BIOL 307 Chp 13

Striated Muscle

Muscle tissue made of long thin cells with many nuclei, mostly under voluntary control; Skeletal; Light and dark bands.

Tendon

Where skeletal muscles attach to bones; Made of collagen.

Origin

The end of a muscle that is attached to the trunk or the more stationary bone.

Insertion

The more distal or mobile attachment of the muscle to a movable bond; Moves during contraction of muscle.

Antagonistic Muscles

“Flexor-extensor pairs," antagonistic muscle groups which exert opposite effects on the same set of bones.

Fascicle

Bundle of muscle fibers surrounded by connective tissue.

Sarcomere

The higher level packaging of repeating myofibrils; Specialized endoplasmic reticulum in muscle fibers.

Myofibrils

Highly organized bundles of contractile and elastic proteins that carry out the work of muscle contraction.

Sarcoplasmic Reticulum

A modified endoplasmic reticulum which wraps around the myofibril like a piece of lace, which concentrates, sequesters, and release Ca2+ for muscle contraction; Extend from one Z disk to the next.

T-tubule

Extension of the muscle cell membrane that associate with the ends of the sarcoplasmic reticulum; Carry electrical signals deep into muscle fibers.

Myosin

A motor protein with the ability to create movement. Contains two light chains and one heavy chain; Can hydrolyze ATP.

Actin

Globular proteins that makes up the thin filaments of muscle fiber; Binds to myosin.

Crossbridge

What connects the parallel thick myosin and thin actin filaments.

Power Stroke

Calcium moves troponin. When myosin heads with hydrolyzed ATP bind to actin molecules, causing the actin to be pushed towards the center of the sarcomere. Then, myosin releases actin when the hydrolyzed ATP is released and binds another ATP molecule to cause another contractile cycle.

Troponin

Complex of three proteins associated with tropomyosin.

Tropomyosin

Regulatory protein that blocks the myosin binding site on actin.

Ryanodine Receptor

Another name for the sarcoplasmic reticulum channels that release Ca2+.

Myoglobin

A red oxygen binding pigment which can bring oxygen more rapidly to the interior of the fibers for glucose metabolism.

Motor Unit

The basic unit of contraction in an intact skeletal muscle; Consists of one somatic motor neuron and all of the muscle fibers it innervates.

Summation

A more forceful contraction occurring because there is a short interval of time between action potentials, not giving the muscle cells enough time to relax.

Tetanus

A state of maximal contraction that occurs when action potentials continue to stimulate muscle fiber repeatedly.

Recruitment

The recruitment of additional motor units of skeletal muscle in order to increase force of attraction.

Pacemaker Potential

Found in cardiac muscle; depolarizations which create regular rhythms of contraction due to ion channels in the cell membrane which spontaneously open and close; Have regular depolarizations and always reach a threshold and fire an action potential.

Distinguish among skeletal, cardiac and smooth muscle in location and function.

Skeletal:

• Positioned on the bones of the skeleton, attached by tendons.

• Make up 40% body weight.

• Function to move the skeleton.

• Voluntary

• Regulated by somatic motor neurons.

Cardiac:

• Has features of both smooth and skeletal muscle.

• Located on the heart wall.

• Involved in pumping blood.

• Involuntary

• Regulated by pacemaker cells and autonomic nervous system.

Smooth:

• Important for maintaining homeostasis, through the movement of fluids and constructing tubes.

• Located in blood vessel walls, walls of digestive tract, walls of bladder and ureters, airway passages, uterus, and eyes.

• Involuntary

Recognize that a “muscle fiber” is simply a muscle cell.

Recognize that thin filaments are made of actin and thick filaments are made of myosin.

Describe the structure of a sarcomere including drawing the relationship among the thick and thin fibers and explaining/drawing how that relationship changes in a contracting muscle (sliding filament theory).

• The contractile unit of the myofibril.

• Contains regions with only actin on the ends, only myosin towards the center, and regions where these two overlap in between to contract the muscle.

• Sliding Filament Theory: overlapping actin and myosin filaments of fixed length slide past each other in an energy requiring process, resulting in muscle contraction.

  • Ca 2+ binds to troponin, moving tropomyosin away from the binding sites on actin.

  • Myosin heads attached to actin to form a crossbridge.

  • Power stroke pulls thin filaments towards the M line, shortening the sarcomere.

  • ATP binds to myosin to cause crossbridge detachment.

  • ATP hydrolysis re-cocks myosin heads for another cycle.

Explain the steps of what happens in the contraction cycle (Fig 13.9) and predict what would happen in response to a particular change.

• ATP binds to myosin, which causes myosin to release actin.

• Myosin hydrolyzes ATP, and the energy from ATP rotates myosin heads toward cocked position and myosin weakly binds to actin.

• Power stroke begins when tropomyosin moves off binding site.

• Myosin releases ADP at end of power stroke.

Explain the steps of what happens in excitation-contraction coupling (Fig 13.10) and predict what would happen in response to a particular change.

1. Somatic motor neuron releases Ach at neuromuscular junction.

2. Net entry of Na+ through Ach receptor channel initiates muscle action potential.

3. Action potential in T-tubule alters conformation of DHP receptor.

4. DHP receptor opens RyR and Ca2+ release channels in SR open causing Ca2+ to enter cytoplasm.

5. Ca2+ binds to troponin allowing actin-myosin binding.

6. Myosin heads cause power stroke.

7. Actin filaments slide towards center of sarcomere.

8. Sarcoplasmic Ca2+ ATPase pumps Ca2+ back into SR.

9. Decrease in free cytosolic Ca2+ causes Ca2+ to unbind from troponin.

10. Tropomyosin recovers binding site when myosin heads release.

Identify which processes in muscle contraction require ATP.

• Crossbridge detachment.

• Cocking of myosin heads.

• Pumping of Ca2+ into SR.

Recognize that muscle cells generally use a mixture of glucose and fatty acids to

produce ATP.

Describe several differences between slow-twitch oxidative and fast-twitch glycolytic muscle fibers, explaining how these differences relate to function.

Slow:

• Smaller in diameter.

• Darker due to more myoglobin.

• Many mitochondria.

• Extensive capillary blood supply.

• Fatigue resistant.

• For more low intensity/leisurely activities.

• Involved in endurance.

• Aerobic for ATP production.

Fast:

• Larger diameter.

• Few mitochondria.

• Pale in color.

• Easily fatigued.

• For more high intensity activities.

• Short, powerful bursts.

Describe the length-tension relationship (Fig 13.15) and explain why this relationship exists.

• Too much or too little overlap of thick and thin filaments in resting muscle results in decreased tension.

  • Too little overlap: Few cross bridges formed, sliding filaments only interact minimally

  • Too much overlap: Thick filaments can only move the thin filaments a short distance, preventing crossbridge formation

Explain how contraction force can be varied within a single muscle fiber (summation/tetanus).

• Increase of stimulation frequency increases intracellular Ca2+ resulting in greater force produced.

  • Unfused Tetanus: Tension oscillates but stays elevated (partial relaxation between stimuli).

  • Fused Tetanus: Stimuli are so rapid Ca2+ stays high constantly leading to relaxation to reach maximal steady force generation.

• Prolonged tetanus can lead to fatigue as ATP is depleted leading to loss of force over time.

Explain how recruitment provides different levels of force within the a whole muscle.

• It allows additional motor units to be recruited to increase the force of contraction.

• Muscle contraction is caused by two variables, the types of motor subunits active, and the number of motor subunits responding at a given time.

• Weak stimulus only activates low-threshold, fatigue resistant slow twitch fibers. → if the stimulus increases in strength, higher threshold motor units with fatigue resistant fast twitch fibers are activated → if even more stimulus is present, glycolytic fast twitch fibers are activated.

Give examples of several locations in the body where smooth muscle is located, providing its function at that location (usually squeezing some kind of fluid around!)

• Walls of Digestive Tract: Peristalsis (wavelike contraction and loosening to push contents out).

• Blood vessels: Regulation of blood pressure.

• Uterus: Contractions during child birth to push baby out.

Recognize that smooth muscle does not have sarcomeres because of its need to be able to contract when stretched.

Recognize that smooth muscle generally contracts without nervous system input, but can be regulated by the autonomic nervous system.

Provide the basic steps of contraction in smooth muscle Fig 13.26a).

• Ca2+ enters cell from ECF or SR.

• Ca2+ binds to calmodulin (CaM).

• Binding activates myosin light chain kinase (MLCK).

• MLCK phosphorylates light chains in myosin heads and increases myosin ATPase activity.

  • Results in increased contraction ability.

Recognize that cardiac muscle has sarcomeres like skeletal muscle, but generally contracts without nervous system input and is regulated by the ANS, like smooth muscle.