The Heart's Electrical System: Intrinsic and Extrinsic Regulation

Exam Tip: Remember that the intrinsic system is made of specialized muscle cells, not nerves. If a question asks what generates the heartbeat without nervous system input, the answer is the intrinsic conduction system.

The Extrinsic Conduction System: External Regulation

While the heart can beat on its own, it does not always beat at the same rate. To meet the physiological demands of the body—such as increasing oxygen delivery during a "fight or flight" response—the extrinsic conduction system provides external modulation via the autonomic nervous system.

Sympathetic vs. Parasympathetic Input

The extrinsic system works through two opposing branches:

• Sympathetic System: This is the "accelerator." When activated, sympathetic nerves release the neurotransmitter norepinephrine, which increases both the heart rate and the strength of the myocardial contraction.
• Parasympathetic System: This is the "brake." Controlled by the vagus nerve (CN X), this system releases acetylcholine, an inhibitory neurotransmitter that decreases the heart rate and relaxes the strength of contractions.

Clinical Monitoring: The Electrocardiogram (ECG)

Because the heart's contractions are driven by electrical impulses, clinicians can monitor cardiac health by recording these electrical signals using an Electrocardiogram (ECG). An ECG trace displays specific waves that correspond to different stages of the cardiac cycle:

• P wave: Represents atrial depolarization, which triggers the contraction of the atria.
• QRS complex: Represents ventricular depolarization. This is the large electrical spike that triggers ventricular systole (contraction).
• T wave: Represents ventricular repolarization, the period where the ventricles reset electrically to prepare for the next beat.

Heart Rate Deviations and Interventions

A normal resting heart rate typically falls between $60\text{--}100$ beats per minute (BPM). When the heart rate falls outside this range, it is classified as follows:

• Bradycardia: A heart rate that is too slow ($< 60$ BPM).
• Tachycardia: A heart rate that is too fast ($> 100$ BPM).

To manage these conditions, medical technology provides life-saving interventions. Pacemakers are electronic devices used to treat bradycardia or other conduction issues by sending electrical impulses to maintain a steady rhythm. In contrast, defibrillators are used to treat tachycardia and dangerous arrhythmias by delivering a controlled electrical shock to reset the heart's rhythm.

Clinical Note: If the coronary arteries (which supply blood to the myocardium) become blocked by a thrombus or embolus, the heart muscle can die in a myocardial infarction (heart attack). Because cardiac muscle cannot regenerate, it is replaced by scar tissue, which cannot conduct electrical impulses or contract, potentially disrupting the conduction system.