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