Muscular System and Muscular Tissue Review

Muscular System and Muscular Tissue Review I. Muscular Tissue: Muscular tissue is one of the 4 primary tissue classes: 1. Muscle 2. Epithelial 3. Connective 4. Nervous Functions of Muscular Tissue (5): 1. Movement – the movement of body parts and body contents in breathing, circulation, feeding and digestion, defecation, urination, and childbirth 2. Stability – prevent unwanted movements; help maintain body posture; maintain tension on tendons and bones to prevent joints from hyperextending and moving 3. Control of body openings and passages – regulate food intake and movement; movement of bile, blood, and other materials 4. Heat production – generates as much as 85% of a person’s body heat 5. Glycemic control – regulation of blood glucose concentrations Universal Characteristics of Muscle Cells (5): 1. Responsiveness – “excitability”; response of muscle cells to electrical changes across plasma membrane 2. Conductivity- electrical change triggers a wave of excitation that travels along the muscle cells 3. Contractility – the ability of a muscle cell to shorten when stimulated in order to create movement 4. Extensibility – the ability of a muscle cell to stretch again between contractions 5. Elasticity - the ability of a muscle cell to return to its original length after a muscle cell is stretched and then released Muscle tissue types (3): 1. Cardiac (heart) muscle 2. smooth muscle (walls of viscera or hollow organs) 3. skeletal muscles (bones) II. Cardiac Muscle Tissue: Properties Located in heart only Cells are called myocytes or cardiocytes Striated like skeletal muscle, but cardiocytes are shorter and thicker Uni-nucleated near center of cell Each myocyte is joined to several others at the uneven, notched linkages—intercalated discs Involuntary control with regular rhythm Can contract without need for nervous stimulation Contractions in unison must last long enough to expel blood Must be highly resistant to fatigue III. Smooth Muscle Tissue: Properties Located in hollow organs - viscera (e.g., stomach, urinary bladder) Non-striated; no visible striations Thick and thin filaments are present, but not aligned with each other Involuntary control; can contract without nervous stimulation Most smooth muscle is innervated by autonomic nerve fibers Uni-nucleated; there is only one nucleus, located near the middle of the cell Ca2+ needed for muscle contraction comes by way of Ca2+ channels in the sarcolemma Contraction and relaxation of smooth muscle very slow in comparison to skeletal muscle Plasticity—the ability to adjust its tension to the degree of stretch *Injured smooth muscle regenerates well and has the greatest ability to regenerate IV. Skeletal Muscle Tissue: Properties Striated; has bands Under voluntary control Multi-nucleated located near plasma membrane Usually attached to one or more bones by ligaments Skeletal muscle cells called muscle fibers comprise skeletal muscle tissue V. Skeletal Muscle Tissue: Anatomy Hierarchy (small to large): Muscle Fibers (cells) - Fascicles – Muscles – Muscle Compartments Skeletal Muscle Cells = Muscle Fibers. Comprise muscular tissue. Muscle cells are called muscle fibers because of their elongated shape. Anatomy of a muscle fiber or cell. There are many parts or structures of the muscle cell or muscle fiber. o Sarcolemma – the plasma membrane of the muscle cell/muscle fiber o Sarcoplasm - the cytoplasm of the muscle fiber o Sarcoplasmic Reticulum (SR) – the smooth endoplasmic reticulum of a muscle fiber; stores Calcium (Ca++) o T-tubules – tunnel-like extensions of the sarcolemma that convey electrical signals from the cell’s surface to its interior o Striations – alternating light and dark transverse bands across a myofibril o Myofibrils – bundles of protein myofilaments contained in the sarcoplasm o Myofilaments - thread-like complex of contractile protein molecules 1. Thick filament – bundled myosin protein molecules; dark bands; contractile protein; binds with actin during muscle contraction 2. Thin filament - composed of three protein molecules; light bands a) actin – contractile protein; binds with myosin during muscle contraction b) tropomyosin - regulatory protein; blocks the myosin-binding active sites during muscle relaxation so that a muscle can’t contract c) troponin – regulatory protein; associated with tropomyosin; acts as a calcium receptor; interferes with tropomyosin’s ability to block the myosin-binding active sites during muscle relaxation to allow myosin to bind with actin so that a muscle can contract 3. Elastic filament – titin protein molecules; flanks a thick filament and anchors it to a Z-disc o Sarcomere – the contractile unit of a muscle fiber/cell; the distance from one Zdisc to the next; muscles shorten because sarcomeres shorten o Z-disc – protein disc to which thin filaments and elastic filaments are anchored at each end of a sarcomere o Endomysium – thin sleeve of loose connective tissue that surrounds muscle fibers; creates room for capillaries and nerve fibers to stimulate and nourish each muscle cell Muscle Fascicles: o Fascicles – bundles of muscle fibers o A muscle’s strength and the direction of its pull are determined by the orientation of muscle fascicles. o Perimysium – thicker connective tissue sheath that surrounds muscle fascicles; carries the larger nerves and blood vessels and stretch receptors Muscles: o Muscles are groups of fascicles. o Epimysium – fibrous sheath that surrounds the entire muscle o Muscles are grouped in compartments separated from one another by fascia. o Fascia – sheet of connective tissue that separates individual muscles from one another. o Muscles are classified according to their shapes as determined by the orientation of its fascicles. o Muscle shapes (5): 1. Fusiform – thick in the middle and tapered at each end (ex. Biceps brachii; gastrocnemius) 2. Parallel – uniform width and parallel fascicles (ex. Rectus abdominis; Sartorius) 3. Triangular – fan-shaped; broad at one end and narrower at the other end (ex. Pectoralis major; temporalis) 4. Pennate – feather-shaped (ex. Semimembranous; deltoid) 5. Circular – sphincters or rings (ex. Anal sphincter; orbicularis oculi) Muscle Compartment: Group of functionally related muscles enclosed and separated from other muscles by fascia o Functional groups of muscles work together to produce movement. o Categories of Muscle Action or Movement (4): 1. Agonist or prime mover – muscle that produces most of the force or movement during a joint action 2. Synergist – muscle that aids or assists the agonist or prime mover 3. Antagonist – muscle that opposes the prime mover 4. Fixator – muscle that prevents a bone from moving o Intrinsic muscles – origin and insertion of a muscle are entirely contained within a particular region o Extrinsic muscles – origin and insertion of a muscle are not contained within a particular region o Origin – stationary end of muscle attached to bone o Insertion – mobile end of muscle attached to bone o Muscle attachments to bones: 1. Indirect – large separation between muscle and bone; muscle attaches indirectly to bone by tendon 2. Direct – little separation between muscle and bone; muscle attaches to bone by collagen fibers o Behavior of Skeletal Muscles - Major phases (4) of contraction and relaxation 1. Excitation - The process in which nerve action potentials lead to muscle action potentials 2. Excitation–contraction coupling - Events that link the action potentials on the sarcolemma to activation of the myofilaments, thereby preparing them to contract 3. Contraction - Step in which the muscle fiber develops tension and may shorten 4. Relaxation - When its work is done, a muscle fiber relaxes and returns to its resting length o Nerve – Muscle Relationship: Skeletal muscle never contracts unless it is stimulated by a nerve.  Somatic motor neurons — Serve skeletal muscles; Nerve cells whose cell bodies are in the brainstem and spinal cord  Somatic motor fibers — axons (nerve fibers) that lead to the skeletal muscle; Each nerve fiber branches out to a number of muscle fibers; Each muscle fiber is supplied by only one motor neuron  Motor unit—one nerve fiber and all the muscle fibers innervated by it; Average motor unit—200 muscle fibers for each motor unit Small motor units— Fine degree of control; Three to six muscle fibers per neuron; Ex. Eye and hand muscles Large motor units— More strength than control; Powerful contractions supplied by large motor units (e.g., gastrocnemius has 1,000 muscle fibers per neuron); Many muscle fibers per motor unit If nerve connections are severed or poisoned by toxins, a muscle is paralyzed.  Spastic paralysis - a state of continual contraction of the muscles; possible suffocation.  Tetanus (lockjaw) - a form of spastic paralysis caused by toxin Clostridium tetani  Flaccid paralysis —a state in which the muscles are limp and cannot contract  Botulism —type of food poisoning caused by a neuromuscular toxin secreted by the bacterium Clostridium botulinum o Common Skeletal Muscle Disorders:  Muscular Dystrophy ― group of hereditary diseases in which skeletal muscles degenerate and weaken, and are replaced with fat and fibrous scar tissue  Myasthenia Gravis - autoimmune disease in which antibodies attack neuromuscular junctions and bind ACh receptors together in clusters; disease of women between 20 and 40  Fibromyalgia – Diffuse, chronic muscular pain and tenderness; most common in women 30 – 50 y.o. o The Length–Tension Relationship of Skeletal Muscles and Muscle Tone:      o The amount of tension generated by a muscle and the force of contraction of a muscle depends on how stretched or contracted the muscle was before it was stimulated. If a muscle is overly contracted at rest, a weak contraction results. If muscle is too stretched before it is stimulated, a weak contraction results. The optimum resting length produces the greatest force when a muscle contracts. Muscle tone - a state of partial contraction maintained by muscles which makes the muscles ideally ready for action. Behavior of Skeletal Muscles: Threshold, Latent Period, and Twitch  Threshold —the minimum voltage necessary to generate an action potential in the muscle fiber and produce a contraction  Twitch—a quick cycle of contraction when the stimulus is at threshold or higher  Latent period— delay between the onset of stimulus and the onset of twitch response  Contraction phase—phase in which the myofilaments slide and the muscle shortens  Relaxation phase —the muscle returns to its resting length o Muscle Contraction: Isometric v. Isotonic Contraction  Isometric muscle contraction  No movement occurs.  *Muscle changes in tension, with no change in length.  Important in postural muscle function and antagonistic muscle joint stabilization.  Isotonic muscle contraction –  Produces movement.  *Muscle changes in length, with no change in tension.  There are 2 forms of isotonic contraction: 1. Concentric contraction: a muscle shortens as it maintains tension. 2. Eccentric contraction : a muscle lengthens as it maintains tension.  o Note: At the beginning of muscle contraction—the isometric phase- muscle tension rises, but the muscle does not shorten or change length. When tension overcomes the resistance of the load, tension levels off and the muscle begins to shorten and move the load—isotonic phase. Muscle Metabolism:       o All muscle contraction depends on ATP. ATP supply depends on the availability of oxygen and organic energy sources (e.g., glucose and fatty acids). Two main pathways of ATP synthesis: 1. Anaerobic fermentation  Enables cells to produce ATP in the absence of oxygen.  Yields little ATP.  Produces toxic lactic acid, a major factor in muscle fatigue. 2. Aerobic respiration Produces far more ATP. Yields less toxic end products (e.g., CO2 and water). Requires a continual supply of oxygen. Energy Demands: Immediate, Short-Term, & Long-Term  Immediate Energy : Short, intense exercise, when brief but maximum effort is needed (i.e., 100 m dash; weight lifting). Oxygen need is briefly supplied by myoglobin (muscle protein) for a limited amount of aerobic respiration at onset, but is rapidly depleted. Muscles meet most of ATP demand by borrowing phosphate groups from other molecules and transferring them to ADP. ATP and creatine phosphate collectively called the phosphagen system provide nearly all energy used for short bursts of intense activity.  Short-Term Energy : As the phosphagen system is exhausted, muscles shift to anaerobic fermentation. Muscles obtain glucose from blood and their own stored glycogen. In the absence of oxygen, glycolysis can generate a net gain of 2 ATP for every glucose molecule consumed, as it converts glucose to lactic acid. The glycogen-lactic acid system produces enough ATP for 30 to 40 seconds of maximum activity.  Long-Term Energy : After 40 seconds or so, the respiratory and cardiovascular systems “catch up” and deliver oxygen to the muscles fast enough for aerobic respiration to meet most of the ATP demands. *Aerobic respiration produces far more ATP than glycolysis does = 36 ATP per glucose molecule. A very efficient means of meeting the ATP demands of prolonged exercise. One’s rate of oxygen consumption rises for 3 to 4 minutes and then levels off to a steady state in which aerobic ATP production keeps pace with demand. Little lactic acid accumulates under steady-state conditions. Depletion of glycogen and blood glucose, together with the loss of fluid and electrolytes through sweating, set limits on endurance and performance even when lactic acid does not.  Muscle Fatigue and Endurance:  Muscle fatigue — A progressive weakness and loss of contractility from prolonged use of the muscles.  Fatigue is thought to result from: ATP synthesis declines as glycogen is consumed ATP shortage slows down the Na+–K+ pumps Lactic acid lowers pH of the sarcoplasm Release of K+ with each action potential causes the accumulation of extracellular K+. Motor nerve fibers use up their ACh. Central nervous system, where all motor commands originate, fatigues by unknown processes, so there is less signal output to the skeletal muscles.  Endurance – the ability to maintain high-intensity exercise for more than 4 to 5 minutes. Determined in large part by one’s maximum oxygen uptake - the point at which the rate of oxygen consumption reaches a plateau and does not increase further with an added workload. Proportional to body size. Peaks at around age 20. Usually greater in males than females. Can be twice as great in trained endurance athletes as in untrained persons. Results in twice the ATP production.  Oxygen Debt (Deficit):  *Oxygen debt - The difference between the amount of oxygen needed for totally aerobic muscle activity and the amount actually used.  Heavy breathing continues after strenuous exercise.  Excess post-exercise oxygen consumption (EPOC) : the difference between the resting rate of oxygen consumption and the elevated rate following exercise.  Typically about 11 L extra of oxygen is needed after strenuous exercise.  Skeletal Muscle Fiber Classifications: Slow Twitch v. Fast Twitch 1. Slow oxidative (SO), slow-twitch, red, or Type I fibers  Abundant mitochondria, myoglobin, and capillaries  Deep red color    Adapted for aerobic respiration and fatigue resistance Relative long twitch Location: Soleus of calf and postural muscles of the back 2. Fast glycolytic (FG), fast-twitch, white, or type II fibers  Fibers are well adapted for quick responses, but not for fatigue resistance.  Rich in enzymes of phosphagen and glycogen–lactic acid systems generate lactic acid, causing fatigue.  Poor in mitochondria, myoglobin, and blood capillaries which gives pale appearance.  Location: Extrinsic eye muscles, gastrocnemius, and biceps brachii   Muscular Strength and Conditioning: *Dependent on the following factors:  Muscle size (primary factor) - muscle exerts a tension on cross-sectional area  Fascicle arrangement - Pennate are stronger than parallel, and parallel stronger than circular  Motor units size - the larger the motor unit the stronger the contraction  Multiple motor unit summation : when a stronger contraction is required, the nervous system activates more motor units called recruitment.  Temporal summation -the greater the frequency of stimulation, the more strongly a muscle contracts.  Length–tension relationship - a muscle resting at optimal length is prepared to contract more forcefully than a muscle that is excessively contracted or stretched.  Fatigue - fatigued muscles contract more weakly than rested muscles. Exercise Training: Resistance v. Endurance  Resistance training (e.g. weightlifting) Contraction of a muscle against a load that resists movement. A few minutes of resistance exercise a few times a week is enough to stimulate muscle growth. Growth is from cellular enlargement. Muscle fibers synthesize more myofilaments and myofibrils and grow thicker.  Endurance training (aerobic exercise) Improves fatigue-resistant muscles. Improves skeletal strength. Increases the red blood cell count and oxygen transport capacity of the blood. Enhances the function of the cardiovascular, respiratory, and nervous systems. Study Guide: Muscular System and Muscle Tissue Ch. 10 & 11 Chapter 10 Number of skeletal muscles Muscle shapes Muscle movements Muscle functions Agonists, antagonists, synergist, fixator Origin, insertion Tendons, ligaments Chapter 11 Universal characteristics of muscle (p. 402) 3 Types of muscle tissue and their characteristics Structural components of a muscle fiber (table 11.1) Muscle contraction process Functions of contractile proteins: myosin and actin; and regulatory proteins: tropomyosin and troponin Motor unit, action potential Phases (4) of muscle contraction and relaxation Length-tension relationship of muscle (p. 416) Muscle recruitment (p. 420) Isometric and Isotonic contraction (p. 421-422) ATP and muscle metabolism (p. 423) Energy: Immediate, Short- and Long-term (p. 424) Fatigue and Endurance (p. 424-425) Oxygen debt (p. 425) Type 1 and Type 2 muscle fibers – characteristics and functions (p. 426) Muscular Strength – factors (7) (p. 427) Exercise training (resistance, endurance) (p. 427) Comparison of Skeletal, Cardiac, and Smooth muscle (table 429) Muscular System Disorders: contracture, cramps, delayed-onset muscle soreness, fibromyalgia, myositis, muscular dystrophy, myasthenia gravis (p. 433)