Ch. 17 The Cardiovascular System I: The Heart

Amerman - Human Anatomy & Physiology 2nd Edition

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Amerman 2nd Edition

Ch. 17 The Cardiovascular System I: The Heart

Problem L3.4

Chapter 17, Problem L3.4

An experimental toxin makes the refractory period of cardiac muscle cells equal in length to that of skeletal muscle fibers. Predict the consequences of this toxin.

Verified step by step guidance

Understand the refractory period: The refractory period is the time during which a cell cannot initiate another action potential. In cardiac muscle cells, the refractory period is much longer than in skeletal muscle fibers, ensuring that the heart has time to contract and relax fully before the next contraction.

Compare cardiac and skeletal muscle refractory periods: Cardiac muscle cells have a refractory period lasting almost as long as the contraction itself, preventing tetanus (sustained contraction). Skeletal muscle fibers, on the other hand, have a much shorter refractory period, allowing for rapid and repeated contractions.

Predict the effect of the toxin: If the toxin shortens the refractory period of cardiac muscle cells to match that of skeletal muscle fibers, the heart could potentially experience tetanic contractions. This would disrupt the normal rhythmic pumping action of the heart, as the heart would not have time to relax between contractions.

Consider physiological consequences: Tetanic contractions in the heart would impair its ability to pump blood effectively, leading to reduced oxygen and nutrient delivery to tissues and potentially causing severe cardiovascular complications, such as heart failure or arrhythmias.

Relate to clinical implications: This scenario highlights the importance of the long refractory period in cardiac muscle cells for maintaining proper heart function. Any disruption to this mechanism, such as through toxins or diseases, could have life-threatening consequences.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Refractory Period

The refractory period is the time during which a cell is unable to respond to a second stimulus after an initial action potential. In cardiac muscle cells, this period is longer than in skeletal muscle fibers, preventing tetanus and allowing the heart to fill with blood between beats. If the refractory period is equalized with that of skeletal muscle, it could lead to continuous contractions, compromising the heart's ability to pump effectively.

Cardiac Muscle Physiology

Cardiac muscle is specialized for continuous, rhythmic contractions and has unique properties, including automaticity and a prolonged refractory period. This allows the heart to maintain a steady rhythm and prevents arrhythmias. Altering the refractory period to match that of skeletal muscle could disrupt this rhythm, leading to potentially fatal arrhythmias or ineffective heart function.

Tetanus in Muscle Contraction

Tetanus refers to a sustained muscle contraction resulting from rapid stimulation without relaxation. In skeletal muscle, this can enhance force production, but in cardiac muscle, it is detrimental. If cardiac muscle cells experience tetanus due to a shortened refractory period, the heart would not relax properly, leading to inadequate blood flow and severe cardiovascular complications.

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How do pacemaker cardiac muscle cells differ from contractile cardiac muscle cells? What is autorhythmicity?

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Which of the following statements is true?

a. The tricuspid valve is located between the right atrium and the right ventricle.

b. The mitral valve is located between the pulmonary veins and the left atrium.

c. The pulmonary valve is located between the pulmonary artery and the pulmonary veins.

d. The aortic valve is located between the right ventricle and the aorta.

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A newer drug, ivabradine, lowers the heart rate by blocking the nonselective HCN cation channels. Why would this action decrease the heart rate? Would this drug have an effect on pacemaker cells, contractile cells, or both? Explain.

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Cardiac muscle cells are joined by structures called:

a. T-tubules.

b. tight junctions.

c. sarcoplasmic reticulum.

d. intercalated discs.

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You are an athletic trainer who is working with someone planning to run a marathon. Your trainee tells you to give him a workout that will make his heart 'beat faster than ever before.' What do you tell him about the effects of too rapid a heart rate?

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