 2. pulling of muscle against its attachments B. Role of Myosin and Actin 1. Myosin: 2/3 protein within skeletal muscle. 2 twisted protein strands with globular protein parts called crossbridges projecting outward along the strands 2. Actin globular protein with myosin binding sites; *2 other important proteins: tropomyosin & troponin associated with surface of actin filaments 3. Sliding filament theory of muscle contraction: a. needs: calcium b. theory: when calcium ions are present, binding sites on the actin filament are exposed. Cross bridges on a myosin filament form linkages by attaching to the binding sites on the actin, and bend, pulling on the actin filament using energy from ATP. The linkage breaks, then the myosin cross bridge forms a linkage with the next binding site c. Active sites for cross bridges: ADP molecules attached to surface of actin 4. Where does the energy come from? conversion of ATP to ADP + P is catalyzed by enzyme ATPase; energy is provided to crossbridges & causes them to be in cocked position C. Stimulus for Contraction: 1. to initiate a muscle contraction, motor neuron must: release neurotransmitter acetylcholine from its synaptic vesicles into the synaptic cleft 3

4 2. protein receptors in motor end plate detect the neurotransmitters; muscle impulse spreads over the surface of the sarcolemma & into the T tubules, where it reaches the SR 3. cisternae of SR release stored calcium to sarcoplasm of muscle fiber (SR has high concentration of calcium d/t active transport) 4. High concentration of calcium in sarcoplasm interacts with troponin & tropomyosin; they move aside & expose the myosin binding sites on the actin filaments 5. sarcomeres shorten because:myosin cross bridges bind & pull on actin filaments 6. Acetylcholinesterase: enzyme that decomposes acetylcholine rapidly after nervous impulse is received 7. Calcium returns to SR; linkages between myosin & actin are broken. D. Energy Sources for Contraction: 1. comes from: ATP. Limited supply must be regenerated constantly 2. creatine phosphate: 4-6X more abundant than ATP: stores excess energy released by mitochondria; regenerates ATP from ADP & P. 3. When there s enough ATP, creatine phosphokinase: promotes synthesis of creatine phosphate 4. As ATP breaks down: energy from creatine phosphate can be transferred to ADP molecules, converting them back to ATP. *Supply of creatine phosphate quickly exhausted in active muscles* E. Oxygen Supply and Cellular Respiration 1. review of CR: oxygen enables the complete breakdown of glucose in mitochondria to release energy to form ATP 2. early phase of CR: little ATP made 3. muscle has high requirement for oxygen 4. Hemoglobin: pigment in RBC that carries oxygen 5. Myoglobin pigment produced in muscles stores oxygen temporarily 4

5 F. Oxygen Debt 1. may develop when: there s strenuous exercise 2. Lactic acid accumulation: a. lactic acid: accumulates as an end product of anaerobic respiration b. carried in blood to liver 3. Oxygen Debt: = the amount of oxygen that the liver needs to change the accumulated lactic acid to glucose + the amount of oxygen muscle cells need to resynthesize ATP & Creatine Phosphate to the levels they were before 4. Repaying debt: takes several hours G. Muscle Fatigue 1. definition when a muscle loses its ability to contract during strenuous exercise 2. usually arises from:accumulation of lactic acid in the muscle (decrease ph from lactic acid) (prevents muscle from contracting) 3. Lack of ATP leads to: muscle cramp due to inability to return calcium ions back to the sarcoplasmic reticulum so muscle fiber can relax H. Heat Production 1. Skeletal muscle contraction: important heat source for body; heat is transported by blood to maintain body temperature 2. CR: also source of heat.only about 25% of energy released by CR is available IV. Muscular Responses A. How can we study muscle function? take out a single fiber, connect it to a device that records its responsiveness to electrical stimulation 5

6 B. Threshold Stimulus definition: minimal strength of a stimulus to cause a fiber to contract (a muscle fiber remains unresponsive to stimulation until the stimulus gets to a certain strength) C. All-or-None Response: when a muscle fiber contracts, it contracts to its full extent D. Recording a muscular contraction: 1. myogram recording of an electrically stimulated muscle contraction 2. twitch single, short contraction that only involves a few motor units 3. latent period time delay between the application of the stimulus, and muscle contraction; <0.01sec 4. latent period followed by: period of contraction, and a period of relaxation E. Summation 1. process in which: muscle fiber receives a series of stimuli of increasing frequency & reaches a point where it can t relax completely (force of individual twitches combine) 2. Tetanic contraction if the sustained contraction has NO relaxation, its called tetany F. Recruitment of Motor Units 1. What is recruitment? increase in the number of activated motor units within a muscle at higher intensities of stimulation 2. Whole muscle has many motor units a. each responds to:different threshold b. lower intensities: stimulate fewer motor units to contract c. higher intensities stimulate more motor units to contract G. Sustained Contractions occur when: 1. muscle tone: achieved by continuous state of sustained contraction of motor units in a muscle 6

7 2. features of muscle tone a. important in posture b. totally lost with loss of consciousness H. Use and Disuse 1. hypertrophy size of fibers increases; not the number! 2. atrophy decrease in size V. Smooth Muscles A. Smooth Muscle Fibers 1. What does a smooth muscle cell look like? elongated tapered ends, no striations, underdeveloped sarcoplasmic reticulum 2. 2 types: a. multiunit blood vessels & iris of eye; fibers occur separately; not in sheets (not well organized b. visceral sheets; found in walls of hollow organs examples: stomach, intestine, bladder, uterus; fibers can stimulate each other & display rhythmicity. Responsible for peristalsis in hollow organs & tubes. B. Smooth Muscle Contraction 1. myosin binding to actin mechanism: similar to skeletal contraction 2. 2 neurotransmitters stimulate & inhibit contraction, depending on target muscle a. acetylcholine b. norepinephrine 3. Hormones stimulate or inhibit contraction 4. Comparison to skeletal muscle: smooth muscle is slower to contract & relax; but it can contract longer using the same amount of ATP VI. Cardiac Muscle A. Mechanism of contraction: similar to that of skeletal & smooth muscle BUT: 7

8 B. Can contract for longer periods than skeletal or smooth muscle because: transverse tubules supply extra calcium; leads to longer lasting contraction C. Intercalated disks 1. join cells 2. transmit force of contraction from one cell to the next. Also, help in rapid transmission of impulses throughout the heart D. Features of Cardiac Muscle: 1. self exciting 2. rhythmic 3. whole structure contracts as a unit VII. Skeletal Muscle Actions A. Origin & insertion some muscles have more than one insertion or origin 1. immovable end = origin. moveable end is the insertion 2. contraction pulls insertion TOWARDS origin B. Interaction of Skeletal Muscles 1. Prime mover of a group of muscles, the one doing the majority of the work 2. synergists helper muscles 3. antagonists opposing muscles 8