QUESTION: 213

Which of the following statements concerning skeletal muscle is correct?

active tension depends on the extent of overlap between thick and thin filaments
during contraction, thin filaments shorten and thick filaments maintain constant length
significant passive tension is generated at normal rest fiber length
tension generation depends on myosin attachment to the z line
velocity of shortening is independent of fiber load

Answer(s): A

Explanation:

Tension generation depends on attachment of the myosin head group to actin, this is referred to as cross bridge formation. The amount of tension that can be generated in response to a stimulus depends, in part, on the number of cross bridge attachments that can be formed. Since myosin molecules form the thick filament and actin comprises the thin filament as overlap between thick and thin filaments increases more myosin head groups can bind to actin and more cross bridges can form. Because of this, active tension generation depends on the extent of overlap of thick and thin filaments. Because skeletal muscle is quite compliant, very little passive tension is encountered until the muscle is stretched almost to its maximal length (choice C). During contraction neither the thick nor the thin filaments shortens (choice B).
Contraction occurs as thick and thin filaments slide past each other. Velocity of shortening decreases with increasing load (choice E). In skeletal muscle, thin filaments attach to the Z line (choice D).

Reveal Solution Next Question

QUESTION: 214

A 55-year-old male with a history of smoking presents with a chronic cough with expectoration. He exhibits dyspnea on exertion, has a stocky build, and shows some cyanosis. Which of the following receptors is involved in the cough reflex?

carotid body chemoreceptors
central chemoreceptors
irritant receptors
lung stretch receptors
muscle spindles

Answer(s): C

Explanation:

Foreign agents react with pulmonary irritant receptors, which participate in the cough reflex. Carotid body and central chemoreceptors (choices A and B) supply sensory information on arterial blood gas ( and C ) composition and thus modulate respiratory rate. Stretch receptors in the lung (choice D) provide afferent information on the depth of breathing and can also influence respiration. Information from peripheral muscle spindles (choice E) can also modulate ventilation and is probably very important in setting the level of increased ventilation seen in moderate physical activity.

Reveal Solution Next Question

QUESTION: 215

Which of the following is an adaptive response to moving from sea level to higher elevation?

bronchial relaxation
decreased cardiac output
decreased circulating levels of EPO
decreased levels of 2, 3-diphosphoglycerate (2, 3-DPG) in erythrocytes
hyperventilation

Answer(s): E

Explanation:

At high elevation the decreased atmospheric pressure means that there is less available . Decreased arterial P is sensed by peripheral chemoreceptors with a resultant increase in ventilation. Hyperventilation helps to bring alveolar and thus arterial levels back toward normal. In hypoxic conditions, erythrocyte 2, 3-DPG levels (choice D) increase. This causes the oxygen-hemoglobin dissociation curve to shift to the right--thereby delivering a greater fraction of hemoglobin bound to the tissues. At high elevation EPO levels (choice C) increase because the kidneys sense low-tissue oxygenation and respond by releasing EPO into the circulation. This hormone stimulates erythrocyte production thereby increasing total blood hemoglobin levels, helping to offset the hypoxia. In low-conditions slight bronchoconstriction (choice A) can occur This is an adaptive response to hypoxia, since it will decrease dead space volume. With decreased dead space volume, a greater fraction of each breath contributes to alveolar ventilation, with more delivered to the system. In hypoxic conditions cardiac output (choice B) increases to deliver more blood.

Reveal Solution Next Question

QUESTION: 216

A 20-year-old female tennis player has just won a tennis match on a warm summer day. Her blood pressure at this time is 135/70 with a heart rate of 140 beats per minute and a respiratory rate of 25 per minute. She is flushed and sweating profusely. Compared to the resting state, what can be said about the level of activity of sympathetic nerves to her heart and to her cutaneous vasculature?

both are increased
both are decreased
neither is different from at rest
sympathetic activity to the heart is decreased while that to the cutaneous vasculature is increased
sympathetic activity to the heart is increased while that to the cutaneous vasculature is decreased

Answer(s): E

Explanation:

During exercise in the heat, blood flow must increase to the actively contracting muscles, as well as the cutaneous vessels to shunt heat from the interior of the body to the skin at the surface of the body.
Activation of sweat glands allows cooling via evaporation from the surface of the skin. To increase blood flow to the muscles and the skin, it is necessary to increase sympathetic tone to the heart to increase cardiac output. Most sweat glands are activated by sympathetic cholinergic nerves that are activated during sweating. However, the cutaneous vasculature involved in bringing warm blood to the surface of the skin for cooling is constricted by a strong sympathetic tone at rest. During exercise, when body cooling is necessary, sympathetic tone to these cutaneous capillary loops is reduced, causing vasodilation and increased cutaneous blood flow, thus choice E is appropriate.

Reveal Solution Next Question

Free STEP1 Exam Braindumps (page: 54)

QUESTION: 213

Explanation:

QUESTION: 214

Explanation:

QUESTION: 215

Explanation:

QUESTION: 216

Explanation: