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Last updated date: 26th Nov 2023
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MVSAT Dec 2023

Identify the correct statement regarding cardiac activity
- Normal activities of the human heart are regulated intrinsically, hence it is neurogenic.
- A special neural centre in the medulla oblongata can moderate the cardiac function through CNS.
- Parasympathetic neural signals increase the rate of heartbeat.
- Adrenal medullary hormones can increase cardiac output.
- The end of the T-wave marks the end of diastole.

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Hint: Heart is made up of cardiac muscle. It contains a network of specialised cardiac muscle fibres called autorhythmic fibers because they are self-excitable. These fibers repeatedly generate action potentials that trigger heart contractions. The extrinsic control of heartbeat is done by neural and hormonal regulation.

Complete Answer:
- The components of the conduction system that are: sinoatrial node (SA), atrioventricular node (AV), bundle of His, and Purkinje fibers ensure that the chambers of heart contract in a coordinated manner. The rhythmical contraction of the atria and ventricles is because of the intrinsic conduction system of the heart. Hence, the human heart is a myogenic heart which does not require outside stimulations.
- The extrinsic control of the heartbeat is regulated by neural signals and hormones. Medulla oblongata is the centre of heart beat regulation in the brain. Neural signals through sympathetic nerves can increase the rate of heartbeat and through parasympathetic nerves, the rate of heart beat can be decreased. Both these nerves are a part of the autonomic nervous system (ANS).
- Hormonal regulation is done by thyroxine and by hormones of adrenal medulla. Thyroxine, secreted by thyroid gland, increases the heartbeat indirectly by increasing the basal metabolic rate which requires more oxygen. Adrenal medulla secretes adrenaline and noradrenaline hormone.
- Adrenaline increases the heartbeat during an emergency whereas noradrenaline increases heartbeat under normal condition. Both ways, the rate of heartbeat is increased, the strength of ventricular contraction and thereby the cardiac output.

Hence, the correct option is Option D i.e. ‘Adrenal medullary hormones can increase cardiac output’.

The rate and rhythm of the heartbeat can be measured by an electrocardiogram. The ECG waves produced by each cardiac cycle are P, ORS, and T. The P wave represents atrial depolarization, QRS shows the onset of contraction of the ventricles and the T wave shows the ventricular repolarization.

Note: The hormones secreted by the adrenal medulla has the same effect as the sympathetic signals i.e., they increase the heartbeat. Adrenaline has a greater effect on the heart while nor-adrenaline regulates the blood vessel diameter.