The Muscular System

Contraction of muscles **Note: An increase in the Ca2+ level in the sarcoplasm STARTS the sliding of thin filaments. (starts contraction)

1. A muscle action potential propagating along a transverse tubule opens Ca2+ release channels in the sarcoplasmic reticulum (SR), calcium ions flow into cytosol, and contraction begins. 2. Ca2+ release channels are open 3. Ca2+ binds to troponin, which changes the shape of the troponin-tropomyosin complex and uncovers the myosin-binding sites on actin.

Which proteins are present in the I band?

1. Actin 2. Troponin 3. Tropomyosin 4. Titin

What are three functions of ATP in muscle contraction?

1. Its hydrolysis by an ATPase activates the myosin head so it can bind to actin and rotate. 2. Its binding to myosin causes detachment from actin after the power stroke 3. It powers the pumps that transport Ca2+ from the cytosol back into the sarcoplasmic reticulum (SR).

Which proteins are present in the A band?

1. Myosin 2. Actin 3. Troponin 4. Tropomyosin 5. Titin

Relaxation of muscles **Note: A decrease in the Ca2+ level in the sarcoplasm STOPS the sliding of thin filaments. (stops contraction)

1. the level of Ca2+ in the sarcoplasm is low, because calcium ions are pumped into the sarcoplasmic reticulum (SR) by Ca2+ active transport pumps. 2. Ca2+ release channels are closed 3. Troponin holds tropomyosin in position to block myosin-binding sites on actin.

Which proteins connect into the Z disc?

Actin and Titin anchor into the Z disc.

What type of contraction occurs in your neck muscles while you are walking?

Holding your head upright without movement involves mainly isometric contractions.

Why is tension maximal at a sarcomere length of 2.2 u.m?

Gives a generous zone of overlap between the parts of the thick and thin filaments without the overlap being so extensive that sarcomere shortening is limited.

(Figure 10.11) Where inside a skeletal muscle fiber are the events shown here occurring? (Pg. 309)

Glycolysis, exchange of phosphate between creatine phosphate & ADP, and glycogen breakdown occur in the cytosol. Oxidation of pyruvic acid, amino acids, and fatty acids (aerobic respiration) occurs in mitochondria.

What would happen if ATP suddenly were not available after the sarcomere had started to shorten?

If ATP were not available, the cross-bridges would not be able to detach from actin. The muscle would remain in a state of rigidity, as occurs in rigor mortis.

(Figure 10.14) Would the peak force of the second contraction be larger or smaller if the second stimulus wave were applied a few milliseconds later? (Pg. 313)

If the second stimulus were applied a little later, the second contraction would be smaller.

What part of the sarcolemma contains acetylcholine receptors (ACh)?

Motor end plate

What is the effect of the size of a motor unit on its strength of contraction? **Note: Assume that each muscle fiber can generate about the same amount of tension.

Motor units having many muscle fibers are capable of more forceful contractions than those having only a few fibers.

Which of the following is largest: muscle fiber, thick filament, or myofibril?

Muscle Fiber

Which connective tissue coat surrounds groups of muscle fibers, separating them from fascicles?

Perimysium bundles groups of muscle fibers into fascicles.

(Figure 10.10) Which numbered steps in this figure are part of excitation-contraction coupling? (Pg. 308)

Steps 4-6. (Muscle action potential through binding of myosin head to actin). 4. Muscle AP traveling along transverse tubule opens Ca2+ release channels in the sarcoplasmic reticulum (SR) membrane, which allows calcium ions to flood into the sarcoplasm. 5. Ca2+ binds to troponin on the thin filament, exposing the binding sites for myosin. 6. Contraction: powers strokes use ATP; myosin heads bind to actin, swivel, & release; thin filaments are pulled toward center of sarcomere.

What happens to the I band and H zone as muscle contracts?

The I bands and H zones disappear during muscle contraction.

Do the lengths of thick and thin filaments change when a muscle contracts?

The lengths of thin & thick filaments DO NOT CHANGE. **Note: Thin & Thick filaments ONLY overlap each other.

What events occur during the latent period?

The muscle action potential sweeps over the sarcolemma and calcium ions are released from the sarcoplasmic reticulum (SR).

Which part of a somite differentiates into skeletal muscle?

The myotome of a somite differentiates into skeletal muscle.

Which structure releases calcium ions to trigger muscle contraction?

The sarcoplasmic reticulum (SR)

Which of the following is the smallest: muscle fiber, thick filament, or myofibril?

Thick Filament

Which type of smooth muscle is more like cardiac muscle, with respect to both its structure and function?

Visceral smooth muscle is more like cardiac muscle; both contain gap junctions, which allow action potentials to spread from each cell to its neighboring.