Distinguish between the internal and external controls of the heartbeat. Explain how an ECG relates to the cardiac cycle. (pages \(97-99\) )

Internal controls are mechanisms within the heart tissue itself that regulate the heartbeat. In contrast, external controls are factors from outside the heart that affect its rate and force of contraction (such as hormones or signals from the nervous system).

The main internal control of the heartbeat is the Sinoatrial (SA) node, also known as the pacemaker of the heart. The SA node spontaneously generates electrical signals that spread throughout the heart muscle, causing it to contract in a coordinated manner. Another internal control is the Atrioventricular (AV) node, which briefly delays the electrical signal from the SA node before it reaches the ventricles. This delay ensures that the atria contract before the ventricles, allowing the blood to flow in the correct direction.

External controls of heartbeat can be classified into two categories: neural and hormonal. 1. Neural controls: The autonomic nervous system, which consists of the sympathetic and parasympathetic divisions, regulates the heartbeat. The sympathetic nervous system increases heart rate and force of contraction in response to stress or increased physical activity, while the parasympathetic nervous system slows down the heart rate during rest and recovery. 2. Hormonal controls: Various hormones, such as adrenaline (epinephrine), norepinephrine, and thyroid hormones, can influence the heart rate and force of contraction. These hormones affect the heart's sensitivity to the signals from the nervous system and help in maintaining an appropriate heart rate in response to changing body demands.

An Electrocardiogram (ECG) is a graphical representation of the electrical activity of the heart during one heartbeat. The cardiac cycle refers to the sequence of mechanical events that take place during a complete heartbeat, including the contraction (systole) and relaxation (diastole) of the heart chambers.

Each waveform on an ECG corresponds to a specific event in the cardiac cycle. The P wave represents atrial depolarization (atrial contraction), the QRS complex represents ventricular depolarization (ventricular contraction), and the T wave represents ventricular repolarization (ventricular relaxation). By interpreting the ECG waveform, one can determine the heart's rhythm, rate, and how efficiently the electrical signals are propagating throughout the heart muscle. This information helps in identifying any abnormalities or dysfunction in the cardiac cycle, which could be an indication of underlying heart problems.