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DIRECTIONS: Each of the numbered items or incomplete statements in this section is followed by answers or completions of the statement. Select the ONE lettered answer or completion that is BEST in each case.

1. Skeletal, smooth, and cardiac muscle all have which of the following in common?

(A) Their cellular structure is based on repeating sarcomeres

(B) The contractile cells are large relative to the size of the organ they comprise.

(C) The contractile system is based on an enzymatic interaction of actin and myosin.

(D) Initiation of contraction requires the binding of calcium ions to actin filaments

2. During the shortening of skeletal muscle,

(A) The distance between Z lines stays the same

(B) The width of the I band changes

(C) The width of the A band changes

(D) All internal spacings between repeating structures change proportionately

3. The compound ATP provides the energy for muscle contraction during the crossbridge cycle. A second important function for ATP in the cycle is to

(A) Provide the energy for relaxation

(B) Allow the thick and thin filaments to detach from each other during the crossbridge cycle

(C) Maintain the separation of thick and thin filaments when the muscle is at rest

(D) Promote the binding of calcium ions to the regulatory proteins

4. Calcium ions are required for the normal activation of all muscle types. Which statement below most closely describes the role of calcium ions in the control of skeletal muscle contraction?

(A) The binding of calcium ions to regulatory proteins on the thin filaments removes the inhibition of actin-myosin interaction

(B) The binding of calcium ions to the thick filament regulatory proteins activates the enzymatic activity of the myosin molecules

(C) Calcium ions serve as an inhibitor of the interaction of thick and thin filaments

(D) A high concentration of calcium ions in the myofilament space is required to maintain muscle in a relaxed state.

5. The normal process of relaxation in skeletal muscle depends on

(A) A sudden reduction in the amount of ATP available for the crossbridge interactions

(B) Metabolically supported pumping of calcium out of the cells when the membrane potential repolarizes

(C) A rapid reuptake of calcium into the sarcoplasmic reticulum

(D) An external force to separate the interacting myofilaments

6. When an isolated skeletal muscle is stretched beyond its optimal length (but not to the point where damage occurs), the reduction in contractile force is due to

(A) Lengthening of the myofilaments so that crossbridges become spaced farther apart and can interact less readily

(B) Decreased overlap between thick and thin filaments, which reduces the number of crossbridges that interact

(C) The thinning of the muscle, which reduces its cross-sectional area and, hence, the force that it can produce

(D) A proportional reduction in the amount of calcium released from the sarcoplasmic reticulum

7. The major immediate source of calcium for the initiation of skeletal muscle contraction is

(A) Calcium entry through the sarcolemma during the passage of an action potential

(B) A rapid release of calcium from its storage sites in the T tubules

(C) A rapid release of calcium from the terminal cisternae of the sarcoplasmic reticulum

(D) A release of calcium that is bound to cytoplasmic proteins in the region of the myofilaments

8. The relaxation of skeletal muscle is associated with a reduction in free intracellular calcium ion concentration. The effect of this reduction is

(A) A reestablishment of the inhibition of the actin-myosin interaction

(B) Deactivation of the enzymatic activity of the individual actin molecules

(continued)

(C) A change in the chemical nature of the myosin molecules, reducing their enzymatic activity

(D) Reduced contractile interaction by the binding of calcium to the active sites of the myosin molecules

9. The chemical energy source that most directly supports muscle contraction is

(A) Creatine phosphate

(B) Glucose

(D) Free fatty acids

10.In the absence of an adequate supply of ATP for skeletal muscle contraction,

(A) Myofilament interaction ceases, and the muscle relaxes

(B) Actin and myosin filaments cannot separate, and the muscle stiffens

(C) Creatine phosphate can directly support myofilament interaction, although less efficiently

(D) The lower energy form, ADP, can support contraction at a reduced rate 11.In the face of insufficient oxygen to meet its current metabolic requirements, skeletal muscle

(A) Quickly loses its ability to contract and relaxes until oxygen is again available

(B) Maintains contraction by using metabolic pathways that do not require oxygen consumption

(C) Maintains contraction by using a large internal store of ATP that is kept in reserve

(D) Contracts more slowly at a given force, resulting in a saving of energy

12.If the calcium pumping ability of the sarcoplasmic reticulum were impaired (but not abolished),

(A) Muscles would relax more quickly because less calcium would be pumped

(B) Contraction would be slowed, but the muscle would relax normally

(C) The muscle would continue to develop force, but its relaxation would be slowed

(D) Activation of the muscle would no longer be possible

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