9-1

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.

9-2

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)

9-3

9-4

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

9-5

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

9-6

Contraction of the Whole Muscle

9-7

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

9-8

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

9-9

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

9-10

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.

9-11

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

9-12

Slow and Fast Fibers• Slow-twitch oxidative

– 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

9-13

9-14

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

9-15

9-16

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)

9-17

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

9-18

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

9-19

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