The period of time between the effective refractory period and phase 4 is the ‘relative refractory period’. The refractory period of cardiac muscle is about 250 milliseconds—almost as long as the contraction of the heart.
Summation does not occur in the heart muscle contraction. The refractory period of the heart muscles is longer than that in the skeletal muscles, therefore heart twitches do not overlap and hence increased (summed) ventricular contraction is not seen.
Summation would result in more powerful contraction of the ventricle causing irregular high cardiac output. Heart might find it diffult to recvoer from this ‘double-twitch’, i.e. repolarization might take a long time. This irregular pumping of heart would cause dysrhythmia.
Vagus nerves are the part of para-sympathetic nervous system and act by decreasing the heart rate by direct effect on the SA and AV nodes in the heart atrium. Parasympathetic stimulation by the vagal nerve endings in the atrium results in slowing down of the heart rate and can bring the cardiac output to almost zero.
Vagus nerves are parasympathetic and supply to AV and SA nodes. Parasympathetic stimulation result in decrease in the heart rate and cardiac output. Since the vagal fibers are primarily distributed in the atria, they result in a decrease in the heart rate. Decrease in the heart rate along with slight decrease in ventricular contraction can decrease the cardiac output by 50%.
The cardiac muscle has inherent autorhythmicity, i.e. they can generate the contraction activity automatically. However, the un-stimulated heart beats at a much slower rate and needs continuous sypathetic stimulation to maintain the nornal heart rate, i.e. 70 pulses a minute. Also, sympathetic stimulation increases the power of the ventricular contraction to as much as double the normal, thereby increasing the volume of blood pumped and increasing the ejection pressure. The vagal nerve endings