Which statement is NOT true regarding cardiac muscle? A. Intercalated disks cont

Question

Which statement is NOT true regarding cardiac muscle? A. Intercalated disks contain both desmosomes for firm attachment and gap junctions to allow communication. B. Cardiac muscle cells rely on a sarcoplasmic reticulum for storage of Ca^2+ ions. C. Cardiac muscle cells must obey the all-or-none law of contraction. D. The oxygen demand of cardiac muscle cells is high because they have many mitochondria. E. Calcium-induced calcium release is the process by which extracellular Ca^2+ triggers the release of stored Ca^2+ from the sarcoplasmic reticulum. Which of the following negatively influence the contractility of the myocardium, thus decreasing stroke volume? A. Increased preload primarily to an increase in end diastolic volume B. Increased epinephrine release C. Decreased sympathetic activity D. None of the above We can use an ECG to predict the mechanical events of the heart. The T-wave corresponds to which of the following mechanical events? A. Ventricular depolarization B. Relaxation of the ventricular muscle C. Ventricular repolarization D. B and C E. A and C

Explanation / Answer

1. Which statement is NOT true regarding cardiac muscle?

Ans: Cardiac muscle cells must obey the all-or-none law of contraction.

2. A) Preload is the "initial stretch" on a muscle fiber:
Try 2 preload experiments on your hand muscles:
1. (low preload) hold a grape, squeeze it.
2. (high preload) hold a mellon squeeze it.

In which of these two examples were you able to generate the greater difference between the initial vs. final stretch of your muscle fibers? I'll bet you could squeeze the grape more!

NOW if a heart is in CHF, Congestive Heart FAILURE (i.e. high preload state, thus high end diastolic volume) will it be able to generate a more or less powerful contraction vs. if the heart is operating under NORMAL filling pressures. (a lower end diastolic volume) --> (It will be a lower contractility in CHF because the myocardial fibers are in such a high initial stretch)

B) Epinephrine is a beta agonist... positive chronotropy (increase heart rate); positive ionotropy (increase contractility)

C) Decreased sympathetic activity = decreased epinephrine

Ans: Hence, option (A) is correct.

3. In electrocardiography, the T wave is referred to as the repolarization period of the ventricles because it represents the period in which the cardiac muscle cells of the heart can relax and prepare for the next contraction. It is essential for correct heart function and rhythm.

Ans: Hence, the correct option is (D) B and C.

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