Muscular System: Histology and Physiology Chapter 9 Muscular System: Histology and Physiology

Functions of the Muscular System Body movement (skeletal muscles attached to bones) Maintenance of posture Respiration (skeletal muscles of thorax are responsible for the movement necessary for respiration) Production of body heat (when skeletal muscles contact, heat is given off as a by-product) Communication (speaking, writing) Constriction of organs and vessels (contraction of smooth muscle) Heart beat (contraction of cardiac muscle)

General Functional Characteristics of Muscle Contractility: ability of a muscle to shorten with force Excitability: capacity of muscle to respond to a stimulus (by nerve or hormone) Extensibility: muscle can be stretched to its normal resting length and beyond to a limited degree Elasticity: ability of muscle to recoil to original resting length after stretched

Types of Muscle Tissue Skeletal Smooth Cardiac Responsible for locomotion, facial expressions, posture, respiratory movements, other types of body movement Voluntary Smooth Walls of hollow organs, blood vessels, eye, glands, skin Some functions: propel urine, mix food in digestive tract, dilating/constricting pupils, regulating blood flow In some locations, autorhythmic Controlled involuntarily by endocrine and autonomic nervous systems Cardiac Heart: major source of movement of blood Autorhythmic

Skeletal Muscle Structure Composed of muscle cells (fibers), connective tissue, blood vessels, nerves Fibers are long, cylindrical, multinucleated Tend to be smaller diameter in small muscles and larger in large muscles. 1 mm- 4 cm in length Develop from myoblasts (they are converted to muscle fibers as contractile proteins accumulate within their cytoplasm); numbers remain constant (# of muscle fibers remain constant after birth----so, enlargement of muscles is an increase in size rather than #) Striated appearance due to light and dark banding

Connective Tissue Coverings of Muscle Layers External lamina. Delicate, reticular fibers. Surrounds sarcolemma (P.M.) Endomysium. Loose C.T. with reticular fibers. Perimysium. Denser C.T. surrounding a group of muscle fibers. Each group called a fasciculus Epimysium. C.T. that surrounds a whole muscle (many fascicles) Fascia: connective tissue sheet Forms layer under the skin Holds muscles together and separates them into functional groups. Allows free movements of muscles. Carries nerves (motor neurons, sensory neurons), blood vessels, and lymphatics. Continuous with connective tissue of tendons and periosteum.

Nerves and Blood Vessel Supply Motor neurons: stimulate muscle fibers to contract. Nerve cells with cell bodies in brain or spinal cord; axons extend to skeletal muscle fibers through nerves Axons branch so that each muscle fiber is innervated Capillary beds surround muscle fibers

Muscle Fibers Nuclei just inside sarcolemma Cell packed with myofibrils within cytoplasm (sarcoplasm = cytoplasm without myofibrils) Threadlike (extends from one end of muscle fiber to the other) Composed of protein threads called myofilaments: thin (actin 8nm) and thick (myosin 12nm) Sarcomeres: actin & myosin myofilaments form highly ordered units called sarcomeres. They are joined end to end to form the myofibrils.

Parts of a Muscle

Actin and Myosin Myofilaments

Actin (Thin) Myofilaments Two strands of fibrous (F) actin form a double helix extending the length of the myofilament; attached at either end at sarcomere. Composed of G actin monomers each of which has an active site Actin site can bind myosin during muscle contraction. Tropomyosin: an elongated protein winds along the groove of the F actin double helix. Troponin is composed of three subunits: one that binds to actin, a second that binds to tropomyosin, and a third that binds to calcium ions. Spaced between the ends of the tropomyosin molecules in the groove between the F actin strands. The tropomyosin/troponin complex regulates the interaction between active sites on G actin and myosin.

Myosin (Thick) Myofilament Many elongated myosin molecules shaped like golf clubs. Molecule consists of two heavy myosin molecules wound together to form a rod portion lying parallel to the myosin myofilament and two heads that extend laterally. Myosin heads Can bind to active sites on the actin molecules to form cross-bridges. Attached to the rod portion by a hinge region that can bend and straighten during contraction. Have ATPase activity: activity that breaks down adenosine triphosphate (ATP), releasing energy. Part of the energy is used to bend the hinge region of the myosin molecule during contraction

Sarcomeres: Z Disk to Z Disk Z disk: filamentous network of protein. Serves as attachment for actin myofilaments Striated appearance I bands: from Z disks to ends of thick filaments A bands: length of thick filaments H zone: region in A band where actin and myosin do not overlap M line: middle of H zone; delicate filaments holding myosin in place In muscle fibers, A and I bands of parallel myofibrils are aligned. Titin filaments: elastic chains of amino acids; make muscles extensible and elastic

Sliding Filament Model Actin myofilaments sliding over myosin to shorten sarcomeres Actin and myosin do not change length Shortening sarcomeres responsible for skeletal muscle contraction During relaxation, sarcomeres lengthen because of some external force, like forces produced by other muscles (contraction of antagonistic muscles) or by gravity. - agonist = muscle that accomplishes a certain movement, such as flexion. - antagonist = muscle acting in opposition to agonist.

Sarcomere Shortening

Physiology of Skeletal Muscle Fibers Nervous system controls muscle contractions through action potentials Resting membrane potentials Membrane voltage difference across membranes (polarized) Inside cell more negative due to accumulation of large protein molecules. More K+ on inside than outside. K+ leaks out (through leak channels) but not completely because negative molecules hold some back. Outside cell more positive and more Na+ on outside than inside. Na+ /K+ pump maintains this situation. Must exist for action potential to occur

Ion Channels Types Each is specific for one type of ion Ligand-gated. Ligands are molecules that bind to receptors. Receptor: protein or glycoprotein with a receptor site Example: neurotransmitters Gate is closed until neurotransmitter attaches to receptor molecule. When Ach (acetylcholine) attaches to receptor on muscle cell, Na gate opens. Na moves into cell due to concentration gradient Voltage-gated Open and close in response to small voltage changes across plasma membrane Each is specific for one type of ion

Action Potentials Phases Depolarization: Inside of plasma membrane becomes less negative. If change reaches threshold, depolarization occurs Repolarization: return of resting membrane potential. Note that during repolarization, the membrane potential drops lower than its original resting potential, then rebounds. This is because Na plus K together are higher, but then Na/K pump restores the resting potential All-or-none principle: like camera flash system Propagate: Spread from one location to another. Action potential does not move along the membrane: new action potential at each successive location. Frequency: number of action potential produced per unit of time

Gated Ion Channels and the Action Potential

Action Potential Propagation

Neuromuscular Junction Synapse: axon terminal resting in an invagination of the sarcolemma Neuromuscular junction (NMJ): Presynaptic terminal: axon terminal with synaptic vesicles Synaptic cleft: space Postsynaptic membrane or motor end-plate

Function of Neuromuscular Junction Synaptic vesicles Neurotransmitter: substance released from a presynaptic membrane that diffuses across the synaptic cleft and stimulates (or inhibits) the production of an action potential in the postsynaptic membrane. Acetylcholine Acetylcholinesterase: A degrading enzyme in synaptic cleft. Prevents accumulation of ACh

Excitation-Contraction Coupling Mechanism by which an action potential causes muscle fiber contraction Involves Sarcolemma Transverse (T) tubules: invaginations of sarcolemma Terminal cisternae Sarcoplasmic reticulum: smooth ER Triad: T tubule, two adjacent terminal cisternae Ca2+ Troponin

Action Potentials and Muscle Contraction

Cross-Bridge Movement

Relaxation Ca2+ moves back into sarcoplasmic reticulum by active transport. Requires energy Ca2+ moves away from troponin-tropomyosin complex Complex re-establishes its position and blocks binding sites.

Muscle Twitch Muscle contraction in response to a stimulus that causes action potential in one or more muscle fibers Muscle contraction measures as force, also called tension. Requires up to a second to occur. Phases Lag or latent (neuromuscular junction & step #1 of cross-bridge movement) Contraction (step #2 - #6 of cross-bridge movement) Relaxation (powerpoint slide # 28)

Stimulus Strength and Muscle Contraction All-or-none law for muscle fibers Contraction of equal force in response to each action potential Sub-threshold stimulus: no action potential; no contraction Threshold stimulus: action potential; contraction Stronger than threshold; action potential; contraction equal to that with threshold stimulus Motor units: a single motor neuron and all muscle fibers innervated by it

Contraction of the Whole Muscle Whole muscles exhibit characteristics that are more complex than those of individual muscle fibers or motor units. Instead of responding in an all-or-none fashion, whole muscles respond to stimuli in a graded fashion, which means that the strength of the contractions can range from weak to strong. Remember: There are many muscle fibers in one fasciculi and many fasciculi in one whole muscle. Strength of contraction in whole muscle is graded: ranges from weak to strong depending on stimulus strength Multiple motor unit summation: the force in which a whole muscle contracts depends on the number of motor units stimulated to contract. (force of contraction increases as more & more motor units are stimulated). A muscle has many motor units Submaximal stimuli Maximal stimulus Supramaximal stimuli

Contraction of the Whole Muscle

Stimulus Frequency and Muscle Contraction Relaxation of a muscle fiber is not required before a second action potential can stimulate a second contraction. As the frequency of action potentials increase, the frequency of contraction increases Incomplete tetanus: muscle fibers partially relax between contraction Complete tetanus: no relaxation between contractions Multiple-wave summation: muscle tension increases as contraction frequencies increase

Types of Muscle Contractions Isometric: no change in length of muscle but tension increases during contraction Postural muscles of body ex: muscles hold spine erect while person is sitting or standing Isotonic: change in length but tension constant ex: waving using computer keyboard Concentric: tension is so great it overcomes opposing resistance and muscle shortens ex: raising of a weight during a bicep curl. Eccentric: tension maintained but muscle lengthens ex: person slowly lowers a heavy weight Muscle tone: constant tension by muscles for long periods of time

Fatigue Decreased capacity to work and reduced efficiency of performance Types Psychological: depends on emotional state of individual ex: burst of activity in tired athlete in response to encouragement from spectators shows how psychological fatigue can be overcome Muscular: results from ATP depletion ex: fatigue in lower limbs of marathon runners or in upper & lower limbs of swimmers Synaptic: occurs in NMJ due to lack of acetylcholine ex: rare-----only under extreme exertion

Physiological Contracture and Rigor Mortis Physiological contracture: state of extreme fatigue (extreme exercise) where due to lack of ATP neither contraction nor relaxation can occur Rigor mortis: development of rigid muscles several hours after death. Ca2+ leaks into sarcoplasm and attaches to myosin heads and crossbridges form but no ATP available to bind to myosin---------so the cross-bridges are unable to release. Rigor ends as tissues start to deteriorate.

Energy Sources ATP provides immediate energy for muscle contractions. Produced from three sources Creatine phosphate During resting conditions stores energy to synthesize ATP ADP + Creatine phosphate------------------ Creatine + 1ATP (Creatine Kinase) Anaerobic respiration Occurs in absence of oxygen and results in breakdown of glucose to yield ATP and lactic acid Aerobic respiration Requires oxygen and breaks down glucose to produce ATP, carbon dioxide and water More efficient than anaerobic

Slow and Fast Fibers Slow-twitch oxidative Fast-twitch Contract more slowly, smaller in diameter, better blood supply, more mitochondria (also called oxidative because carry out aerobic respiration), more fatigue-resistant than fast-twitch, large amount of myoglobin (dark pigment which binds oxygen & acts as a muscle reservoir for oxygen when blood does not supply adequate amount). Postural muscles, more in lower than upper limbs. Dark meat of chicken. Functions: Maintenance of posture & performance in endurance activities. Fast-twitch Respond rapidly to nervous stimulation, contain myosin that can break down ATP more rapidly than that in Type I, less blood supply, fewer and smaller mitochondria than slow-twitch (adapted to perform anaerobic respiration) Lower limbs in sprinter, upper limbs of most people. White meat in chicken. Comes in oxidative and glycolytic forms Functions: Rapid, intense movements of short duration Distribution of fast-twitch and slow-twitch Most muscles have both but varies for each muscle Exercise: weight lifting enlarges fast-twitch & aerobic training enlarges slow-twitch Effects of exercise: change in size of muscle fibers Hypertrophy: increase in muscle size Increase in myofibrils Increase in nuclei due to fusion of satellite cells Increase in strength Atrophy: decrease in muscle size Reverse except in severe situations where cells die

Smooth Muscle Not striated, fibers smaller than those in skeletal muscle Spindle-shaped; single, central nucleus More actin than myosin Caveolae: indentations in sarcolemma; may act like T tubules Dense bodies instead of Z disks as in skeletal muscle; have noncontractile intermediate filaments Ca2+ required to initiate contractions; binds to calmodulin (protein). Calmodulin molecules with Ca++ bound to them activate an enzyme called myosin kinase, which transfers a phosphate group from ATP to heads of myosin molecules. Cross-bridging occurs Relaxation: caused by enzyme myosin phosphatase

Electrical Properties of Smooth Muscle Slow waves of depolarization and repolarization transferred from cell to cell Depolarization caused by spontaneous diffusion of Na+ and Ca2+ into cell Does not follow all-or-none law Contraction regulated by nervous system and by hormones (ex: epinephrine)

Regulation of Smooth Muscle Innervated by autonomic nervous system (composed of nerve fibers that send impulses from CNS to smooth muscle, cardiac muscle, glands) Neurotransmitters are acetylcholine and norepinephrine (increases cardiac output, blood glucose levels) Hormones important as epinephrine and oxytocin Receptors present on plasma membrane; which neurotransmitters or hormones bind determines response

Cardiac Muscle Found only in heart Striated Each cell usually has one nucleus Has intercalated disks and gap junctions Autorhythmic cells Action potentials of longer duration The depolarization of cardiac muscle results from influx of Na+ and Ca2+ across the plasma membrane

Effects of Aging on Skeletal Muscle Reduced muscle mass Increased time for muscle to contract in response to nervous stimuli Reduced stamina Increased recovery time Loss of muscle fibers