How does the length of time of the absolute refractory period of cardiac muscle

Question

How does the length of time of the absolute refractory period of cardiac muscle compare to skeletal muscle and why? a. The absolute refractory period is much longer in cardiac muscle compared to skeletal muscle. This is because the heart cannot produce tetanus and cannot fatigue as a result of the longer absolute refractory period. b. The absolute refractory period is much longer in skeletal muscle compared to cardiac muscle because skeletal muscle must rest in between contractions. c. The absolute refractory period is the same in skeletal and cardiac muscle because they are both muscle types and have the same properties. d. The absolute refractory period is much longer in cardiac muscle compared to skeletal muscle because this allows the heart to be stimulated faster if necessary for the sympathetic nervous system. During atrial systole: a. The atrial pressure exceeds ventricular pressure. b. The AV valves are shut. c. Valves prevent backflow into the vena cava. What is the effect of vagal nerve stimulation on heart rate? Does the vagal nerve represent parasympathetic or sympathetic nervous system influence on the heart? a. Decrease in heart rate, Parasympathetic b. Increase in heart rate, Sympathetic c. Decrease in heart rate, Sympathetic d. Increase in heart rate, Parasympathetic

Explanation / Answer

Question 1 ans- a. The absolute refractory period is much longer in cardiac muscle compared to skeletal muscle. This is because the heart cannot produce tetany and fatigue as a result of the longer refractory period.

Explanation- Refractory period in excitable cells mean: Inability of the cell to respond to any stimulus (abolishing of excitability) during a period of time. In skeletal muscle cells, refractory period is quite short (1-2 ms). In cardiac muscle cells, there are absolute and relative refractory periods . The cardiac excitability is totally cancelled in absolute refractory period. It prolongs from the beginning of the depolarization phase till the end of plateau phase ( about 250 -300 ms ) about hundred times longer that of the skeletal muscle`s refractory period. Thus, this period coincides with the systole (contraction) of the cardiac muscle. And due to which in absolute period an other contraction would never occur except after completion of the systole. Hence, tetanisation of cardiac muscle is impossible .

Question 2 ans- a. The atrial pressure exceeds ventricular pressure.

Explanation- The AV valves are open. They are one-way valves that prevents backflow of blood into the atrium but allows blood to flow into the ventricle . This maintains the movement of blood forward, toward the systemic circulation.

Question 3 ans- a. Decrease in heart rate, Parasympathetic

Parasympathetic innervation of the heart is partially controlled by the vagus nerve and is shared by the thoracic ganglia. Vagal nerves as well as spinal ganglionic nerves help by lowering heart rate.

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